| Abstract : | The origin of heavy-elements in universe has long been a mystery until the detection of kilonova(KN) SSS17a/AT2017gfo, an optical-counterpart of GW170817 event. The presence of heavy-elements in AT2017gfo spectrum indicates that neutron-star merger events might be the sites where very heavy-elements take birth. In search of KNe, I undertook a program during LIGO-Virgo-Collboration(LVC) observing run 3(O3) with the GROWTH-India Telescope(GIT). During this work, I automated the GIT operation, scheduling, developed data reduction pipelines and follow-up on six GW triggers during O3 run of LVC, in particular, the S190426c event. Our study of S190426c constrained the ejecta-mass from the merger events of neutron-stars, ruling out high ejecta-mass models (M_ej > 0.3 M_Sun). Furthermore, combined data from GIT and other telescopes from GROWTH-collaboration put even stricter constraints and estimate the joint-probability of detecting zero KNe from the combined data is ~4.2%.
In addition, I studied the closely-related phenomena of gamma-ray bursts (GRBs). My work on GIT automation enabled fast triggering of GRBs and detected early-time rise/flattening in ~35% of the GRB optical afterglow lightcurves. Furthermore, we followed a few bright afterglow candidates until late-time and detected the most-delayed flaring ever in GRB-210204A. Our study implies that the flaring was caused by a refreshed shock. The early-time afterglow dataset obtained with GIT, thanks to the automation work of this thesis, will pave way to understand the reverse-shock phenomenon.
The automation work and rapid response of GIT has already resulted into discoveries of rare optical transients: Fast blue optical transients(AT2018cow, AT2020xnd, AT2022tsd), jetted tidal disruption event (AT2022cmc) and shortest-collapsar (GRB200826A). The current work will be crucial in the Rubin-observatory era to enable the rapid follow-up of large number of transients, thus opening doors to unprecedented astrophysics. |
