Thermal X-ray signatures in late-stage unequal-mass massive black hole binary mergers

TL;DR

This study investigates electromagnetic signatures from late-stage unequal-mass massive black hole binaries embedded in circumbinary disks using high-resolution 2D hydrodynamic simulations with a gamma-law equation of state, viscous heating, shock heating, and radiative cooling. By varying mass ratios q = 0.1, 0.3, 0.5 and evolving from pre-decoupling to merger, it uncovers a new pre-merger X-ray spike for lower-q systems, followed by a pronounced X-ray drop near merger, and reveals a role-reversal in accretion that can suppress Doppler modulation signatures. These results extend prior equal-mass findings and suggest an observable electromagnetic signature that could assist LISA in identifying MBHB host galaxies hours before merger. The work highlights the dependence on mass ratio and numerical choices, emphasizing the need for broader parameter studies and observational follow-up with X-ray facilities.

Abstract

The multi-messenger combination of gravitational waves (GWs) from merging massive black hole binaries (MBHBs) and the electromagnetic (EM) counterpart from the surrounding circumbinary disk (CBD) will open avenues to new scientific pursuits. In order to realize this science, we need to correctly localize the host galaxy of the merging MBHB. Multi-wavelength, time-dependent electromagnetic (EM) signatures can greatly facilitate the identification of the unique EM counterpart among many sources in LISA's localization volume. To this end, we studied merging unequal-mass MBHBs embedded in a CBD using high-resolution 2D simulations, with a $Γ$-law equation of state, incorporating viscous heating, shock heating and radiative cooling. We simulate each binary starting from before it decouples from the CBD until just after the merger. We compute EM signatures and identify distinct features before, during, and after the merger. We corroborate previous findings of a several order of magnitude drop in the thermal X-ray luminosity near the time of merger, but with delayed timing compared to an equal-mass system. The source remains X-ray dark for hours post-merger. Our main results are a potential new signature of a sharp spike in the thermal X-ray emission just before the tell-tale steep drop occurs. This feature may further help to identify EM counterparts of LISA's unequal MBHBs before merger without the need for extensive pre-merger monitoring. Additionally, we find a role-reversal, in which the primary out-accretes the secondary during late inspiral, which may diminish signatures originating from Doppler modulation.

Figures (20)

• Figure 1: Zoomed-in snapshots of the logarithmic surface density. From left to right, we show models $q=1$Krauth2023, $q=0.5$, $q=0.3$, and $q=0.1$ (from this work). The total mass of the binary $M_1+M_2$ is the same in all cases. From top to bottom, we show snapshots at different times: at the initiation of inspiral, 1 day before merger, and at merger.
• Figure 2: Black hole accretion rates in the $q=1$ fiducial model before and after the time of merger, $t_m$, for the case with no post-merger recoil or mass loss. Accretion rates are normalized by the steady Shakura-Sunyaev value around a single BH Krauth2023.
• Figure 3: Model $q=0.5$: Black hole accretion rates $\sim1$ viscous time before the time of merger, $t_m$, and a short while after. The inset is zoomed-in on the time of merger. Accretion rates are normalized by the steady Shakura-Sunyaev value around a single BH. While we see the secondary initially out-accretes the primary, as we approach merger that dominance switches as the secondary force-feeds the primary. We see the accretion for both BHs begins dropping before merger occurs, and plummets several orders of magnitude into post-merger.
• Figure 4: As in Fig. \ref{['fig:mod6_acc']}, but for $q=0.3$: while we see the secondary initially out-accretes the primary, as we approach merger that dominance switches as the secondary force-feeds the primary. With decreased $q$, we see this feeding is even stronger with a sharper rise in the primary accretion rate pre-merger. Additionally, even the secondary has a brief but sharp spike before merger, showing increased accretion shortly before merger. However, we still see the accretion for both BHs begins dropping before merger occurs. It still plummets several orders of magnitude into post-merger, but less so than in the $q=0.5$ case.
• Figure 5: As in Fig. \ref{['fig:mod6_acc']}, but for $q=0.1$: while we see the secondary initially out-accretes the primary, as we approach merger that dominance switches as the secondary force-feeds the primary. With further decreased $q$, we see this force-feeding is even more dramatic, once again with the secondary spiking shortly before merger. Still though, we see the accretion for both BHs begins dropping before merger occurs. Accretion again plummets several orders of magnitude into post-merger, but even less so than even the $q=0.3$ case now.