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Title: Minidisks in Binary Black Hole Accretion

Abstract

Newtonian simulations have demonstrated that accretion onto binary black holes produces accretion disks around each black hole (“minidisks”), fed by gas streams flowing through the circumbinary cavity from the surrounding circumbinary disk. We study the dynamics and radiation of an individual black hole minidisk using 2D hydrodynamical simulations performed with a new general relativistic version of the moving-mesh code Disco. We introduce a comoving energy variable that enables highly accurate integration of these high Mach number flows. Tidally induced spiral shock waves are excited in the disk and propagate through the innermost stable circular orbit, providing a Reynolds stress that causes efficient accretion by purely hydrodynamic means and producing a radiative signature brighter in hard X-rays than the Novikov–Thorne model. Disk cooling is provided by a local blackbody prescription that allows the disk to evolve self-consistently to a temperature profile where hydrodynamic heating is balanced by radiative cooling. We find that the spiral shock structure is in agreement with the relativistic dispersion relation for tightly wound linear waves. We measure the shock-induced dissipation and find outward angular momentum transport corresponding to an effective alpha parameter of order 0.01. We perform ray-tracing image calculations from the simulations to produce theoretical minidiskmore » spectra and viewing-angle-dependent images for comparison with observations.« less

Authors:
;  [1]
  1. Center for Cosmology and Particle Physics, Physics Department, New York University, New York, NY 10003 (United States)
Publication Date:
OSTI Identifier:
22663886
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 835; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ACCRETION DISKS; ANGULAR MOMENTUM; BINARY STARS; BLACK HOLES; COMPARATIVE EVALUATIONS; DISPERSION RELATIONS; DISPERSIONS; HARD X RADIATION; HEATING; HYDRODYNAMICS; MACH NUMBER; RADIATIVE COOLING; RELATIVISTIC RANGE; REYNOLDS NUMBER; SHOCK WAVES; SIMULATION; SPECTRA; STARS; STREAMS

Citation Formats

Ryan, Geoffrey, and MacFadyen, Andrew, E-mail: gsr257@nyu.edu. Minidisks in Binary Black Hole Accretion. United States: N. p., 2017. Web. doi:10.3847/1538-4357/835/2/199.
Ryan, Geoffrey, & MacFadyen, Andrew, E-mail: gsr257@nyu.edu. Minidisks in Binary Black Hole Accretion. United States. doi:10.3847/1538-4357/835/2/199.
Ryan, Geoffrey, and MacFadyen, Andrew, E-mail: gsr257@nyu.edu. Wed . "Minidisks in Binary Black Hole Accretion". United States. doi:10.3847/1538-4357/835/2/199.
@article{osti_22663886,
title = {Minidisks in Binary Black Hole Accretion},
author = {Ryan, Geoffrey and MacFadyen, Andrew, E-mail: gsr257@nyu.edu},
abstractNote = {Newtonian simulations have demonstrated that accretion onto binary black holes produces accretion disks around each black hole (“minidisks”), fed by gas streams flowing through the circumbinary cavity from the surrounding circumbinary disk. We study the dynamics and radiation of an individual black hole minidisk using 2D hydrodynamical simulations performed with a new general relativistic version of the moving-mesh code Disco. We introduce a comoving energy variable that enables highly accurate integration of these high Mach number flows. Tidally induced spiral shock waves are excited in the disk and propagate through the innermost stable circular orbit, providing a Reynolds stress that causes efficient accretion by purely hydrodynamic means and producing a radiative signature brighter in hard X-rays than the Novikov–Thorne model. Disk cooling is provided by a local blackbody prescription that allows the disk to evolve self-consistently to a temperature profile where hydrodynamic heating is balanced by radiative cooling. We find that the spiral shock structure is in agreement with the relativistic dispersion relation for tightly wound linear waves. We measure the shock-induced dissipation and find outward angular momentum transport corresponding to an effective alpha parameter of order 0.01. We perform ray-tracing image calculations from the simulations to produce theoretical minidisk spectra and viewing-angle-dependent images for comparison with observations.},
doi = {10.3847/1538-4357/835/2/199},
journal = {Astrophysical Journal},
number = 2,
volume = 835,
place = {United States},
year = {Wed Feb 01 00:00:00 EST 2017},
month = {Wed Feb 01 00:00:00 EST 2017}
}
  • Merging supermassive black hole-black hole binaries produced in galaxy mergers are promising sources of detectable gravitational waves. If such a merger takes place in a gaseous environment, there is a possibility of a simultaneous detection of electromagnetic and gravitational radiation, as the stirring, shock heating, and accretion of the gas may produce variability and enhancements in the electromagnetic flux. Such a simultaneous detection can provide a wealth of opportunities to study gravitational physics, accretion physics, and cosmology. We investigate this scenario by performing fully general-relativistic, hydrodynamic simulations of merging, equal-mass, nonspinning black hole-black hole binaries embedded in gas clouds. Wemore » evolve the metric using the Baumgarte-Shapiro-Shibata-Nakamura formulation with standard moving puncture gauge conditions and handle the hydrodynamics via a high-resolution shock-capturing scheme. We consider both 'binary Bondi accretion' in which the binary is at rest relative to the ambient gas cloud, as well as 'binary Bondi-Hoyle-Lyttleton accretion' in which the binary moves relative to the gas cloud. The gas cloud is assumed to be homogeneous far from the binary and governed by a {Gamma}-law equation of state. We vary {Gamma} between 4/3 and 5/3. For each simulation, we compute the gas flow and accretion rate and estimate the electromagnetic luminosity due to bremsstrahlung and synchrotron emission. We find evidence for significant enhancements in both the accretion rate and luminosity over values for a single black hole of the same mass as the binary. We estimate that this luminosity enhancement should be detectable by the Large Synoptic Survey Telescope for a 10{sup 6}M{sub {center_dot}}binary in a hot gas cloud of density n{approx}10 cm{sup -3} and temperature T{approx}10{sup 6} K at z=1, reaching a maximum of L{approx}3x10{sup 43} erg s{sup -1}, with the emission peaking in the visible band, and lasting for {approx}1 hour.« less
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  • The association of an electromagnetic signal with the merger of a pair of supermassive black holes would have many important implications. For example, it would provide new information about gas and magnetic field interactions in dynamical spacetimes as well as a combination of redshift and luminosity distance that would enable precise cosmological tests. A proposal first made by Bode and Phinney is that because radiation of gravitational waves during the final inspiral and merger of the holes is abrupt and decreases the mass of the central object by a few percent, there will be waves in the disk that canmore » steepen into shocks and thus increase the disk luminosity in a characteristic way. We evaluate this process analytically and numerically. We find that shocks only occur when the fractional mass loss exceeds the half-thickness of the disk, hence significant energy release only occurs for geometrically thin disks which are thus at low Eddington ratios. This strongly limits the effective energy release, and in fact our simulations show that the natural variations in disk luminosity are likely to obscure this effect entirely. However, we demonstrate that the reduction of luminosity caused by the retreat of the inner edge of the disk following mass loss is potentially detectable. This decrease occurs even if the disk is geometrically thick, and lasts for a duration on the order of the viscous time of the modified disk. Observationally, the best prospect for detection would be a sensitive future X-ray instrument with a field of view of the order of a square degree, or possibly a wide-field radio array such as the Square Kilometer Array, if the disk changes produce or interrupt radio emission from a jet.« less
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