Single-Pulse Gamma-Ray Bursts have Prevalent Hard-to-Soft Spectral Evolution

TL;DR

This study analyzes the spectral evolution of 62 bright gamma-ray bursts, revealing that single-peaked bursts tend to exhibit a hard-to-soft spectral evolution, contrasting with multi-peaked bursts.

Contribution

A new algorithm for identifying single-pulse morphology in gamma-ray bursts and a citizen science approach to assess light curve morphology and spectral evolution.

Findings

Single-peaked bursts often show hard-to-soft spectral evolution.

Multi-peaked bursts tend to have non-monotonically decreasing peak frequencies.

The results support models involving high-density environments for some single-pulse bursts.

Abstract

We analyze the spectral evolution of 62 bright Fermi gamma-ray bursts with large enough signal to noise to allow for time resolved spectral analysis. We develop a new algorithm to test for single-pulse morphology that is insensitive to the specific shape of pulses. Instead, it only checks whether or not there are multiple, isolated, statistical significant peaks in the light curve. In addition, we carry out a citizen science test to assess light curve morphology and spectral evolution. We find that, no matter the adopted assessment method, bursts characterized by single-peaked prompt emission light curves have a greater tendency to also have a consistently decaying peak energy, or hard-to-soft spectral evolution. This contrasts the behavior of multi-peaked bursts, for which the tendency is to have a peak frequency that is not monotonically decreasing. We discuss this finding in the theoretical framework of internal/external shocks, and find it to be consistent with at least some single pulse bursts being associated with particularly high-density environments.