High-Energy Astrophysics American and Soviet Perspectives/Proceedings from the U.S.-U.S.S.R. Workshop on High-Energy Astrophysics (1991) / Chapter Skim
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COSMIC GAMMA-RAY BURSTS
COSMIC GAMMA-RAY BURSTS
Pages 204-217
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From page 204... ...
Both the light curies and the energy spectra of these short transient events display a great diversity. However, rapid rise times and periodiaties sometimes observed in the light curies suggest a compact object origin.
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From page 205... ...
Despite numerous deep searches for burster counterparts at many wavelengths, no unambiguous candidate for a quiescent counterpart has yet been identified; as a result, the distances to bursters are completely unlmown It now seems indisputable that some fraction, perhaps 20% at least, of the GRB sources are neutron stars, most likely in our galaxy. This conclusion follows from the observation of line features in GRB energy spectra.
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From page 206... ...
382 C.~-298 KElI l rll . 0 0.1 0.2 0.3 TIME , ~ o FIGURES lam Alma histories of three gamma bursts observed By Franco-Soviet expenments aboard the interplanetary Venera 13 and 14 spacecraft.
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From page 207... ...
HIGH-ENERGY ASTROPHYSICS 2~) 7 1 1 i ~0 (A 60 Car o 50 GO 30 20 10 o Figure lb 800 400 Go ( )
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From page 208... ...
Thus there is circumstantial evidence in the time histories that GRBs may originate on or near neutron stars. ENERGY SPECTRA GRB energy spectra tend to be almost as diverse in some respects as the time histories.
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From page 209... ...
HIGH-ENERGY ASTROPHYSICS 1 10-2 > Y _4 t`, 1 0 Cot 0 10-6 o I 10-8 10 209 I ~ l I I I I i I T I - \ \ SOFT GAkl~lA REPEATER X-RAY \ \ SGR 1806- 20 BURSTER \ \ kT~35 keV ~ kT~lkeV \ \ 3UR~ 20 a 40 keV ~_ - CREATURES \ CRAB \\ \ ~400 keV (TOTAL) \ \ EM ISSIObJ -FEATURE ~ \ \ E-1 e-E/kT \ \ kT~200 keV \ \P0~4ER LAW - \ \ TAIL lO ~ 1 ~ 1 ~ ~ ~ ~ ~ ~ 10 100 0.001 0.0 1 .1 ENERGY, MeV FIGURE 2 Apical photon number spectra of some astrophysical objects.
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From page 210... ...
This interpretation raises the interesting question of how the lines can remain relatively narrow in the presence of a hot continuum. In the case of the cyclotron features, the answer appears to be that the absorbing medium is optically thin to the continuum but, due to the magnetic field, thick to photons at resonant energies (Wang et al.
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From page 211... ...
1987~. The lack of positive identification with any known objects makes it practically impossible to establish the distances to gamma-bursters, even if one accepts the galactic neutron star hypothesis.
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From page 212... ...
Since the lack of GRB counterparts leaves the distance scale undetermined, much effort has been focused on understanding the spatial distribution of bursters, in the hope that, for example, an underlying galactic distribution might become evident at some level. It is possible to quantity the isotropy or anisotropy in a coordinate system-independent fashion, by first expanding the source distinction in spherical harmonics, and then calculating moments of the distribution (Hartmann and Epstein 1989~.
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From page 213... ...
Although instrumental effects distort this portion of the curve, the same data may be used in a different formulation to test the hypothesis that GRB sources are drawn from a uniform space distribution This is the V/Vma~ test, first used to test the distribution of quasars (Schmidt 1968) , and recently revised for GRB sources (Schmidt et al.
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From page 214... ...
If all GRBs are indeed due to galactic neutron stars, they will yield a considerable amount of information on a number of topics of current interest: the behavior of matter, including its radiation mechanisms, under extreme conditions of gravitational and magnetic fields and temperatures, the evolutionary path followed by the neutron star which leads to the burster phase, and the retention of a strong magnetic field as it evolves are just a few examples. Future missions will deploy new, sensitive, high resolution and multiwavelength experiments to resolve the issues surrounding ORB ongin.
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From page 215... ...
Using a novel "message-forwarding" capability, the experiment will transmit information to simple ground receiving stations around the globe, to petit rapid searches for counterparts with short-lived emission at, e.g., optical and radio wavelengths. It is anticipated that such an experiment will revolutionize GRB studies and resolve many outstanding problems, by providing the long-sought burster identifications.
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From page 216... ...
Wefel (eds.~. Cosmic Gamma-Rays and Cosmic Neutrinos Reidel Pub.
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From page 217... ...
HIGH-ENERGY ~TROP~ICS 217 Motch, C., H Pedersen, S
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