A Comptonized Fireball Bubble: Physical Origin of Magnetar Giant Flares

Abstract

Magnetar giant flares (MGFs) have been long proposed to contribute at least a sub-sample of the observed short gamma-ray bursts (GRBs). The recent discovery of the short GRB 200415A in the nearby galaxy NGC 253 established a textbook-version connection between these two phenomena. Unlike previous observations of the Galactic MGFs, the unsaturated instrument spectra of GRB 200415A provide for the first time an opportunity to test the theoretical models with the observed $\gamma$-ray photons. This paper proposed a new readily fit-able model for the MGFs, which invokes an expanding fireball Comptonized by the relativistic magnetar wind at photosphere radius. In this model, a large amount of energy is released from the magnetar crust due to the magnetic reconnection or the starquakes of the star surface and is injected into confined field lines, forming a trapped fireball bubble. After breaking through the shackles and expanding to the photospheric radius, the thermal photons of the fireball are eventually Comptonized by the relativistic $e^{\pm }$ pairs in the magnetar wind region, which produces additional higher-energy gamma-ray emission. The model predicts a modified thermal-like spectrum characterized by a low-energy component in the Rayleigh-Jeans regime, a smooth component affected by coherent Compton scattering (CC) in the intermediate energy range, and a high-energy tail due to the inverse Compton process. By performing a Monte-Carlo fit to the observational spectra of GRB 200415A, we found that the observation of the burst is entirely consistent with our model predictions.

22 pages, 9 figures, 1 table

References in corpus (10)

• The strongest cosmic magnets: Soft Gamma-ray Repeaters and Anomalous X-ray Pulsars
• Fast radio bursts as synchrotron maser emission from decelerating relativistic blast waves
• A Comprehensive Analysis of Fermi Gamma-Ray Burst Data. I. Spectral Components and Their Possible Physical Origins of LAT/GBM GRBs
• Magnetars: Properties, Origin and Evolution
• The Physics of Fast Radio Bursts
• A bright gamma-ray flare interpreted as a giant magnetar flare in NGC 253
• Rapid Spectral Variability of a Giant Flare from a Magnetar in NGC 253
• The Photosphere Emission Spectrum of Hybrid Relativistic Outflow for Gamma-ray Bursts
• Polarized Radiation Transfer in Neutron Star Surface Layers
• Evidence of Photosphere Emission Origin for Gamma-Ray Burst Prompt Emission

Cited by in corpus (3)

• A Comptonized Fireball Bubble Fits the Second Extragalactic Magnetar Giant Flare GRB 231115A
• Eclipsed X-ray Bursts from Magnetar SGR J1935+2154 and the Fireball Measurements
• Spectral Evidence of Heavy Nuclei from the Neutron Star Crust in Magnetar Bursts