Blindly detecting orbital modulations of jets from merging supermassive black holes

TL;DR

This paper predicts that merging supermassive black hole binaries produce detectable, modulated radio signals due to orbital and precession effects, which can be observed in wide-field radio surveys, providing electromagnetic signatures of gravitational wave events.

Contribution

It introduces a model for detecting orbital modulations of jets from merging SMBH binaries via radio surveys, highlighting the potential for electromagnetic counterparts to gravitational wave signals.

Findings

Ubiquitous predicted transients unless suppressed by low efficiency or obscuration.

Detectable signals depend on geometric and efficiency factors, with a threshold of f_{geo} ε > 2×10^{-4}.

Future wide-field radio surveys can potentially observe these modulated signals.

Abstract

In the last few years before merger, supermassive black hole binaries will rapidly inspiral and precess in a magnetic field imposed by a surrounding circumbinary disk. Multiple simulations suggest this relative motion will convert some of the local energy to a Poynting-dominated outflow, with a luminosity 10^{43} erg/s * (B/10^4 G)^2(M/10^8 Msun)^2 (v/0.4 c)^2, some of which may emerge as synchrotron emission at frequencies near 1 GHz where current and planned wide-field radio surveys will operate. On top of a secular increase in power on the gravitational wave inspiral timescale, orbital motion will produce significant, detectable modulations, both on orbital periods and (if black hole spins are not aligned with the binary's total angular momenta) spin-orbit precession timescales. Because the gravitational wave merger time increases rapidly with separation, we find vast numbers of these transients are ubiquitously predicted, unless explicitly ruled out (by low efficiency $\epsilon$) or obscured (by accretion geometry f_{geo}). If the fraction of Poynting flux converted to radio emission times the fraction of lines of sight accessible $f_{geo}$ is sufficiently large (f_{geo} \epsilon > 2\times 10^{-4} for a 1 year orbital period), at least one event is accessible to future blind surveys at a nominal 10^4 {deg}^2 with 0.5 mJy sensitivity. Our procedure generalizes to other flux-limited surveys designed to investigate EM signatures associated with many modulations produced by merging SMBH binaries.