2a) together provide a unique opportunity to perform deep and longer follow-up observations of the optical counterparts/ associated host galaxies of GRBs.īased on the optical brightness, some of the optical afterglows of GRBs are defined as Dark GRBs. The longitudinal advantage of the Devasthal observatory (which lies in between the Canary Islands and Eastern Australia) and deep imaging/spectroscopy capabilities of back-end instruments (at visible and near-infrared bands) of the 3.6m DOT telescope (see Fig. Specifically, optical observations play a crucial role in constraining the redshift, energetics, environments, and geometry of these powerful astronomical sources. Soon after the GRB-prompt discovery, ground-based multi-wavelength observational facilities (including ARIES telescopes in the optical/NIR bands) started searching for the associated afterglows across the electromagnetic spectrum. 1).įigure 1: The theoretical understanding of prompt emission and afterglow of GRBs (Credits: Dr. Later on, this material expands and interacts with the pre-existing material surrounding the GRB events (external shock), producing the broadband synchrotron radiation, which attributes to the afterglow emission (see Fig. The ejected out-flowing can have various shells that can collide with each other creating internal shock, which is responsible for the prompt gamma-ray emission. GRBs are believed to originate from a “jet” moving at a relativistic speed, launched by a fast rotating stellar-mass black hole or magnetized neutron star formed at the core collapse of a massive star (long GRBs, duration > 2 sec) or in the merger of compact objects (short GRBs, duration < 2 sec). Their duration is associated with their origin: long GRBs occur with the death of massive stars, while short GRBs have been proven to be the electromagnetic counterparts of the gravitational wave (originated due to the merger of two compact objects) signals discovered recently by LIGO/Virgo. Based on the observed time scale of prompt emission, astronomers classify GRBs as long or short, whether the event lasts more or less than two seconds. The prompt emission (initial gamma-ray emission) of GRBs is automatically discovered by space-based gamma-ray missions such as NASA’s Fermi Gamma-ray Space Telescope, Neil Gehrels Swift Observatory, India’s AstroSat, and many more. GRB events have two distinct emission phases: one is the short-lived prompt emission (the initial burst phase, peak at sub-MeV energy range), followed by a long-lived multiwavelength afterglow phase. GRB emits more energy in a second than our Sun will emit in its lifetime. GRBs are powerful bursts of high-energy gamma-ray radiation and are among the most exotic phenomena studied in modern astronomy. The death of massive stars produces the brightest and most explosive astronomical sources known as gamma-ray bursts (GRBs). High mass (more massive than eight solar mass) and low mass (typical to solar mass) stars follow a different life cycle. They are born, live, and at some point, they die. Similar to humans, stars also have a life cycle. This article briefly summarizes the recent discovery of dark (GRB 210205A) and orphan (AT2021any/ZTF21aaeyldq) afterglows of GRBs using 3.6m DOT + 4K × 4K CCD Imager. In a short period of the proposed target of opportunity (ToO) observations (since cycle 2020-C2), DOT discovered many interesting results such as the detection of long GRB (GRB 211211A) from a neutron star-white dwarf merger, the detection of host galaxies of peculiar GRBs, the detection of most delayed optical flare (GRB 210204A) observed from any GRB so far, etc. In recent times, astronomers started exploring these exciting and explosive astronomical sources using recently commissioned (since 2016) India’s largest optical telescope, i.e., 3.6m Devasthal Optical Telescope (DOT) at Devasthal observatory of Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital. India has a long history of more than two decades of observations of optical counterparts of gamma-ray bursts (GRBs) using Indian meter-class telescopes like the 1.04m Sampurnanand Telescope (ST), 1.3m Devasthal Fast Optical Telescope (DFOT), 2m Himalayan Chandra Telescope (HCT), and 2.34m Vainu Bappu Telescope (VBT) utilizing the longitudinal advantage of the place.
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