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

TL;DR

The paper investigates whether non-thermal X-ray emission from magnetic reconnection could obscure the disappearing thermal X-ray signature of merging MBHBs in circumbinary disks, potentially affecting LISA counterpart identification. It develops semi-analytic models for thermal X-rays from minidiscs, non-thermal synchrotron emission from reconnection, and magnetospheric balding decay, and tests them across the LISA MBHB mass range with MAD-inspired parameter ranges. The main finding is that the non-thermal component is several orders of magnitude fainter than the thermal emission during the inspiral and decays quickly after flux disruption, leaving the thermal X-ray drop robust. This strengthens the case for the disappearing thermal X-ray signature as a clean electromagnetic counterpart for multi-messenger studies, while noting caveats and the need for future, more self-consistent simulations to quantify additional emission channels.

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. Even under optimistic assumptions that enhance the non-thermal emission, it remains significantly subdominant. 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.

Figures (2)

• Figure 1: X-ray luminosity $L_{\mathrm{X}}$ as a function of black hole mass for thermal (blue) and synchrotron (green) emission models. Solid and dashed blue curves show the thermal scaling for non-spinning ($a=0$) and rapidly spinning ($a=0.99$) black holes, respectively. The green solid curve denotes the fiducial synchrotron model, while dotted and dashed curves illustrate variations with magnetic field strength and accretion rate scaled by factors of $10$ and $10^{-1}$ relative to the fiducial values.
• Figure 2: Time required for the non-thermal X-ray luminosity to decay by one e-fold due to magnetospheric balding, shown as a function of black hole mass. Coloured curves correspond to decay timescales $\tau = 100\,r_g/c$, $300\,r_g/c$, and $500\,r_g/c$. The black dash–dotted curve indicates the time required for the thermal X-ray luminosity to recover to within two orders of magnitude of its pre-merger level.