DR. KUNAL MOOLEY

On August 17, 2017, the LIGO and Virgo observatories announced the detection of the binary neutron star merger GW170817. The gravitational wave signal was accompanied by a low-luminosity burst of gamma-rays. Within 12 hours of the merger an optical counterpart, powered by r-process nucleosynthesis of ~0.05 M_sun of neutron-rich ejecta, was discovered and localized to a spiral galaxy NGC 4993 at a distance of 40 Mpc. X-ray and radio signals, consistent with the synchrotron afterglow, were detected 1-2 weeks after the merger. The radio afterglow of GW170817 gave key insights into the energetics, morphology of the ejecta and the circum-merger environment. Radio interferometric observations have confirmed that an energetic and narrowly-collimated jet interacted with the kilonova ejecta to produce a wide angle outflow (cocoon) that dominated the radio afterglow at early times. The jet successfully penetrated the ejecta and dominated the emission at late times. Radio VLBI observations together with state-of-the-art hydrodynamical simulations suggest that the jet had an opening angle of less than 5 degrees and was observed from a viewing angle of about 20 degrees. The strong constraint on the geometry allows us to precisely measure the rate of expansion of the nearby Universe (the Hubble's constant through the "standard siren" technique). The radio afterglow of GW170817 has provided one of the strongest observational evidence, so far, that binary neutron star mergers produce short-hard GRBs. Although a unique event, GW170817 represents only an initial exploration of a rich scientific landscape populated by the stellar evolution, explosion and eventual merger of massive binary systems. The study of the EM counterparts (especially radio afterglows) of neutron star mergers detected by LIGO/Virgo over the coming years will be able to address many unsolved questions pertaining to central engines, fundamental physics, cosmology, and binary stellar evolution.