Study of GRBs-on Aspects of Multi-Wavelength Emission, Environment and Host Galaxies

Abstract

Gamma-ray Bursts (GRBs) are powerful astronomical transient events emitting enormous energy in -rays within a short period. They provide a unique laboratory for studying relativistic e ects such as beaming, jets, shocks and blastwaves to radiation mechanisms such as synchrotron radiation to galactic and stellar populations. GRBs can be traditionally classi ed into two sub-groups: short/hard and long/soft. There are two distinct phases of the GRBs: 1. The prompt emission (emitting mainly in -rays, sometimes in X-ray and optical), which arises because of the internal shocks within the jet, and 2. the afterglow emission (emission from X-ray to radio) occurs due to the interaction between the jet and the ambient medium. As GRBs are multi-wavelength phenomena, it is mandatory to carry out panchromatic follow-up observations using di erent space and ground-based telescopes. The data and extensive modelling provide insight into the radiation mechanisms and the progenitor and environment properties. This thesis tries to bring up di erent aspects of GRB emissions. So far, the progenitors of short GRBs are very elusive. The merger of two neutron stars can produce a rapidly rotating and highly magnetised millisecond magnetar. A signi cant proportion of the rotational energy deposited to the emerging ejecta can produce a late-time radio brightening from its interaction with the ambient medium. Detection of this latetime radio emission from short GRBs can have profound implications for understanding the physics of the progenitor. We report the radio observations of ve short GRBs - 050709, 061210, 100625A, 140903A, and 160821B using the Giant Metrewave Radio Telescope (GMRT) at 1250, 610, and 325 MHz frequencies after 2 ð€€€ 11 years from the time of the burst. The GMRT observations at low frequencies are crucial to detect the signature of merger ejecta emission at the peak. These observations are the most delayed searches associated with some of these GRBs for any late-time low-frequency emission. We nd no evidence for such an emission. We nd that none of these GRBs is consistent with maximally rotating magnetar with a rotational energy of 1053 erg. We nd no evidence for such an emission. Despite the non-detection, our study underscores the power of radio observations in the search for magnetar signatures associated with short GRBs. In this chapter, a detailed multi-wavelength analysis of GRB 200524A is carried out. It comes in the list of one of the brightest GRBs detected by Fermi. GRB 200524A is the most well-sampled and highest energetic GRB of the year 2020. An in-depth analysis of this GRB in the prompt and afterglow phase reveals several interesting features. GRB 200524A is a single peak GRB with no precursor emission. Timeresolved spectral analysis with Fermi GBM exhibits intensity tracking kind of burst behaviour. Despite being a long GRB, it shows almost zero spectral lag as GRB 200524A has multiple overlapping pulses. The power law tting of the light curves indicates a smooth, decaying kind of behaviour. With the help of temporal and spectral slopes, we could nd the spectral regime and the medium associated with GRB 200524A. Multi-wavelength modelling suggests that GRB 200524A is a very high energetic GRB (Eiso = 3 1053 erg) occuring in a very low-density ambient medium (3:5 10ð€€€3cmð€€€3). This unique combination can be why the jet break e ect is absent in the afterglow light curves. We started a comprehensive long-time GRB radio afterglow monitoring of GRB 171205A up to 1500 days since the burst. This is the most extended afterglow observation till now in the history of GRBs. Because of its enigmatic or complex afterglow evolution, any physical blastwave model fails to explain its afterglow emission. Several power-law components are required to justify the complete evolution of the GRB. The low luminosity and high peak radio ux make this GRB unique. Finally, we summarise the results of di erent aspects of long and short GRBs covered in this thesis. Apart from that, we highlight the future prospects of ongoing and upcoming projects. We aim to implement the magnetar model on GRB 170817A and the whole short GRB sample having a magnetar signature. The late-time radio observations of GRB 171205A and other bright radio GRBs will be used to perform reball calorimetry to estimate exact blastwave energy. We also highlight the importance of future-generation sensitive radio telescopes in e ciently detecting radio afterglows and late-time merger ejecta emissions.