Interpretations of gamma-ray burst spectroscopy

Abstract

Abstract. We describe the strong spectral evolution that occurs during a gamma-ray burst (GRB) pulse and the means by which it can be analyzed. In particular, we discuss the change of the light curve as a function of energy and the spectral lag. Based on observed empirical correlations, an analytical model is constructed which is used to describe the pulse shape and quantize the spectral lags and their dependences on the spectral evolution parameters. Using this model, we find that the spectral lag depends mainly on the pulse-decay time-scale and that hard spectra (with large spectral power-law indices α) give the largest lags. Similarly, large initial peak-energies, E 0 , lead to large lags, except in the case of very soft spectra. The hardness ratio is found to depend only weakly on α and the hardness-intensity-correlation index, η. In particular, for low E 0 , it is practically independent, and is determined mainly by E 0 . The relation between the hardness ratio and the lags, for a certain E 0 are described by power-laws, as α varies. These results are the consequences of the empirical description of the spectral evolution in pulses and can be used as a reference in analyses of observed pulses. We also discuss the expected signatures of a sample of hard spectral pulses (e.g. thermal or small pitch-angle synchrotron emission) versus soft spectral pulses (e.g. optically-thin synchrotron emission). Also the expected differences between a sample of low energetic bursts (such as X-ray flashes) and of high energetic bursts (classical bursts) are discussed.

SPECTRAL LAGS AND THE LAG-LUMINOSITY RELATION: AN INVESTIGATION WITHSWIFTBAT GAMMA-RAY BURSTS

“…While these negative lags are not shown in the plots, it is worth noting that negative lags are not necessarily unphysical (Ryde 2005). Moreover, in the few cases where the uncertainty is large, i.e., the extracted lags are consistent with zero, these points are not plotted either but are listed in Table 3.…”

Luminosity Correlations for Gamma-Ray Bursts and Implications for Their Prompt and Afterglow Emission Mechanisms

“…Two of the GRBs (GRB 060206 and GRB 080603B) have negative spectral lags, meaning that the time of arrival of low-energy photons precedes that of high-energy photons. Although negative lags are not necessarily unphysical (Ryde 2005), we chose to exclude them due to the logarithmic nature of the lag-luminosity relation in our plots. This was also done in previous studies of this relation by Ukwatta et al (2010Ukwatta et al ( , 2012 and others.…”

“…In the case of the spectral-lag-luminosity relation, one possible explanation for the observed lags involves the spectral evolution during the prompt phase (Dermer 1998;Kocevski & Liang 2003;Ryde 2005) in which, due to cooling effects, E pk shifts toward a lower energy band so that the temporal peak of the corresponding light curve will also shift to lower energies, thereby resulting in the observed lag. Another explanation for the spectral-lag-luminosity relation is based purely on kinematic effects (Salmonson 2000;Salmonson & Galama 2002;Ioka & Nakamura 2001;Dermer 2004;Shen et al 2005;Lu et al 2006), where the peak luminosity L pk and spectral lag τ depend on a single kinematic variable…”

Correlations between Lag, Luminosity, and Duration in Gamma-Ray Burst Pulses

“…Time intervals containing flux increases (pulses) exhibit some general behaviors, including (1) longer decay than rise rates, (2) hard-to-soft spectral evolution, and (3) broadening at lower energies (e.g. Norris et al (1996); Ryde (2005)). …”