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