摘要:

We have investigated two biases in the BATSE 1024 ms burst trigger using Monte Carlo simulations of burst time histories. The peak counts bias, which arises from statistical fluctuations in the count rates, results in an average overestimate of the on-board peak rate by ∼8&percent;. The slow riser bias, which arises from errors in the on-board calculation of background, results in an average underestimate of the on-board peak rate by ∼5&percent;. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1361506

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

We use the BD2 sample of gamma-ray bursts (GRBs) based on 5.9 years of BATSE DISCLA data with a variety of models of the luminosity function to derive characteristic GRB luminosities, space densities and redshift distributions. Previously published results for an open universe and modest density evolution of the GRBs showed characteristic peak luminosities around5×1051&hthinsp;ergs&hthinsp;s−1in the 50–300 keV band if the emission is isotropic, and local space densities around 0.2 Gpc−3&hthinsp;y−1. In this paper, we illustrate for several luminosity function models the predicted distributions of peak flux, luminosity and redshifts. We use the luminosity function models also to address the connection between supernovae and GRB. If all supernovae of type Ib/c harbor a GRB, the beaming fraction would have to be in the range10−5–10−3.5.We find that GRB 980425, if correctly identified with SN 1998bw, has to be part of a population different from that of the bulk of GRBs. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1361507

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

We constrain the luminosity function of gamma ray bursts (GRBs) by testing several cosmological models versus a combination of (a) observed time dilation in the peak-to-peak time scales of GRB light curves, (b) theLogN−LogPnumber counts relation, and (c) GRB luminosities directly determined from recent redshift measurements and apparent GRB brightness. A power law luminosity function&fgr;(L)=C⋅L−&bgr;(Lmin<L<Lmax)is examined within three cosmological models, with cosmological parameters(&OHgr;0,&OHgr;&Lgr;)=(1.0,0.0),(0.3, 0.0), or (0.3, 0.7). It is found that (a) GRB models with constant comoving density rate do not fit the data while models with GRB density evolution tracing the observed star formation rate can accommodate all the observed data; (b) the widthK=Lmax/Lminof the luminosity function is found to beK>100at the 1 &sgr; level; (c) theintrinsicaverage luminosity ⟨L⟩ is found to be2∼6×1051&hthinsp;ergs⋅s−1at the 3&sgr; level, this implies that bursts at the BATSE 100&percent; efficiency threshold have typical red shifts of⟨z0.42⟩=2∼3.©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1361508

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

The smallreferencegroup of BATSE gamma-ray bursts with measured red-factors of optical afterglows is compared with the largecomparisongroup of brightest BATSE bursts. These two groups are shown to be similar both in respect with distributions of measuredEpparameters in the Observer frame and also in respect with frame-independent distributions of newly implemented cosmological invariant parameters (CIP). Using the known values of red-factors Z for thereferencegroup, the distribution ofEp(RF)is built for the rest frames of their sources. De-redshifted statistics ofEp(RF)are compared with observed distributions ofEpfor thecomparisongroup of bursts. From this comparison the collective estimation of red-shiftsZ=2.5–3.5has been performed for the group of brightest BATSE bursts. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1361509

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

A physically-based flux characteristic for gamma-ray bursts is proposed that consists of the photon flux in a particular energy range positioned at the peak of the energy spectrum accumulated during the sum of emission time intervals. LogN-LogF distributions are investigated using these fluxes for bright BATSE bursts with peak fluxes on the 64 ms time scale larger than 2 ph/cm2⋅s. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1361510

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

For the set of six gamma-ray bursts with redshifts observed by BeppoSAX and CGRO/BATSE, their isotropic peak luminosities and spectral lags are anti-correlated. In this small set of bursts, the relationship is approximated by a power-law,L53≈1.3×(&tgr;/0.01&hthinsp;s)−1.15.While GRB 980425 (if associated with SN 1998bw) would appear to extend this trend qualitatively, it falls below the power-law relationship by a factor of several hundred. The same underlying anti-correlation, but apparently convolved with redshift, appears to be manifest in the 30&percent; brightest BATSE bursts—dimmer bursts tend to have longer spectral lags. These results appear to lend at least empirical meaning to the GRB pulse paradigm:short spectral lag and therefore narrow pulse width are somehow related to high luminosity. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1361511

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

We introduce a definition of gamma-ray burst (GRB) duty cycle that describes the GRB’s efficiency as an emitter; it is the GRB’s average flux relative to the peak flux. This GRB duty cycle is easily described in terms of measured BATSE parameters; it is essentially fluence divided by the quantity peak flux times duration. Since fluence and duration are two of the three defining characteristics of the GRB classes identified by statistical clustering techniques (the other is spectral hardness), duty cycle is a potentially valuable probe for studying properties of these classes. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1361512

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

We analyze the distribution of total energy of bursts with optical afterglows. Our sample contains eleven bursts for which there are BATSE data allowing detailed fits to the spectra. Six of these bursts have measured redshifts while five have host galaxies whose redshifts are unknown. Using a new technique based on the distribution of magnitudes of observed high redshift galaxies, we bracket the expected redshifts of these host galaxies. We perform a maximum likelihood fit for the energy distribution of these GRBs. Assuming that the total energy emitted has a log-normal distribution, we find that the average isotropic energy emitted isE&ggr;=1.5−0.6+1.5×1053&hthinsp;ergs(forH0=65&hthinsp;km&hthinsp;s−1&hthinsp;Mpc−1,&OHgr;m=0.3and&OHgr;A=0.7) with a standard deviation&sgr;&ggr;=1.2−0.4+0.6.The corresponding distribution of X-ray afterglow energy (for seven bursts) is significantly narrower with&sgr;x=0.5−0.25+0.4andEx=3.8−1.5+2×1051&hthinsp;ergs,in agreement with the prediction of the patchy shell model of Kumar and Piran [1]. We also give a table with the detailed fits to the spectra of the 11 GRBs with optical afterglow with BATSE data. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1361513

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

The concept of Pure Physical Parameters (PPP, orP3) is presented, as the instrument-independent parameterization for the statistical search for basic physical signatures of gamma-ray bursts. The emission time of photon light curves and the peak of photon spectra of high power emission are proposed for each burst characterization, asP3parameters for time variability and for spectral energy of gamma-rays. TheP3parameters are presented for 6 gamma-ray bursts in their rest frames with known red-shifts of optical afterglows. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1361514

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

While the bulk of the gamma-ray bursts (GRBs) are now believed to be from cosmological distances, the origin of very short-time GRBs is still not known. In the past, we have shown that GRBs with time duration less than 100 ms may form a separate class of GRBs based on the hardness and time distribution of these events. We also showed that theln&hthinsp;N−ln&hthinsp;Sdistribution is consistent with the expectation of a quasi-Euclidean distribution of sources. In this letter, we report the study of the angular location of these GRBs that show a strong deviation from isotropy within the Galactic coordinates of+180°<longitude<+90°and−30°<latitude<30°.We have studied the rest of the GRBs and do not find a similar deviation. This further indicates that the very-short GRBs likely form a separate class of GRBs, most likely from sources of Galactic or near solar origin. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1361515

出版商:AIP    

年代:1900

数据来源: AIP