The late inspiral of supermassive black hole binaries with circumbinary gas discs in the LISA band

Abstract

We present the results of 2D, moving-mesh, viscous hydrodynamical simulations of an accretion disc around a merging supermassive black hole binary (SMBHB). The simulation is pseudo-Newtonian, with the BHs modeled as point masses with a Paczynski-Wiita potential, and includes viscous heating, shock heating, and radiative cooling. We follow the gravitational inspiral of an equal-mass binary with a component mass M bh = 10 6 M from an initial separation of 60r g (where r g ≡ GM bh /c 2 is the gravitational radius) to the merger. We find that a central, low-density cavity forms around the binary, as in previous work, but that the BHs capture gas from the circumbinary disc and accrete efficiently via their own minidiscs, well after their inspiral outpaces the viscous evolution of the disc. The system remains luminous, displaying strong periodicity at twice the binary orbital frequency throughout the entire inspiral process, all the way to the merger. In the soft X-ray band, the thermal emission is dominated by the inner edge of the circumbinary disc with especially clear periodicity in the early inspiral. By comparison, harder X-ray emission is dominated by the minidiscs, and the light curve is initially more noisy but develops a clear periodicity in the late inspiral stage. This variability pattern should help identify the EM counterparts of SMBHBs detected by the space-based gravitational-wave detector LISA.

A population of luminous accreting black holes with hidden mergers

“…With the discovery of gravitational waves emitted from the merger of stellar-mass black holes, interest in understanding gravitational waves produced from the merger of supermassive black holes has increased considerably. The study of nuclear mergers is therefore critical for comparison with cosmological merger-rate models, because it can help constrain the timescales for supermassive-black-hole inspiral and the rate of such events, which are likely to be found with gravitational wave detectors, such as pulsar timing arrays 22 and the Laser Interferometer Space Antenna 23 . Predictions of the detection rates for these instruments are based on parameterizations of the merger rates and the supermassive-black hole-population 24 , but these are highly uncertain and vary by orders of magnitude 25 .…”

Population Estimates for Electromagnetically Distinguishable Supermassive Binary Black Holes

“…Initial work on this topic (Roedig et al 2012;Tanaka et al 2012;Gültekin & Miller 2012;Kocsis et al 2012) downplayed light from the minidisks, either because they were omitted from consideration or because they were thought to be dim. However, more recent simulational work (Ryan & MacFadyen 2017;D'Ascoli et al 2018;Tang et al 2018;Bowen et al 2019) has confirmed the view of Roedig et al (2014) that they can radiate with substantial luminosity. In an ordinary accretion disk, the light in the total disk spectrum at frequency ν is emitted primarily by the band of radii in the disk at which the temperature T ∼ hν/k B .…”

Spectral energy distributions of candidate periodically variable quasars: testing the binary black hole hypothesis

“…CIRCUMBINARY ACCRETION DISK SEDS Figure 1 illustrates theoretical SEDs of BSBH circumbinary accretion disks in the IR-optical-UV. Models of BSBH circumbinary accretion disks predict two characteristic morphologies that may indicate the presence of BSBHs through abnormalities in their IR-optical-UV SEDs (e.g., Roedig et al 2014;Foord et al 2017;Tang et al 2018). One is a central cavity, where the inner region of the circumbinary disk is almost emptied by the secondary BH.…”

“…For BSBHs with near-equal mass ratios (e.g., q>0.1), the emission would be truncated blueward of the wavelength that corresponds to the temperature of the innermost disk edge (e.g., Gültekin & Miller 2012;D'Orazio et al 2013), producing a sharp exponential cutoff in the IR-optical-UV SED as illustrated in Figure 1 (the blue dashed curve). The other is minidisks, where there is substantial accretion onto one or both BHs, each with their own shock-heated thin disk (e.g., Yan et al 2015;Ryan & MacFadyen 2017;Tang et al 2018). The minidisks emit high energy radiation analogous to a single BH with a geometrically thin and optically thick disk (the red dashed and dotted curves in Figure 1).…”

“…Periodic quasar light curves have long been suggested as indirect evidence for candidate millipc BSBHs. The optical flux periodicity may be caused by accretion rate changes due to the intrinsic binary orbital tidal torque modulation (e.g., MacFadyen & Milosavljević 2008;Shi et al 2012;Roedig et al 2012;D'Orazio et al 2013;Farris et al 2014;Tang et al 2018), and/or the apparent Doppler boost modulation from the highly relativistic motion of gas in the mini accretion disk around the secondary BH (e.g., D'Orazio et al 2015b;Charisi et al 2018). While 100 quasars with candidate periodicity have been proposed as evidence for BS-BHs (e.g., Valtonen et al 2008;Graham et al 2015a,b;Liu et al 2015Liu et al , 2016Bon et al 2016;Charisi et al 2016;Zheng et al 2016;Li et al 2019), even the strongest candidate periodicity has been shown to be subject to false positives due to stochastic quasar variability given the uneven sampling, limited time baseline, and/or relatively low sensitivity.…”