Multiwavelength Observations of Gamma Ray Bursts

Abstract

Gamma-ray bursts (GRBs) are fascinating sources studied in modern astronomy. They are extremely luminous electromagnetic (Lγ,iso ∼ 1048 − 1054 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 are the possible progenitors? What powers the GRBs jets/central engine? 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 problems using multi-wavelength observations.