Temporal and Spectral Evolution of Gamma-Ray Burst Broad Pulses: Identification of High-latitude Emission in the Prompt Emission

• Published in: The Astrophysical journal Vol. 949; no. 2; pp. 110 - 120
• Main Authors: Tak, Donggeun, Uhm, Z. Lucas, Racusin, Judith, Zhang, Bing, Guiriec, Sylvain, Kocevski, Daniel, Zhang, Bin-Bin, McEnery, Julie
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.06.2023; IOP Publishing
• Subjects: Astrophysics; Emission analysis; Evolution; Fermi Gamma-ray Space Telescope; Gamma ray astronomy; Gamma ray bursts; Gamma rays; Relativistic jets; Space telescopes; Spectral analysis; Spectrum analysis
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/acc581

Abstract We perform a detailed analysis of broad pulses in bright gamma-ray bursts (GRBs) to understand the evolution of GRB broad pulses. Using the temporal and spectral properties, we test the high-latitude emission (HLE) scenario in the decaying phase of broad pulses. The HLE originates from the curvature effect of a relativistic spherical jet, where higher-latitude photons are delayed and softer than the observer’s line-of-sight emission. The signature of HLE has not yet been identified undisputedly during the prompt emission of GRBs. The HLE theory predicts a specific relation, F ν , E p ∝ E p 2 , between the peak energy E p in ν F ν spectra and the spectral flux F ν measured at E p , F ν , E p . We search for evidence of this relation in 2157 GRBs detected by the Gamma-ray Burst Monitor on board the Fermi Gamma-ray Space Telescope from 2008 to 2017. After imposing unbiased selection criteria in order to minimize contamination in a signal by background and overlaps of pulses, we build a sample of 32 broad pulses in 32 GRBs. We perform a time-resolved spectral analysis on each of these 32 broad pulses and find that the evolution of 18 pulses (56%) is clearly consistent with the HLE relation. For the 18 broad pulses, the exponent δ in the relation of F ν , E p ∝ E p δ is distributed as a Gaussian function with a median and width of 1.99 and 0.34, respectively. This result provides a constraint on the emission radius of GRBs with the HLE signature.