GRB 240715A: Revealing Novel Intrinsic Mechanism by Different Individual Pulse

TL;DR

This study analyzes the prompt emission of GRB 240715A, revealing unique spectral lag behaviors and suggesting an intrinsic mechanism involving electron cooling, with implications for understanding gamma-ray burst emission processes.

Contribution

It uncovers novel spectral lag evolutions and proposes a new intrinsic mechanism involving electron cooling, challenging previous models of GRB emission.

Findings

Opposite spectral lag evolutions in different pulses.

Large negative lag in the first pulse is an outlier.

Third pulse likely involves photospheric emission.

Abstract

The Space-based multiband astronomical Variable Objects Monitor (SVOM), detected its first short gamma-ray burst, GRB 240715A, in-flight, which was jointly observed by Fermi. Based on observational data of SVOM/GRM and Fermi/GBM, we perform a comprehensive temporal and spectral analysis for individual pulse in the prompt emission of this burst, and novel characteristics are revealed. Firstly, opposite evolutions of spectral lag are found in the first and third pulse of this burst. Second, the large negative lag of the first pulse is an outlier in short GRB sample, especially when the pulse duration is considered. Spectral analysis shows that the negative lag of the first pulse is caused by the evolution of spectrum index, and is irrelevant to Epeak, which is inconsistent with the previous study. The intrinsic mechanism is probably attributed to electron cooling in the decaying magnetic field, which leads to the continuous hardening of the spectrum index and results in negative lag. Furthermore, spectral analysis also shows that the third pulse is more likely to be described by a quasi-thermal spectrum, indicating the existence of photospheric emission. It is difficult to explain how the synchrotron radiation appears before photospheric emission in a single GRB and some assumptions are discussed.