Blindly Detecting Merging Supermassive Black Holes with Radio Surveys

TL;DR

This paper explores the potential for detecting merging supermassive black hole binaries using wide-field radio surveys, focusing on their electromagnetic signatures and the limitations imposed by short merger timescales.

Contribution

It introduces a model for radio emission from merging supermassive black holes and assesses the detectability with current and future radio surveys, highlighting the challenges and prospects.

Findings

Detection rate <1 per year with blind surveys

Radio emission is weak and often undetectable without prior localization

Pre-merger signals may be detectable through modulation effects

Abstract

Supermassive black holes presumably grow through numerous mergers throughout cosmic time. During each merger, supermassive black hole binaries are surrounded by a circumbinary accretion disk that imposes a significant (~1e4 G for a binary of 1e8 Msun) magnetic field. The motion of the binary through that field will convert the field energy to Poynting flux, with a luminosity ~1e43 erg/s (B/1e4 G)^2 (M/1e8 Msun)^2, some of which may emerge as synchrotron emission at frequencies near 1 GHz where current and planned wide-field radio surveys will operate. We find that the short timescales of many mergers will limit their detectability with most planned blind surveys to <1 per year over the whole sky, independent of the details of the emission process and flux distribution. Including an optimistic estimate for the radio flux makes detection even less likely, with <1 mergers per year over the whole sky. However, wide-field radio instruments may be able to localize systems identified in advance of merger by gravitational waves. Further, radio surveys may be able to detect the weaker emission produced by the binary's motion as it is modulated by spin-orbit precession and inspiral well in advance of merger.