X-ray and gamma-ray bursts

Nuclear Physics B (Proc . Suppl .) 14B (1990) 129-146 North-Holland

ray

129

rstst

Melville P. Ulmer and Fulvio Melia* Department of Physics and Astronomy Northwestern University, Evanston, IL 60208, U .S .A . Cti Intense bursts of X-rays and gamma-rays have now been monitored by space borne detectors for over a decade. By its very nature, a "burst" implies some type of energy storage and trigger mechanism . These in turn must be combined with a geometry and a means of radiation transport to produce viable models . Although we feel confident about certain aspects of our understanding of these phenomena, some fea-

tures remain tantalizingly puzzling. The goal of this

bursts detected from three separate sources.

The salient features of these events include burst rise times of - 1 s, decay time scales of - 3 -100 s, peak luminosities of - 16 38 ergs s -1 , and total emitted energies of _ 10 39 ergs per burst . The distribution of X-ray burst sources on the celestial sphere is strongly concentrated in the direction

of the Galactic Center, implying an average source distance of ti 10 kiloparsec . Many of these objects

have now been identified with binary systems that also emit X-rays in their quiescent state . The optical

paper is not necessarily to give a comprehensive review of these transient events, but rather to highlight

companions appear to be "low mass" (,,S 1 MO) main sequence dwarfs or degenerate dwarfs orbiting a com-

astrophysical context by providing us with the opportunity of learning something about the physical

surface of a neutron starr, and the X-ray spectra can generally be well fitted by blackbody emission with a peak temperature of - 3 x 107 K .

some of the interesting topics and the problems that remain unsolved . We also demonstrate how the study of X-ray and Gamma-ray bursts fits into a broader

characteristics of neutron stars : (I.) their equation of state; (2) their magnetic field strength, and (3) their birth rate and evolution .

rsts We begin this section my summarizing a few "facts" and then discuss storage mechanisms, energy sources, models, and puzzles . Other reviews of this subject

have been given by references 1, 2, 3, 4, and 5 . Xray bursts typically have light curves like that shown

schematically in Figure 1 . Of course, not all bursts have this canonic 1 shape, nor do they look the same in all energy bands . For example, Figure 2 shows the

pact object with a comparable mass (- 1 MO ; e.g., reference 6) . The overall size of the emitting region is consistent with the burst originating over the entire

Thermonuclear flashes in the hydrogen-rich material of an accreting neutron star were first proposed by Woosley and Taame, Maraschi and Cavaliere9 , and Jossl° as a model for X-ray burst sources . Over the

past decade, tae preponderance of accumulating evidence has come to strongly favor this picture . Figure 3 shows an illustrative calculation of the burst intensity as a function of time", including the expected temperature at several epochal stages of its evolution, under the assumption that the area of the emitting region is constant in time (cf. reference 12) . In general, the predicted cooling rate matches quite well with the observations 7 .

temporal structure in different energy channels for

NSF grant t This work was supported in part by NASA contract S-10987-C (MPU), NASA grant NAGW-1609, Sloan Foundation (FM) . PHY 88-57218, and the Alfred P. *Presidential Young Investigator and Alfred P . Sloan Fellow . 0920-5632/90/$03 .50 © Elsevier Science Publishers B .V . (North-Holland)

M.P. Ulmer, F. Melia/X-ray and gamma-ray bursts

130 120

100

80 N N i

60

O U

40

20

50

100

Time (sec)

200

150

Figure 1: Schematic diagram showing the history and salient features of a typical X-ray burst . In this example, the rise and (exponential) decay times are 1 s and 30 s, respectively. In the following, we assume that the basic thermonuclear flash model is correct and we present some heuristic arguments to estimate such things as the neutronstar's magnetic field, the expected burst recurrence time scale, and the allowed range of M/R, where are M and R are, respectively, the mass and radius of the neutron star.

etic fiel Detailed calculations show that the hydrogen/heliumrich shell is unstable to thermonuclear flashes over a wide range of accretion rates M, although the properties of such flashes vary substantially with varying M. Indeed, at very high accretion rates, such as would occur locally on the polar cap if the magnetic field were sufficiently strong to funnel the inflow, the nuclear burning shells are stabilized against explosive bursts 13,14 and this argument can then, in principle, be used to place an upper limit on the magnetic field

strength such that the accreted material is spread over the entire stellar surface . Let us assume that the accretion column contains enough material to produce a ti 6 x 1039 ergs burst (i .e., about 1038 ergs s-1 for 60 s) via nuclear burning . If we simplistically assume the material is degenerate and that it forms a cube (of side length 1 km), we can relate the Fermi pressure Pf ne 1013 x ( p )5/3 dynes cm-' , PC

(1)

to the magnetic field pressure P,

=

s

8a

(2)

to get an order of magnitude estimate of the field strength needed to confine the column. In so doing, we find that

M.P. Ulmer, F. Melia/X-ray and gamma-ray bursts MXB 1735 - 44 t t

10

1.3-3 keV

cts/0.4 sec

6-1

3-6 keV

15

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-

5

y

5-12 keV

30-

8-19 keV

30-

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.

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8

19-27 keV cts/0.8 sec

4 I ~ O r

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~ jj -' ßl I

50 Time in Seconds

Figure 2: Profiles of typical X-ray bursts from three different sources in five X-ray energy channels (from reference 1). The gradual decay persists longer at low X-ray energies than at high energies, indicating cooling of the burst emission region . B .. 1012 G x ( M°" )s~e 10 21 g

(3)

where Ma. is the accreted mass. We can also see how the size of the polar cap region scales with B by equating the gravitational force on the accreted material to the total stress in the magnetic field, i .e., GMM R2 ar e

~ .,

L2 B2 g7r

where L is a characteristic scale length across the column. Thus, for the canonical values M = 1 .4 Me and R = 20 km, B must be 51011 gauss in order for the entire surface to be covered (i.e., L 100 km) .

These estimates can be reconciled with observations of X-ray pulsars, which are known to contain accreting, strongly-magnetized neutron stars with fields B Z 1012 gauss . That none of these systems has ever been seen to produce X-ray bursts supports the hypothesis that bursting neutron stars are weakly magnetized . 21 .2

The recurrence time scale

A lower limit to the inter-burst interval At, is the time it takes to store the - 1021 grams of hydrogen and helium needed for a thermonuclear runaway on the stellar surface. 4s it accretes, the material releases gravitational energy which is thermalized and

M.P.

132

Ulmer, F.

Melia/X-ray and gamma-ray bursts

T® =1 .76 keV

Figure 3: The calculated temporal variation of the surface luminosity for a theoretical X-ray burst (from reference 4). Here, to is the recurrence time scale equal to 11.7 hr. The spectral temperature of the burst (Te) is given at the peak of the burst and after an e-folding decay time. appears as a steady flux of X-rays between the transient nuclear events . In principle, we should therefore be able to infer a value for At, by comparing the total energy emitted as a result of nuclear burning during the burst to that released in the quiescent state. Let L. be the steady X-ray luminosity, Lb the burst luminosity, Ata the burst duration (- 608), eb the conversion factor of grams to ergs during the nuclear burning (;t: 6 x 1018/cr ergs g-1 x 0.007), and Ea,c the conversion factor of gravitational energy per unit mass to ergs (= GM/R62 _ 0 .15). Then, putting Lb x Atb ^ E6

and

c2Atp

(5)

L. = c«.,, Ùc2 ,

6

we get At'

=

LbEaee X Atb . Lob

7

Thus, since L6/L. .,Ï 10, At, P-- 12000 s x (

L6 Lb

Atb ) - )(6o s

We see that bursts with higher ratios of Lb to L$ have longer recurrence time scales, and that typical interburst intervals should be on the order of hours for X-ray sources with a detectable, steady X-ray flux.

M.P. Ulmer, F.

Melia/X-ray and

gamma-ray bursts

133

t

Figure 4: Energy spectrum of an X-ray burst from X1636-536 (reference 62) . The dip at ";: 4.1 keV can be interpreted as gravitationally redshifted K-shell absorption by a heavy element, such as iron. ®3

uation o state

In principle, we can also use X-ray bursts to learn about the equation of state of neutron-star matter . We demonstrate here how this might be accomplished, although (as we shall see) the interpretation of the data is still open to question . The idea is based on a feature (at - 4.1 keV) that is sometimes seen in the X-ray spectra (Figure 4) of at least 4 different sourcesis , is,s, and has been interpreted as a gravitationally-reshifted iron line. Cyclotron absorption is unlikely to be the cause of the spectral dip since, as discussed above, X-ray burst cources are apparently weakly magnetized neutron stars (B « 10" gauss) and the field strength needed to produce a feature with central energy E is (9) B ;~% 3 x 101, ( E ) gauss . 4.1 keV For a stationary emitting surface area, the radiative

flux is limited by the value (the so-called "Eddington" limit) at which the outwardly directed pressure due to momentum transfer from scattering interactions between the photons and particles is balanced by the inwardly directed force per unit area due to gravity. If we know that the observed flux is Eddington-limited, the total himinesity can then be directly related to the mass of the underlying neutron star, which results in the expression _

öcs r,2_ Las/2 4x aG(R) (1 R)

() 10

where ae is the electron scattering opacity of pure hydrogen, a is the Stefan-Boltzmann constant, G is the gravitational constant, c is the speed of light, re - 2GM/c® is the gravitational radius, and 8 is a complex function of the color and effective temperatures that depends on measurements of the flux, the spectral shape, and the source distanceis,is . A second relation between the neutron-star mass and

M.P. Ulmer, F. Melia/X-ray and gamma-ray bursts

13 4

N

O

2

L

O O W O

'0

Rad 1 us (km) Figure 5: Error domains of mass and radius of the neutron star in 1606-52, for five candidate elements of the 4.1 keV absorption line, superposed on six theoretical neutron-star mass-radius relations. The region with Fe* corresponds to the model that includes the transverse Doppler effect" . The solid curves correspond to calculations where the velocity doppler shifts are ignored . (Adapted from reference 5.) radius follows from tae assumption that the feature is redshifted by the gravitational field at the stellar surface, which gives

where co and e are the natural and gravitationally redshifted line energies, respectively. This equation includes the effect due to the special relativistic doppler shift resulting from the I{eplerian motion of the absorbing material. We note, however, that a doppler shift due to high velocities may be inconsistent with the observed narrowness of the line. In addition, all 4 sources display lines with energies that are within - 5% of each other, which would require a unique value of the observation angle relative to the orbital plane ofthe absorbing material in every source . (That these energies are so close to each other also raises a concern about the basic interpretation of the origin

of these lines since the surface gravities of all these different sources would then have to be the same.) With these caveats in mind and given the uncertainties in the relevant input parameters, we can use these relations to delimit the range of allowed values of M and R, and M/R. Figure 5 (from reference 16, as modified by reference 5) shows the M - R relation for different equations of state, together with the results of the above calculations. The short diagonal lines, for all but Fe*, correspond to calculations where the velocity doppler shifts are ignored, assuming the line is due to elements other than iron. Of course, the hypothesis that the absorbing element is not. iron has its problems also, since a single element must be produced and transported to just the right optical depth to cause the observed effect .

M.P. Ulmer, F. Melia/X-ray and gamma-ray bursts

135

Figure 6: X-ray bursts from the Rapid Burster MXB1730-335 (adapted from reference 1). These pulses show no spectral softening during decay, indicating a non-thermal emission process-perhaps due to sporadic accretion .

2.4

The

apid

rster

The behavior of the rapidly bursting X-ray source MXB1730-335 (reference 17) is unique, in that it produces two distinctly different types of bursts" ,la,ao~ one like the classical bursts discussed above, and a second more common type that can occur in quick succession with intervals as short as a few seconds shown (see Figure 6). Yet this object has never been observed to emit a steady flux of X-rays. Interestingly, bursts of the second type do not show a significant spectral softening during decay, which suggests that they operate via a different mechanism from the explosive nuclear burning believed to be responsible for the first type. Instead, these transient events are likely due to sporadic accretion in which the material is temporarily stored and then accreted as discrete

"blobs" rather than in a continuous flow. It is still a puzzle why this behavior occurs only in this source and what produces this apparent sporadic accretion.

.5

Summary o

-ray

ursts

In summary, we have a standard model for X-ray bursts in which the underlying source is a (weakly magnetized, i.e., B « 1011 gauss) neutron star, burning fuel that is rich in hydrogen and helium on its surface. In some cases, the burst spectrum shows a feature (at ti 4.1 keV) that has been interpreted as a gravitationally redshifted photoelectric absorption line. In principle, this feature can allow us to derive limits on the stellar mass and radius . Two of the remaining puzzles are: why do all the absorption fea-

136

M.P. Ulmer, F. Melia/X-ray and gamma-ray bursts

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1

1 1

1--.1-I-,-I -1

d

ENERGY,

eV

Figure 7: Three spectra taken at different time intervals during a Gamma-ray burst displaying emission up to 100 MeV . (Adapted from reference 25.) tares have an energy of 4.1 keV? and what makes the unique rapid burster MXE1730-335 burst?

S S

As observed from Earth, Gamma-ray bursts se,'-- in -pore powerful and more exotic than X-ray bursts . Cammaray burst spectral measurements display a rich phe. romenology that perhaps belies the uniquene "-. of an underlying physical mechanism (see Figures 7, 10, and 11 for examples; reviews of spectral data have

been given by, references 21 and 22) . Line-like features have been observed in the ti 20 - 80 keV range as well as in the 400 - 500 keV range. The former occur in ti 20% and the lat" ?r in ti 5 - 10% of all bursts observed with KONUS". In addition, significant .y-radiation has sometimes been detected at energies well in excess of 10 MeV (reference 24), extending up to 100 MeV in at least one burst2 $, compared with X--: a-d bursts whose spectra turn over at - 10 - 20 keV . In addition, peak Gamma-ray burst fluxes can be as high as ti 10-4 ergs s-1 cm-a, four to five orders of magnitude greater than those of X-ray bursts .

M.P. Ulmer, F. Melia/X-ray and gamma-ray bursts

13 7

1200 1000® v r W

Soo

Q

600 e X 400 g_ W Z 200 0

Figure 8: Time history of a Gamma-ray burst detected on March 7, 1979, showing the count rate in X-rays (filled circles, 3 -10 keV) and gamma-rays (histogram, > 100 keV). (From reference 22.) Gamma-ray burst time histories display a wide variety of shapes and durations, ranging from 50 ms events with a single peak, to events with a complex time structure lasting many tens of seconds (e.g., Figures 8 and 9). In some cases, there have been hints of periodicities in the range of - 5 s (e.g., reference 26), but this is quite rare. With the exception of only 3 objects, whose unique spectral characteristics imply a separate class (sometimes known as "Soft Gamma Repeaters"), no Gammaray Burst sources with accurate localization have been seen to repeat . Thus, a lower limit to the recurrence time scale between bursts from a single source is the observation time, spanning some 10 -15 years . However, this estimate is somewhat mitigated by the current (high) detection thresholds, which does not allow us to observe bursts with lower intensities . The distribution of bursts displays no clustering about the galactic plane, or near the Galactic Center, which would be strong evidence for a galactic origin . But Gamma-ray burst sources could still be in the galactic

disk if current instruments are not sufficiently sensitive to sample beyond 500 - 1000 pc. However, the apparent isotropy seen in the distribution does not rule out an extended galactic halo population, nor a distant extragalactic one . One of the biggest problems with source determination is that in spite of direct searches for quiescent low-energy counterparts, no Gamma-ray burst sources have ever been convincingly identified in the radio27, infrared2s,ae, optical (e.g., reference 30), and X-ray regimes31,3a, except for a possible association of the unique 1979 March 5 GRB with the N49 supernova remnant in the Large Magellanic Cloud33 . As such, the distance to these sources is unknown, so that Gamma-ray bursts could originate anywhere from 10 parsecs to 1000's of megaparsec away. We must therefore resort to indirect arguments and intuition in our attempt to understand the underlying physical processes responsible for this phenomenon. There is, however, some evidence that the sources

M.P. Ulmer, F. Melia/~l-ray and gamma-ray bursts

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BSEE-3 REAL TIME

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f~ti~~ 10

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. 30

40 50 60 ELAPSED SECOt~OS

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Figure 9: Time history of a Gamma-ray burst detected on August 1, 1983, in the energy band 7 -10 keV . (From reference 22.) may be neutron stars, and we will examine some of the arguments in favor of this hypothesis in the following sections. The points wN plan to address, at least briefly, are: the geometry of the emitting region, the storage mechanism, the energy output, the lack of optical/radio identification, and the distributlcn of these objects in the Galaxy. We can also use the observations to tell us implicitly something about neutron-star evolution and the number and spatial distribut:or~ of neutron stars. .1

t®ragp/Energy C®nsi erati®ns

If Gamma-ray bursts originate from nearby sources (~1 kiloparsec), the implied burst energy is ti 103T ergs, which is released over roughly a 10 s interval. T'r_is is typical ofthe power emitted during other transient events associated with neutron stars (e.g., X-ray bursts), and together with the rapid variability seen in typical Gamma-ray burst light curves (~ î - 10 ms), which is consistent with the dynamical timescale

at the surface of a solar mass object with a radius 10 - 20 km, suggests that these events too are due to the release ofenergy in a neutron-star environment . There exist several mechanisms by which this power may be generated, including (1) star quakes due to adjustments in the equation of state (e.g., as a result of "phase transitions" ; reference 34). A typical radial contraction of about 10_s cm for a rotating neutron star (period ti 1 s) will induce a change in the moment of inertia corresponding to roughly 103T ergs of rotational energy. (2) Thermonuclear flashes in the Hydrogen-rich material accreted on the stellar surface3 s~38, which is known to work in the case of Xray bursts (see above) . (3) Accretion processes (e.g., reference 37), which have been observed in accreting neutron stars within binary systems . (4) Magnetospheric flares3s~39 ~ 4° , which are implied by the strong magnetic fields (z101 ~ gauss) inferred in some bursts (see below). Thus, storing the requisite amount of energy in a magnetized, rotating neutron star is not dif&cult.

M .P. Ulmer, F. Melia/X-ray and gamma-ray bursts

3.

Spatial istri ution, Identi cations, and istances

As noted above, the distribution of Gamma-ray bursts in the sky is apparently isotropic (see, for example, reference 41) . This implies relatively nearby (- 10 500 parsecs) or very distant (i .e., either in the halo of our own galaxy, or cosmological) localizations . The extragalactic interpretation requires an extremely energetic phenomenon that we would probably classify as "exotic," for example, cosmic strings or primeval black holes . If we ignore the extra-galactic possibil-

ity, the implication of isotropy is that neutron stars might exist well out of the galactic plane (e .g ., in the halo) . Of course, if we could identify these sources with known astronomical objects whose intrinsic luminos-

ity and distance were measurable, we would be able to say much more about the origin of the Gamma-

ray bursts themselves . However, as we have already remarked, very deep searches of relatively small error boxes have not yielded any conclusive identifications, though some hope along these lines may have been offered by the recent association of an optical counter-

part with the (steady) gamma-ray source Geminga42.43 . This object, which is itself believed to be a lone neutron star, has spectral colors that are similar to those

139

sume the Gamma-ray bursts originate from neutron stars at a distance D ;!~ 1 kiloparsec, the observational

constraints on the optical flux imply that the neutron star cannot have a main sequence companion with a mass (M z 0 .08 o ) . The neutron star must either be in a very low mass binazy,4or 9t mast be isolated (possibly with a degenerate accretion disk43-

).

Moreover, no burst source has ever been associated with known pulsars, even though large magnetic fields (z10" gauss) are inferred in many cases (see below) .

This suggests either that the magnetic field in these sources is aligned with the spin axis or that the spin rate is so low that the system no longer pulses . An independent upper limit to the distance comes from the argument that copious sources of photons with energies above - 1 MeV cannot be too far away,

otherwise the large rate of e+e - pair creation due to -y-1 interactions in an intrinsically intense radiation

field would render the gamma-ray emitting region too optically thick. That we see emission at least out to 100 MeV, implies the optical depth to -y-r interactions is probably small, so that (in the absence of beaming) the distance should be

F, Ichar i/$ kp c . (12) D S 2 (10-6 ergs s-1 cm-2 )( 108 cm)

expected of Gamma-ray burst sources 83 .44,4s, and therefore suggests that an analogous search ir. the error boxes of known burst locations might result in the

We assume here that the most rapid variation observed gives us the size of the emitting region, i .e ., 3 x 10-3 s, c is the speed of Id,., = At c, where ®t

pectation that Gamma-ray bursts might also be accompanied by transient events at longer wavelengths (e .g., optical flashes) (e .g ., reference 46) . In this way,

Using a constraint such as this, Hartmann et al. (reference 50) concluded that the known number of neutron stars within -!~2 kpc is insufficient to generate all of the Gamma-ray bursts if the radiation is emitted isotropically from a 1 km2 polar cap area . Thus, it seems that at least partial beaming of the highenergy component may actually be required, since a

detection of (faint) optical counterparts. Another promising avenue has been one of searching archival photographic images of these error boxes with the ex-

several possible candidates have been identified, but the question of whether the association is real still remains controversial (see references 43, 44, 45, 47, and 48 for relevant discussions) .

Even though these searches have not yet been successful in identifying the Gamma-ray burst sources themselves, they have nonethelss set stringent upper limits on their quiescent fluxes . For example, if we as-

light, and F,, is the observed flux.

larger distance to the burst locations increases the amount of space sampled . Invoking beaming will not necessarily help this problem, since tight beaming requires that there be many more burst sources than

are actually observed . However, beaming does seem to be a promising way to explain the general spectral

M.P. Ulrr!er, F. lilelia/X-ray and gamma-ray bursts

140

E (keV)

Figure 10: Gamma-ray burst spectra with absorption lines detected by the Soviet KONUS experimentssl . features of bursts (see below for further discussion). 3.3

etr

sorption Features

e®

The case for a neutron-star origin of Gamma-ray bursts has been strengthened by the discovery of low-energy spectral features with KONUS (reference 51), IIEAO1 (reference 52), and GINGA (reference 53) (see Figures 10 and 11 for examples) . The observational status of the earlier events had remained controversial, in part because of the possibility that they were due to fast spectral variation or low temporal and spectral resolution of the instruments . These results were confirmed, however, by observations made with the Gamma-ray burst detector (GBD) on board the GINGA satellite . These differ from the previous measurments in that (i) the GBD proportional and scintillation

counters together cover a wide energy range from 1.5 to 375 keV, and (ii) for the first time two absorption lines were seen in the same burst . These features (at ,. 20 and 40 keV) have been interpreted as the fundamental and first harmonic of a cyclotron resonance, implying a magnetic field strength of ti 2 x 1012 gauss . These lines are very narrow, indicating that the mean energy per particle where these features are produced is low, and that the magnetic field variation is not sufficient to smear out the harmonic structure . These conditions require an absorption region no larger' han - 0.07R (for a dipole field near the surface of a neutron star of radius R) at a temperature kT AS' 5 keV (references 54, 55) . Thus, it is likely that the spectral component associated with the low energy features (- 10 - 100 keV) originates very close to the stellar surface . Several inferences concerning the geometry can be

M.P. Ulmer, F. Melia/X-ray and gamma-ray bursts 2 .0

0 -2.0

-3 .0

H

0 .0

I

10

100

r 1000

Energy (KeV)

Figure 11 : Spectrum of a Gamma-ray burst showing two absorption lines (at ti 20 and 40 keV), interpreted as the fundamental and first harmonic of a cyclotron resonance in a magnetic field .;: 2 x 1012 gauss . (Adapted from reference 53.) made from these observations. First, models requiring accretion onto a polar cap as the primary release of energy in Gammaray bursts seem to be ruled out because at large viewing angles - a/2 to the magnetic axis the projected polar cap area would be too small, whereas gamma-rays emitted at small viewing angles 0° would be absorbed by the overlying material. Second, only a fraction of the bursts are associated with absorption lines, so that either (1) not all burst sources have 1012 gauss fields, (2) not all bursts originate close to the stellar surface, or (3) the emission is anisotropic so that the spectrum is dependent on observation angle and the low-energy features are sometimes obscured by other spectral componentss s- 57-58. Third, photons with energy Z 2 MeV materialize in the presence of a 1012 G field transverse to their direction of propagation . Thus, in bursts that exhibit b-ath N

cyclotron lines and high energy gamma-ray emission, the high-energy component must either be beamed along the magnetic field, or it must originate far from the stellar surface (or both) . Fourth, as is the case with X-ray bursts, it might be difficult to understand why all the absorption features have energies that are consistent (within observational error) with the same value in all bursts . Instrumental effects seem to be ruled out as the cause of the features since the lines have now been observed by three different detectors . Yet another indication that the gamma-rays are either preferentially beamed away from the stellar surface and/or are produced well above it is the so-called "X-ray paucity constraint." Very few Gamma-ray bursts have been observed (usually serendipitously) with X-ray instruments (see Figure 12 for one exam-

M.P Ulmer, F. Melia/X-ray and gamma-ray bursts

14 2

1972 May 14 First Pulse

v

ao a~

s~o 0

~II~II!I~IIIiIII~IIIIIII~IIIIIII

0

2 4 6 log lo (Photon energy / eV)

8

Figure 12: Spectrum observed during the first pulse of the 1972 M~,y 14 eventb9. Although the error-bars associated with the X-ray point at ~ 10 keV are large, the data suggest an excess below this energy. Compare with the theoretical curve shown in Figure 14. ple), but in bursts whose spectrum below ~ 10 keV is known, the emitted power in the ti 3 - 10 keV range is seldom more than a few percent of the total power in gamma-rays. Thus, only a small fraction of the gamma-radiation may be incident on nearby matters, since the deposited gamma-ray energy is reprocessed and reemitted as ±hermal radiation at UV and X-ray energies. Many of these characteristics may be understood in the context of a model wherein the overall spectrum changes with aspect angle as a result of the superposition of several components with different angular distributions . For example, in the magnetospheric plasma oscillation (MPO) model4o~se, the incipient gamma-rays (resulting from first and higher order Co~npton events) are produced when soft photons emitted by the stellar surface are upscattered by the relativistic particles in an oscillating region above the polar cap (Figure 13). The thermal emission is due in turn to the reprocessing of gamma-ray energy ab-

sorbed by the neutron star, and the fourth component results fro~nä the reflection of gamma-rays at the reprocessing boundary. Although all four components have a common physical origin, the incipient gammarays are preferentially beamed in the di-ection of the magnetic field within the oscillating region, whereas the reflected radiation and (especially) the thermal emission are distributed more isotropicaLl_y. Thus, for example, it is possible în this picture to observe large gamma-ray fluxes while at the same time the X-ray temperature is relatively low because the total burst power is concentrate ~ in a narrow cone of high flux. Figure 14 shows that the MP® model can successfully reproduce typical Gamma-ray burst spectra (such as that observed during the 1972 May 14 event ; references 59, 60, 61), demonstrating the viability of models in which the radiation is emitted anisotropically. Beaming such as this seems to be a natural ingredient in many models of Gamma-ray bursts since it can be coupled directly to the emission mechanism

M.P. Ulmer, F. Melia/X-ray and gamma-ray bursts

143

Figure 13: Schematic diagram showing the geometry of the oscillating plasma above the polar cap region" . The particle motion is damped by inverse Compton scattering interactions with thermal photons originating from the stellar atmosphere . Photons (1)-(4) represent the main spectral components: (1) once-scattered radiation, (2) radiation undergoing multiple-scattering events (mostly second-order), (3) ,y-rays reflected by vhe star, and (4) the self-consistently calculated thermal emission due to the deposition of , y-ray energy below the photosphere . itself. In addition, there exists a precedent for a beaming hypothesis from several. other astronomical sources, ranging from galaxies with radio jets, down to neutron-star systems such as SS 433 . Even so, until an optical or radio identification of a gammaray burst source is made, we can only lean heavily on our preconceived notions and guess that neutron stars have all the "right" ingredients to produce the bursts.

We note, finally, that although the emission-Lke feature seen in some bursts by KONUShas not yet been confirmed (see above), its interpretation as a gravitationally redshifted e+e - annihilation lire demonstrates that, in principle, a study of Gamma-ray bursts may also also lead to a better understanding of the neutron-star equation of state.

3.4

Summary of Gamma-

ursts

Although none of the Gamma-ray burst sources has yet been firmly identified with an optical or radio counterpart, there is circumstantial evidence that point to a neutron-star origin: (1) A fraction of the bursts display low-energy features that may be attributed to cyclotron absorption in a strong magnetic field

M.P. Ulmer, F. Melia/X-ray and gamma-ray bursts

144

-2 r CM

v

1

0

St

order

-toal

reflected

0

ao

0

A x

a -. ô 1-4

-6

_8

0

2 4 6 log io (Photon en2rgy / eV)

8

Figure 14: Theoretical spectrum (ergs s-1 cm2) arising from the superposition of the 4 components shown schematically in Fig . 13, for a viewing angle ® 20 - 30° with respect to the field direction in the oscillating region: dashed curve - first-order scattering, thin solid curve - reflected ,y-rays, thick solid curve - overall spectrum. I,6o is the source distance in units of 50 pc. Compare with the observed spectrum shown in Figure 12. (From reference 56.) (~ 1011 gauss); (2) The energetics are favorable if the objects are galactic ; (3) unless relativistic beaming is important (which may indeed be the case), the presently observed objects must be AS2 kpc away to explain the detection of significant flux above - 1 1VIeV . From the study of Gamma-ray bursts, then, we can hope to learn something about the Galactic distribution and number of neutron stars, and how neutron stars evolve and cease pulsing, yet retain strong magnetic fields--either through alignment of the magnetic field axis with the rotation axis or by spinning down below the rate at which they can pulse. We may also be able to use these objects to learn about the M/R relation of a neutron star. But perhaps the most exciting possibility is that the current circumstantial

evidence is misleading and neutron stars are not involved . For example, if Gamma-ray bursts originate outside of our galaxy, the underlying physics would undoubtedly be new and exotic . Time will tell.

We conclude by making the following comparisons between X-ray bursts and Gamma-ray bursts . Both of these transient phenomena are produced by neutronstar systems . X-ray bursts originate from neutron stars with weak (< 1011 gauss) magnetic fields whereas Gamma-ray bursts apparently are produced in environments with strong magnetic fields (z101 a gauss) .

M.P. Ulmer, F. Melia/X-ray and gamma-ray bursts

X-ray bursts result from the explosive nuclear burning of hydrogen and helium stored on the stellar surface . As yet, there is no consensus about the mechanism for generating Gamma-ray bursts, but current models involving plasma oscillations seem promising. Some puzzles remain . The rapid burster is uniquewe would like to know why it bursts and why its behavior is distinctly different from other X-ray burst sources . We also would like to understand what causes the absorption lines in X-ray bursts, and why these features always seem to have the same energy. We would like to understand the origin of the absorption lines in Gamma-ray bursts as well, and why these too seem to occur within a narrow range of energies. Most importantly, we need to have optical and/or radio identifications before we can reasonably expect to make significant progress in our understanding of the Gamma-ray burst sources and the underlying physical processes . In conclusion, X-ray and Gamma-ray bursts continue to be intriguing phenomena, and we await with excitement the development of new insights provided by future observational and theoretical work. We gratefully acknowledge discussions with Ron Taam and Steve Matz, who provided us with many useful insights.

ere ces 1 Lewin, W. H. G., and Joss, P., Sp. Sci. Rev., 28, (1981) 3. 2 Joss, P. C. and Rappaport, S. A., Ann . Rev. Aston. Astro., 22, (1984) 537 .

3 Melia, F., and Joss, P. C., in Radiation Hydrodynamics in Stars and Compact Objects, Eds. D. Mihalas and K.-H. A. Winkler (Berlin: SpringerVerlag) (1985) p. 283 . 4 Taam, R. E., Ann. Rev. Nucl. Part. Sci., 35, (1985) 1. 5 Inoue, H., in Physics of Neutron Stars and Black Holes, ed. Y. Tanaka (Tokyo : Universal Academy), (1988) p 235 . 6 Stella, L., Priedorsky, W. and White, N. E.,

145

Ap. J., 312, (1987) L17. 7 Swank, J. H., Becker, R. H., Boldt, E. A., Holt, S. S., Pravdo, S. H., and Selemitsos, P. J., Ap. J. (Letters), 212, (1977), L73.

8 Woosley S. E. and Taam, R. E., Nature, 263, (1976) 101. 9 Maraschi, L., and Cavaliere, A., in Highlights in Astronomy, Ed. E. A. Mûller (Dordrecht: Reidel), (1977) p. 127 . 10 Joss, P. C., Nature, 270, (1977) 310.

11 Taam, R. E., Ap. J., 258, (1982) 761. 12 Melia, F., and Joss, P. C., in Nigh Energy Transients in Astrophysics, ed. S. E. Woosley (New York: AIP), (1984) p. 330 . 13 Joss, P. C.y Ap. .T jQetters), 225, (1978) L123. 14 Taam, R. E., and Pickluro, R. E., Ap. J., 224, (l978) 2l0 . 15 Fujimoto, M. Y., Ap. J. (Letters), 293), (1985) L19 . 16 Fujimoto, M. Y., and Taam, R. E.,Ap. J. , 05 (1986) 246 . 17 Lewin, W. H. G., Doty, J., Clark, G. W., et al., Ap. J (Letters), 207, (1976) L95 .

18 Ulmer, M. P., Lewin, W. H. G., Hoffman, J. A., Doty, J. and Marshaal, H. Ap. J. (Letters), 214, (1978) L11. 19 Hoffman, J. A., Marshall, H. M. and Lewin, W. H. G., Nature, 271, (1978) 630 . 20 Marshall, H. L., Ulmer, M. P., Joffman, J. A., Doty, J., and Lewin, W. H. G., 1979, Ap. J., 227, 555 . 21 Teegarden, B. J., in High Energy Transients in Astrophysics, ed. S. E. Woosley (New York: AIP), (1984) p. 352 . 22 Liang, E. P., and Petrosian, V., AIP Conference Proceedings 141 (New York: AIP) (1984) . 23 Mazets, E. P., et al., Space Sei., 82, (1982) 261 .

24 Matz, S. M., Forrest, D. J., Vestrand, W. T., Chupp, E. L., Share, G. H., and Rieger, E., Ap. J. (Letters), 288, (1985) L37 .

M.P. Ulmer, F. Melia/X-ray and gamma-ray bursts

146

25 Share, G. H., Matz, S . M ., Messina, D. C., Nolan, P. L., Chupp, E. L., Forrest, D. J., and Cooper, J. F., Adv . Space Sci., 6, (1987) 15.

46 Schaefer, B . E., Nature, 302, (1981) 43 .

26 Wood, K., Byram, E., Chubb, T., Friedman, Meekins, J., Share, G., and Yentis, D ., Ap. J., 247, (1-981) 632 .

48 Melia, F., Ap. J., 335, (1988) 965.

27 Hjellming, R. M., and Ewald, S. P., Ap. J. (Letters), 246, (1981) L137. 28 Apparao, K. M. V. . and Allen, D., Astron . Ap., 107, (1982) L5. 29 Schaefer, B ., et al., Ap. J., 313, (1987) 226 . 30 Terrell, J., Fenimore, E., Klebesadel, R., and Desai, U., Ap. J., 254, (1982) 279.

31 Grindlay, J. E., et al., Nature, 300, (1983) 730 . 32 Pizzichini, G ., et al., Ap. ., 301, (1986) 641 . 33 Evans, W. D., et al., Ap. J. (Leiters), 237, (1980) L7. 34 Ramaty, R., et al., Nature, 287, (1`-80) 122 . 35 Woosley, S . E., in High Energy Transients in Astrophysics, ed. S. E. Woosley (New York: AIP), (1984) p. 485.

36 Bonazzola, S., Hameury, J. M., Heyvaerts, J., and Lasota, J. P., Astron. Ap., 136, (1984) 89. 37 Colgate, S. A., Petschek, A. G ., and Sarracino, R., in High Energy Transients in Astrophysics, ed. S. E. Woosley (New York: AIP), (1984) p. 548 . 38 Liang, E. P., Ap. J. (Leiters), 283, (1984) L21 . 39 Melia, F., and Fatuzzo, M., Ap. J., in press (1989) .

40 Melia, F., Ap. J., ia press (1989) . 41 Hurley, K., in High Energy Phenomena Around Collasped Stars, ed. F. Pacini (Reidel:Dordretch) (1987) p. 317 . 42 Halpern, J. P., and Tytler, D., Ap. J., 330, (1988) 201 . 43 Melia, F., Nature, 338, (1989) 322. 44 Melia, F., Ap . J. (Letters), 324, (1988) L21.

45 Melia, F., Proc . Sofia COSPAR Symp ., Ed. N. White, (1988) p. 641 .

47 Melia, F., Rappaport, S., and Joss, P. C., Ap. J. (Letters), 305, (1986) L51. 49 Michel, F. C., Ap. J., 290, (1985) 721.

50 Hartmann, D. et al., Ap. J., in press (1989) . 51 Mazets, E. P., Golenetskii, S. V., Aptekar', R. L., Gur'yan, Yu. A., and Il'inskii, V. N., Nature, 290, (1981) 378.

52 Hueter, G. J., in High Energy Transients in Astrophysics, ed. S. E. Woosley (New York : AIP), (1984) p. 373.

53 Murakami, T., et al., Nature, 335, (1989) 234. 54 Melia, F., Ap. J. (Letters), 334, (1988) L9 . 55 Melia, F., Nature, 336, (1988) 658. 56 Melia, F., Ap. J., submitted (1989) . 57 Melia, F., Astron. Ap., submitted (1989) . 58 Melia, F., Pub. Astron. Soc. Japan, submitted (1989) . 59 Wheaton, W. A., Ulmer, M. P., Baity, W. A. et al. , Ap. J. (Letters), 185, (1973) L57. 60 Ulmer, M. P., 1974, in Proceedings of the International Conference on X-rays in Space, ed. D. Venkatesan, (Univ. of Calgary : Calgary), P. 163. 61 Ulmer, M. P. 1973, in Proceedings of Conference on Transient Cosmic Gamma- and X-ray Sources (LA-5505-C), ed. I. B. Strong, (LANL:Los Alamos), p 33 . 62 Waki, I. Inoue, H., Koyama, K ., et al., Pub. Astron . Soci. Japan, 36, (1984) 819.