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 jets. Radio is key to probing the structure of such jets 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.


Top: Stokes Q intensity map of the co-added observations of the GW170817 field carried out in S-band between March 25 and May 12. Stokes I contours of GW170817 radio counterpart are also shown (white; 20%, 40%, 60%, and 80% relative emission contours). The Stokes I intensity contours of the host galaxy of GW170817 are also overlaid (bottom-right portion of the panel). The FWHM synthesized beam ellipse is shown in magenta. BOTTOM: Same as the top panel, but for the Stokes U intensity map. These Figures demonstrate how upper-limits on the linear polarization fraction of GW170817 radio afterglow were derived. Figure from Corsi et al. 2018.

Left: Simulated ngVLA observations of GW170817-like radio afterglows. The real part of the measured visibility as a function of baseline for two models (isotropic and off-axis collimated fireballs at 150 days since explosion when the 2.4 GHz flux density is of order 100μJy, comparable to the 3 GHz peak flux density of GW170817) compared to that of a uniform disk of 2 marcsec diameter and total flux density of 100μJy. We assume a 4 hr-long observation at a central frequency of 2.4 GHz with an ngVLA-like radio array which could reach a noise rms of about 0.2μJy at this frequency. An ngVLA could directly resolve and distinguish different ejecta structures, something currently inaccessible to the VLA. Figure from Corsi el al. 2020