Multiwavelength study of the luminous GRB 210619B observed with Fermi and ASIM

We report on detailed multiwavelength observations and analysis of the very bright and long GRB 210619B, detected by the Atmosphere-Space Interactions Monitor installed on the International Space Station 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 index (alpha(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(pt) exhibit the flux tracking pattern. The late time broad-band photometric data set 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 +/- 0.01 mag for the small magellanic cloud extinction law. In addition, we also report late-time optical observations with the 10.4m Gran Telescopio de Canarias placing deep upper limits for the host galaxy (z = 1.937), favouring a faint, dwarf host for the burst.

Automated summary

We present detailed observations and analysis of GRB 210619B, a very bright and long gamma-ray burst. The energy-resolved prompt emission light curve shows a bright hard emission pulse followed by softer/longer pulses. Spectral analysis suggests a transition from thermal to non-thermal outflow. The late-time data supports a rare hard electron energy index.