The Low-Energy Spectral Index of Gamma-Ray Burst Prompt Emission from Internal Shocks

Abstract

The prompt emission of most gamma-ray bursts (GRBs) typically exhibits a non-thermal Band component. The synchrotron radiation in the popular internal shock model is generally put forward to explain such a non-thermal component. However, the low-energy photon index $\alpha \sim -1.5$ predicted by the synchrotron radiation is inconsistent with the observed value $\alpha \sim -1$. Here, we investigate the evolution of a magnetic field during propagation of internal shocks within an ultrarelativistic outflow, and revisit the fast cooling of shock-accelerated electrons via synchrotron radiation for this evolutional magnetic field. We find that the magnetic field is first nearly constant and then decays as $B^{\prime }\propto t^{-1}$, which leads to a reasonable range of the low-energy photon index, $-3/2<\alpha <-2/3$. In addition, if a rising electron injection rate during a GRB is introduced, we find that $\alpha$ reaches $-2/3$ more easily. We thus fit the prompt emission spectra of GRB 080916c and GRB~080825c.

14 pages, 6 figures, 1 table; Invited research article in Galaxies special issue "Gamma-Ray Burst Science in 2030", final edited version

References in corpus (10)

• The Physics of Gamma-Ray Bursts and Relativistic Jets
• The Internal-Collision-Induced Magnetic Reconnection and Turbulence (ICMART) Model of Gamma-Ray Bursts
• Detection of a Thermal Spectral Component in the Prompt Emission of GRB 100724B
• Prompt GRB emission from gradual energy dissipation
• Temporal Evolution Of Thermal Emission From Relativistically Expanding Plasma
• Synchrotron emission in small scale magnetic field as possible explanation for prompt emission spectra of gamma-ray bursts
• Detection of Low-energy Breaks in Gamma-Ray Burst Prompt Emission Spectra
• Observational Limits on Inverse Compton Processes in GRBs
• Low-energy Spectra of Gamma-ray Bursts from Cooling Electrons
• Spectral evolution in gamma-ray bursts: predictions of the internal shock model and comparison to observations

Cited by in corpus (3)

• Implication of GRB 221009A: Can TeV Emission Come from the GRB Prompt Phase?
• Gamma-Ray Burst prompt emission from the synchrotron radiation of relativistic electrons in a rapidly decaying magnetic field
• Detection of Prompt Fast-Variable Thermal Component in Multi-Pulse Short Gamma-Ray Burst 170206A