336 research outputs found
On the Shape of Pulse Spectra in Gamma-Ray Bursts
The discovery (Liang & Kargatis 1996), that the peak energy of time-resolved
spectra of gamma-ray burst (GRB) pulses decays exponentially with fluence, is
analytically shown to imply that the time-integrated photon number spectrum of
a pulse should have a unique shape, given by an underlying E^-1 behavior. We
also show that the asymptotic low energy normalization of the time-integrated
spectrum is equal to the exponential decay constant. We study analytically how
this general behavior is modified in more realistic situations and show that
diversity is then introduced in the properties of time-integrated GRB pulse
spectra. We argue that further diversity will occur in time-integrated
multi-pulse (complex) GRB spectra. The total energy received per cm^2 is
approximately the decay constant times the maximum peak energy of the pulse.
Our analytical results connect the properties of the time-integrated pulse
spectrum with those of the time-resolved spectra, and can thus be used when
studying observed GRB pulse spectra. We illustrate with the bright burst GRB
910807 and comment on GRB 910525 and GRB 921207.Comment: 7 pages, 6 postscript figures, accepted by the Astrophysical Journa
On the Time Evolution of Gamma-Ray Burst Pulses: A Self-Consistent Description
For the first time, the consequences of combining two well-established
empirical relations, describing different aspects of the spectral evolution of
observed gamma-ray burst (GRB) pulses, are explored. These empirical relations
are: i) the hardness-intensity correlation, and ii) the hardness-photon fluence
correlation. From these we find a self-consistent, quantitative, and compact
description for the temporal evolution of pulse decay phases within a GRB light
curve. In particular, we show that in the case of the two empirical relations
both being valid, the instantaneous photon flux (intensity) must behave as
1/(1+ t/\tau) where \tau is a time constant that can be expressed in terms of
the parameters of the two empirical relations. The time evolution is fully
defined by two initial constants, and two parameters. We study a complete
sample of 83 bright GRB pulses observed by the Compton Gamma-Ray Observatory
and identify a major subgroup of GRB pulses (~45 %), which satisfy the
spectral-temporal behavior described above. In particular, the decay phase
follows a reciprocal law in time. It is unclear what physics causes such a
decay phase.Comment: 4 pages, 1 figure, 2 tables, to appear in ApJ
Can the cosmic x ray and gamma ray background be due to reflection of a steep power law spectrum and Compton scattering by relativistic electrons?
We reconsider the recent model for the origin in the cosmic X-ray and gamma-ray background by Rogers and Field. The background in the model is due to an unresolved population of AGNs. An individual AGN spectrum contains three components: a power law with the energy index of alpha = 1.1, an enhanced reflection component, and a component from Compton scattering by relativistic electrons with a low energy cutoff at some minimum Lorentz factor, gamma(sub min) much greater than 1. The MeV bump seen in the gamma-ray background is then explained by inverse Compton emission by the electrons. We show that the model does not reproduce the shape of the observed X-ray and gamma-ray background below 10 MeV and that it overproduces the background at larger energies. Furthermore, we find the assumptions made for the Compton component to be physically inconsistent. Relaxing the inconsistent assumptions leads to model spectra even more different from that of the observed cosmic background. Thus, we can reject the hypothesis that the high-energy cosmic background is due to the described model
Determining Bolometric Corrections for BATSE Burst Observations
We compare the energy and count fluxes obtained by integrating over the
finite bandwidth of BATSE with a measure proportional to the bolometric energy
flux, the phi-measure, introduced by Borgonovo & Ryde. We do this on a sample
of 74 bright, long, and smooth pulses from 55 GRBs. The correction factors show
a fairly constant behavior over the whole sample, when the
signal-to-noise-ratio is high enough. We present the averaged spectral
bolometric correction for the sample, which can be used to correct flux data.Comment: 3 pages, 3 figures, to appear in AIP proc. "Gamma-Ray Burst and
Afterglow Astronomy 2001" Woods Hole, Massachusett
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