Presentation #109.16 in the session “Multi-Messenger Astrophysics (Session)”.
Gravitational waves (GWs) from compact object binary coalescences are now regularly observed by a growing detector network that includes Advanced LIGO, Advanced VIRGO, and KAGRA. While much has been learned purely from the GW signals, the identification of an electromagnetic (EM) counterpart is the only way to completely understand the astrophysical context of a GW event, from its relativistic outflows to its host environment. One promising EM signature of GW mergers is radio synchrotron emission from the shocks formed between fast-moving ejecta and the surrounding medium, expected on timescales of days, months, or years post-merger. Modeling this radio emission constrains the merger dynamics, the propagation and structure of relativistic jets and other ejecta, particle acceleration in shocks, and the progenitor environment; radio searches are also the only viable method of localizing GW events that occur in the daytime sky where optical telescopes cannot observe. I will present lessons learned from our radio follow-up of GW190814, a GW event that garnered exceptional community interest due to its excellent localization and the uncertain nature of the binary’s lighter-mass component (either the heaviest known neutron star, or the lightest known black hole). We observed 75 galaxies containing ~32% of the total stellar luminosity in GW190814’s final localization volume at t = 35–266 days post-merger and assessed all detected radio sources for variability. Ultimately, we do not find any viable radio counterparts, although our results suggest that the background of radio variables may be higher in galaxy-targeted searches than in blind wide-field searches. For a viewing angle of ~46 deg (the best-fit binary inclination derived from the GW signal) and assumed electron and magnetic field energy fractions of εe = 0.1 and εB = 0.01, we can rule out a typical short gamma-ray burst-like Gaussian jet with an opening angle of 15 deg and isotropic-equivalent kinetic energy 2e51 erg propagating into a constant-density medium n > 0.1 cm3 within the searched area. These are the first limits resulting from a galaxy-targeted search for a radio counterpart to a GW event, and I will discuss the challenges—and possible advantages—of applying similar search strategies to future events using current and upcoming radio facilities.