Low frequency view of the first binary neutron star merger GW 170817/GRB 170817A with the Giant Metrewave Radio Telescope

Abstract

On 17th August 2017, Gravitational Wave (GW) detector network AdvLIGO/VIRGO reported the first ever detection of a merging neutron star binary, GW170817 [1]. A spatially and temporally coincident gamma-ray signal, short Gamma Ray Burst (GRB) 170817A, was observed by the Fermi space telescope, kickstarting the era of multi-messenger astronomy [2]. Further electromagnetic follow-up observations revealed associated emission in X-ray, UV/Optical/IR, and radio wavelengths [3]. Our collaboration has used the Atacama Large Millimeter Array (ALMA) and the Giant Metrewave Radio Telescope (GMRT) to follow-up the event and made several successful detections of the low-frequency radio transient with the GMRT [4,5]. Multi-wavelength electromagnetic observations of the GW event corroborated the hypothesis that short GRBs originate from neutron star mergers. Yet some puzzles remained. The prompt gamma-ray signal was exceptionally under-luminous compared to the classical short GRB population. In addition, the non-thermal X-ray and radio emission had a distinctly unique time evolution in comparison to typical short GRB afterglows. Currently there are two competing models capable of explaining all the electromagnetic observations: (i) a relativistic jet viewed away from its axis (off-axis jet) or (ii) a quasi-spherical cocoon of sub-relativistically moving material. Both models are motivated from numerical general relativistic hydrodynamical simulations of neutron star mergers.