Multi-wavelength study of the luminous GRB 210619B observed with Fermi and ASIM

TL;DR

This study presents a comprehensive multi-wavelength analysis of the luminous GRB 210619B, revealing insights into its emission mechanisms, jet composition, and host galaxy characteristics, with detailed spectral and temporal analysis supporting a complex radiation model.

Contribution

It provides the first detailed multi-wavelength observational analysis of GRB 210619B, highlighting its high luminosity, spectral evolution, and host galaxy properties, advancing understanding of GRB emission mechanisms.

Findings

GRB 210619B is among the top 10 most luminous Fermi GRBs.

Spectral analysis indicates a transition from thermal to non-thermal emission.

The afterglow fits an external forward shock model with a hard electron energy index.

Abstract

We report on detailed multi-wavelength observations and analysis of the very bright and long GRB 210619B, detected by the Atmosphere-Space Interactions Monitor (ASIM) installed on the International Space Station (ISS) and the Gamma-ray Burst Monitor (GBM) on-board the Fermi mission. Our main goal is to understand the radiation mechanisms and jet composition of GRB 210619B. With a measured redshift of $z$ = 1.937, we find that GRB 210619B falls within the 10 most luminous bursts observed by Fermi so far. The energy-resolved prompt emission light curve of GRB 210619B exhibits an extremely bright hard emission pulse followed by softer/longer emission pulses. The low-energy photon indices ($\alpha_{\rm pt}$) values obtained using the time-resolved spectral analysis of the burst suggest a transition between the thermal (during harder pulse) to non-thermal (during softer pulse) outflow. We examine the correlation between spectral parameters and find that both peak energy and $\alpha_{\rm pt}$ exhibit the flux tracking pattern. The late time broadband photometric dataset can be explained within the framework of the external forward shock model with $\nu_m$ $< \nu_c$ $< \nu_{x}$ (where $\nu_m$, $\nu_c$, and $\nu_{x}$ are the synchrotron peak, cooling-break, and X-ray frequencies, respectively) spectral regime supporting a rarely observed hard electron energy index ($p<$ 2). We find moderate values of host extinction of E(B-V) = 0.14 $\pm$ 0.01 mag for the Small Magellanic Cloud (SMC) extinction law. In addition, we also report late-time optical observations with the 10.4 m GTC placing deep upper limits for the host galaxy ($z$=1.937), favouring a faint, dwarf host for the burst.