Non-thermal X-ray Emission from Merging Massive Black Hole Binaries

TL;DR

This study models thermal and non-thermal X-ray emissions from merging massive black hole binaries, concluding that non-thermal emission is too weak to obscure the thermal X-ray drop, supporting its use as an electromagnetic merger signature.

Contribution

It provides a semi-analytic assessment of non-thermal X-ray emission's impact on thermal signatures in MBHB mergers, confirming the thermal drop's robustness as an electromagnetic counterpart.

Findings

Non-thermal X-ray emission remains much weaker than thermal emission during inspiral.

Even with optimistic assumptions, non-thermal emission does not erase the thermal drop.

Non-thermal emission fades rapidly near merger once magnetic flux supply is disrupted.

Abstract

Recent hydrodynamical simulations have identified a disappearing thermal X-ray signature in massive black hole binaries (MBHBs) embedded in circumbinary disks, arising from the tidal truncation and depletion of minidiscs shortly before merger. This feature has been proposed as a promising electromagnetic counterpart to MBHB mergers detectable by LISA. In this work, we examine whether non-thermal X-ray emission powered by magnetic reconnection could obscure or modify this thermal X-ray drop. We construct semi-analytic models for both the thermal X-ray emission from minidiscs and the non-thermal synchrotron emission produced by reconnection in magnetically dominated black hole magnetospheres. Evaluating these models across the MBHB mass range relevant for LISA, we find that for physically motivated magnetic field strengths and accretion rates, the non-thermal X-ray luminosity remains several orders of magnitude below the thermal component throughout the inspiral, particularly in the soft X-ray band where the thermal emission is concentrated. Even under optimistic assumptions that enhance the non-thermal emission, it remains insufficient to erase the characteristic thermal drop, though a transient hard X-ray enhancement may arise near merger. We further incorporate the magnetospheric balding framework to model the decay of non-thermal emission near merger, finding that reconnection-powered X-ray emission fades on short, mass-scaled timescales once the external magnetic flux supply is disrupted. Taken together, our results indicate that non-thermal emission is unlikely to mask the disappearing thermal X-ray signature, reinforcing its robustness as an electromagnetic counterpart to MBHB mergers and its potential utility for multi-messenger studies with LISA.