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Register a new userWarping and tearing of misaligned circumbinary disks around eccentric SMBH binaries
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We study the warping and tearing of a geometrically thin, non-self-gravitating disk surrounding binary supermassive black holes on an eccentric orbit. The circumbinary disk is significantly misaligned with the binary orbital plane, and is subject to the time-dependent tidal torques. In principle, such a disk is warped and precesses, and is torn into mutually misaligned rings in the region, where the tidal precession torques are stronger than the local viscous torques. We derive the tidal-warp and tearing radii of the misaligned circumbinary disks around eccentric SMBH binaries. We find that in disks with the viscosity parameter, alpha, larger than a critical value depending on the disk aspect ratio, the disk warping appears outside the tearing radius. This condition is expressed as alpha > sqrt{H/3r} for H/r ~<0.1, where H is the disk scale height. If alpha < sqrt{H/3r}, only the disk tearing occurs because the tidal warp radius is inside the tearing radius, where most of disk material is likely to rapidly accrete onto SMBHs. In warped and torn disks, both the tidal-warp and the tearing radii most strongly depend on the binary semi-major axis, although they also mildly depend on the other orbital and disk parameters. This strong dependence enables us to estimate the semi-major axis, once the tidal warp or tearing radius is determined observationally: For the tidal warp radius of 0.1 pc, the semi-major axis is estimated to be ~10^{-2} pc for 10^7 Msun black hole with typical orbital and disk parameters. We also briefly discuss the possibility that central objects of observed warped maser disks in active galactic nuclei are supermassive black hole binaries.
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We study a warping instability of a geometrically thin, non-self-gravitating, circumbinary disk around young binary stars on an eccentric orbit. Such a disk is subject to both the tidal torques due to a time-dependent binary potential and the radiative torques due to radiation emitted from each star. The tilt angle between the circumbinary disk plane and the binary orbital plane is assumed to be very small. We find that there is a radius within/beyond which the circumbinary disk is unstable to radiation-driven warping, depending on the disk density and temperature gradient indices. This marginally stable warping radius is very sensitive to viscosity parameters, a fiducial disk radius and the temperature measured there, the stellar luminosity, and the disk surface density at a radius where the disk changes from the optically thick to thin for the irradiation from the central stars. On the other hand, it is insensitive to the orbital eccentricity and binary irradiation parameter, which is a function of the binary mass ratio and luminosity of each star. Since the tidal torques can suppress the warping in the inner part of the circumbinary disk, the disk starts to be warped in the outer part. While the circumbinary disks are most likely to be subject to the radiation-driven warping on a AU to kilo-AU scale for binaries with young massive stars more luminous than 10^4Lsun, the radiation driven warping does not work for those around young binaries with the luminosity comparable to the solar luminosity.
It is usually thought that viscous torque works to align a circumbinary disk with the binarys orbital plane. However, recent numerical simulations suggest that the disk may evolve to a configuration perpendicular to the binary orbit (polar alignment) if the binary is eccentric and the initial disk-binary inclination is sufficiently large. We carry out a theoretical study on the long-term evolution of inclined disks around eccentric binaries, calculating the disk warp profile and dissipative torque acting on the disk. For disks with aspect ratio $H/r$ larger than the viscosity parameter $alpha$, bending wave propagation effectively makes the disk precess as a quasi-rigid body, while viscosity acts on the disk warp and twist to drive secular evolution of the disk-binary inclination. We derive a simple analytic criterion (in terms of the binary eccentricity and initial disk orientation) for the disk to evolve toward polar alignment with the eccentric binary. When the disk has a non-negligible angular momentum compared to the binary, the final polar alignment inclination angle is reduced from $90^circ$. For typical protoplanetary disk parameters, the timescale of the inclination evolution is shorter than the disk lifetime, suggesting that highly-inclined disks and planets may exist orbiting eccentric binaries.
We examine the light curves of two quasars, motivated by recent suggestions that a supermassive black hole binary (SMBHB) can exhibit sharp lensing spikes. We model the variability of each light curve as due to a combination of two relativistic effects: the orbital relativistic Doppler boost and gravitational binary self-lensing. In order to model each system we extend previous Doppler plus self-lensing models to include eccentricity. The first quasar is identified in optical data as a binary candidate with a 20-yr period (Ark 120), and shows a prominent spike. For this source, we rule out the lensing hypothesis and disfavor the Doppler-boost hypothesis due to discrepancies in the measured vs. recovered values of the binary mass and optical spectral slope. The second source, which we nickname Spikey, is the rare case of an active galactic nucleus (AGN) identified in Keplers high-quality, high-cadence photometric data. For this source, we find a model, consisting of a combination of a Doppler modulation and a narrow symmetric lensing spike, consistent with an eccentric SMBHB with a total mass of approximately 30 million solar masses, rest-frame orbital period T=418 days, eccentricity e=0.5, and seen at an inclination of 8 degrees from edge-on. This interpretation can be tested by monitoring Spikey for periodic behavior and recurring flares in the next few years. In preparation for such monitoring we present the first X-ray observations of this object taken by the Neil Gehrels Swift observatory.
In this paper, we explore the mechanisms that regulate the formation and evolution of stellar black hole binaries (BHBs) around supermassive black holes (SMBHs). We show that dynamical interactions can efficiently drive in-situ BHB formation if the SMBH is surrounded by a massive nuclear cluster (NC), while orbitally segregated star clusters can replenish the BHB reservoir in SMBH-dominated nuclei. We discuss how the combined action of stellar hardening and mass segregation sculpts the BHB orbital properties. We use direct N-body simulations including post-Newtonian corrections up to 2.5 order to study the BHB-SMBH interplay, showing that the Kozai-Lidov mechanism plays a crucial role in shortening binaries lifetime. We find that the merging probability weakly depends on the SMBH mass in the $10^6-10^9{rm ~M}_odot$ mass range, leading to a merger rate $Gamma simeq 3-8$ yr$^{-1}$ Gpc$^{-3}$ at redshift zero. Nearly $40%$ of the mergers have masses in the BH mass gap, $50-140{rm ~M}_odot$, thus indicating that galactic nuclei are ideal places to form BHs in this mass range. We argue that gravitational wave (GW) sources with components mass $m_1>40{rm ~M}_odot$ and $m_2<30{rm ~M}_odot$ would represent a strong indicator of a galactic nuclei origin. The majority of these mergers could be multiband GW sources in the local Universe: nearly $40%$ might be seen by LISA as eccentric sources and, a few years later, as circular sources by LIGO and the Einstein Telescope, making decihertz observatories like DECIGO unique instruments to bridge the observations during the binary inspiral.
The new generation of VLTI instruments (GRAVITY, MATISSE) aims to produce routinely interferometric images to uncover the morphological complexity of different objects at high angular resolution. Image reconstruction is, however, not a fully automated process. Here we focus on a specific science case, namely the complex circumbinary environments of a subset of evolved binaries, for which interferometric imaging provides the spatial resolution required to resolve the immediate circumbinary environment. Indeed, many binaries where the main star is in the post-asymptotic giant branch (post-AGB) phase are surrounded by circumbinary disks. Those disks were first inferred from the infrared excess produced by dust. Snapshot interferometric observations in the infrared confirmed disk-like morphology and revealed high spatial complexity of the emission that the use of geometrical models could not recover without being strongly biased. Arguably, the most convincing proof of the disk-like shape of the circumbinary environment came from the first interferometric image of such a system (IRAS08544-4431) using the PIONIER instrument at the VLTI. This image was obtained using the SPARCO image reconstruction approach that enables to subtract a model of a component of the image and reconstruct an image of its environment only. In the case of IRAS08544-4431, the model involved a binary and the image of the remaining signal revealed several unexpected features. Then, a second image revealed a different but also complex circumstellar morphology around HD101584 that was well studied by ALMA. To exploit the VLTI imaging capability to understand these targets, we started a large program at the VLTI to image post-AGB binary systems using both PIONIER and GRAVITY instruments.
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