Very high energy gamma-ray astronomy and the origin of cosmic rays

LJCLEARPHYSICS

EI~NliVII-R

Nuclear Physics B (Proc. Suppl.) 39A (1995) 193 206

PROCEEDINGS SUPPLEMENTS

Very high energy gamma-ray astronomy and the origin of cosmic rays F.A.Aharonian Max-Planck-Institut ffir Kernphysik, Saupfercheehkweg 1, D-69029 Heidelberg, Germany The paper highlights the status and motivations of very high energy (E > 100 GeV) "r-ray astronomy in the era of the Compton GRO. I discuss the potential of future ground-based 7-ray observations with emphasis on objectives connected with the general problem of the origin of galactic cosmic rays.

1. I N T R O D U C T I O N It is difficult to overestimate the significance of the study of primary cosmic "r-rays by groundbased detectors. The outstanding success of the C o m p t o n G a m m a - R a y Observatory, in particular the results obtained by the Energetic G a m m a Ray Experiment Telescope ( E G R E T ) [1], indicate an obvious necessity for a new generation of satellite-borne high energy 7-ray detectors. The primary aim of this activity seems to be in performing deep sky surveys in 7-rays at energies E < 10GeV. Furthermore, since most of the E G R E T sources do not exhibit spectral cutoffs in the 1-10 GeV region, the extension of investigations into the unexplored region beyond 10 GeV seems to be the second important issue, ttoweveL for any practicable effective area of space-based 7ray telescopes (S < 10 m 2) the very high energy (VHE) region above 100 GeV will remain, at least in the foreseeable future, the province of groundbased "r-ray detectors. Moreover, it is hkely that the ground-based detectors will fill (partially, of course) the " v a c u u m " of the high energy 'r-ray observations from space which is unfortunately expected during at least several years after the expiration of the G R O mission. The status of ground-based "r-ray astronomy may be characterized as controversial and ambiguous activity in the past, with modest bnt firmly estabhshed results at the present, and with good prospects for the future. Numerous claims by m a n y groups have been reported during the last 10 years of detections of signals from more than 25 astrophysical objects 0920-5632/95/$09.50© 1995 Elsevier Science B.V. SSDI 0920-5632(95)00022-4

All

rights reserved.

at TeV a n d / o r PeV energies. At first sight, the picture seems rather impressive. However, closer examination of these results shows that most of them have marginal statistical significance [2]. In fact, there are only 3 undisputed DC sources of VHE 7-rays associated with the Crab Nebula, the active galaxy Markarian 421, and the pulsar P S R B1706-44 (see e.g. [3]). Also, tens of episodic events reported by several groups from X-ray binaries and cataclysmic variables like Her X-l, Vela X-1 and AE Aq (for review see [2],[4]) perhaps could be added to this "fist" of V I l E "r-ray emitters. And finally, it should be also mentioned that Cyg X-3 has been claimed by m a n y groups as an emitter of neutral particles at GeV, TeV, PeV and EeV energies (for review see [2],[5]). Cyg X3 has played perhaps the most i m p o r t a n t role in the 80s in the renewed interest in ground based "r-ray observations, but ironically this very source has created, to some extent, a certain doubt concerning the credibility of most results of groundbased 7-ray astronomy in the past. Most of experts treat these d a t a with a healthy degree of scepticism due to the low confidence level of the experimental results and the reported extraordinary characteristics of the primary radiation . One of the principal reasons for the slow development of ground-based "r-ray astronomy is connected perhaps with the fact that the community has only recently reahzed that the proper detection/identification of the primary "r-rays needs much more sensitive and sophisticated detectors and methods than the ones used in the past in traditional cosmic ray experiments. Indeed, the noticeable progress in the so-called Imaging

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F.A. Aharonian/Nuclear Physics B (Proc. Suppl.) 39A (1995) 193-206

Atmospheric Cherenkov Technique immediately yielded the first high confident DC detections of TeV 7-rays from the Crab Nebula [2] and PSR B1706-44 [6]. It is worth noting that the current sensitivity of the Whipple Observatory 10 m telescope is already sufficient to give a detection of the Crab Nebula at the ~ 5o" signal level after only 1 hour of observation which sounds very impressive even by the standards of the satelliteborne 7-ray detectors. In this paper I do not plan to discuss the history of the controversy of the field, and refer the reader to the recent comprehensive review of T.Weekes [2] on that subject. Also, rather than attempting to cover all objectives of 7-ray astronomy, I will discuss only one specific aspect directly connected with the problem of the origin of galactic cosmic rays (CRs). I will try to bring forward some convincing arguments showing that the extension of study to the poorly explored energy domain E > 100 GeV opens a new and very promising p a t h towards the understanding of processes and sites of acceleration of galactic CRs.

2. T H E P O T E N T I A L OF THE DETECTION TECHNIQUE

Various methods have been applied to the detection of primary 7-rays by ground-based instruments [7] but at present the Imaging Atmospheric Cherenkov Technique is considered to be the most promising approach for significant improvement of the detector sensitivities below 10TeV. The high 7-ray detection rate and the effective CR background rejection ability are the most remarkable features of Imaging Atmospheric Cherenkov Telescopes (IACTs). The high efficiency of the technique predicted by Hillas [8] was first demonstrated by the Whipple collaboration using a single 10 m diameter reflector at Mt.Hopkins, Arizona (see e.g. [9]). The further development of the technique over the next few years is likely to be in two directions: (i) Determination of the energy and arrival direction of individual 7-ray primaries, with an essential improvement in the CR background rejection efficiency; (ii) Reduction of the energy threshold

of detectors towards < 100 GeV. The most effective way of achieving the first goal seems to operate several telescopes as a system. Although the Cherenkov images obtained by one telescope contain some information about the direction and energy of showers, an unambiguous determination of the energy and arrival direction of primary particles on an event-by-event basis requires the registration of each shower by at least two separate telescopes. The distance between telescopes should be large enough to give low correlation between the images of a given shower seen by each telescope (and therefore higher angular resolution and 7 / P separation efficiency) but at the same time they have to be close enough to provide a high rate of coincidence; depending on the angular, spectral and temporal features of the flux from a given source, the optimal distance between telescopes is estimated to be between 50 and 100 m. It is expected that I A C T systems like H E G R A , consisting of several imaging telescopes with reasonably high resolution cameras (pixel size 0.25 °) and moderately sized optical mirrors (Sr, ir "" 1 0 m 2) can provide an energy threshold as low as 300 GeV, angular and energy resolutions of about 6¢ ~ 0.10 and 6 E / E ~ 25%, respectively, and a minimum detectable (almost "backgroundfree") flux from point sources F,,i,~(> 1 TeV) ~-. 10 -12 ph/cm2s [10]. Future I A C T arrays covering an area > I km z, as proposed for the ambitious Telescope Array project [11] would further improve the sensitivity to F,-,,i,~ ~ 10 - l z ph/cm2s. An energy threshold of Cherenkov telescopes of around 100 GeV can be achieved with still reasonably sized reflectors, Smi r > 5 0 m 2, in combination with very high resolution (pixel size ~ 0.1 °) cameras a n d / o r by linking a few telescopes to form a system. The strong activity of some groups ( C A N G A R O O , CAT, Durham, Whipple) in this area will result, hopefully within the next few years, in the appearance of several "100 GeV" threshold 7-ray telescopes. To summarize, the planned systems of I A C T s sustain an optimistic view for the future of VHE 7-ray observations. In particular: (1) The minimum detectable V t t E flux could be be reduced down to u < 10 -12 erg/cm~s. This

EA. Aharonian/Nuclear Physics B (Proc. Suppl.) 39A (1995) 193-206

will allow one to probe the intrinsic (47r) luminosities of 7-ray sources at the level LVHE < 10a~(D/lkpc) 2 erg/s which is ~ 102 times lower than the relevant luminosities of the typical E G R E T sources at GeV energies; (2) For power-law 7-ray spectra extending into the VHE region ( d N ~ / d E o¢ E - a ) detection rates at TeV energies typically are expected to be higher than at GeV energies up to a ~ 2.5; (3) The IACT systems will provide better accuracy of determination of direction of individual 7ray primaries (5¢ < 0.1 °) than E G R E T . This, in combination with high detection rates, will allow one to study the angular distribution of moderately extended diffuse 7-ray sources; (4) The expected energy resolution, A E / E < 25%, coupled with high detection rates, will make possible good spectroscopic measurements. The atmospheric Cherenkov telescopes are oriented to the detection of 7-rays from point sources. In fact, the IACT systems provide the potential to search for 7-ray emission from extended regions. The relatively large fields of view of the H E G R A (~ 50 ) [10], TACTIC (~ 7 ° ) [12] and Telescope Array telescopes (..- 10 °) [11] will allow a purposeful search for moderately extended sources. It is believed that the ability of TeV instruments to detect 7-rays from ~ 1 ° sources could dramatically extend the observable classes of both galactic and extragalactic VHE 7-ray emitters. Meanwhile, studies of the largescale (>> 1°) diffuse sources as well as all-sky surveys cannot be effectively provided by IACTs. It should be noted nevertheless that the high sensitivity of the planned stereoscopic IACT systems will partially compensate for this disadvantage, at least for strong 7-ray sources. For example, they will be able to probe potential VHE emitters for 7-ray signals at the level of the " ~ C r a b " flux during only one night of observations. Another hope of performing all sky surveys is raised by the new type of "low threshold" air shower detectors represented by AIROBICC [13] and MILAGRO [14]. Though it is still not clear that these detectors will be sufficiently sensitive at energies well below 10 TeV, they, together with "100 TeV" air shower detectors like CASA-MIA [15], could en-

195

able effective searches for episodic serendipitous sources [2], i.e. sources without any firm a priori basis for their selection as possible 7-ray emitters. Furthermore, construction of such detectors on much larger scales (S > 1 km 2) will allow purposeful study of diffuse 7-radiation from different regions of the galactic disk at energies above 10 TeV. 3. S E A R C H I N G F O R S I T E S O F A C C E L ERATION OF COSMIC RAYS

Historically, the first interest to the 7-ray domain of the cosmic electromagnetic spectrum appeared more than 40 years ago (e.g. [16],[17],[18],[19]) in the context of the general problem of the origin of CRs. Later, experimental and theoretical investigations have revealed new exciting aspects of 7-ray astronomy , but the basic idea on the crucial role of 7-ray observations for solving the problem of origin of galactic CRs is still considered as one of the most important aims ofT-ray astronomy [20]. Detection of high energy ( E > 100 MeV) diffuse 7-radiation from different regions of the interstellar medium (ISM) by SAS-2 and COS B satellite telescopes have already made an essential contribution to the current knowledge of the distribution of CRs in the Galaxy [20],[21]. It is expected that the E G R E T observations will provide a deeper insight into the study of the diffuse radiation of the galactic disk, in particular the separation of different components of 7-radiation produced by electronic and nucleonic components of CRs. Furthermore, the first E G R E T source catalog [22] which covers the first 18 months of the all-sky survey, contains almost 50 galactic 7ray sources. It is remarkable that many representatives of several classes of potential CR accelerators are presented in this catalog: pulsars (Crab, Vela, Geminga, PSR B1706-44, and PSR B105552), the Crab Nebula, and molecular clouds/star formation regions (Orion and Ophinchus complexes). Most low latitude E G R E T sources have still not been identified, but it is likely that an essential portion of these sources are associated with SNRs [23]. It should be noted however that the limited angular resolution of the E G R E T in-

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strument, combined with high 'y-ray background from the CR irradiation of the ISM unfortunately makes it rather difficult to unambiguously identify GeV "y-ray sources in the galactic disk, and correspondingly to study acceleration scenarios in these sources. On the other hand, space-based 'y-ray observations alone are not sufficient to answer the question: are GeV 'y-ray sources suppliers of galactic CRs in the whole energy range from 109 eV to 1015 eV or they are accelerators of only relatively low energy particles? This principal issue should be addressed by ground-based VtIE "y-ray observations. 4. V H E

GAMMA-RAYS

FROM

SNRs

Although it is commonly believed that the nuclear component of CR below the so-called 'knee' around of E ~ 1015 eV is produced in the galactic SNRs, the arguments leading to this statement are indirect and rather circumstantial. Actually, Wolfendale and coworkers [24],[25] using the SAS-2 and COS B data have found some evidence for a CR excess inside the nearby supernova remnant Loop 1, which can be interpreted as CR acceleration by a SNR shock. The possible association of more than 10 unidentified E G R E T sources with galactic SNRs [23] supports this suggestion. However, the claim concerns only low energy, E <_ 100 GeV, protons a n d / o r electrons, and therefore it cannot be considered as a conclusive argument in favour of SNRs as a sources of galactic CRs up to 1015 eV. The straightforward proof of this hypothesis can be the detection of VHE 'y-rays from SNRs [26],[27]. The principal conclusion of our recent study of production and the observational possibilities for detection of ~r°-decay 7-rays from SNRs [26] can be summarized as follows: If SNRs are responsible for acceleration of most galactic CRs, they should be detectable 7-ray sources, first of all in the T e V energy region. The flux of'y-rays from a-° decays at E 7 ~ 300 MeV due to the interactions of the accelerated protons with the ambient matter are almost independent, for a given energy density of accelerated particles, of the particle spectral index: F 7 (_> 300 MeV) ~ 3 . 1 0 - T d cm 2s-1,

(1)

where

is a dimensionless amplitude scaling factor; n is the ambient density, d is the distance to the source, and 0 is the fraction of the total SN explosion energy, ESN, converted into CR energy. The 7-ray flux at energies E >> 100 MeV is FT( ~ E) = l O - l ° E T e a T 1 A f a

c m - 2 s -1,

(3)

where f~ ~ 0.9; 0.43; and 0.19 for the spectral index of the accelerated protons a --- 2.1, 2.2, and 2.3, respectively; ETeV ------E / 1 TeV. The observational possibilities of current satelrite borne and ground-based 'y-ray detectors are limited to A _~ 1 and A _~ 0.1, respectively. Note that in the case of a SN explosion in a conventional region of the ISM with n .~ l c m - a (and for the typical parameters ESN ~ 1051 erg and 0 ~ 0.1), the scaling factor A exceeds 0.1 only for relatively close SNRs (d _< l k p c ) . However these SNRs seem to be hardly observable due to the large angular size of the "y-ray emitting regions. Indeed, SNR interiors, taken as diffusive shock accelerators, should start to be visible in 'y-rays after the so-called Sedov phase (typically t >_ 10a - 104yr), when the radius of SNRs exceeds 10 pc [28]. Thus the angular size of the 'y-ray source becomes less than 1° only for SNRs beyond 1 kpc. This conflict between the angular size and the absolute flux makes detection of "y-rays, both in GeV and TeV regions, very difficult by current 'y-ray detectors. However, SNRs exploded in or near dense regions like giant molecular clouds (GMC) may be detectable [29]. Pollock [30] has discussed the possible identification of two COS B sources, 2CG078+01 and 2CG006-00, confirmed recently by E G R E T [22], with the radio bright SNRs "y Cygni and W28, respectively. The possible interaction of the "y Cygni with the dense CO cloud, if confirmed, may be interpreted as a realization of the Blandford and Cowie model [31] which suggests that 'y-rays are produced in dense clouds, overtaken by SNRs, due to bremsstrahlung of the electrons which are responsible also for the synchrotron radio emission at GtIz frequences.

EA. Aharonian/NuclearPhysicsB (Proc. Suppl.) 39A (1995) 193-206 Meanwhile, since it is believed t h a t SN shocks accelerate effectively protons a n d nuclei up to energies ~ 1015 eV, for conservative values of the total energy in the accelerated CR, W¢~ -- 0EsN 10 ~° erg, and the gas density in nearby molecular clouds, u > 1 0 0 c m -3, the observed 7-ray flux F-c(_> 3 0 0 M e V ) ~ 5 . 1 0 - 7 c m - 2 s -1 can be explained equally well by a'°-decay ~/-rays. In this case the hard energy s p e c t r u m of ")'-rays (with a power-low index a ~ 2.1 - 2.3) extends bey o n d 1 TeV, providing observable fluxes at the level _> 10 -11 p h / c m 2 s [29]. We believe t h a t purposeful V H E observations of SNRs will be started soon. Due to the extended character of the 7-ray p r o d u c t i o n regions, observations of these sources will not be easy. However, the search for "/-rays from sources like 7 Cygni , W 2 8 and IC 443, having dense gas clouds nearby seem to be promising. 5. G I A N T M O L E C U L A R CLOUDS A S T R A C E R S OF C O S M I C R A Y S T h e role of gas clouds is not limited to the specific case of the ' S N R - G M C ' interacting systems. SNRs are the most probable, hut still only one of the possible classes of potential C R accelerators. T h e existence of an accelerator is by itself not enough for effective -),-ray production; for t h a t one needs a second c o m p o n e n t , namely a target. T h e G M C s in the G a l a x y seem to be ideal objects to play t h a t role. These objects are genetically connected with star formation regions which are strongly believed to be most probable settings (with or w i t h o u t SNRs) of C R production. 5.1. D i f f u s e e m i s s i o n f r o m ' p a s s i v e ' G M C s Assuming t h a t 7-rays in the molecular cloud are p r o d u c e d by interaction of C R with the ambient gas, the expected 7-ray flux is

F-C(~E)~-IO-Z(M~k~c)~t.f(~_E),

(4)

where q-c is the so-called 7-ray emissivity (q't ~

q-c~10-2~ ( H - a t o m ) - l s - 1 ) , M is the mass (Ms --M/105M®), and d is the distance to G M C (dkp~ ~ d/lkpc). For the local (observed) C R s p e c t r u m the ~-°-decay ?,-ray emissivity above

197

100MeV, ~(_> 100MeV), varies within 1 and 2, depending on the chosen d e m o d u l a t e d s p e c t r u m and mass composition of CRs (e.g. [32]). In G M C s the 7-ray emissivity m a y differ from the local one, namely q-c = ~;cRq®, where ~ c a is the ratio of C R intensities in the cloud and near the Earth, ~CR = Icloud/I®. If for some reason C R penetration into G M C is hampered, e.g. due to d a m p i n g on some kind of plasma instabilities ("self-shielding" effect, e.g. [33]), then ~ c a < 1. If there are C R accelerators in or near G M C , ~ c a _~ 1. A n d only for the case, t h a t the cloud is a 'passive' target for CRs, is ~ c a unity. T h e last case is p r o b a b l y realized in some local GMCs. For example, the 7-ray fluxes from Orion and Ophiuchus complexes detected by COS B and E G R E T at the level ~ 10 - 8 cm -2 s -1, are in general agreement with the assumption t h a t the C R density in these objects is nearly same as the local (observed) flux. Indeed, from Eq.(4) we have ~CR " Ms/d~pc ".~ 10, and as the p a r a m e t e r M5/d~p c is a b o u t 10 for b o t h objects, we conelude t h a t ~CR ~ 1. Note t h a t an uncertainty of up to a factor of 2 is possible in this kind of estimate due to the u n k n o w n contribution of the bremsstrahlung and IC channels to the 7-ray emissivity as well as due to the combined uncertainty of mass and distance to the GMCs. T h e 7-ray flux from a G M C expected for the observed C R proton spectrum

Jp(E)

= 2 . 2 . 1 0 4 E -2"75 c m - 2 s - l s r - l G e V -1, (5)

at E ~ 100 MeV is [34] F-c(> -1.7s 2 _ E ) ~ 10 - l a ET~ v (Ms/dkp~)cm-2s

-1.

(6)

It should be noted t h a t to account for the heavy nuclei contribution to the 7r° production it is necessary to multiply F~(_> E) by a factor o f p which in case of " s t a n d a r d " C R and ambient gas compositions is of a b o u t p ~ 1.5 [32]. According to Eq.(6) the flux of 7-rays above 1 TeV cannot exceed the level of 1 0 - 1 2 c m - 2 s - 1 even for several galactic G M C s for which the parameter Ma/d~p¢ ~ 10. T h o u g h this kind of massive extended (> few degrees) regions (like Taurus, Orion, Lupus, etc), taken as 'passive' targets for galactic CRs, should be easily detected [35]

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EA. Aharonian/Nuclear Physics B (Proc. Suppl,) 39A (1995) 193 206

by E G R E T (the expected flux above 100 MeV is about ~ 1 0 - ~ c m - ~ s - ~ ; see Eq.(4)), their detection at TeV energies could be realized only with future huge installations like the Telescope Array [11]. Nevertheless, the search for VIIE "r-rays from GMCs with current detectors seems very i m p o r t a n t due to the possible existence of highenergy C R accelerators nearby [34]. Gamma-rays from 'active' GMCs The intense-star-formation regions are believed to be 7-ray emitters. The presence of potential accelerators (supersonic stellar winds a n d / o r SNRs) and targets (dense gas clouds) creates favourable conditions for 7-ray production in these regions (e.g. [36],[37]). Indeed, the analysis of "f-ray fluxes from the direction of 13 local clouds on the basis of the COS B data done by Issa and Wolfendale [38] indicates a possible variation of the enhancement factor tcca between 0.2 and 20. Despite the large uncertainties in GMC mass and distance estimates, there is evidence in a few cases (Cas OB6, Car Neb) that the CR intensity does need to exceed essentially the value near the Earth to explain the ")'-ray fluxes detected. If the association of ")'-ray sources with these clouds is genuine, a natural interpretation of the CR enhancement could be the presence of relatively young and powerful particle accelerators near GMCs. Such a possibility is demonstrated in Fig.1 [39] where the fluxes of protons at distances r =10 pc, 30 pc, and 100 pc from the 'impulsive' source are shown at different stages after their injection into the ISM: t = 102 yr (solid lines), 10 a yr (dashed fines), 104 yr (dash-dotted lines), 105 yr (dotted fines), and l0 s yr (fancy). For comparison, the spectrum of galactic C R protons presented in the form of Eq.(5) is shown by dashed region. The results presented in Fig.1 are calculated for a total energy W c a = 10S°erg released in the form of relativistic particles with power-law spectral index ~ = 2.1. Generally, the spect r u m of CRs at the given time and coordinates could noticeably differ from the primary (source) spectrum. While the energy-independent diffusion leads to a time variation of the C R flux without change of the spectral form, the energy-

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dependent diffusion results, in addition, in a modification of the particle spectrum. At the distance from the source the m a x i m u m C R flux is reached, for a given energy, at instant t t m a x ( E ) = r 2 / f D ( E ) , where D ( E ) is the C R diffusion coefficient. At t << tmax(E) the particles have not yet reached the point r, while at t >> t m = ( E ) the C R flux decreases due to the spherical expansion as o~ t -~/2. Both the absolute CR flux and the energy modification of the spectrum depend strongly on the diffusion coefficient D ( E ) , which is a rather uncertain parame-

EA. Aharonian/Nuclear Physics B (Proc. Suppl.) 39A (1995) 193-206 ........ i

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,/,,,,,,iJ . . . . . . . .i , , ,,,,,,i . . . . . . 1000 104 105 106 Time [yr]

Figure 2. The time dependence of the v-ray emissivity (the left-hand ordinate axis) and fluxes in the 0.3-3 GeV and 1-10 TeV intervals from GMC for Ms/d~p¢ = 1 (the right-hand ordinate axis). The distances of clouds from the accelerator are: 10 pc (solid curves), 30 pc (dashed curves), 100 pc (dash-dotted curves). The dashed regions indicate the corresponding emissivities of v-rays induced by galactic CRs. The parameters assumed for the primary spectrum and the CR diffusion are the same as in Fig.1.

ter. It is believed that D ( E ) has a power-law behaviour, D ( E ) o¢ E~; 6 ~ 0.5, with possible tendency to energy-independence below ~ 10 GeV (e.g. [40]). The commonly used values of the diffusion coefficient in the ISM at E ---- 10 GeV is of about D10 - D(10GeV) ~ 102acm2/s (e.g. [40],[41]). However, much smaller values, e.g. Dr0 ~ 1028-1027 cm2/s, cannot be excluded [42], in particular in dense star formation regions. The results presented in Fig.1 were calculated for parameters D10 = 1027cm2/s and 6 = 0.5. It is seen from Fig.1 that the impulsive acceleration of particles with total energy of W c a = 10S°erg can lead to an essential enhancement of the C R fluxes within R < 100 pc from the source. Correspondingly, it results in much higher pro-

199

duction rate of zc°-decay 7-rays than the emissivity provided by the bulk of the galactic CRs in the ISM (see Fig.2). Note that the 'impulsive particle acceleration' should be treated as an idealized working hypothesis for illustration of the variation (in space and time) of the CR flux in the near vicinity of the accelerator. This assumption, in fact, could not be fulfilled even in the case of SN explosions. Indeed, though the possibility of the C R production at early stages of supernova evolution cannot be excluded [20], the more realistic model of 'diffusive shock acceleration' (e.g. [43])in SNRs predicts CR production during the so-called Sedov phase with a typical duration ~ 104yr. At the same time 'impulsive acceleration' could be applied to the classical 'continuous' sources like pulsars, if the bulk of particles is produced at the early stages, e.g. before the magnetic dipole braking of the pulsar [44]. The expanding "bubble" of CRs penetrating the possible molecular clouds nearby will initiate an intensive 7-ray production. It can be seen from Fig.2 that some clouds become visible, in certain intervals of times, in GeV a n d / o r TeV energies ( ~ 1 0 - 7 c m - 2 s -1 and ~ 10-12cm-2s -1, respectively) provided that M~/d~p c > 0.1 [39]. (Note that the visibility of 'passive' clouds under b o m b a r d m e n t of diffuse galactic CRs requires Ms/d~p c _> 1 and _> 10 at GeV and TeV energies, respectively). Therefore, it seems rather challenging to apply such a scenario for explanation of several "extended or multiple" galactic 3'-ray sources reported by the E G R E T t e a m [22]. The particle spectrum in the first stages of propagation (for example, t < 103 yr at r < 30 pc) is very hard due both to the hard source spectrum and the energy-dependent diffusion. If the shape of the C R spectrum does not strongly affect the production rate of GeV 1'-rays (for the given C R energy density), it turns out to be crucial for production of VHE 7-rays. Therefore, it is quite possible that at some stages the C R irradiated GMCs could be detected at TeV energies rather than at GeV energies. It is worth noticing that the ratio of !,-ray fluxes at GeV and TeV energies contains valuable information about the energy dependence of the diffusion coef-

200

EA. Aharonian/Nuclear Physics B (Proc. Suppl.) 39A (1995) 193-206

ficient. In addition, detection of v-rays from different clouds at different (known) distances from the C R source provides an information about the absolute value of the diffusion coefficient (if, of course, the age of the accelerator, and masses of clouds are known). Thus, future combined radio, IR, and v-ray observations could provide a unique possibility for the experimental measurements of the C R diffusion coefficient in different regions of the ISM. Above I have discussed only the production of 7r°-decay v-rays. In fact, at p-p interactions, as a result of the decay of charged 7r+ mesons, secondary electrons will also be produced in comparable amounts and with a similar spectrum as v-rays. The IC scattering of these secondary electrons on different ambient photon fields results in production of additional v-rays, but it does not change noticeably the v-ray luminosity of the source. However, the IC mechanism plays very i m p o r t a n t role in production of VHE v-rays in the environments of objects accelerating electrons. This kind of scenario is realized effectively in supernova remnants powered by a central pulsar, plerions [45]. The Crab Nebula is the classical representative of this class of objects.

6. T H E

CRAB

NEBULA

The undisputed synchrotron origin of the radiation of the Crab Nebula from radio wavelengths to hard X-rays [46] and possibly even _< 1 GeV v-rays [47] implies the existence of relativistic electrons of energies _> 1015 eV. Actually it is likely that there are two different populations of accelerated electrons in the nebula: (1) the old 'radio electrons' below ~ 300 GeV with powerlaw index _~ 1.5 (the radiative cooling time of these electrons is more than the age of the pulsar); (2) the "freshly accelerated" high energy electrons ( E _> 300 GeV). It can be seen from Fig.3 that the synchrotron radiation calculated [48] for the magnetic field distribution derived from the MHD model of Kennel and Coroniti [46] is in a good agreement with the observed spect r u m of the Crab Nebula over 16 decades of frequency from 107 to 1023 Hz. Note that the ten-

dency of the observed v-ray spectrum to steepen above 1 MeV is explained by assuming power-law injection of electrons with an exponential cutoff at Ec ~ 3. 101SeV which is in good agreement with theoretical arguments [47] that the acceleration of electrons in the Crab Nebula should drop above ~ 1015 eV. It should be noted however that even in the case of acceleration of electrons well beyond 1015 eV the synchrotron radiation cannot explain the flattening of the v-ray spectrum at E > 1 GeV. The fluxes measured by E G R E T at energies of several GeV [49] require another radiation mechanism. If this unpulsed radiation is really associated with the Crab Nebula and not with the pulsar itself (e.g. as discussed in [50]), there are 3 conventional mechanisms of v-ray production in the Crab Nebula: ,r°-deeay; bremsstrahlung and IC scattering. For the m a x i m u m energy which in principle could be released in relativistic particles during t ~ 103yr of existence of the Crab pulsar, W _< L0 x t ~ 1.5. 1049erg, and the average gas density in the nebula n ~ 5era -a, the first two processes provide < 10% of the observed flux above 1 GeV, if one suggests that the accelerated particles are uniformly distributed in the nebula. The contribution of the IC mechanism is expected to be higher. Despite the different accuracies of the process cross-section, as well as different spatial distributions of the nebular magnetic field used in the past [51],[52], [53],[54], the unambiguous conclusion of the theoretical predictions was that one has to expect high (detectable) fluxes of v-rays at TeV energies. The detection of VHE v-rays from the Crab at very high confidence level has stimulated detailed investigation of the problem by de Jager and Harding [47] and recently also by Aharonian and Atoyan [48] and Gallant and Kirk [55]. There are 3 different photon fields which play principal role in the production of IC v-rays in the Crab Nebula: (i) the synchrotron radiation of the nebula, (ii) the extra (probably dust associated) IR radiation, and (iii) 2.7K microwave background radiation (MBR). The existence of the synchrotron radiation implies a realization of the so-called synchrotron-self-Compton (SSC) model in the Crab Nebula [51]. However, due

201

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to the existence of the extra I R field which contributes as much as the synchrotron radiation [47],[48],[55] to the production of v-rays, it is more proper to say that the production of Vrays in the Crab Nebula can be regarded as a realization of an ordinary IC model rather than the SSC model. Meanwhile, the contribution of the IC scattering on the MBR photons to the Vradiation of the Crab Nebula has been surprisingly neglected in the past. In fact, if the electron spectrum extends beyond 100 TeV, which is unavoidable if the observed unpulsed hard X-rays and low energy v-rays originate in the nebula, the IC scattering on the M B R becomes noticeable [48]. Since this channel of v-ray production takes place in the T h o m s o n regime, it makes the overall IC spectrum harder for the Crab Nebula (differential spectral index ~ 2.4 at 1 TeV and ~ 2.7 at 10 TeV). It can be seen from Fig.4 that the calculated IC spectrum only marginally agrees with the spectrum reported by the Whipple group [56]. Also, the expected flux at E > 10TeV is very close to the flux upper limits reported by various groups. This limits significantly the possibility of attributing the fluxes measured by E G R E T above 1 GeV [49] to the IC v-rays. At energies below 100 GeV the IC v-rays have very hard spectrum, therefore the expected v-ray flux at E _< 10 GeV is to be below the E G R E T fluxes (see Fig.4). In

I~"

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Figure 4. The inverse Compton, bremsstrahlung and synchrotron radiation of the Crab Nebula. The solid line is the contribution of the IC "y-rays upscattered off the synchrotron, "dust" IR, and 2 . 7 K background photons; the dash-dotted llne is the contribution of the bremsstrah]ung photons, assuming the uniform distribution of electrons in the nebula with n = 5 cm-3; the heavy solid llne is the superposition of the IC and bremsstraldung photons, assuming an "amplification" for the bremsstrahlung contribution of about a factor of 20. The dotted line is the synchrotron 7-radiation of the Crab Nebula. The range of TeV "/-ray fluxes reported by the Whipple group (dashed region) and upper limits above 20 TeV reported by Tibet, HEGRA, Cygnus and CASA-MIA groups are also presented.

fact, it is possible to increase the predicted IC 3'ray fluxes by assuming an average magnetic field in the Crab Nebula of/Y ~ 10 -4 G. (The lower the magnetic field is, the larger is the electron flux needed to explain the synchrotron radiation, giving a correspondingly larger IC V-ray flux). This value is by factor of 2 or 3 less than the characteristic magnetic fields usually discussed but it still cannot be excluded. However, any increase in the IC flux at GeV energies automatically impfies an almost parallel increase in the spectrum at very high energies, which might lead to a conflict with reported fluxes or upper fimits at B > 1TeV (see Fig.4). It should be noted that though the TeV v-ray signals from the Crab Nebula have been detected by the Whipple, ASGAT, Themis-

202

EA. A haronian/Nuclear Physics B (Proc. Suppl.) 39A (1995) 193~06

tocle, HEGRA, Cangaroo and CrAO groups at very high confidence levels, the absolute fluxes and energy spectra still contain large (systematic) uncertainties. Therefore it is premature to exclude the possibility that the "r-rays observed from 1 GeV to ~ 10 TeV may be due to a single IC mechanism. If confirmed, the reported spectral features of the radiation from the Crab Nebula, namely the excess at several GeV [49] and the steep spectrum above 400 GeV [56], will require another "r-ray production mechanism(s) responsible for the radiation of the Crab Nebula at E > 1 GeV to be suggested. The possible identification of the observed GeV fluxes as the bremsstrahlnng of the electrons responsible for the observed radio emission of the Crab Nebula is of special interest. With the existing mean density of the gas, fi ~ 5em -a, and estimated total energy of the 'radio electrons', W _~ (2 - 3) • 104Berg in the Crab Nebula, the bremsstrahlung could provide no more than 10 % of the 7-ray flux reported by E G R E T (see Fig.4) [48]. In fact, in the Crab Nebula the gas is concentrated mainly in filaments, where the gas density is rt ~ 10 a em -a [57]. In the case of uniform distribution of relativistic electrons throughout the nebula the effective gas density is defined by the mean density of the nebula, rtefr ~ ft. However, if electrons are trapped, at least partially, in the regions of higher gas density, i.e. they propagate inside the filaments slower than they would elsewhere (for example, due to the different structure and magnitude of magnetic fields inside and outside of the filaments), then neff >> fi, and correspondingly the bremsstrahlung contribution becomes significantly higher [48]. The invocation of such a scenario to explain the fluxes at GeV energies would have an important impact also on the spectrum of VHE "r-rays. The hard 7-ray spectrum of the bremsstrahlung radiation of the 'radio electrons' with the index a7 _~ 1.5 is dominant up to E ~ 100GeV, but then it becomes much steeper. In the energy range between 0.1 and 10TeV the superposition of the bremsstrahlnng and the IC radiation would result in almost power-law spectrum with an index a- r ~_ (2.5 - 2.7) extending to E _~ 10 TeV. Meanwhile, the spectrum of pure IC

-/-rays at these energies is qualitatively different: it is hard at E _~ 100 GeV, with the power-law index c~ _~ 2.0, and gradually steepens with increasing energy to the index a _~ 2.7 at E ___ 10 TeV. Note that the "amplification" effect, due to the effective slow-down of the propagation of relativistic particles in the filaments, would work equally well for 7-rays from p - p interactions though our preliminary analysis shows that the rr°-decay "y-rays do not fit the observed spectrum well. At the same time the "bremsstrahlung+IC" spectrum fits the E G R E T and Whipple data quite closely, though, of course, new spectral measurements, both in the GeV and TeV regions, are needed for a more conclusive statement. 7. V H E G A M M A - R A D I A T I O N INITIATED BY PULSARS At present five 7-ray sources from the E G R E T catalog are identified with spin-powered pulsars; Crab, Vela, Geminga, PSR B1706-44, and PSR B1055-52 [22]. The modulation of the 7-ray light curves at the periods known from observations at other wavelengths testifies to the production of'rradiation in the pulsar magnetospheres. G a m m a ray emission by pulsars is interpreted as the result of nonthermal cascade processes supported by curvature radiation, Compton scattering and synchrotron radiation from relativistic electrons (see e.g. [58], either at the polar cap [59], or in the vacuum gaps of the outer magnetosphere [60]. Though the effective production of 7-radiation in pulsar magnetospheres can be extended to very high energies, pair production in magnetic fields strongly prevents the escape of VHE "r-rays. Unfortunately, the predictions of the current models of 7-radiation by pulsars are not sufficiently conclusive, especially for the VHE domain, thus ground-based "l-ray observations of pulsars could be considered rather as a challenge. Note that there is some belief that the fiat spectra of -/-ray pulsars at GeV energies raise the chance for detection of photons at TeV energies as well. However, a very fiat energy spectrum coupled with an extremely high conversion efficiency r/.r = L T / E (L.r is the 7-ray luminosity assuming a beaming solid angle of i steradian, a n d / ~ is the spin-down

KA. Aharonian/Nuclear Physics B (Proc. Suppl.) 39tl (1995) 193-206 1 035

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energy loss rate of the pulsar) which is the case for P S R B1055-52 and Geminga (0.3 and 0.07, respectively) [61] necessarily requires, simply due to the energy conservation law, cutoffs above 10 GeV. Indeed, noticeable roll-offs are observed in the spectra of both pulsars [61]. The search for TeV 7-rays from pulsars was started some 20 years ago, but the observational situation is somewhat confused [2]. Despite several claims of possible detections of pulsed TeV radiation from several objects, in particular from the Crab pulsar and Geminga (see e.g. [4]), the results are, as a rule, not straightforward and in m a n y cases controversial [2]. Whether or not the spectra of 7-ray pulsars continue to the VHE region is a question which remains one of the interesting issues of groundbased 7-ray observations. Besides the pulsed radiation produced in the magnetosphere, one may

203

expect 7-rays from much more extended regions surrounding the pulsars. The pulsars are undisputed sources of relativistic electrons accelerating particles directly in the magnetospheres as well as through the pulsar wind termination shocks (see e.g. [45],[62]). The last mechanism seems likely to be responsible for the injection of electrons up to energies 1015 eV into the Crab Nebula [47]. The interaction of these electrons with the photon fields in the nebula results in the production of observable VI{E 7-radiation (see Sec.6). It might create, to some extent, an impression that the existence of the nebula around the Crab pulsar plays a crucial role in the formation of the VHE 3,-radiation. In fact, the large magnetic field in the Crab Nebula (B > 10 -4 G) only reduces the efficiency of 7-ray production. Indeed, since the energy density of the magnetic field in the Crab Nebula exceeds, at least by 2 orders of magnitude, the energy density of photon fields, only < 1% of the energy of accelerated electrons is converted to the IC "r-rays, the rest being radiated in synchrotron emission mainly in the form of optical, UV and soft X-ray photons (see Fig.3). Thus, the efficiency of IC 7-rays can, in fact, be higher if electrons are injected into the conventional regions of the ISM with typical magnetic field B g 3 # G . The high production rate of IC "r-rays in the ISM is provided predominantly by 2.7K background photons, thus somewhat lower density of I R / O photons in the ISM than in the Crab Nebula, does not significantly reduce the emissivity of IC 7-rays. Indeed, the emissivity of IC 7-rays in the case of blackbody distribution of ambient photons in the Thomson regime is proportional to [63] q(ET) c(

~T(P+5)/2E~cp+1)/2

(7)

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204

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due to the Klein-Nishina effect. The 7-ray luminosities of the expanding cloud of electrons ejected continuously by an isolated pulsar into the ISM are shown in Fig.5 [39]. In calculations of the energy spectrum of electrons, formed in the ISM due to the energy dependent diffusion and radiative energy losses, we have used the formalism developed in [64]. Due to the hard spectrum of the electrons, the high energy 7-ray production through IC scattering on the 2 . 7 K background photons dominates over the other 7-radiation channels. At lower energies, E < 1GeV, the contribution of the bremsstrahlung photons becomes noticeable, especially for steep electron spectra. The results presented in Fig.5 are obtained by integrating the fluxes over all angles from the central source. Therefore they depend on the initial spectrum of electrons only, but not on the diffu-

;

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Figure 7. Gamma-ray fluxes expected within different angles from the Vela pulsar: 0.01° (dashed line), 0.1 ° (sohd line), 1° (dash-dotted line). The parameters assumed are the same as those used in Fig.6 with exception of the electron spectrum (a = 2) and the diffusion coefficient (D10 = 10:s cm2/s and 6 = 0).

sion coefficient. However, the character of diffusion of electrons in the ISM has a crucial impact on the angular distribution of radiation, and thus on the observability of 7-ray fluxes. It is illustrated in Figures 6 and 7 [39], where the 7-ray fluxes expected in the direction of the Vela pulsar are shown. The fluxes are calculated at the present epoch assuming that the half of the spindown energy of the pulsar (E -- 7 . 1 0 ae erg/s) is continuously released, in analog with the Crab, in the form of relativistic electrons during the lifetime of the pulsar t ~ 1.1 • 104 yr. Fig.6 corresponds to the "worst" combination, from the point of view of observability of VHE 7-rays, of the model parameters. The fast expansion of the cloud of electrons due to the large diffusion coefficient and the relatively soft (a = 2.4) initial spectrum of electrons (which during propagation becomes even steeper due to the energydependent diffusion) results in radiation of the bulk photons within large angles and at low en-

205

EA. Aharonian/Nuclear Physics B (Prec. Suppl.) 39A (1995) 193 206

ergies. Correspondingly, the "r-ray fluxes in both GeV and TeV energies turn out to be well below the sensitivities of current 7-ray telescopes. The situation could be quite different if electrons with hard initial spectrum propagate more slowly in the ISM. In particular, for the small and energyindependent diffusion coefficient the bulk of the "r-rays are produced with a very hard spectrum in rather compact regions surrounding the pulsar. As a result of this "optimistic" scenario, the fluxes of VHE "y-rays within small angles reach a level detectable by current Cherenkov telescopes. It is interesting to note that the predicted VHE "r-ray flux within 0.1 ° ( ~ 10 -11 e m - 2 s -1) shown in Fig.7 is comparable with the unpulsed flux observed by the C A N G A R O O collaboration, though the statistical significance of the detection is not very high. The same C A N G A R O O collaboration [6] recently reported a high confidence level (> 10~) detection of a similar flux from another E G R E T pulsar, P S R B1706-44. Due to the relatively large distance (d -- 1.Skpc), and low spin-down luminosity ( E = 3 . 4 . 1 0 zSerg/s) of this young (t ~ 1.7 • 104 yr) pulsar, the constant rate electron injection into the ISM can only marginally provide the observed fluxes, admitting D10 < 10~Scm2/s and Le ~ /~. However, there is another possibility to explain the observed flux, assuming that 1049 erg eleetrons were injected in the ISM at the first stages (before the braking) of operation of the pulsar (see Fig.8). Note that such an assumption was recently invoked for explanation of the high energy electrons ( E > 100 GeV), observed directly in cosmic rays, as being produced by Geminga [64]. Finally, it is worth noting that the predicted fluxes of GeV "r-rays produced during the propagation of the electrons in the conventional regions of the ISM are essentially below the E G R E T flux sensitivity. However, in the case of expansion of the cloud of electrons in possible dense (n >> 1 em -3) regions surrounding the pulsar, the fluxes of the bremsstrahlung photons could be significantly increased to observable levels. It seems a rather promising application of this model to the 6 unidentified E G R E T sources consistent in position with young radio pulsars [61]. The realization of the effeetive injection of VHE

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electrons into the ISM by pulsars (or any other sources of high energy electrons) m a y result in rather high diffuse radiation from some regions of the galactic disk. While detection of the genuine diffuse r°-decay "r-radiation due to interactions of CRs with the interstellar gas is a very difficult task for even large ground-based instruments [34], the superposition of 7-ray fluxes from extended regions, surrounding the pulsars, could be detectable by E G R E T as well as by low threshold air shower detectors like A I R O B I C C [13] and MILAGRO [14]. The region towards the outer galaxy containing 3 -y-ray pulsars, Crab, Vela and Geminga, seems to be of special interest.

EA. AharonhTn/Nuclear Physics B (Proc. Suppl.) 39A (1995) 193 206

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8. S U M M A R Y

20.

To summarize, there is a justifiable hope to believe that the next generation of systems of imaging atmospheric Cherenkov telescopes will raise the status of VHE 7-ray astronomy to the level which has been achieved at GeV energies. It is almost undisputed that one of the traditional aspects of 7-ray astronomy connected with the origin of cosmic rays should be in the list of the priority objectives for future ground-based 7-ray observations. Many potential particle accelerators having solid observational and theoretical background, in particular shell-like SNRs and plerions, are expected to be VHE emitters, and in some cases they can be detected more easily at TeV energies rather than at GeV energies. This sustains an optimistic view that future observations with ground-based 7-ray detectors will provide the crucial insight into the old standing problem of the origin of galactic cosmic rays.

21. 22. 23. 24. 25.

I would like to acknowledge the contributions of A.Atoyan, C.Covault, L.O'C.Drury, P.Duffy, R.Tuffs and H.J.VSlk for m a n y fruitful discussions of different aspects on this paper. I a m indebted to the organizing committee for suggesting this invited talk. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

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