摘要:

Results are presented of a statistical study of spectra of cosmic gamma‐ray bursts detected in the Soviet‐French experiment APEX on the Phobos‐2 spacecraft. Intense events withV/Vmax<0.3 are shown to have harder spectra than the weak ones withV/Vmax≳0.3. The time histories of intense and weak events are also different: the rise fronts of strong bursts are longer and correspond to more energetic photons than for the weaker ones. On the other hand, both for strong and weak events the mean durations and energy spectra of back slopes are similar. The astrophysical consequences of those results are considered for models of sources of gamma‐ray bursts.

ISSN:0094-243X    

DOI:10.1063/1.42819

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

BATSE’s Spectral Detectors provide a series of high resolution spectra over the duration of a gamma‐ray burst; fits to these spectra show the evolution of the continuum as the burst progresses. The burst continuum can usually be fit by the spectral fromAE&agr; exp(−E/kT) from ∼25 keV to more than 3 MeV, with varying trends in the value and evolution of the spectral parameters. As a result of limited statistics forE≳1–2 MeV in the individual spectra, a high energy power law is not required. Only long duration strong bursts can be studied by fitting a series of spectra, and therefore our conclusions concern only this class of burst. The bursts we analyzed tend to be characterized by a hard‐to‐soft trend both for individual intensity spikes and for the burst as a whole: the hardness (e.g.,Twith &agr; fixed) leads the count rate in spectra which resolve the temporal variations, while the hardness of successive spikes decreases. We also summarize the performance of the Spectral Detectors and the development of analysis tools to date.

ISSN:0094-243X    

DOI:10.1063/1.42820

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

We present preliminary analyses of gamma‐ray burst spectra from the BATSE Spectroscopy Detectors. Our conclusions are: (1) No spectral lines have yet been detected in BATSE data from any cosmic gamma‐ray burst. This is not surprising as the data for few bright bursts is available, and previous experiments saw lines in only a small fraction of the bursts. (2) Burst spectra show emission up to 20 MeV, with four of eight examined bursts having significant spectral breaks from 1 to 2 MeV. These breaks are consistent with opacity effects due to the interaction of photons with a high magnetic field. (3) Various distance independent parameters from burst spectra and time histories have no correlation with parameters related to distance. In other words, bright bursts look the same as faint bursts. This places a strong constraint on two population models of bursts. (4) The detection time of individual photons is not correlated from detector to detector, as predicted by Mitrofanov’s pulsed emission model.

ISSN:0094-243X    

DOI:10.1063/1.42821

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Conclusions about the nature of &ggr;‐ray bursts derived from the size‐frequency distribution may be altered if a significant correlation exists between burst intensity and spectral shape. Moreover, if &ggr;‐rays bursts have cosmological origin, such a correlation may be expected to result from the expansion of the universe. We have performed a rudimentary search of the BATSE bursts for hardness/intensity correlations. The range of spectral shapes was determined for each burst by computing the ratio of the intensity in the range 100–300 keV to that in 55–300 keV. We find weak evidence for the existence of a correlation, the strongest effect being present when comparing the maximum hardness ratio for each burst with its maximum rate.

ISSN:0094-243X    

DOI:10.1063/1.42822

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

We report the initial results of an analysis of 19 gamma‐ray burst energy spectra recorded by the Venera‐13 and 14 SIGNE experiments. The complete data‐base includes some 150 events, with 5 channel energy spectra taken in the 50–700 keV range with 0.5 s time resolution, for 64 s. The observed spectra of each event analyzed were fitted with a power law, bremsstrahlung, and thermal synchrotron model, for each time interval containing a statistically significant number of counts. Although specific examples may be found which exhibit hard‐to‐soft evolution, or a correlation between the effective temperature kT of a fit and the time history, cases of ‘‘random’’ spectral evolution are also present, and there appears to be no single way to characterize spectral evolution. There is also no clear preference for a particular model fit in the data analyzed so far.

ISSN:0094-243X    

DOI:10.1063/1.42823

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

We use simple 2‐parameter models to fit gamma‐ray burst spectra from the SIGNE experiments aboard the Venera 13 and 14 spacecraft in order to estimate burst luminosity and spectral hardness. We then look for correlations between these two parameters. The hardness is characterized by kT andEcfor thermal bremsstrahlung and thermal synchrotron fits, respectively. The spectra are integrated over 1/2 s or longer, depending on photon statistics. We find that generally luminosity increases with spectral hardness. We fit power laws of the formL∝kT&ggr;to 12 bursts. A wide range of indices with 0.5<&ggr;<2.5 is found, broader than that reported by Golenetskiietal.1

ISSN:0094-243X    

DOI:10.1063/1.42824

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Time‐integrated spectra from 177 &ggr;‐ray bursts observed by the Gamma Ray Spectrometer (GRS) on NASA’s Solar Maximum Mission satellite (SMM) have been systematically searched for evidence of emission lines ≥300 keV. The distribution of fitted line strengths is consistent with what is expected from a random sample of data, suggesting that there are no statistically significant narrow or moderately broadened line features in any of the bursts.

ISSN:0094-243X    

DOI:10.1063/1.42825

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

An exhaustive search of the Ginga data on &ggr;‐ray burst GB870303 reveals two separate time intervals during which statistically significant line features are evident. One (previously unreported) interval shows a single prominent line feature at ≊20 keV; a second, corresponding to the interval reported by Murakamietal., shows two line features at ≊20 and 40 keV. From model fits to the data, we find that both sets of lines are well‐described by cyclotron resonant scattering in a magnetic fieldB≊1.8×1012G, and that the differences in the line strengths between the two intervals are significant. The variations are qualitatively similar to those produced by a change in the viewing angle &thgr; relative the magnetic field. We conjecture that the change in &thgr; is due to rotation of the neutron star, and derive limits 45 sec ≲P≲180 sec on the rotation periodP.

ISSN:0094-243X    

DOI:10.1063/1.42833

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

We have extended our previous work by exploring several plane‐parallel slab geometries to model the formation of cyclotron line features. We calculated the Compton temperatureTCas a function of column densityNefor each of the new geometries. We then fit the resulting spectra to GB880205 exactly as described in Wangetal. The results show that the addition of column depth below the photon source plane leads to a modest improvement in &khgr;2which, although not statistically significant, is pleasing because these geometries are more physically realistic.

ISSN:0094-243X    

DOI:10.1063/1.42834

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

We explain why the ability to extract physical parameters from cyclotron lines depends crucially on the observation of multiple harmonics. We then describe the cyclotron line signature of neutron star rotation and discuss possible observational evidence of it.

ISSN:0094-243X    

DOI:10.1063/1.42835

出版商:AIP    

年代:1991

数据来源: AIP