摘要:

Due to very successful measurements by the Compton/BATSE instrument, a large volume of GRB data has now become available. These data allow anew statisticsof GRBs focused on the average temporal signatures of fluxes and on spectral variability. We apply these methods to the BATSE GRB data, and ask some new questions. These questions, when properly answered, may lead to a basic concept of the GRB phenomena. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51688

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Understanding the bimodal duration distribution (dynamic range>104) of &ggr;-ray bursts is central to determining if the phenomenon is in fact a singular one. A unifying concept, beyond isotropy and inhomogeneity of the two groups separately, is that bursts consist of pulses, organized in time and energy: wider pulses are more asymmetric, their centroids are shifted to later times at lower energies, and shorter, more symmetric pulses tend to be spectrally harder. Long bursts tend to have many pulses while short bursts usually have few, relatively narrow pulses. Two factors, viewing angle and beaming, may account for pulse asymmetry and the large dynamic range(∼200)in pulse widths. A cosmological time-dilation signature, with an expected dynamic range of order two, would be difficult to measure against these large intrinsic variations and low signal-to-noise levels of dimmer bursts. Some statistics (T90,pulse intervals) are particularly sensitive to brightness bias, noise, and apparently minor variations in definition. Also, spectral redshift would move narrower, high-energy emission from dim bursts into the band of observation, constituting a countering effect to time dilation. With analyses restricted to bursts longer than∼2 s,tests for time dilation that are constructed to be free of brightness bias have yielded time-dilation factors∼2–3,for pulse structures, intervals between structures, and durations. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51731

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Gamma Ray Bursts (GRB) of duration less than about 2 s, detected by the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory have been selected for temporal analysis. These bursts constitute nearly 25&percent; of the total and presumably form a separate class. Several parameters to describe the complexity and rapidity based on the burst temporal structure are derived and their dependence on other temporal and spectral properties are explored. A parameter is derived for each burst to characterize its spectral evolution based on its light curves in 4 energy channels. Bursts detected during April 1991 and March 1993 have been analysed yielding a sample size of 51 bursts. It has been found that the burst complexity is independent of its spectral content. The spectral evolution of short bursts is same as that of longer bursts. Also a systematic search for a coherent emission of &ggr;-rays in short bursts yielded a negative result. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51673

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We present measurements ofT90andT50durations of BATSE gamma-ray bursts in the 3B catalog, based on an automated algorithm designed to reduce brightness bias effects by scaling burst peak intensities and variances to those of the dimmest burst in the sample. A multi-scale threshold is applied to estimate the beginning and end of burst activity.T90andT50measures indicate an observed time-dilation factor of order 2 between average durations of bright and dim bursts. We also estimate the dependence of duration measure on energy band, and find that an increase of order 10–25&percent; would be required if the actual cosmological time-dilation factor were 2–3. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51681

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The short time structure (possibly shot noise) might reveal much about the spatial nature of the GRB source. In the case of GRBs from Primordial Black Holes (PBHs), the initial explosion region is∼10−13 cmin size, much smaller than any other known compact object. The BATSE team has already reported on one interesting event. We report here on the study of the TTE data from three BATSE events that may be PBH candidates. The latest results will be presented at the meeting. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51556

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The observation of subclasses within the classical Gamma-Ray Burst population is the first clue that the bursts may be produced by a small number of distinct mechanisms, sources, or initial conditions. Events in one subclass are characterized by a short duration (less than 2 seconds) and, on average, harder spectra compared to longer bursts. Here, we present a statistical study of GRB properties as observed with the PHEBUS experiment such as intensity, duration, spectral hardness, and the shape of their time profiles. We show which burst characteristics are common to both the long and short populations and which are not. Finally, we compare our results with BATSE observations. Specifically, we discuss the possible causes for the striking difference observed in the hardness-duration diagrams for the two experiments. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51566

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We present here theT90andT50distributions for different trigger time scales from the 3B catalog and confirm the bimodality established before with the 1B data. We show that the Hardness Ratio/duration anti-correlation is present in the 3B data set and we explore Hardness and Intensity correlations by studying various burst subsets. We discuss the impact of these correlations within the framework of a cosmological GRB origin hypothesis. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51695

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The time profiles of many gamma-ray bursts observed with BATSE consist of distinct pulses, which offer the possibility of characterizing the temporal structure of these bursts using a relatively small set of pulse-shape parameters. This pulse analysis has previously been performed on some bright, long-bursts using binned data, and on some short bursts using BATSE Time-Tagged Event (TTE) data. The BATSE Time-to-Spill (TTS) burst data record the times required to accumulate a fixed number of photons, giving variable time resolution. The spill times recorded in the TTS data behave as a gamma distribution. We have developed an interactive pulse-fitting program using the pulse model of Norris &etal; and a maximum-likelihood fitting algorithm to the gamma distribution of the spill times. We then used this program to analyze a number of bright, long bursts for which TTS data are available. We present statistical information on the attributes of pulses comprising these bursts. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51707

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The search for gamma-ray burst counterparts is one of the most important fields of current investigation. Measurements of pre- and post-burst emission can provide crucial constraints on emission processes and physics. OSSE can make deep searches for precursors, recurrent burst emission, and fading or persistent counterparts from 50 keV to 10 MeV for gamma-ray bursts serendipitously in or near the field of view at the time of the burst. We present limits from one such event which was well-localized by COMPTEL/IPN, GRB950421, in terms of absolute flux, and as a fraction of the measured burst flux. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51718

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Gamma Ray Burst light curves have strikingly different morphologies, ranging from a smooth single pulse to a complex series of overlapping pulses. We present multifractal analysis as a method for quantifying the variability in their morphology. Multifractal analysis was chosen because it measures the structure on different time scales which is typical of many GRBs. We provide an estimate of the error on this variability measure and look for correlations with other GRB characteristics including intensity and duration. This new measure may help to classify bursts, as well as identify repeaters and gravitationally lensed partners. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51571

出版商:AIP    

年代:1996

数据来源: AIP