Multiwavelength study of the luminous GRB 210619B observed with <i>Fermi</i> and ASIM
M. D. Caballero‐García, Rahul Gupta, S. B. Pandey, S. R. Oates, M. Marisaldi, Andreas Ramsli, Y. D. Hu, A. J. Castro‐Tirado, R. Sánchez-Ramírez, P. Connell, F. Christiansen, Amit Kumar Ror, Amar Aryan, J-M Bai, M. Á. Castro-Tirado, Y-F Fan, Elena Fernández‐García, Amit Kumar, Anders Lindanger, Andrey Mezentsev, Javier Navarro‐González, Torsten Neubert, Nikolai Østgaard, I. Pérez-García, V. Reglero, David Sarria, T R Sun, D-R Xiong, Jun Yang, Y-H Yang, B. -B. Zhang
Abstract
ABSTRACT 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 (αpt) 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 αpt 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 νm &lt; νc &lt; νx (where νm, νc, and νx are the synchrotron peak, cooling-break, and X-ray frequencies, respectively) spectral regime supporting a rarely observed hard electron energy index (p &lt; 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 10.4 m Gran Telescopio de Canarias placing deep upper limits for the host galaxy (z = 1.937), favouring a faint, dwarf host for the burst.