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

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.