# Radio follow-up of a candidate gamma-ray transient in the sky localization area of GW170608

## TL;DR

This study conducted radio follow-up observations of the GW170608 binary black hole merger to search for electromagnetic counterparts, finding no evidence but demonstrating the feasibility of future radio follow-ups for well-localized events.

## Contribution

First radio follow-up of GW170608 targeting a Fermi LAT transient, establishing observational limits and assessing models for electromagnetic emission from black hole mergers.

## Key findings

- No radio counterpart detected in the GW170608 error region.
- Radio luminosity upper limit set at $6 imes 10^{28}$ erg s$^{-1}$ Hz$^{-1}$.
- Constraints on jet angles for on-axis jets with energy ~$10^{49}$ erg.

## Abstract

After the identification of a candidate $\gamma$-ray transient in the error region of the binary black hole (BBH) merger GW150914 by the \textit{Fermi} satellite, the question of whether BBH mergers can be associated to electromagnetic counterparts remains highly debated. Here, we present radio follow-up observations of GW170608, a BBH merger that occurred during the second observing run (O2) of the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO). Our radio follow up focused on a specific field contained in the GW170608 sky localization area, where a candidate high-energy transient was detected by the \textit{Fermi} Large Area Telescope (LAT). We make use of data collected at 1.4\,GHz with the Karl G. Jansky Very Large Array (VLA), as well as with the VLA Low-band Ionosphere and Transient Experiment (VLITE). Our analysis is sensitive to potential radio afterglows with luminosity densities $L_{\rm 1.4\,GHz}\gtrsim 6\times10^{28}$\,erg\,s$^{-1}$\,Hz$^{-1}$. In the most optimistic theoretical models, $\approx 20\%$ of BBH events occurring in massive hosts could be associated with outflows as radio luminous as this. Although we find no evidence for the presence of a radio counterpart associated with the LAT candidate in the GW170608 error region, our analysis demonstrates the feasibility of future radio follow-up observations of well localized BBHs. Comparing our radio upper-limits with theoretical expectations for the radio afterglows potentially associated with jets launched in BBH mergers, we find that for jets of energy $\approx 10^{49}$\,erg seen on-axis, only jet angles $\theta_{jet}\gtrsim 40^{\circ}$ are compatible with the observations.