Title : Introduction to Gamma Ray Bursts (GRBs)

Abstract

Gamma Ray Bursts (GRBs) are short, intense flashes of gamma rays coming from deep space. They were first discovered (in 1967) by VELA military satellites of the USA launched to govern the 1963 nuclear test-ban treaty. They are observed across the electromagnetic spectrum, from radio to very-high-energy gamma rays. At present, space-based observatories detect, on average, approximately one GRB per day. The extensive observational and theoretical efforts have confirmed them as highly relativistic explosions, confirmed their distance scale, pinpointed their host galaxies, identified the systems that create them and opened them as astrophysical probes throughout the Universe. GRBs are now a major astrophysical tool to probe the final stages of stellar evolution and the creation of supernovae, to examine the creation and propagation of relativistic jets and their impact on the Universe at large, and to use as indicators to identify galaxies across cosmic time-some in the earliest epochs of Universe. The extreme nature of both their progenitors and emission mechanisms makes them the most promising sources for joint electromagnetic and gravitational wave detection. This is demonstrated by the recent (2017) detection of gravitational waves (GW170817) and short GRB (GW170817A) from a merger of the double neutron-star binary system. In this seminar, I will talk about the brief history of GRBs, their observations and properties, different dedicated GRB missions, detection methods, understanding so far, etc., and finally, I will connect you to my work.

Title : Intra-night optical variability of radio-quiet narrow-line Seyfert-1 galaxies

Abstract

Variability studies of Active Galactic Nuclei (AGNs) play a powerful diagnostic tool in understanding the physical processes occurring in objects that are unresolved by direct imaging with currently available techniques. Here, we report the first attempt to systematically characterize intra-night optical variability (INOV) for a sample of seven radio-quiet and/or radio-silent narrow-line Seyfert 1 galaxies (RQNLSy1s) that had shown recurring flaring at 37 GHz in the radio observations at Metsähovi Radio Observatory (MRO), indicating the presence of relativistic jets in them, but no hints of jets in the recent radio observations of Karl G. Jansky Very Large Array (JVLA) at 1.6, 5.2, and 9.0 GHz. A total of 28 intra-night sessions, each lasting > 3 h, was conducted with 1-2.5m class telescopes of ARIES and PRL for the INOV study of this sample. The resulting level of INOV from this sample is found to be statistically comparable to those observed in γ-ray-detected radio-loud NLSy1s (MBH ∼ 10^7 M⊙ ), that display blazar-like INOV. Thus, it appears that even lower-mass (MBH ∼ 10^6 M⊙ ) RQNLSy1 galaxies can maintain blazar-like activities. Furthermore, based on the optical behavior of these seven RQNLSy1s, we infer that INOV from the current sample of RQNLSy1s could be either due to a twisted in-homogeneous jet or magnetic re-connection in the magnetosphere of the black hole. In this talk, I will also discuss the optical study of the most distant gamma-ray-detected NLSy1 (z = 1.344), whose observations were taken from the PRL 2.5m telescope.