This topic presents a brief overview of the achievements of observational gamma-ray astronomy in different energy bands. The status of space-based gamma-ray astronomy in the low energy (LE) and high energy (HE) regimes are discussed in Sections 2.1 and 2.2. Comprehensive description of the results at MeV and GeV energies obtained basically with BATSE, OSSE, COMPTEL and EGRET detectors aboard NASA’s Compton Gamma Ray Observatory can be found in the proceedings of the Fourth (Dermer et al., 1997) and Fifth (McConnell and Ryan, 2000) Compton Symposia, as well as in the proceedings of the Gamma-Ray Astrophysics-2001 symposium (Ritz et al., 2001). The methods of detection of cosmic MeV and GeV Y-rays are described in great details in the topic by Fichtel and Trombka (1997). Therefore, Sections 2.1 and 2.2 will be rather short. Only the major observational results, with brief commentaries on their astrophysical implications, will be highlighted.
Several review articles on VHE gamma-ray astronomy have been written over the last decade – see e.g. papers by Cronin et al. (1993), Aharo-nian and Akerlof (1997), Ong (1998), Hoffman et al. (1999), Catanese and Weekes (1999), as well as the proceedings of the recent two symposia on "High Energy Gamma-Ray Astronomy" and "The Universe Viewed in Gamma-rays" (Enomoto et al., 2003). It should be noticed, however, that currently the field is developing so fast that every year brings new discoveries and surprises, and, consequently, new ideas and models (as well as puzzles), thus review articles quickly become "old" on timescales shorter than the typical time needed for their publication in regular journals or conference proceedings. Most likely (hopefully !) the same will happen with this topic.
This energy regime is uniquely related to several areas of high energy astrophysics, in particular to the phenomena of Gamma Ray Bursts, nucleosynthesis of heavy elements in the Universe, origin of low energy (sub-relativistic) cosmic rays, solar flares, etc. Also, this energy regime provides key observations for understanding of high energy processes in stellar black-holes in our Galaxy, as well as the massive black-holes believed to exist in the centers of AGN. In particular, many popular ideas and models that assume formation of relativistic, both thermal and nonthermal, plasmas around these objects, can be tested via studies of characteristic electron-positron annihilation radiation.
At the same time, this energy region has been and unfortunately remains a challenge for design and construction of adequately sensitive Y-ray detectors that would rise the low-energy gamma-ray astronomy to the level of its immediate neighbours – X ray astronomy and high-energy gamma-ray astronomy. There are several reasons for slow development in instrumentation in this energy region: (i) the small photon interaction cross-section in the transition region from the Compton scattering to pair production; (ii) small energy deposits and large mean free paths of secondary products; (iii) large uncertainties in reconstruction of full kinematics of the first interaction, and correspondingly rather limited angular resolution; (iv) large local backgrounds, especially at MeV energies due to excitation of surrounding materials by cosmic rays. The combination of these factors – low Y-ray detection efficiency, modest angular resolution and high background – severely limits sensitivities of Y-ray detectors operating in this energy region. The minimum detectable energy fluxes at hard X-ray and low-energy Y-rays are, indeed, not very impressive, and even after significant improvements by next generation instruments, they unfortunately will remain relatively modest compared to sensitivities expected in the foreseeable future in the HE and VHE domains (see Fig. 1.2).
Even so, low-energy Y-rays contain invaluable astrophysical information that cannot be obtained by other means. Therefore, any further improvement of detector performance would lead to exciting results in several areas of astrophysics and cosmology.
The COMPTEL source catalog
The imaging Compton telescope COMPTEL provided the first all sky survey in the MeV Y-ray domain (Schonfelder et al., 2000). This instrument was designed to operate in the energy range from 0.75 to 30 MeV. Within its large FoV of about 1 steradian, the source location accuracy was of about 1°. Typically COMPTEL could resolve different sources, provided that they were about 3 to 5 degrees away from each other. With 5 to 10 percent energy resolution, COMPTEL was able to detect and identified several Y-ray lines of extra-solar origin – 1.1809 MeV (26Al), 1.157 (44Ti), 0.847 and 1.238 MeV (56Co), as well as 2.223 MeV (deuterium or neutron-capture line). The sensitivity of COMPTEL was significantly limited due to the instrumental background. Also, for identification of sources in the galactic disk, detailed modelling of the diffuse galactic emission was essential for this experiment. On average, for a 2-week observation period, the source detection threshold was an order of magnitude below the Crab flux, i.e. at the level of « 2 — 3 x 10-10 erg/cm2s (see Fig. 1.2). Unfortunately, the INTEGRAL mission cannot offer better sensitivity above 1 MeV for continuum emission. But INTEGRAL will indeed greatly improve the detection sensitivity for line emission.
The first COMPTEL source catalog includes 32 persistent sources and 31 GRBs reported at > 3a statistical significance level (Schonfelder et al., 2000). The persistent sources of continuum emission belong to 3 types of source populations. In addition Y-ray line emission has been detected from 7 objects. And finally, 9 sources remain unidentified.
• Spin-Down pulsars - Crab, Vela, and PSR1509-58.
While the Crab and Vela pulsars are established as prominent Y-ray emitters detected at MeV/GeV energies, PSR 1509-58 remains up to now as a "MeV" Y-ray pulsar (see Fig 2.1). COMPTEL has detected both pulsed and continuum emission components from the direction of the Crab. The continuum component is presumably associated with the nebula, i.e. has a synchrotron origin. This interpretation agrees with the multiwavelength data obtained from the Crab Nebula (see Fig. 1.2).
• Stellar Black-Hole Candidates - Cyg X-1, Nova Persei 1992 (GRO J0422+32).
In addition to these galactic black-hole candidates with spectra extending beyond 1 MeV, OSSE – another hard X-ray/low energy Y-ray instrument aboard Compton GRO – has detected hard tails of radiation from several other similar objects, presumably microquasars, extending to 1 MeV (Grove et al., 1997). The observations of Cyg X-1 by OSSE and COMPTEL show significant variations in the MeV region between the so-called hard and soft spectral states, that are discovered and well studied in X-rays.
Fig. 2.1 Broad-band spectral energy distributions of pulsed emission of 7-ray pulsars (from Thompson, 1999). The MeV data are from COMPTEL, GeV data – from EGRET, and the TeV upper limits are from observations with the Whipple (pulsars in the Northern hemisphere) and CANGAROO/Durham (pulsars in the Southern hemisphere) telescopes.
The MeV spectra in these two states are shown in Fig. 2.2 together with model curves calculated within the so-called hybrid thermal/nonthermal Comptonization model (McConnell et al., 2002). This model assumes that Y-rays are produced by a non-thermal population of electrons accelerated in the accretion plasma around the black hole. This radiation can originate also in the synchrotron jet recently discovered in several representatives of this source population, including Cyg X-1. MeV radiation from synchrotron radio jets can be result of inverse Compton (Georganopoulos et al., 2002) or synchrotron radiation.In either case, the radiation should be of nonthermal origin associated with relativistic electrons accelerated in the jet.
Until now high energy Y-rays above 100 MeV have not been convincingly detected from a black-hole candidate with hard X-ray/soft gamma-ray spectra. Nevertheless, it is possible that some of the unidentified Y-ray sources are from the same source population (see Sec.2.2).
Fig. 2.2 X- and gamma-ray spectra ofthe black-hole candidate Cyg X-1 in soft ("high") and hard ("low") spectral states as measured by COMPTEL (MeV), OSSE (sub-MeV) and BeppoSAX (X-ray) instruments. The spectral fits are obtained within the hybrid thermal/nonthermal Comtonization model.
• AGN - CTA 102, 3C 454.3, PKS 0528+134, GRO J0516-609, PKS 0208512, 3C 273, PKS 1222+216, 3C 279, Centaurus A, PKS 1622-297.
Except for the radiogalaxy Centaurus A, all these objects are blazars -highly variable AGN with relativistic jets close to the line of sight. Because the spectral energy distributions of high energy branches of these objects peak at MeV energies, they are called MeV blazars, The famous quasar 3C 273 is a prominent representative of this class of objects (Lichti et al., 1995). The broad-band spectral energy distribution of 3C 273 is shown in Fig 2.3.
Fig. 2.3 Flux density and spectral energy distribution of the quasar 3C 273.
MeV blazars have quite steep spectra beyond MeV energies (with photon index greater than 2), in contrast to the so-called GeV blazars with flat Y-ray spectra extending to GeV energies. But, most probably, there is no fundamental difference between MeV and GeV blazars. It is believed that both classes represent the same AGN population – flat spectrum radio quasars (FSRQs). The difference between MeV and GeV blazars can be explained within the so-called external Compton model, assuming that GeV flat spectra originate in the broad emission line (BEL) regions, where their production is dominated by Comptonization of optical-UV emission lines, whereas the spectra of MeV blazars are formed at distances where the target photons are supplied by hot dust. Moreover, it is possible that the MeV and GeV blazar phenomena can appear interchangeably within the same object like in PKS 0208-512 (Sikora et al, 2002).
• Unidentified Sources
The first official COMPTEL catalog contains nine unidentified sources. Five of them are located at low galactic latitudes (|b| < 10°), and four above the galactic plane (|b| > 10°). Three of the five unidentified high-latitude sources are not point-like. Either they have diffuse origin or are result of superposition of several faint objects. Two of the four low-latitude sources possibly coincide with the EGRET unidentified sources 2EG 2227+61 and 2EG0241+6119 (discovered initially by COS B). The nature of all unidentified COMPTEL sources remains highly unknown. One cannot exclude the possibility that some of them are caused by statistical fluctuations.
• Gamma Ray Line Sources
COMPTEL has demonstrated that a variety of astrophysical objects may produce Y-ray line emission. A noticeable success in this regard was generation of a maximum entropy COMPTEL map at 1.809 MeV. It is widely believed that this Y-ray line from 26 Al is mainly produced in Wolf-Rayet stars, therefore it serves as a unique tracer of star formation in our Galaxy over the last several millions of years. The all-sky map in the 1.809 MeV line revealed bright extended regions, in particular in the inner Galaxy, as well as in the Vela, Cygnus and Aquila regions. An excess emission was reported also from the direction of Carina, but in this case the source seems to be point-like, i.e. with an angular extension that does not significantly exceed 1 degree.
Gamma-ray line emission was detected from three other point-like sources, although in different lines. The line 1.157 MeV from 44Ti has been detected from Cas A and, perhaps, also from RX J0852-4621 (a supernova discovered recently in the Vela region). A tentative detection of two coupled Y-ray lines at 0.847 and 1.238 MeV that are associated with 56Co was reported from SN 1991T. If confirmed, these results would have great impact on the theory of nucleosynthesis providing invaluable probes into the inner layers of exploding stars.
Finally, an excess of 2.2 MeV deuterium line emission has been found with ~ 3.7a statistical significance from a region that does not show any remarkable activity at other wavelengths. If true, this would imply a very effective source of neutron production, most likely through spallation of nuclei in a very hot, sub-relativistic plasma or by energetic nonthermal particles interacting with the ambient cold gas. In either case, the deuterium line production region, which generally could be separated from the neutron production region, should be very dense, n > 1016 cm-3, otherwise the neutrons would decay before being captured by thermal protons. This could happen, for example, in a binary system – the neutrons can be produced in the two-temperature accretion plasma around the compact object, a black hole or a neutron star, "evaporate" from the accretion disk, and be captured by a dense atmosphere of the normal star.Due to relatively narrow width of the detected 2.2 MeV line, the latter cannot be explained by neutron capture in the hot accretion plasma. It should be noted, however, that production of free neutrons, and therefore also the 2.2 MeV line is a relatively inefficient mechanism, which makes this (in fact, any) interpretation of the reported 2.2 MeV flux quite problematic, especially given the lack of a prominent object in the error box of COMPTEL.
Unfortunately, the energy threshold of COMPTEL of about 0.75 MeV did not allow studies of the 0.511 MeV electron-positron annihilation line. This line is the longest-known and the most intense extra-solar Y-ray line. The first evidence of positron annihilation radiation from the direction of the Galactic Center was obtained in early 1970s, but it took several years until the signal was confirmed and unambiguously identified, by a high-resolution germanium balloon experiment (Leventhal et al., 1978), with the positron annihilation line.
The history of annihilation line studies over the past 30 years contains many controversial issues like the the uncertainty in the size of the line production region, or claims about an existence of a variable component of radiation. If confirmed, the variable emission would appear to imply production and annihilation of positrons in a compact object(s). However, the observations of recent years, in particular by OSSE, did not succeed in finding any evidence for a point-like source on top of the diffuse 0.511 MeV emission. Also, no evidence has been found also for variability of emission above or below the diffuse flux (Harris, 1997). On the other hand, the OSSE observations brought new puzzles, like a high-latitude, asymmetrically extended feature of annihilation radiation that was claimed to be larger than the Galactic bulge feature (Purcell et al., 1997). However, the history of the extra-solar Y-ray line observations tells us to be cautious about this as well as with all the Y-ray line results mentioned above, given the inadequate angular resolution and the limited capability of the current low-energy Y-ray detectors to suppress and identify reliably the high instrumental background.
A breakthrough is expected in this regard from the new INTEGRAL mission. The significant improvement in the Y-ray line sensitivity, by a factor of five or so, over the Compton GRO detectors, should allow INTEGRAL not only to confirm the results discussed above, but, more importantly, will provide deeper insight into the nuclear processes that take place in different environments in our Galaxy and, hopefully, also beyond. On the other hand, INTEGRAL unfortunately cannot provide deeper Y-ray probes of the continuum component of radiation above 1 MeV. This unfortunate fact, as well as the urgent need for new instruments in this important energy region with more than factor of 10 improvement over the COMPTEL sensitivity, is recognised by the gamma-ray astronomical community. There is a hope that MEGA (Bloser et al., 2002) – likely the first representative of the new generation Compton telescopes – will dramatically change the status of low energy Y-ray astronomy.