Multiwavelength study of the luminous GRB 210619B observed with <i>Fermi</i> and ASIM

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