- Email: [email protected]
Status and future of high energy diffuse gamma-ray astronomy
4, 1983 Adv. Space Res. Vol.3, No.4, pp.5l Printed in Great Britain. All rights reserved.
0273—1177/83 $0.00 Copyright
+ .50 ©COSPAR
STATUS AND FUTURE OF HIGH ENERGY DIFFUSE GAMMA-RAY ASTRONOMY C. E. Fichtel NASA/Goddard Space Flight Center, Greenbelt, MD, U.S.A.
ABSTRACT
There are two distinctly different high energy diffuse 1—ray components, one well correlated with broad galactic features and the other apparently isotropic and presumably extragalac— tic. The observed diffuse galactic high energy y—radiation is generally thought to be produced in interactions between the cosmic rays and the interstellar matter and photons. It should then ultimately be possible to obtain from the diffuse galactic emission a detailed picture of the galactic cosmic—ray distribution, a high contrast view of the general structure of the galaxy, and further insight into molecular clouds. Two of the candidates for the explanation of the extragalactic diffuse radiation are the sum of emission from active galaxies and matter—antimatter annihilation. A major advancement in the study of the properties of both galactic and extragalactic y radiation should occur over the next decade. INTRODUCTION Assuming cosmic rays pervade the Galaxy, they necessarily produce high energy y—rays as they interact with the interstellar matter and photons. The diffuse galactic high energy y radiation thus created may be used to assist in understanding the nature of the interstellar medium, galactic structure, and galactic cosmic rays, as was realized well before the diffuse galactic y radiation was observed. A low level, apparently isotropic and extragalactic radiation, has now also been seen. It is characterized by an intensity and energy spectrum which eliminated all, but a few of the potential theoretical explanations for it. DIFFUSE GALACTIC HIGH ENERGY GA}~MA RADIATION Interaction Processes Within our galaxy, cosmic rays interact with matter, photons, and magnetic fields, in each case producing i rays. The cosmic ray nucleon interactions give rise to y rays primarily through the decay of ir° mesons, giving a unique spectrum with a maximum at approximately 68 MeV. Cosmic ray electrons produce y rays through bremsstrahlung, but with a markedly different energy spectral shape, one which decreases monotonically with energy. Cosmic ray electrons also interact with the interstellar starlight, optical and infrared, and the blackbody radiation through the Compton process. Finally, cosmic ray electrons can interact with magnetic fields giving rise to synchrotron or curvature radiation, but these processes are ns.ich less important than the others previously mentioned for the galactic diffuse radiation and will not be discussed further here. Cosmic ray nucleon, interstellar matter interactions. High energy cosmic ray nuclei interacting with the ir~terstellar medium lead to several secondaries which decay to give high energy i rays. The detailed calculations leading to the predicted intensity and energy spectrum of the ‘~ rays (based on the average numbers of mesons formed in an interaction, their angular distribution, and the resulting energy spectrum) are quite lengthy. This is due to the need to study the many different products, to take into account the different cosmic ray species (protons, helium nuclei, and heavier particles) and interstellar nuclei in the correct proportions, follow their decay, integrate over all angles, and then integrate over the cosmic ray energy spectrum. These calculations have, however, been performed. See, for example, Cavallo and Gould [1], Stecker (2], Badwar and Stephens [3], and Morris [4]. The recent work of Norris [4] appears to be quite thorough, incorporating the existing experimental results into a semi—empirical model in a complete and consistent fashion.
5
6
C.E. Fitchel
Cosmic ray electron interactions. As cosmic ray electrons pass through the interstellar medium they product photons over a wide range of energies. In the case of bremsstrahlung, the radiation from interactions of energetic with matter, calculation the 2 MeV iselectrons rather uncertain eventhe locally in our of galaxy radiation in the region below about i0 because the interstellar cosmic ray electron spectrum is not well known at low energies where the electron spectrum observed near the Earth has undergone strong solar modulation. This problem did not arise in the case of comic ray nucleon matter interactions because there the higher energy cosmic rays are most significant in the production of ~ rays, and their spectrum and general composition are well known and not affected markedly by solar modulation. The cosmic ray electron, matter 1—ray production can be calculated using the bremsstralung cross—section formulas of Koch and Motz [5]. The calculations in general are very complex. Futher, considering the uncertainties in the molecular hydrogen density and in the electron spectrum, it seems possible only to give a range for the local source functions for brems— strahlung, which are estimated as ~b LE > 100 MeV) = ~4 to 8) x io—26 photons cm3s’ and Q (35 < E < 100 14eV) = (6 to 15) x 10 2 photons cm3s (from Fichtel and Trombka, [6], base on the earlier work of Fichtel et al. [7], and Koch and Motz, [5]). Although this source function is smaller than the cosmic ray nucleon one above 100 14eV, it dominates at lower energies and, in fact, becomes increasingly important the lower 1—ray energy. Even for simple models, the cosmic ray electron spectrum is not expected to have exactly the same shape everywhere in the galaxy because more secondary electrons are produced in regions of greater matter density. Cosmic ray electrons also interact with starlight photons, for which both the optical and infrared ranges are important, and with the blackbody radiation to produce Compton ‘r rays. The source functions of these interactions are much smaller in the galactic plane in the vicinity of the solar system. The total contribution to the galactic I radiation, however, is significant because the cosmic ray and stellar photon scale heights above the galactic plane are much greater than those of the matter, and, of course, the blackbody photon density is uniform. Hence, the integral intensity along a line of sight is closer to that of the bremsstrahlung than the source functions above would imply. The calculations associated with the producton of Compton y rays are again quite complex~ however, they have been performed in some detail for the cases of astrophysical interest by Ginzburg and Syrovatskii [8]. Whereas, in the case of bremsstrahlung. the astrophysical I rays come predominantly from electrons of energies similar to those of the I rays, or only a factor of several higher in energy, for the Compton case, the energy of the parent electron is given approximately by the equation E
~
Ee )2 c,
(1)
where C 1, is the photon energy before the electron interaction. Therefore, if the interactions is with starlight where the typical photon energy is a few eV, in order for the I ray to be about a hundred MeV, Ee must,be several GeV. For the 30 blackbody radiation, the typical photon energy is about 8 x 10— eV; therefore, the parent electrons must be in the range of 2 x 10~ MeV to produce one hundred 14ev 1 rays. In the energy range above a GeV, there is no serious uncertainty in the electron spectrum due to there being little solar modulation. However, in the few GeV range, the electron energy spectrum changes shape; so the calculations must be performed carefully. At energies as high as a few hundred thousand MeV, the electron energy spectrum is less well known, and an increased uncertainty in the calculated 1—ray intensity is introduced.
Using the Compton scattering functions of Ginzburg and Syrovatskii [8] and the calculations 5ec (ESt > al. 100 [7],and May) = references (0.3 to 0.6~ x io’~6the photons ~c ~ function 14eV > Eis > estimated 100 14eV) of Fichtel therein), local cm3s’~ Comptonand source to (0.5 = be to 1.0) x 1026 photons cm”3s . The range of values arises partially from uncertainties in the electron spectrum and partially from a lack of knowledge of the photon density. The galactic distribution functions for optical and infrared photons to be used here are those developed by Kniffen and Fichtel [9]. Galactic Matter Distribution The relevant concern here is the galactic diffuse matter in the form of atoms, molecules, ions, and dust with which cosmic rays interact. The primary constituents are atomic and molecular hydrogen. Both are known to be confined to a narrow disk (- 0.1 kpc in scale height for atomic hydrogen) with the molecular hydrogen distribution apparently narrower than that of the atomic hydrogen by a factor of two (e.g. , Gordon and Burton [10] ; Solomon and Sanders [11]). Atomic hydrogen reveals its presence through the emision of the 21 cm line, which is produced by the hyperfine transition of this neutral atom. There remains some uncertainty
High Energy Diffuse Gamma—Ray Astronomy
7
of the density in the inner regions due to the need for the absorption correction. Although the translation of the observations into a galactic spatial distribution is difficult, on a broad scale the density profile shows a general spiral pattern. The density distribution of molecular hydrogen cannot be measured directly, but must be inferred from other measurements. At present), the best approach appears to be through the observations of the 2.6 mm spectral line of laCO, which shows the distribution of cold interstellar matter. The nature of this process makes the molecular hydrogen density distribution less certain than that of the atomic hydrogen. The average galactic radial distribution of molecular and atomic hydrogen deduced by Gordon and Burton [10] shows that the molecular hydrogen to atomic hydrogen ratio is much larger in the inner galaxy that it is in the outer galaxy; however, the absolute intensity of molecular hydrogen is still quite uncertain. For the work here, the molecular hydrogen density normalization is treated as an adjustable parameter in the range from that estimated by Gordon and Burton [10] to a factor of 2.5 smaller. On the basis of a high sample survey and observations in both the first and second quadrants of the glactic plane, Cohen et al. have recently shown the existence of the molecular counterparts of the five classical 21 cm spiral arms segments in these quadrants. The CO observations indicate that the great majority of the molecular hydrogen is in clouds. The recent work of Solomon and Sanders [11] has, in fact, suggested that the interstellar medium is dominated by massive cloud complexes. Galatic Cosmic Ray Distribution The radio continuum measurements and cosmic ray results support the picture of the cosmic rays having a large scale height (approximately 1 kpc) relative to the matter and several fundamental theoretical considerations (e.g., Parker. [13]) suggest that the cosmic ray density throughout the galaxy may generally be as large as could be contained under near— equilibium conditions. The energy density of the cosmic rays should then be larger where the matter density is larger on a coarse scale such as that of the galactic arms. On a smaller scale, the pressures of the cosmic ray gas and magnetic fields cause the cosmic ray gas and field system to expand between the large clouds through which the magnetic fields thread, probably eliminating local nonunformities and anisotropies. Gamma Ray Results and Their Interpretation The most intense celestial high energy I radiation observed is that from the galactic plane. This feature was observed first by the pioneering counter telescope flown on OSO—3 [14] and the major features of this galactic radiation have now been defined by measurements made with the SAS—2 and COS—B satellites [15, 16, 17, 18, 19]. The distribution of high energy (E > 100 14eV) I—ray intensity along the galactic plane summed over the latitude interval from —10°to +10°obtained with the SAS—2 I—ray telescope is shown in Figure 1 in
2.0
I
T1.5
I
‘~-
VELA (PSROh33-45}’
~
I t~I
CYGNUS X—3
CRAB
1~~’1~ I t”j,’ T~1‘~ ~ ii(~t~ WI III
(PSRO53I+21) y195+5
IT
!:‘.0 0.5
Iu~++
~
-
~
0 ~ 150
~
120
~
90
60
I
I
30
0
I
I
330 300 (DEGREES)
I
270
I
240
210
180
150
Flg.1 The high energy E > 100 MeV I—ray intensity as a function of longitude for —10° < b < 10° from the SAS—2 data [17] compared to the model discussed here (9, 20].
8
C.E. Fitchel
bins which are 2.5°wide in latitude.
Notice that the emission from the region 310° < 1 <
50° is particularly intense relative to the remainder of the galactic plane. This was expected on the basis of the known galactic structure and the expected sources of the diffuse I rays. Figure 2.
The results of COS—B have confirmed the same general features, as shown in
I ~__ _ _
_____
0.0 I
150
120
I
90
I
I
I
60
I
I
30
I
I
I
0
I
330
300
I
270
I
I
I
240
I
I
210
180
GALACTIC LONGITUDE I
Fig.2 Gamma ray intensity as a function of longitude averaged over the latitude range —10°< b < 10° from 70 MeV — 150 14eV, 150 MeV — 300 MeV, and 300 14eV — 5000 14eV from the COS—B data [21] compared to the model discussed here shown by the solid line [9, 20] and one example of a constant
cosmic ray distribution
shown by the
dashed
line.
Ibi < 90°) a major portion of the I radiation observed is now believed to be galactic on the basis of its correlation with the matter, galaxy Counts, and synchrotron radiation [22, 23] , as will be discussed in the next section. Even at high latitudes (10° <
Detailed distributions in galactic latitude for I rays above 70 14eV are given in the paper by Mayer—Hasselwander et al. [21]. In the range of galactic latitudes from 330°to 30°an enhancement is visible in the latitude range 5°< b < 20° relative to the range —20°< b < —5°. This enhancement seen already in the early SAS—2 data taken together with the excess seen at negative latitudes in the galactic anticenter has been interpreted as I—ray emission produced in the local concentration of clouds known as Gould’s Belt [16, 24] . The latitude distributions of 90° < 1 < 150° have a peak at about b = 2°, while the distribution for 250° < 1 < 300°has an excess at negative latitudes. This effect is seen in both the COS—B data [21] and SAS—2 data [17]. The offsets in the I—ray data are qualitatively similar to the “hat brim” effect visible In the radio observations and due to the large—scale warping of the galactic disk. The broad distribution in galactic latitude for the longitude intervals away from the galactic center provides strong evidence that the observed I rays in these directions are, for the most part, produced locally (within a few kpc), whereas the additional narrow distribution seen toward the inner parts of the galaxy implies that a large part of the emission comes from more distant (> 3 kpc) features. Except for the four strong sources identified in Figure 1, there is no significant evidence for a variation of the energy spectrum along the galactic plane, in either the SAS—2 [17] or COS—B [21] data. The energy spectrum obtained for the galactic center region is shown in FIgure 3. Before estimating the diffuse galactic I radiation to be expected from cosmic ray interactions with galactic matter and photons, it should be mentioned that there is probably also an unresolved point source contribution to the “diffuse” radiation measured by the SAS—2 and COS—B y—ray instruments since the limited angular resolution of these instruments does not permit the separation of point sources. It Is quite difficult to estimate this contribution; however, several factors suggest that point sources may not be a major contributor. These include the uniformity of the energy spectrum just discussed and, as will be seen, the y—ray luminosity of the galaxy and its distribution being about what would be expected from the diffuse sources. For the purpose of this paper, the reader is simply asked to keep in mind that there is some point source contribution yet to be determined which at least for the moment is assumed to be small, but not zero. Based on the source functions, the matter distributions, and the assumption that the cosmic rays pervade the galaxy, the expected I—ray intensity has been calculated. The results are somewhat model dependent and, after discussing the comparison with one model, some of the differences will be discussed. The model to be used as that of Kniffen and Fichtel [9]with
High Energy Diffuse Gamma—Ray Astronomy
their photon distributions and the assumption that the cosmic rays are correlated with the matter in the plane, but have a larger scale height. The authors of several other papers (See Fichtel and Trombka [6], chapter 5, for a
9
I
summary.) also come to the conclusion that the cosmic ray density is enhanced where the matter density is greatest, in
agreement with the concept of coupling deduced from theoretical considerations by Parker [13]. The spiral arm model of Georgelin and Georgelin [12] is used. The prediction of the I—ray intensity from this model assuming the molecular hydrogen to be approximately one—half that of Gordon and Burton [10] is compared to the SAS—2 and COS—B longitude distributions in Figures 1 and 2 and to the energy spectrum in the galactic center region in Figure 3. Considering the uncertainty in the point source contribution, and the mass distribution, the agreement in Figures 1 and 2 is quite reasonable. It also suggests that there are interesting details to look for in the future. Regarding Figures 2 and 3, there are two comments. First, if the older cosmic ray nucleon source function of Stecker [2] had been used, the agreement in the 300 MeV to 5000 14eV energy interval would be quite poor with the theory predicting 1.6 to 2 times as many I—rays in the center than observed. The second comment is that, although the intensities for the center and the sources (195,5), PSR 0531+21, PSR
id’
BREMSSTRAHLUNG
~,,~SAS-2
1~ ‘~
—
-6 0
\
-
,,~_-COS-D
NUCLEAR
‘~E
INTERACTIONS ~
I~
-
F
-
COMPTON
10
-
0 0
II
IIIiI~2
III~
ENERGY 1MeV)
Fig.3 Energy spectrum of the galactic I radiation for a region near the galactic center. The calculated spectra shown by a dashed line in the figure are based on the work of Kniffen and Fichtel [9] modified to include the recent nucleon—nucleon calculations of Morris [4] . The COS—B data are those of Mayer—Hasselwander et al. [22], and the SAS—2 data are those of
0833—45 measured with the SAS—2 and COS—B Hartman et al. [17]. instruments are in good agreement with each other, the COS—B instrument appears to see a somewhat larger (about 25%) anticenter diffuse intensity. The SAS—2 instrument, which sees the lower diffuse intensity, had essentially no background; the very small extragalactic isotropic background is not substracted in Figure 1. In the case of COS—B, which sees the higher diffuse intensity, the estimated background has been subtracted. In general, the variations with latitude at different longitudes near the plane predicted by the work of Kniffen and Fichtel [9] are in reasonable agreement with the data when all components are considered.
A constant cosmic ray density, as might be predicted in the universal cosmic ray model, predicts a rather small I—ray intensity from the central region as shown in Figure 2 especially if a normalization is chosen to avoid disagreement at intermediate longitudes and the anticenter. Hence, should it be correct, a relatively large point source contribution would be needed in the galactic center region. It has already been noted that the majority of matter is now thought to be in large clouds. If this should be the case, I—ray astronomy should be able to contribute much to our understanding of the subject in the local region of the galaxy. Although there has been substantial theoretical work in the production of I rays by comic rays specifically in clouds (see, for example, Wolfendale [26]) since the paper by Black and Fazio [27], there are only two I—ray identifications which appear to be reasonably firm, namely the Orion Complex [28] and the region near p Oph [29]. Figure 4 compares the COS—B I—ray data with radio millimeter data (in the CO line) of Columbia University for the Orion region. The correlation is clearly encouraging. The intensity is about what is expected on the basis of the local cosmic ray density and the estimated matter density (see, for example, Issa and Wolfendale [30]; and Morris [4]). In addition to individual clouds, there are local belts. Gould’s belt was established with SAS—2 satellite [16]. COS—B results have not only confirmed the existence of this belt in I rays, but also show the existence of the Dolidze [31] belt in I rays [32] These results suggest that I—ray astronomy will not only help in unraveling the general structure of the galaxy, but also in improving the understanding of galactic clouds, their
JASR 3/4—B
10
C.E. Fitchel
relation to cosmic rays, and the origin and propagation of cosmic rays, and in providing a fuller picture of more general features locally in our galaxy. DIFFUSE ISOTROPIC RADIATION Experimental Observations At energies above 10 14eV,
: :.
the first
measurements related an extragalactic diffuse radiation wereto those of Kraushaar and Clark [33] , whose upper limits from Explorer 11 provided an experimental refutation of the steady—state theory of cosmology. Several other upper limits
It)
were diffuse experiments, reported high a energy but from theenergy early flux firstclearly balloon came suggestion from the established OSO—3 final 35 14eV) satellite results I—ray of experiment high experiment the SAS—2 extension high [14]. energy The of ofthe (> a
O’i
diffuse radiation with a steep energy spectrum above 35 14eV.
1o —15~ _______________________________ 20m 40m 20” 511 ‘•‘
The most controversial energy region has been from about 1/2 to 10 14eV, where there is a substantial local background resulting from cosmic ray interactions leading to excited nuclei. The analyses
~~i95o)
of Trombka et al. [34] and Daniel and Lavakare [35] have indicated that many of
the early reported positive results in this energy range were too high because of the failure to eliminate all the background from the measurements. At present, the final results from Apollo 15 and 16 I—ray detectors [34], which were on
Fig. 4 Comparison of the COS—B I—ray data shown by a thin line to the first CO isophote of peak antenna temperature shown by a thick line obtained by Columbia University for the Orion region. The figure is from Caraveo et al. [28].
a boom of variable length, are generally accepted as a good representation of the diffuse energy spectrum in the intermediate energy range. These data together with the recent work
of Sch5nfelder et al. [36] are shown in Figure 5. The high energy (> 35 14eV) region also deserves special mention not because of any significant detector or locally
I&
MARSHALL,
produced background, which can be very
small as in the case of SAS—2, but because of the need to separate the galactic diffuse radiation from the general diffuse radiation being discussed here.
Several
—
10 KINZER, JOHNSON & KURFESS 119781 POWER LAW FIT
~‘:~oMeKA
give
the
same result within the uncertainties,
M at lI~71
J
T~MPSOW& FICHTELII982b1
and the 150 Mhz brightness temperature were compared with the I radiation [22], and a study of the I—ray intensity as a function of galaxy counts and a combination of galaxy counts and a (1/sin All approaches
et at 119801
~
radiation data including a comparison of 1/sin b [39], analyses wherein other galactic radiation such as the 21 cm line
[23] .
01 119801
SCHONFELDER,
different approaches have been used to perform this separation on the SAS—2 I—
b) function
Ct
I
HIGH LATITUDE GALACTIC EMISSION
10°
101
102
103
I0~
I0~
IO~
PHOTON ENERGY (k~Vl
namely a steep spectrum which extrapolates
readily back through the measured low energy I ray intensities to the results determined in the x—ray region,
Fig. 5 Recent diffuse I—ray energy spectral measurements [23, 34, 36, 37, 38].
The last approach to determining the diffuse radiation at high energies was an attempt to approximate the physical situation believed to exist for the galactic diffuse radiation as closely as possible. From work by Puget et al. [40], Lebrun [41], and Lebrun et al. [42], galaxy counts appear to have emerged as a good tracer of the total (atomic and molecular) gas column density in the local region of the galaxy and hence should give a good
High Energy Diffuse Gamma—Ray Astronomy
11
measure at high latitudes of the combined nucleon—nucleon and bremsstrahlung component, assuming the cosmic ray density in the plane is reasonably uniform locally and its scale height is uniform. This latter concept was pursued by Thompson and Fichtel [23], and the results are shown in Figure 6.
10
II
8
I~
II~
0
.0
°
I
20
It
I
I
I
76
~
x
B
E
8
~II
E .
“2
6
/jx
-
TV
5
/xx
E
• .:..-“ 2Dt”~ I
0
III
.
III
0.8
2 0
11111
0.4
~
1.2
1.85—log (N
0)
Fig. 6 Comparison of the I—ray intensity above 100 14eV measured by SAS—2 as a function of galaxy counts (Thompson and Fichtel [23]).
/5
0
-
I
I
I
I
0.5
1.0
1.5
2.0
2.5
[I.85—1ogN61 +0.26/sin (b) Fig. 6b Comparison of the I—ray intensity above 100 14eV measured by SAS—2 [39] as a fonction of [1.85 — log (NG) + O.26/sin(b)] (Thompson and Fichtel [23]).
Although the diffuse spectral measurements are reasonably self—consistent, the degree of spatial isotropy is not well known. At low I—ray energies (‘. 1 MeV), Trombka et al. [34] estimate that the anisotropic component from galactic sources does not exceed 20 percent of the total flux. At high energies (35 to 100 14eV), the center—to—anticenter ratio for the radiation with 20°< IbI < 40°was measured to be 1.10 ±0.19 and the perpendicular to the galactic plane intensity to that in the 20°< bi < 40° region was measured by Fichtel, Simpson, and Thompson [22] to be 0.87 ±0.09; each of these results is, of course, consistent with isotropy to within errors. Although much more precise measures of the isotropy are clearly desired, no evidence for a major anisotropy exists. In particular, the high energy I—rays results just quoted eliminate a spherical galactic halo origin for the radiation in view of the Sun’s great distance from the galactic center. In the future trying to establish the level of isotropy, or deviations there from, on both a coarse scale and a fine scale will be quite important. Possible Origins and Implications A large number of theories predicting a diffuse I—ray background have appeared in the
literature over the years. With the measurements of the spectrum and intensity which now exist and which have been presented here, most of these seem not to be likely candidates for the majority of the diffuse radiations (see, for example, Fichtel and Trombka, [6]). Two possibilities seem to remain at present and will be discussed here. One of these involves a baryon—symmetric universe, containing superciusters of galaxies of matter and others of antimatter. The annihilation of nucleons and anti—nucleons at the boundaries leads to a I—ray spectrum [43] consistent with the observations. The normalization is selected to have the curve agree with the data, but it is consistent with the currently accepted possible range of densities between clusters. The other possibility for the explaination of the diffuse extragalactic radiation which seems to be a reasonable candidate is the sum of the radiation from active galaxies integrated over cosmological times. An accurate estimate of the expected radiation is hampered by the lack of measured 1—ray spectrum from active galaxies. To date, only one Quasar, one Seyfert galaxy, and one radio galaxy have been seen in I rays; numerous upper limits exist for BL Lacertae Oblects (for a summary, see Fichtel and Trombka [6], Chapter
7).
However, using the data on the few known objects, Bignami
St
al [44] and Fichtel and
12
C.E.
Fitchel
Trombka [6] conclude that it is quite conceivable that Seyfert galaxies and Quasars could account for the diffuse I radiation using conservative evolutionary models. Not only the intensity, but also the spectral shape are quite reasonable. Leiter and Boldt [45] have even proposed a specific model based on supermassive Schwarzschild black holes with accretion disks radiating near the Eddington luminosity limit. The I—ray emission would have two components, a continuous synchrotron seif—Compton one and a transient Penrose Compton component. The latter is of special interest because it could lead to detectable 2) over several days variations in the diffuse radiation in small elements of the sky (10 deg in the 1/2 to 3 MeV region, giving a specific test for this theory. FUTURE OF DIFFUSE HIGH ENERGY GANMA RAY ASTROPHYSICS In the preceeding sections, an attempt has already been made to indicate the very significant scientific return that will result from future measurements by instruments which
will not only have a significantly larger sensitivity, but also improved energy and angular resolution. Hence, in this section, only the known future opportunities to make improved measurements will be discussed. Gamma—I The next high energy I—ray satellite expected to fly is Gamma—I, which is a joint effort of four Soviet and two French laboratories and is to be launched in the early 198Os on a Soviet satellite. It is similar to SAS—2 and COS—B in the sense that its central element is a multilayer spark chamber system, triggered by a directional counter telescope, and surrounded on the upper end by an anticoincidence system. The sensitive area is larger than that of SAS—2 or COS—B; the area solid angle factor is about the same because the viewing angle is smaller. It has an energy—measuring calorimeter which should be able to measure energies with significantly better accuracy than the energy—measuring element on COS—B. The I—ray arrival direction will also be measured with greater accuracy. The upper spark chamber system is a twelve—level wide—gap Vidicon system. The directionality of the electront is determined by a time—of—flight system rather than a directional Cherenkov counter. The improved sensitivity and I—ray direction accuracy should allow better definition of the characteristics of the galactic plane and provide better position information on many or most of the localized excesses already observed, as well as adding to the number of observed extragalactic sources. CR0 The Gamma—Ray Observatory (GRO) will be a shuttle—launched, free—flyer satellite.
The
nominal circular orbit will be about 400 kilometers with an inclination of 28.5°. The radius will remain below 450 kilometers to prevent the high trapped particle intensities dosages during passage through the South Atlantic Anomally. Knowledge of the pointing direction will be determined to an accuracy of 2 arc minutes so that this error contributes negligibly to the overall determination of the direction of I—ray source. Absolute time will be accurate to 0.1 milliseconds to allow precise comparisons of pulsars and other time varying sources with observations at other wavelengths from ground observations and other satellites. The spacecraft will carry four instruments, two of which cover the high energy
I ray portion of the spectrum. The CR0 Imaging Compton Telecope (COMPTEL) instrument covers the 1—30 14eV range. It employs the unique signature of a two—step absorption of the I ray, i.e., Compton collision in the first detector followed by total absorption in a second detector element. This method, in combination with effective charged particle shield detectors, results in a more efficient suppression of the otherwise inherent instrumental background. Spatial resolution in the two detectors together with the well defined geometry of the Compton interaction permits the reconstruction of the sky image over a wide field of view (~-1 steradian) with a resolution
of a few degrees for individual i rays and about 10 arc mm
for strong sources.
The High Energy Gamma Ray Telescope on GRO is designed to cover the energy range from 20 14eV to 30 x lO~ 14eV. The instrument uses a multi—thin—plate spark chamber to detect gamma rays by the electron—positron pair process. A total energy counter using NaI(Ti) is placed beneath the instrument to provide good energy resolution over a wide dynamic range. The
instrument is covered by a plastic scintillator anticoincidence dome to prevent readout on events not associated with I rays. The combination of high energies and good spatial resolution in this instrument provides the best source positions of any GRO instrument.
The
detector system has over twenty times the area, solid angle, efficiency factor of the SAS—2 or COS—B instrument.
For a more full discussion of CR0, particularly from a science point—of—view, see Kniffen et al.
[46].
High Energy Diffuse Gamma—Ray Astronomy
13
Space Station Looking further into the future NASA is currently considering a Space Station which would be a free—flying structure designed for a long lifetime in space. Whereas this program is not now approved, I—ray astronomy, with future requirements for long exposures of large payloads
could clearly benefit from such a Station. With the I—ray sky surveyed in some depth with CR0 for example, it would be possible to concentrate on the detailed features of discrete sources and to study carefully limited regions such as clouds, galactic arms, and nearby galaxies. References 1.
C. Cavallo and R.J. Could, Nuovo Cimento2B, 77 (1971).
2.
F.W. Stecker, Cosmic Gamma Rays, NASA SP—249 (1971).
3.
G.D. Badhwar and S.A. Stephens, Proc. (1977).
4.
D.J. Morris,
15’th
mt.
Cosmic Ray Conf., Plovdiv 1, 198,
High Energy Gamma—Ray Production in Gases and Solids, Ph. D. Thesis,
University of Maryland, 1982. 5.
H.W. Koch and J.W. Motz, Rev. Mod. Phys. 31, 920 (1959).
6.
C.E. Fichtel and J.I. Trombka, Gamma Ray Astrophysics, New Insight into the Universe,
NASA SP—453, 1981. 7.
C.E. Fichtel, D.A. Kniffen, D.J. Thompson, C.F. Bignami, and C.Y. Cheung, Ap. J. 208, 211 (1976).
8.
V.L. Ginzburg and S.I. Syrovatskii, The Origin of Cosmic Rays, Oxford: 1964.
9.
D.A. Kniffen and C.E. Fichtel Ap. J. 250, 389 (1981). Gordon and W.B.
Burton, Ap.
Pergamon Press,
10.
M.A.
J. 208, 346 (1976).
11.
P.M. Solomon and D.B. Sanders, “Giant Molecular Clouds as the Dominant Component of Interstellar Matter in the Galaxy”, in: Molecular Clouds in the Galaxy, ed. P.M. Solomon and 14. Edmunds, Oxford: Pergamon Press, 1980, p. 41.
12.
Y.M. Georgelin and Y.P. Georgelin, Astron. Astrophys. 49, 57 (1976).
13.
E.N. Parker, “Cosmic Ray Propagation and Containment”: in The Structure and Content of the Galaxy and Galactic Gamma Rays, NASA CP—OO2, Washington, DC: Government Printing
Office, 1977, p. 283. 14.
W.L. Kraushaar, G.W. Clark, G.P. Garmire, R. Borken, P. Higbie, C. Leong, and T. Thorsos, Ap. J., 1.77, 341 (1972).
15.
D.A. Kniffen, R.C. Hartman, D.J. Thompson, and C.E. Fichtel, Ap. J. (Letters), 186, Ll05 (1973).
16.
q.E. Fichtel, R.C. Hartman, D.A. Kniffen, D.J. Thompson, G.F. Bignami, H. Ogelman, M.E. Ozel, and T. Tilmer, Ap. J. 198, 163 (1975).
17.
R.C. Hartman, D.A. Kniffen, D.J. Thompson, C.E. Fichtel, H.B. Ogelman, T. TUner, and N.E. Ozel, Ap. J. 230, 597 (1979).
18.
K. Bennett, G.F. Bignami, R. Buceheri, W. Hermsen, G. Kanbach, F. Lebrun, H.A. Mayer— Hasselwander, J.A. Paul, C. Piccinotti, L. Scarsi, F. Soroka, B.N., Swanenburg, and R.D. Willis, in: Recent Advances in Gamma Ray Astronomy, ESA SP—124, p. 83, 1977.
19.
H.A. Mayer—Hasselwander, K. Bennett, G.F. Bignami, R. Buccheri, N. D’Amico, W. Hermsen, G. Kanbach, F. Lebrun, G.G. Lichti, J.L. Masnou, J.A. Paul, K. Pinkau, L. Scarsi, B.N. Swanenburg, and R.D. Willis, Annals of the New York Academy of Sciences, Proc. of the 9th Texas Symp., ed. J. Ehlers, J.J. Perry, and M. Walker, 336, 211, (1980).
20.
C.E. Fichtel and D.A. Kniffen, work in progress (1982).
.
14
C.E. Fitchel
21.
H.A. Mayer—Hasselwander, K. Bennett, G.F. Bignami, R. Buccheri, P.A. Caraveo, W. Hermsen, C. Kanbach, F. Lebrun, C.G. Lichti, J.L. Masnou, J.A. Paul, K, Pinkau, B. Sacco, L. Scarsi, B.N. Swanenburg, and R.D. Wills, Astron. Astrophys. 105, 164 (1982).
22.
C.E. Fichtel,
23.
D.J. Thompson and C.E. Fichtel, to be published in Astron. Astrophys., (1982).
24.
D.J. Thompson, C.E. Fichtel, R.C. Hartman, D.A. Kniffen, R.C. Lamb, (1977).
25.
D.A. Kniffen, C.E. Fichtel, and D.J. Thompson Ap. J. 215, 765 (1977).
26.
A.W. Wolfendale, Submitt. Astro. Conf., Queen Mary College, Sept.
G.A. Simpson, and D.J. Thompson, Ap. J. 222, 833 (1978).
Ap. J. 213, 252
1981, preprint
(1981). 27.
H.J. Black and C. Fazio, Ap. J. 195, L23 (1973).
28.
P.A. Caraveo, L. Barbareschi, K. Bennett, G.F. Bignami, W. Hermsen, C. Kanbach, F. Lebrun, J.L. Masnou, H. Mayer—Hasaelwander, B. Sacco, A.W. Strong, and R.D. wills, 17’th Int. Cos. Ray Conf. Vol. 1, 139 (1981).
29.
G.F. Bignami and C. Morfill, Astro. Astrophys.
30.
M.R. Issa and A.W. Wolfendale, Nature 292, 430 (1981).
31.
M.V. Dolidze, Soviet Astron. Lett. 61, 51 (1980).
32.
G.F. Bignami, L. Barbareachi, J.B.G.M. Bloemen, R. Buccheri, P.A. Caraveo, W. Hermaen,
87, 85 (1980).
C. Kanbach, F. Lebrun, H. Mayer—Hasselwader, J.A. Paul, A.W. Strong, and R.D. Wills, 17th Int. Cos. Ray Conf., Vol. 1, 182 (1981). 33.
W.L. Kraushaar and G.W. Clark, Phys.
34.
J.I. Trombka, C.S. Dyer, L.C. Evans, J.M. Bielefeld, Ap. J., 212, 925 (1977).
35.
R.R. Daniel and P.J. Lavakare, Proc.
36.
V. Sch’dnfelder, F. Graml, and F.P. Penningsfeld, Ap. J. 240, 330 (1980).
37.
F. Marshall, E. Boldt.,
S. Holt,
Rev. Letters 8, 106 (1962). S.M.
Seltzer, and A.E. Metzger,
of the 14th mt. Cosmic Ray Conf., 1, 23 (1975).
R. Miller, R. Mushotzky, L. Rose,
R. Rothschild, and
P. Serlemitsos, Ap. J. 235 (1980). and J. Kurfess, Ap. J. 222. 370 (1978).
38.
R. Kinzer, W. Johnson,
39.
C.E. Fichtel, R.C. Hartman, D.A. Kniffen, D.J. Thompson, H.B. Ogelman, M.E. Ozel, and T. Ttlmer, Ap. J., 217, L9 (1977).
40.
J.L. Puget, C. Ryter, C. Serra, G. Bignami, Aatron. and Astrophys. 50, 247 (1976).
41.
F. Lebrun, Astron. Astrophysics, 79, 153 (1979).
42.
F. Lebrun, G.F. Bignami, R. Buccheri, P.A. Caraveo, W. Hermsen, C. Kanbach, H.A. Mayer— Hasselwander, J.A. Paul, A.W. Strong, and R.D. Wills, to be published in Astron Astrophys. (1981).
43.
F.W. Stecker, D.L. Morgan, and J. Bredekamp, Phys. Rev. Letters, 27, 1469 (1971).
44.
G.F. Bignami, C.E. Fichtel, R.C. Hartman, D.J. Thompson, Ap. J. 232, 649 (1979).
45.
D. Leiter and E. Boldt, NASA TM 83892, 1982.
46.
D. Kniffen, J. Kurfess, V.Sch~nfelder, C. Fichtel, R. Hofstadter, K. Pinkau, and G. Fishman, The Gamma Ray Observatory Science Plan, 1981.
0273—1177/83 $0.00 Copyright
+ .50 ©COSPAR
STATUS AND FUTURE OF HIGH ENERGY DIFFUSE GAMMA-RAY ASTRONOMY C. E. Fichtel NASA/Goddard Space Flight Center, Greenbelt, MD, U.S.A.
ABSTRACT
There are two distinctly different high energy diffuse 1—ray components, one well correlated with broad galactic features and the other apparently isotropic and presumably extragalac— tic. The observed diffuse galactic high energy y—radiation is generally thought to be produced in interactions between the cosmic rays and the interstellar matter and photons. It should then ultimately be possible to obtain from the diffuse galactic emission a detailed picture of the galactic cosmic—ray distribution, a high contrast view of the general structure of the galaxy, and further insight into molecular clouds. Two of the candidates for the explanation of the extragalactic diffuse radiation are the sum of emission from active galaxies and matter—antimatter annihilation. A major advancement in the study of the properties of both galactic and extragalactic y radiation should occur over the next decade. INTRODUCTION Assuming cosmic rays pervade the Galaxy, they necessarily produce high energy y—rays as they interact with the interstellar matter and photons. The diffuse galactic high energy y radiation thus created may be used to assist in understanding the nature of the interstellar medium, galactic structure, and galactic cosmic rays, as was realized well before the diffuse galactic y radiation was observed. A low level, apparently isotropic and extragalactic radiation, has now also been seen. It is characterized by an intensity and energy spectrum which eliminated all, but a few of the potential theoretical explanations for it. DIFFUSE GALACTIC HIGH ENERGY GA}~MA RADIATION Interaction Processes Within our galaxy, cosmic rays interact with matter, photons, and magnetic fields, in each case producing i rays. The cosmic ray nucleon interactions give rise to y rays primarily through the decay of ir° mesons, giving a unique spectrum with a maximum at approximately 68 MeV. Cosmic ray electrons produce y rays through bremsstrahlung, but with a markedly different energy spectral shape, one which decreases monotonically with energy. Cosmic ray electrons also interact with the interstellar starlight, optical and infrared, and the blackbody radiation through the Compton process. Finally, cosmic ray electrons can interact with magnetic fields giving rise to synchrotron or curvature radiation, but these processes are ns.ich less important than the others previously mentioned for the galactic diffuse radiation and will not be discussed further here. Cosmic ray nucleon, interstellar matter interactions. High energy cosmic ray nuclei interacting with the ir~terstellar medium lead to several secondaries which decay to give high energy i rays. The detailed calculations leading to the predicted intensity and energy spectrum of the ‘~ rays (based on the average numbers of mesons formed in an interaction, their angular distribution, and the resulting energy spectrum) are quite lengthy. This is due to the need to study the many different products, to take into account the different cosmic ray species (protons, helium nuclei, and heavier particles) and interstellar nuclei in the correct proportions, follow their decay, integrate over all angles, and then integrate over the cosmic ray energy spectrum. These calculations have, however, been performed. See, for example, Cavallo and Gould [1], Stecker (2], Badwar and Stephens [3], and Morris [4]. The recent work of Norris [4] appears to be quite thorough, incorporating the existing experimental results into a semi—empirical model in a complete and consistent fashion.
5
6
C.E. Fitchel
Cosmic ray electron interactions. As cosmic ray electrons pass through the interstellar medium they product photons over a wide range of energies. In the case of bremsstrahlung, the radiation from interactions of energetic with matter, calculation the 2 MeV iselectrons rather uncertain eventhe locally in our of galaxy radiation in the region below about i0 because the interstellar cosmic ray electron spectrum is not well known at low energies where the electron spectrum observed near the Earth has undergone strong solar modulation. This problem did not arise in the case of comic ray nucleon matter interactions because there the higher energy cosmic rays are most significant in the production of ~ rays, and their spectrum and general composition are well known and not affected markedly by solar modulation. The cosmic ray electron, matter 1—ray production can be calculated using the bremsstralung cross—section formulas of Koch and Motz [5]. The calculations in general are very complex. Futher, considering the uncertainties in the molecular hydrogen density and in the electron spectrum, it seems possible only to give a range for the local source functions for brems— strahlung, which are estimated as ~b LE > 100 MeV) = ~4 to 8) x io—26 photons cm3s’ and Q (35 < E < 100 14eV) = (6 to 15) x 10 2 photons cm3s (from Fichtel and Trombka, [6], base on the earlier work of Fichtel et al. [7], and Koch and Motz, [5]). Although this source function is smaller than the cosmic ray nucleon one above 100 14eV, it dominates at lower energies and, in fact, becomes increasingly important the lower 1—ray energy. Even for simple models, the cosmic ray electron spectrum is not expected to have exactly the same shape everywhere in the galaxy because more secondary electrons are produced in regions of greater matter density. Cosmic ray electrons also interact with starlight photons, for which both the optical and infrared ranges are important, and with the blackbody radiation to produce Compton ‘r rays. The source functions of these interactions are much smaller in the galactic plane in the vicinity of the solar system. The total contribution to the galactic I radiation, however, is significant because the cosmic ray and stellar photon scale heights above the galactic plane are much greater than those of the matter, and, of course, the blackbody photon density is uniform. Hence, the integral intensity along a line of sight is closer to that of the bremsstrahlung than the source functions above would imply. The calculations associated with the producton of Compton y rays are again quite complex~ however, they have been performed in some detail for the cases of astrophysical interest by Ginzburg and Syrovatskii [8]. Whereas, in the case of bremsstrahlung. the astrophysical I rays come predominantly from electrons of energies similar to those of the I rays, or only a factor of several higher in energy, for the Compton case, the energy of the parent electron is given approximately by the equation E
~
Ee )2 c,
(1)
where C 1, is the photon energy before the electron interaction. Therefore, if the interactions is with starlight where the typical photon energy is a few eV, in order for the I ray to be about a hundred MeV, Ee must,be several GeV. For the 30 blackbody radiation, the typical photon energy is about 8 x 10— eV; therefore, the parent electrons must be in the range of 2 x 10~ MeV to produce one hundred 14ev 1 rays. In the energy range above a GeV, there is no serious uncertainty in the electron spectrum due to there being little solar modulation. However, in the few GeV range, the electron energy spectrum changes shape; so the calculations must be performed carefully. At energies as high as a few hundred thousand MeV, the electron energy spectrum is less well known, and an increased uncertainty in the calculated 1—ray intensity is introduced.
Using the Compton scattering functions of Ginzburg and Syrovatskii [8] and the calculations 5ec (ESt > al. 100 [7],and May) = references (0.3 to 0.6~ x io’~6the photons ~c ~ function 14eV > Eis > estimated 100 14eV) of Fichtel therein), local cm3s’~ Comptonand source to (0.5 = be to 1.0) x 1026 photons cm”3s . The range of values arises partially from uncertainties in the electron spectrum and partially from a lack of knowledge of the photon density. The galactic distribution functions for optical and infrared photons to be used here are those developed by Kniffen and Fichtel [9]. Galactic Matter Distribution The relevant concern here is the galactic diffuse matter in the form of atoms, molecules, ions, and dust with which cosmic rays interact. The primary constituents are atomic and molecular hydrogen. Both are known to be confined to a narrow disk (- 0.1 kpc in scale height for atomic hydrogen) with the molecular hydrogen distribution apparently narrower than that of the atomic hydrogen by a factor of two (e.g. , Gordon and Burton [10] ; Solomon and Sanders [11]). Atomic hydrogen reveals its presence through the emision of the 21 cm line, which is produced by the hyperfine transition of this neutral atom. There remains some uncertainty
High Energy Diffuse Gamma—Ray Astronomy
7
of the density in the inner regions due to the need for the absorption correction. Although the translation of the observations into a galactic spatial distribution is difficult, on a broad scale the density profile shows a general spiral pattern. The density distribution of molecular hydrogen cannot be measured directly, but must be inferred from other measurements. At present), the best approach appears to be through the observations of the 2.6 mm spectral line of laCO, which shows the distribution of cold interstellar matter. The nature of this process makes the molecular hydrogen density distribution less certain than that of the atomic hydrogen. The average galactic radial distribution of molecular and atomic hydrogen deduced by Gordon and Burton [10] shows that the molecular hydrogen to atomic hydrogen ratio is much larger in the inner galaxy that it is in the outer galaxy; however, the absolute intensity of molecular hydrogen is still quite uncertain. For the work here, the molecular hydrogen density normalization is treated as an adjustable parameter in the range from that estimated by Gordon and Burton [10] to a factor of 2.5 smaller. On the basis of a high sample survey and observations in both the first and second quadrants of the glactic plane, Cohen et al. have recently shown the existence of the molecular counterparts of the five classical 21 cm spiral arms segments in these quadrants. The CO observations indicate that the great majority of the molecular hydrogen is in clouds. The recent work of Solomon and Sanders [11] has, in fact, suggested that the interstellar medium is dominated by massive cloud complexes. Galatic Cosmic Ray Distribution The radio continuum measurements and cosmic ray results support the picture of the cosmic rays having a large scale height (approximately 1 kpc) relative to the matter and several fundamental theoretical considerations (e.g., Parker. [13]) suggest that the cosmic ray density throughout the galaxy may generally be as large as could be contained under near— equilibium conditions. The energy density of the cosmic rays should then be larger where the matter density is larger on a coarse scale such as that of the galactic arms. On a smaller scale, the pressures of the cosmic ray gas and magnetic fields cause the cosmic ray gas and field system to expand between the large clouds through which the magnetic fields thread, probably eliminating local nonunformities and anisotropies. Gamma Ray Results and Their Interpretation The most intense celestial high energy I radiation observed is that from the galactic plane. This feature was observed first by the pioneering counter telescope flown on OSO—3 [14] and the major features of this galactic radiation have now been defined by measurements made with the SAS—2 and COS—B satellites [15, 16, 17, 18, 19]. The distribution of high energy (E > 100 14eV) I—ray intensity along the galactic plane summed over the latitude interval from —10°to +10°obtained with the SAS—2 I—ray telescope is shown in Figure 1 in
2.0
I
T1.5
I
‘~-
VELA (PSROh33-45}’
~
I t~I
CYGNUS X—3
CRAB
1~~’1~ I t”j,’ T~1‘~ ~ ii(~t~ WI III
(PSRO53I+21) y195+5
IT
!:‘.0 0.5
Iu~++
~
-
~
0 ~ 150
~
120
~
90
60
I
I
30
0
I
I
330 300 (DEGREES)
I
270
I
240
210
180
150
Flg.1 The high energy E > 100 MeV I—ray intensity as a function of longitude for —10° < b < 10° from the SAS—2 data [17] compared to the model discussed here (9, 20].
8
C.E. Fitchel
bins which are 2.5°wide in latitude.
Notice that the emission from the region 310° < 1 <
50° is particularly intense relative to the remainder of the galactic plane. This was expected on the basis of the known galactic structure and the expected sources of the diffuse I rays. Figure 2.
The results of COS—B have confirmed the same general features, as shown in
I ~__ _ _
_____
0.0 I
150
120
I
90
I
I
I
60
I
I
30
I
I
I
0
I
330
300
I
270
I
I
I
240
I
I
210
180
GALACTIC LONGITUDE I
Fig.2 Gamma ray intensity as a function of longitude averaged over the latitude range —10°< b < 10° from 70 MeV — 150 14eV, 150 MeV — 300 MeV, and 300 14eV — 5000 14eV from the COS—B data [21] compared to the model discussed here shown by the solid line [9, 20] and one example of a constant
cosmic ray distribution
shown by the
dashed
line.
Ibi < 90°) a major portion of the I radiation observed is now believed to be galactic on the basis of its correlation with the matter, galaxy Counts, and synchrotron radiation [22, 23] , as will be discussed in the next section. Even at high latitudes (10° <
Detailed distributions in galactic latitude for I rays above 70 14eV are given in the paper by Mayer—Hasselwander et al. [21]. In the range of galactic latitudes from 330°to 30°an enhancement is visible in the latitude range 5°< b < 20° relative to the range —20°< b < —5°. This enhancement seen already in the early SAS—2 data taken together with the excess seen at negative latitudes in the galactic anticenter has been interpreted as I—ray emission produced in the local concentration of clouds known as Gould’s Belt [16, 24] . The latitude distributions of 90° < 1 < 150° have a peak at about b = 2°, while the distribution for 250° < 1 < 300°has an excess at negative latitudes. This effect is seen in both the COS—B data [21] and SAS—2 data [17]. The offsets in the I—ray data are qualitatively similar to the “hat brim” effect visible In the radio observations and due to the large—scale warping of the galactic disk. The broad distribution in galactic latitude for the longitude intervals away from the galactic center provides strong evidence that the observed I rays in these directions are, for the most part, produced locally (within a few kpc), whereas the additional narrow distribution seen toward the inner parts of the galaxy implies that a large part of the emission comes from more distant (> 3 kpc) features. Except for the four strong sources identified in Figure 1, there is no significant evidence for a variation of the energy spectrum along the galactic plane, in either the SAS—2 [17] or COS—B [21] data. The energy spectrum obtained for the galactic center region is shown in FIgure 3. Before estimating the diffuse galactic I radiation to be expected from cosmic ray interactions with galactic matter and photons, it should be mentioned that there is probably also an unresolved point source contribution to the “diffuse” radiation measured by the SAS—2 and COS—B y—ray instruments since the limited angular resolution of these instruments does not permit the separation of point sources. It Is quite difficult to estimate this contribution; however, several factors suggest that point sources may not be a major contributor. These include the uniformity of the energy spectrum just discussed and, as will be seen, the y—ray luminosity of the galaxy and its distribution being about what would be expected from the diffuse sources. For the purpose of this paper, the reader is simply asked to keep in mind that there is some point source contribution yet to be determined which at least for the moment is assumed to be small, but not zero. Based on the source functions, the matter distributions, and the assumption that the cosmic rays pervade the galaxy, the expected I—ray intensity has been calculated. The results are somewhat model dependent and, after discussing the comparison with one model, some of the differences will be discussed. The model to be used as that of Kniffen and Fichtel [9]with
High Energy Diffuse Gamma—Ray Astronomy
their photon distributions and the assumption that the cosmic rays are correlated with the matter in the plane, but have a larger scale height. The authors of several other papers (See Fichtel and Trombka [6], chapter 5, for a
9
I
summary.) also come to the conclusion that the cosmic ray density is enhanced where the matter density is greatest, in
agreement with the concept of coupling deduced from theoretical considerations by Parker [13]. The spiral arm model of Georgelin and Georgelin [12] is used. The prediction of the I—ray intensity from this model assuming the molecular hydrogen to be approximately one—half that of Gordon and Burton [10] is compared to the SAS—2 and COS—B longitude distributions in Figures 1 and 2 and to the energy spectrum in the galactic center region in Figure 3. Considering the uncertainty in the point source contribution, and the mass distribution, the agreement in Figures 1 and 2 is quite reasonable. It also suggests that there are interesting details to look for in the future. Regarding Figures 2 and 3, there are two comments. First, if the older cosmic ray nucleon source function of Stecker [2] had been used, the agreement in the 300 MeV to 5000 14eV energy interval would be quite poor with the theory predicting 1.6 to 2 times as many I—rays in the center than observed. The second comment is that, although the intensities for the center and the sources (195,5), PSR 0531+21, PSR
id’
BREMSSTRAHLUNG
~,,~SAS-2
1~ ‘~
—
-6 0
\
-
,,~_-COS-D
NUCLEAR
‘~E
INTERACTIONS ~
I~
-
F
-
COMPTON
10
-
0 0
II
IIIiI~2
III~
ENERGY 1MeV)
Fig.3 Energy spectrum of the galactic I radiation for a region near the galactic center. The calculated spectra shown by a dashed line in the figure are based on the work of Kniffen and Fichtel [9] modified to include the recent nucleon—nucleon calculations of Morris [4] . The COS—B data are those of Mayer—Hasselwander et al. [22], and the SAS—2 data are those of
0833—45 measured with the SAS—2 and COS—B Hartman et al. [17]. instruments are in good agreement with each other, the COS—B instrument appears to see a somewhat larger (about 25%) anticenter diffuse intensity. The SAS—2 instrument, which sees the lower diffuse intensity, had essentially no background; the very small extragalactic isotropic background is not substracted in Figure 1. In the case of COS—B, which sees the higher diffuse intensity, the estimated background has been subtracted. In general, the variations with latitude at different longitudes near the plane predicted by the work of Kniffen and Fichtel [9] are in reasonable agreement with the data when all components are considered.
A constant cosmic ray density, as might be predicted in the universal cosmic ray model, predicts a rather small I—ray intensity from the central region as shown in Figure 2 especially if a normalization is chosen to avoid disagreement at intermediate longitudes and the anticenter. Hence, should it be correct, a relatively large point source contribution would be needed in the galactic center region. It has already been noted that the majority of matter is now thought to be in large clouds. If this should be the case, I—ray astronomy should be able to contribute much to our understanding of the subject in the local region of the galaxy. Although there has been substantial theoretical work in the production of I rays by comic rays specifically in clouds (see, for example, Wolfendale [26]) since the paper by Black and Fazio [27], there are only two I—ray identifications which appear to be reasonably firm, namely the Orion Complex [28] and the region near p Oph [29]. Figure 4 compares the COS—B I—ray data with radio millimeter data (in the CO line) of Columbia University for the Orion region. The correlation is clearly encouraging. The intensity is about what is expected on the basis of the local cosmic ray density and the estimated matter density (see, for example, Issa and Wolfendale [30]; and Morris [4]). In addition to individual clouds, there are local belts. Gould’s belt was established with SAS—2 satellite [16]. COS—B results have not only confirmed the existence of this belt in I rays, but also show the existence of the Dolidze [31] belt in I rays [32] These results suggest that I—ray astronomy will not only help in unraveling the general structure of the galaxy, but also in improving the understanding of galactic clouds, their
JASR 3/4—B
10
C.E. Fitchel
relation to cosmic rays, and the origin and propagation of cosmic rays, and in providing a fuller picture of more general features locally in our galaxy. DIFFUSE ISOTROPIC RADIATION Experimental Observations At energies above 10 14eV,
: :.
the first
measurements related an extragalactic diffuse radiation wereto those of Kraushaar and Clark [33] , whose upper limits from Explorer 11 provided an experimental refutation of the steady—state theory of cosmology. Several other upper limits
It)
were diffuse experiments, reported high a energy but from theenergy early flux firstclearly balloon came suggestion from the established OSO—3 final 35 14eV) satellite results I—ray of experiment high experiment the SAS—2 extension high [14]. energy The of ofthe (> a
O’i
diffuse radiation with a steep energy spectrum above 35 14eV.
1o —15~ _______________________________ 20m 40m 20” 511 ‘•‘
The most controversial energy region has been from about 1/2 to 10 14eV, where there is a substantial local background resulting from cosmic ray interactions leading to excited nuclei. The analyses
~~i95o)
of Trombka et al. [34] and Daniel and Lavakare [35] have indicated that many of
the early reported positive results in this energy range were too high because of the failure to eliminate all the background from the measurements. At present, the final results from Apollo 15 and 16 I—ray detectors [34], which were on
Fig. 4 Comparison of the COS—B I—ray data shown by a thin line to the first CO isophote of peak antenna temperature shown by a thick line obtained by Columbia University for the Orion region. The figure is from Caraveo et al. [28].
a boom of variable length, are generally accepted as a good representation of the diffuse energy spectrum in the intermediate energy range. These data together with the recent work
of Sch5nfelder et al. [36] are shown in Figure 5. The high energy (> 35 14eV) region also deserves special mention not because of any significant detector or locally
I&
MARSHALL,
produced background, which can be very
small as in the case of SAS—2, but because of the need to separate the galactic diffuse radiation from the general diffuse radiation being discussed here.
Several
—
10 KINZER, JOHNSON & KURFESS 119781 POWER LAW FIT
~‘:~oMeKA
give
the
same result within the uncertainties,
M at lI~71
J
T~MPSOW& FICHTELII982b1
and the 150 Mhz brightness temperature were compared with the I radiation [22], and a study of the I—ray intensity as a function of galaxy counts and a combination of galaxy counts and a (1/sin All approaches
et at 119801
~
radiation data including a comparison of 1/sin b [39], analyses wherein other galactic radiation such as the 21 cm line
[23] .
01 119801
SCHONFELDER,
different approaches have been used to perform this separation on the SAS—2 I—
b) function
Ct
I
HIGH LATITUDE GALACTIC EMISSION
10°
101
102
103
I0~
I0~
IO~
PHOTON ENERGY (k~Vl
namely a steep spectrum which extrapolates
readily back through the measured low energy I ray intensities to the results determined in the x—ray region,
Fig. 5 Recent diffuse I—ray energy spectral measurements [23, 34, 36, 37, 38].
The last approach to determining the diffuse radiation at high energies was an attempt to approximate the physical situation believed to exist for the galactic diffuse radiation as closely as possible. From work by Puget et al. [40], Lebrun [41], and Lebrun et al. [42], galaxy counts appear to have emerged as a good tracer of the total (atomic and molecular) gas column density in the local region of the galaxy and hence should give a good
High Energy Diffuse Gamma—Ray Astronomy
11
measure at high latitudes of the combined nucleon—nucleon and bremsstrahlung component, assuming the cosmic ray density in the plane is reasonably uniform locally and its scale height is uniform. This latter concept was pursued by Thompson and Fichtel [23], and the results are shown in Figure 6.
10
II
8
I~
II~
0
.0
°
I
20
It
I
I
I
76
~
x
B
E
8
~II
E .
“2
6
/jx
-
TV
5
/xx
E
• .:..-“ 2Dt”~ I
0
III
.
III
0.8
2 0
11111
0.4
~
1.2
1.85—log (N
0)
Fig. 6 Comparison of the I—ray intensity above 100 14eV measured by SAS—2 as a function of galaxy counts (Thompson and Fichtel [23]).
/5
0
-
I
I
I
I
0.5
1.0
1.5
2.0
2.5
[I.85—1ogN61 +0.26/sin (b) Fig. 6b Comparison of the I—ray intensity above 100 14eV measured by SAS—2 [39] as a fonction of [1.85 — log (NG) + O.26/sin(b)] (Thompson and Fichtel [23]).
Although the diffuse spectral measurements are reasonably self—consistent, the degree of spatial isotropy is not well known. At low I—ray energies (‘. 1 MeV), Trombka et al. [34] estimate that the anisotropic component from galactic sources does not exceed 20 percent of the total flux. At high energies (35 to 100 14eV), the center—to—anticenter ratio for the radiation with 20°< IbI < 40°was measured to be 1.10 ±0.19 and the perpendicular to the galactic plane intensity to that in the 20°< bi < 40° region was measured by Fichtel, Simpson, and Thompson [22] to be 0.87 ±0.09; each of these results is, of course, consistent with isotropy to within errors. Although much more precise measures of the isotropy are clearly desired, no evidence for a major anisotropy exists. In particular, the high energy I—rays results just quoted eliminate a spherical galactic halo origin for the radiation in view of the Sun’s great distance from the galactic center. In the future trying to establish the level of isotropy, or deviations there from, on both a coarse scale and a fine scale will be quite important. Possible Origins and Implications A large number of theories predicting a diffuse I—ray background have appeared in the
literature over the years. With the measurements of the spectrum and intensity which now exist and which have been presented here, most of these seem not to be likely candidates for the majority of the diffuse radiations (see, for example, Fichtel and Trombka, [6]). Two possibilities seem to remain at present and will be discussed here. One of these involves a baryon—symmetric universe, containing superciusters of galaxies of matter and others of antimatter. The annihilation of nucleons and anti—nucleons at the boundaries leads to a I—ray spectrum [43] consistent with the observations. The normalization is selected to have the curve agree with the data, but it is consistent with the currently accepted possible range of densities between clusters. The other possibility for the explaination of the diffuse extragalactic radiation which seems to be a reasonable candidate is the sum of the radiation from active galaxies integrated over cosmological times. An accurate estimate of the expected radiation is hampered by the lack of measured 1—ray spectrum from active galaxies. To date, only one Quasar, one Seyfert galaxy, and one radio galaxy have been seen in I rays; numerous upper limits exist for BL Lacertae Oblects (for a summary, see Fichtel and Trombka [6], Chapter
7).
However, using the data on the few known objects, Bignami
St
al [44] and Fichtel and
12
C.E.
Fitchel
Trombka [6] conclude that it is quite conceivable that Seyfert galaxies and Quasars could account for the diffuse I radiation using conservative evolutionary models. Not only the intensity, but also the spectral shape are quite reasonable. Leiter and Boldt [45] have even proposed a specific model based on supermassive Schwarzschild black holes with accretion disks radiating near the Eddington luminosity limit. The I—ray emission would have two components, a continuous synchrotron seif—Compton one and a transient Penrose Compton component. The latter is of special interest because it could lead to detectable 2) over several days variations in the diffuse radiation in small elements of the sky (10 deg in the 1/2 to 3 MeV region, giving a specific test for this theory. FUTURE OF DIFFUSE HIGH ENERGY GANMA RAY ASTROPHYSICS In the preceeding sections, an attempt has already been made to indicate the very significant scientific return that will result from future measurements by instruments which
will not only have a significantly larger sensitivity, but also improved energy and angular resolution. Hence, in this section, only the known future opportunities to make improved measurements will be discussed. Gamma—I The next high energy I—ray satellite expected to fly is Gamma—I, which is a joint effort of four Soviet and two French laboratories and is to be launched in the early 198Os on a Soviet satellite. It is similar to SAS—2 and COS—B in the sense that its central element is a multilayer spark chamber system, triggered by a directional counter telescope, and surrounded on the upper end by an anticoincidence system. The sensitive area is larger than that of SAS—2 or COS—B; the area solid angle factor is about the same because the viewing angle is smaller. It has an energy—measuring calorimeter which should be able to measure energies with significantly better accuracy than the energy—measuring element on COS—B. The I—ray arrival direction will also be measured with greater accuracy. The upper spark chamber system is a twelve—level wide—gap Vidicon system. The directionality of the electront is determined by a time—of—flight system rather than a directional Cherenkov counter. The improved sensitivity and I—ray direction accuracy should allow better definition of the characteristics of the galactic plane and provide better position information on many or most of the localized excesses already observed, as well as adding to the number of observed extragalactic sources. CR0 The Gamma—Ray Observatory (GRO) will be a shuttle—launched, free—flyer satellite.
The
nominal circular orbit will be about 400 kilometers with an inclination of 28.5°. The radius will remain below 450 kilometers to prevent the high trapped particle intensities dosages during passage through the South Atlantic Anomally. Knowledge of the pointing direction will be determined to an accuracy of 2 arc minutes so that this error contributes negligibly to the overall determination of the direction of I—ray source. Absolute time will be accurate to 0.1 milliseconds to allow precise comparisons of pulsars and other time varying sources with observations at other wavelengths from ground observations and other satellites. The spacecraft will carry four instruments, two of which cover the high energy
I ray portion of the spectrum. The CR0 Imaging Compton Telecope (COMPTEL) instrument covers the 1—30 14eV range. It employs the unique signature of a two—step absorption of the I ray, i.e., Compton collision in the first detector followed by total absorption in a second detector element. This method, in combination with effective charged particle shield detectors, results in a more efficient suppression of the otherwise inherent instrumental background. Spatial resolution in the two detectors together with the well defined geometry of the Compton interaction permits the reconstruction of the sky image over a wide field of view (~-1 steradian) with a resolution
of a few degrees for individual i rays and about 10 arc mm
for strong sources.
The High Energy Gamma Ray Telescope on GRO is designed to cover the energy range from 20 14eV to 30 x lO~ 14eV. The instrument uses a multi—thin—plate spark chamber to detect gamma rays by the electron—positron pair process. A total energy counter using NaI(Ti) is placed beneath the instrument to provide good energy resolution over a wide dynamic range. The
instrument is covered by a plastic scintillator anticoincidence dome to prevent readout on events not associated with I rays. The combination of high energies and good spatial resolution in this instrument provides the best source positions of any GRO instrument.
The
detector system has over twenty times the area, solid angle, efficiency factor of the SAS—2 or COS—B instrument.
For a more full discussion of CR0, particularly from a science point—of—view, see Kniffen et al.
[46].
High Energy Diffuse Gamma—Ray Astronomy
13
Space Station Looking further into the future NASA is currently considering a Space Station which would be a free—flying structure designed for a long lifetime in space. Whereas this program is not now approved, I—ray astronomy, with future requirements for long exposures of large payloads
could clearly benefit from such a Station. With the I—ray sky surveyed in some depth with CR0 for example, it would be possible to concentrate on the detailed features of discrete sources and to study carefully limited regions such as clouds, galactic arms, and nearby galaxies. References 1.
C. Cavallo and R.J. Could, Nuovo Cimento2B, 77 (1971).
2.
F.W. Stecker, Cosmic Gamma Rays, NASA SP—249 (1971).
3.
G.D. Badhwar and S.A. Stephens, Proc. (1977).
4.
D.J. Morris,
15’th
mt.
Cosmic Ray Conf., Plovdiv 1, 198,
High Energy Gamma—Ray Production in Gases and Solids, Ph. D. Thesis,
University of Maryland, 1982. 5.
H.W. Koch and J.W. Motz, Rev. Mod. Phys. 31, 920 (1959).
6.
C.E. Fichtel and J.I. Trombka, Gamma Ray Astrophysics, New Insight into the Universe,
NASA SP—453, 1981. 7.
C.E. Fichtel, D.A. Kniffen, D.J. Thompson, C.F. Bignami, and C.Y. Cheung, Ap. J. 208, 211 (1976).
8.
V.L. Ginzburg and S.I. Syrovatskii, The Origin of Cosmic Rays, Oxford: 1964.
9.
D.A. Kniffen and C.E. Fichtel Ap. J. 250, 389 (1981). Gordon and W.B.
Burton, Ap.
Pergamon Press,
10.
M.A.
J. 208, 346 (1976).
11.
P.M. Solomon and D.B. Sanders, “Giant Molecular Clouds as the Dominant Component of Interstellar Matter in the Galaxy”, in: Molecular Clouds in the Galaxy, ed. P.M. Solomon and 14. Edmunds, Oxford: Pergamon Press, 1980, p. 41.
12.
Y.M. Georgelin and Y.P. Georgelin, Astron. Astrophys. 49, 57 (1976).
13.
E.N. Parker, “Cosmic Ray Propagation and Containment”: in The Structure and Content of the Galaxy and Galactic Gamma Rays, NASA CP—OO2, Washington, DC: Government Printing
Office, 1977, p. 283. 14.
W.L. Kraushaar, G.W. Clark, G.P. Garmire, R. Borken, P. Higbie, C. Leong, and T. Thorsos, Ap. J., 1.77, 341 (1972).
15.
D.A. Kniffen, R.C. Hartman, D.J. Thompson, and C.E. Fichtel, Ap. J. (Letters), 186, Ll05 (1973).
16.
q.E. Fichtel, R.C. Hartman, D.A. Kniffen, D.J. Thompson, G.F. Bignami, H. Ogelman, M.E. Ozel, and T. Tilmer, Ap. J. 198, 163 (1975).
17.
R.C. Hartman, D.A. Kniffen, D.J. Thompson, C.E. Fichtel, H.B. Ogelman, T. TUner, and N.E. Ozel, Ap. J. 230, 597 (1979).
18.
K. Bennett, G.F. Bignami, R. Buceheri, W. Hermsen, G. Kanbach, F. Lebrun, H.A. Mayer— Hasselwander, J.A. Paul, C. Piccinotti, L. Scarsi, F. Soroka, B.N., Swanenburg, and R.D. Willis, in: Recent Advances in Gamma Ray Astronomy, ESA SP—124, p. 83, 1977.
19.
H.A. Mayer—Hasselwander, K. Bennett, G.F. Bignami, R. Buccheri, N. D’Amico, W. Hermsen, G. Kanbach, F. Lebrun, G.G. Lichti, J.L. Masnou, J.A. Paul, K. Pinkau, L. Scarsi, B.N. Swanenburg, and R.D. Willis, Annals of the New York Academy of Sciences, Proc. of the 9th Texas Symp., ed. J. Ehlers, J.J. Perry, and M. Walker, 336, 211, (1980).
20.
C.E. Fichtel and D.A. Kniffen, work in progress (1982).
.
14
C.E. Fitchel
21.
H.A. Mayer—Hasselwander, K. Bennett, G.F. Bignami, R. Buccheri, P.A. Caraveo, W. Hermsen, C. Kanbach, F. Lebrun, C.G. Lichti, J.L. Masnou, J.A. Paul, K, Pinkau, B. Sacco, L. Scarsi, B.N. Swanenburg, and R.D. Wills, Astron. Astrophys. 105, 164 (1982).
22.
C.E. Fichtel,
23.
D.J. Thompson and C.E. Fichtel, to be published in Astron. Astrophys., (1982).
24.
D.J. Thompson, C.E. Fichtel, R.C. Hartman, D.A. Kniffen, R.C. Lamb, (1977).
25.
D.A. Kniffen, C.E. Fichtel, and D.J. Thompson Ap. J. 215, 765 (1977).
26.
A.W. Wolfendale, Submitt. Astro. Conf., Queen Mary College, Sept.
G.A. Simpson, and D.J. Thompson, Ap. J. 222, 833 (1978).
Ap. J. 213, 252
1981, preprint
(1981). 27.
H.J. Black and C. Fazio, Ap. J. 195, L23 (1973).
28.
P.A. Caraveo, L. Barbareschi, K. Bennett, G.F. Bignami, W. Hermsen, C. Kanbach, F. Lebrun, J.L. Masnou, H. Mayer—Hasaelwander, B. Sacco, A.W. Strong, and R.D. wills, 17’th Int. Cos. Ray Conf. Vol. 1, 139 (1981).
29.
G.F. Bignami and C. Morfill, Astro. Astrophys.
30.
M.R. Issa and A.W. Wolfendale, Nature 292, 430 (1981).
31.
M.V. Dolidze, Soviet Astron. Lett. 61, 51 (1980).
32.
G.F. Bignami, L. Barbareachi, J.B.G.M. Bloemen, R. Buccheri, P.A. Caraveo, W. Hermaen,
87, 85 (1980).
C. Kanbach, F. Lebrun, H. Mayer—Hasselwader, J.A. Paul, A.W. Strong, and R.D. Wills, 17th Int. Cos. Ray Conf., Vol. 1, 182 (1981). 33.
W.L. Kraushaar and G.W. Clark, Phys.
34.
J.I. Trombka, C.S. Dyer, L.C. Evans, J.M. Bielefeld, Ap. J., 212, 925 (1977).
35.
R.R. Daniel and P.J. Lavakare, Proc.
36.
V. Sch’dnfelder, F. Graml, and F.P. Penningsfeld, Ap. J. 240, 330 (1980).
37.
F. Marshall, E. Boldt.,
S. Holt,
Rev. Letters 8, 106 (1962). S.M.
Seltzer, and A.E. Metzger,
of the 14th mt. Cosmic Ray Conf., 1, 23 (1975).
R. Miller, R. Mushotzky, L. Rose,
R. Rothschild, and
P. Serlemitsos, Ap. J. 235 (1980). and J. Kurfess, Ap. J. 222. 370 (1978).
38.
R. Kinzer, W. Johnson,
39.
C.E. Fichtel, R.C. Hartman, D.A. Kniffen, D.J. Thompson, H.B. Ogelman, M.E. Ozel, and T. Ttlmer, Ap. J., 217, L9 (1977).
40.
J.L. Puget, C. Ryter, C. Serra, G. Bignami, Aatron. and Astrophys. 50, 247 (1976).
41.
F. Lebrun, Astron. Astrophysics, 79, 153 (1979).
42.
F. Lebrun, G.F. Bignami, R. Buccheri, P.A. Caraveo, W. Hermsen, C. Kanbach, H.A. Mayer— Hasselwander, J.A. Paul, A.W. Strong, and R.D. Wills, to be published in Astron Astrophys. (1981).
43.
F.W. Stecker, D.L. Morgan, and J. Bredekamp, Phys. Rev. Letters, 27, 1469 (1971).
44.
G.F. Bignami, C.E. Fichtel, R.C. Hartman, D.J. Thompson, Ap. J. 232, 649 (1979).
45.
D. Leiter and E. Boldt, NASA TM 83892, 1982.
46.
D. Kniffen, J. Kurfess, V.Sch~nfelder, C. Fichtel, R. Hofstadter, K. Pinkau, and G. Fishman, The Gamma Ray Observatory Science Plan, 1981.