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Type of talk
Rahul Gupta
Date and Time of Talk
Gamma-ray bursts (GRBs) are fascinating sources studied in modern astronomy. They are extremely luminous electromagnetic (Liso ~ 10^{48} - 10^{54} erg/s) explosions in the Universe observed from cosmological distances. These unique characteristics provide a marvellous chance to study the evolution of massive stars and probe the rarely explored early Universe. In addition, the central source's compactness and the high bulk Lorentz factor in GRB's ultra-relativistic jets make them efficient laboratories for studying high-energy astrophysics. GRBs are the only astrophysical sources observed in two distinct signals: gravitational and electromagnetic waves. GRBs are believed to be produced from a "fireball" moving at a relativistic speed, launched by a fast-rotating black hole or magnetar. GRBs emit radiation in two phases: the initial gamma/hard X-rays prompt emission, the duration of which ranges from a few seconds to hours, followed by the multi-wavelength and long-lived afterglow phase. Based on the observed time frame of GRB prompt emission, astronomers have generally categorized GRBs into two groups: long (> 2 s) and short (< 2 s) bursts. Short GRBs are typically produced when two compact objects merge, while long GRBs could result from a collapsing massive star. Despite the discovery of GRBs in the late 1960s, their origin is still a great mystery. There are several open questions related to GRBs, such as: What powers the GRBs jets/central engine? What are the possible progenitors? How to classify them? The short bursts originating from the collapse or long bursts presenting features characteristic of compact binary mergers have posed a new challenge to our understanding of possible progenitors and the origin of these events. What is the jet composition? Is it a baryon-dominated or Poynting-flux-dominated outflow? What is the underlying emission process that gives rise to observed radiation? Where and how does the energy dissipation occur in the outflow? Is it via internal shocks or magnetic reconnections? How to solve the radiative efficiency problem? What are the possible causes of Dark GRBs and orphan afterglows? How to investigate the local environment of GRBs? etc. In this thesis, we explored some of these open enigmas (progenitor, emission mechanisms, jet composition and environment) using multi-wavelength observations obtained using space and ground-based facilities.

To examine the jet composition and radiation physics of prompt emission, we have used a unique time-resolved spectro-polarimetric technique using the prompt observations of some of the bright GRBs discovered using Fermi and AstroSat. Further, we have used very early optical afterglow follow-up observations and detailed modelling of rarely observed reverse shock emissions in earlier phases to explore the low-efficiency problem. We also studied the dark GRBs (GRB 150309A and GRB 210205A) and orphan afterglow (AT20221any) properties using late-time afterglow data taken using 3.6m Devasthal Optical Telescope (DOT) of ARIES and 10.4m GTC. Additionally, we studied a sample of host galaxies of five peculiar GRBs observed using 3.6m DOT to explore the environment of GRBs. We compared the physical parameters of these galaxies with well-studied host galaxies of long and short GRBs. Our study demonstrated the capabilities of 3.6m DOT for faint sources observations such as host galaxies of GRBs.
The community is actively developing several larger optical telescopes, such as the Extremely Large Telescope (ELT) and Thirty Meter Telescope (TMT), alongside currently operational 10-meter class telescopes like 10.4m GTC. Simultaneously, on the high-energy front, the community is working towards the next gamma-ray missions (e.g., COSI, POLAR2, DAKSHA). The synchronous observations of GRBs and related transients utilizing these upcoming major ground and space-based facilities will play a pivotal role in unravelling the intricate details of these energetic and cosmic sources. Our study on the prompt emission of GRBs, as well as the comprehensive analysis of their early-to-late time afterglow observations, modelling, and host galaxy investigations, provides valuable insights for future observations of similar sources using upcoming larger telescopes. Overall, the research presented in this thesis contributes significantly to the field of GRB and paves the way for further progress in our understanding of this extraordinary cosmic phenomenon under a larger perspective of time-domain astronomy. Looking ahead, the combined observations of GRBs across various messengers hold immense potential for unravelling the mysteries of GRBs. These multi-messenger observations of GRBs and related transients will shed light on the fundamental processes occurring in the Universe, opening new avenues of exploration and discovery.
Email Speaker
About Speaker
Rahul is a fifth-year PhD student in the astronomy division at ARIES. This is his pre-thesis submission talk. 
Email Host
Host Name
Narendra Singh
Host Phone (ext/mob)
759 /807