Spectral Evolutions in Gamma-Ray Burst Exponential Decays Observed with Suzaku WAM

TL;DR

This study analyzes the spectral evolution of gamma-ray bursts observed with Suzaku WAM, revealing different emission mechanisms through time-resolved spectral analysis and power-law decay behavior.

Contribution

It provides the first detailed spectral evolution analysis of GRBs with Suzaku WAM, identifying distinct emission origins via time-resolved spectra and decay time constants.

Findings

Spectral evolution follows a power-law dependence with gamma ~ -0.34 on average.

Two GRBs show different emission origins: thermal blackbody and Band model.

Decay time constants suggest synchrotron or inverse-Compton cooling mechanisms.

Abstract

This paper presents a study on the spectral evolution of gamma-ray burst (GRB) prompt emissions observed with the Suzaku Wide-band All-sky Monitor (WAM). By making use of the WAM data archive, 6 bright GRBs exhibiting 7 well-separated fast-rise-exponential-decay (FRED) shaped light curves are presented and the evaluated exponential decay time constants of the energy-resolved light curves from these FRED peak light curves are shown to indicate significant spectral evolution. The energy dependence of the time constants is well described with a power-law function tau(E) ~ E^gamma, where gamma ~ -(0.34 +/- 0.12) in average, although 5 FRED peaks show consistent value of gamma = -1/2 which is expected in synchrotron or inverse-Compton cooling models. In particular, 2 of the GRBs were located with accuracy sufficient to evaluate the time-resolved spectra with precise energy response matrices. Their behavior in spectral evolution suggests two different origins of emissions. In the case of GRB081224, the derived 1-s time-resolved spectra are well described by a blackbody radiation model with a power-law component. The derived behavior of cooling is consistent with that expected from radiative cooling or expansion of the emission region. On the other hand, the other 1-s time-resolved spectra from GRB100707A is well described by a Band GRB model as well as with the thermal model. Although relative poor statistics prevent us to conclude, the energy dependence in decaying light curve is consistent with that expected in the former emission mechanism model.