Radio counterparts of GW transients

With the direct detection of Gravitational waves (GWs) from merging black holes (BHs) and neutron stars (NSs) by the Laser Interferometer Gravitational wave Observatory (LIGO), GWs are opening a new window onto the universe. Indeed, GWs are providing us a unique opportunity to answer fundamental questions on some of the most fascinating objects in the stellar graveyard. GW170817, in particular, has brought us the very first direct detection of GWs from two merging NSs, officially marking the start of a new era in astronomy!

 

Motivated by these exciting results, and as a member of the LIGO Scientific Collaboration, I am working on the hunt for electromagnetic counterparts of GW events, particularly focusing on the radio emission associated with the ejection of fast ejecta. Radio is key to probing the structure of such jejecta regardless of observing geometry, and can be used to potentially constrain the structure of magnetic fields via polarization measurements. I am also interested in probing whether radio emission from fast dynamical ejecta by NS mergers can be detected at very late times and used as a tool to further understand the dynamics of merging NSs.

Currently most of my work dedicated to the hunt for radio afterglows is carried out using the Karl G. Jansky Very Large Array. With an eye toward the future, I also dedicate part of my time in helping to shape the science case and technical requirements for the next generation Very Large Array (ngVLA), which would be a real game changer facility for multi-messenger radio astronomy.

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Figure from Balasubramanian et al. 2021: 3 GHz radio light curve of GW170817 with our recent radio measurement (red star) along with predictions for the rising part of the kilonova afterglow light curve as a function of the index α of the ejecta energy-speed distribution (larger values of α correspond to steeper distributions), with the assumption that the minimum speed of the ejecta is β0 = 0.3. The solid lines assume ejecta parameters as in Makhathini et al. 2020 and various values of α. These values of α are all compatible with our latest radio measurement (red star), while smaller values of α would produce radio emission in excess to it. For comparison, the dashed lines show the case for ejecta parameters as in Kathirgamaraju et al. 2019, with α = 20, 30.