摘要:

We examine the geocenter angle distribution of the 1637 bursts contained within the 4B catalog. The distribution is modelled using an isotropic distribution of gamma-ray bursts and a sharp cutoff due to the presence of the earth. A comparison between the observed distribution and the model is made, which provides an independent assessment of the BATSE location errors. Examination of the error distribution near the earth cutoff is presented. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.55313

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

BATSE, Ulysses, and TGRS and KONUS on WIND detected four gamma-ray events within 1.8 days during 1996 October 27–29, consistent with coming from the same location on the sky. We assess the evidence that these events may be due to a series of bursts from a single source by calculating the probability that such a clustering in position and in time of occurrence might happen by chance. The calculation of this probability is afflicted by the usual problem ofa posterioristatistics. We introduce a clustering statistic, which is formed from the “minimum circle radius” (i.e. the radius of the smallest circle that just encloses the positions of all the events) and the minimum time lapse (i.e. the time elapsed between the first and last event). We also introduce a second clustering statistic, which is formed from the “cluster likelihood function” and the minimum time lapse. We show that the use of these statistics largely eliminates the “a posteriori” nature of the problem. The two statistics yield significances of the clustering of3.3×10−4and3.1×10−5,respectively, if we interpret the four events as four bursts, whereas the clustering is not significant if we interpret the four events as only three bursts. However, in the latter case one of the bursts is the longest ever observed by BATSE. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.55314

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Different types of gravitational lensing may affect GRBs if they lie at cosmological distances. Given the non-detection of lensing events due to the known galaxy field and assuming that QSO images are distorted by the same lens field, we have derived a very conservative upper limit on the redshift ofzgrbmax∼4.06.If dark matter comprises a certain fraction of the mass of the universe and is composed by individual objects, the lack of mega-, micro-, pico-, and femto-lensing events is also used to set limits on the cosmological abundance of these compact objects. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.55315

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Recent detections of gamma-ray burst (GRB) counterparts confirm their great distances and consequent potential use as cosmic probes. However, GRB diversity may thwart this idea. The scatter of intrinsic GRB properties could easily mask the extrinsic effect of cosmological time dilation. Current investigations examine the question of temporal self-similarity: Extrinsic time dilation must be manifest with the same factor on all GRB timescales as a function of distance. Here we show that time-reversal-independent analysis of GRB time profiles using a peak alignment methodology reveals average profiles with approximately equal rise and decay timescales, per peak-flux group, except for the dimmest group. This departure from self-similarity is consistent with a selection effect: At sufficiently low peak fluxes, the BATSE causal trigger misses dim, slowly rising bursts. Interestingly, for GRB970508, the redshift-corrected time-dilation factor inferred for its peak flux is consistent with the measured redshift,z=0.835,if bright bursts lie at redshifts of a few tenths. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.55316

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The stretching parameters of GRB time profiles are estimated at their rise fronts and back slopes, comparing dim bursts to bright events. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.55300

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Bayesian Blocks is a new time series algorithm for detecting localized structures (spikesorshots), revealing pulse shapes, and generally characterizing intensity variations. It maps raw counting data into a maximum likelihood piecewise constant representation of the underlying signal. This bin-free method imposes no lower limit on measurable time scales. Applied to BATSE TTE data, it reveals the shortest known burst structure—a spike superimposed on the main burst in GRB920229 (BATSE trigger 1453), with rise and decaytimescales≈few100 &mgr;s. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.55317

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Normalized flux averaging (NFA) is a simple method of pulse structure analysis. The equivalent pulse width of the main pulse and the time intervals between pulses are defined at the same dimensionless level for every burst time history. Distributions of parameters are compared for different brightness groups of GRBs. If gamma-ray bursts of the same particular brightness group are at the same cosmological distance then all time scales resulting from the emission mechanism must be stretched by the same amount in the observer’s reference frame. Time intervals between separate pulses and equivalent pulse widths represent different physical time scales of the emitters. Using these NFA based parameters, the stretch factors for different brightness groups of bursts are found. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.55318

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A wavelet analysis has been performed on 80 intense gamma-ray bursts (GRBs) from the BATSE 3B catalog with durations longer than 2 seconds. The wavelet analysis applied novel features developed for edge detection in image processing and this filtering process was used to extract a fit to the irregular GRB profile from the background. A straightforward algorithm was subsequently used to identify statistically significant peaks in this profile. The areas and FWHM of 270 peaks that were characterised as isolated were found to be consistent with lognormal distributions. The distribution of time intervals between peak maxima for all 963 identified peaks in the GRBs is also presented. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.55319

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We apply Redundant Wavelet Transforms (RWTs) to GRB time histories in order to identify significant structures on various time scales. While the classical Discrete Wavelet Transform (DWT) also carries out a data compression and a denoising, it does not lead to a description as a set of peaks. On the contrary, the so-calleda` trousalgorithm (that is a redundant wavelet transform often being considered as a fast continuous wavelet transform for a real wavelet) carries out a time scale analysis which easily allows us to decompose the signal into peaks on different scales. After neglecting insignificant coefficients of the RWT the signal is easily restored while noise is suppressed. Not only is the quality of the smoothing better than the one we get with DWT, but also the thresholded wavelet coefficients contain directly the peak decomposition. It is then possible to use a Multiscale Vision Model that has been originally developed for 2D images that allows one to build oriented trees from the neighboring of significant wavelet coefficients. Each tree is then divided into subtrees taking into account the maxima along the scale axis. This leads to the identification of objects in the scale space, that could be further restored by classical inverse methods. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.55320

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Multiscale edge detection using wavelet transform maxima provides a robust method to reduce noise, quantify variability, and compress information in a transient signal. We have applied this method to gamma-ray burst (GRB) time series data from the Compton Gamma-Ray Observatory (CGRO). We define a variability parameter that provides another way to describe a GRB in a manner similar to duration or peak intensity. We present a study of correlations between our variability parameter and other burst parameters. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.55321

出版商:AIP    

年代:1998

数据来源: AIP