Closure Relations of Gamma-Ray Bursts in High Energy Emission

• Published in: The Astrophysical journal Vol. 883; no. 2; pp. 134 - 143
• Main Authors: Tak, Donggeun, Omodei, Nicola, Lucas Uhm, Z., Racusin, Judith, Asano, Katsuaki, McEnery, Julie
• Format: Journal Article
• Language: English
• Published: Goddard Space Flight Center The American Astronomical Society 27.09.2019; American Astronomical Society; IOP Publishing; Institute of Physics (IOP)
• Subjects: ASTRONOMY AND ASTROPHYSICS; Astrophysics; Cooling; Density; Electrons; Emission; Flux density; Gamma ray bursts; Gamma rays; gamma-ray burst: general; Magnetic fields; Spectra; Gamma-Ray Bursts
• ISSN: 0004-637X; 1538-4357; 1538-4357
• DOI: 10.3847/1538-4357/ab3982

The synchrotron external shock model predicts the evolution of the spectral (β) and temporal ( ) indices during the gamma-ray burst (GRB) afterglow for different environmental density profiles, electron spectral indices, electron cooling regimes, and regions of the spectrum. We study the relationship between and β, the so-called "closure relations" with GRBs detected by Fermi Large Area Telescope (Fermi-LAT) from 2008 August to 2018 August. The spectral and temporal indices for the >100 MeV emission from the Fermi-LAT as determined in the Second Fermi-LAT Gamma-Ray Burst Catalog (2FLGC) are used in this work. We select GRBs whose spectral and temporal indices are well constrained (58 long-duration GRBs and 1 short-duration GRBs) and classify each GRB into the best-matched relation. As a result, we found that a number of GRBs require a very small fraction of the total energy density contained in the magnetic field (ϵB 10−7). The estimated mean and standard deviation of electron spectral index p are 2.40 and 0.44, respectively. The GRBs satisfying a closure relation of the slow cooling tend to have a softer p value compared to those of the fast cooling. Moreover, the Kolmogorov-Smirnov test of the two p distributions from the fast and slow coolings rejects a hypothesis that the two distributions are drawn from the single reference distribution with a significance of 3.2 . Lastly, the uniform density medium is preferred over the medium that decreases like the inverse of distance squared for long-duration GRBs.