Constraints on LIGO/Virgo compact object mergers from late-time radio observations

ABSTRACT We present results from a search for radio afterglows of compact object mergers conducted with the Australian SKA Pathfinder (ASKAP). We used data from four epochs of the Rapid ASKAP Continuum Survey to search compact binary merger localization regions observed during the LIGO/Virgo O2, and O3 observing runs. Our investigation focused on 11 events (published in the GWTC-1, GWTC-2, and GWTC-3 catalogues of gravitational-wave events) with 90 per cent posterior localizations smaller than $150\, \deg ^2$ and $\ge$99 per cent probabilities of being of astrophysical origin, to identify potential radio afterglow-like transients up to $\lesssim$1500 d post-merger. We identified candidate afterglow-type variable sources in the 90 per cent localization for events GW190503, GW200202, and GW200208, which were ruled out as unlikely to be related to the corresponding GW event on further analysis. Since we find no likely candidate counterparts, we constrain the inclination angle and the circum-merger density at isotropic equivalent energies in the range $2\times 10^{51}\!-\!1\times 10^{54}\rm \:erg$. These constraints are based on the assumption that the electron energy distribution in the associated jets follows a power-law index of $p = 2.2$, with 1 per cent of the shock energy in the magnetic field ($\epsilon _B = 0.01$) and 10 per cent in the electrons ($\epsilon _e = 0.1$). We discuss the detectability of late-time afterglows as a function of merger distance and inclination angles with millijansky surveys.