TY - JOUR
T1 - Structure and instability of the ionization fronts around moving black holes
AU - Sugimura, Kazuyuki
AU - Ricotti, Massimo
N1 - Funding Information:
The authors thank Takashi Hosokawa, Daisuke Toyouchi, and Hide-nobu Yajima for fruitful discussions and comments. KS appreciates the support by the Fellowship of the Japan Society for the Promotion of Science for Research Abroad. MR acknowledges the support by NASA grant 80NSSC18K0527. The numerical simulations were performed on the Cray XC50 at CfCA of the National Astronomical Observatory of Japan, as well as on the computer cluster, Draco, at Frontier Research Institute for Interdisciplinary Sciences of Tohoku University, and on the Cray XC40 at Yukawa Institute for Theoretical Physics in Kyoto University. The authors also acknowledge the University of Maryland supercomputing resources (http://hpcc.umd.edu) made available for conducting the research reported in this paper. We thank the anonymous reviewer for helping improve the quality of this paper.
Publisher Copyright:
© 2020 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.
PY - 2020/5/14
Y1 - 2020/5/14
N2 - In this paper, we focus on understanding the physical processes that lead to stable or unstable ionization fronts (I-fronts) observed in simulations of moving black holes (BHs). The front instability may trigger bursts of gas accretion, rendering the BH significantly more luminous than at steady state. We perform a series of idealized three-dimensional radiation hydrodynamics simulations resolving the I-fronts around BHs of mass MBH and velocity $v$∞ accreting from a medium of density nH. The I-front, with radius RI, transitions from D-Type to R-Type as the BH velocity becomes larger than a critical value $v_\mathrm{R}\sim 40\, \mathrm{km\,s}^{-1}$. The D-Type front is preceded by a bow-shock of thickness ΔRI that decreases as $v$∞ approaches $v$R. We find that both D-Type and R-Type fronts can be unstable given the following two conditions: (i) for D-Type fronts the shell thickness must be ΔRI/RI < 0.05 (i.e. $v_\infty \gtrsim 20\, \mathrm{km\,s}^{-1}$), while no similar restriction holds for R-Type fronts; (ii) the temperature jump across the I-front must be TII/TI > 3. This second condition is satisfied if $T_\mathrm{I}\lt 5000\, \mathrm{K}$ or if $n_\mathrm{H}\, M_\mathrm{BH} \gtrsim 10^6\, M_\odot \, \mathrm{cm^{-3}}$. Due to X-ray pre-heating typically $T_\mathrm{I} \sim 10^4\, \mathrm{K}$, unless the D-Type shell is optically thick to X-rays, which also happens when $n_\mathrm{H}\, M_\mathrm{BH}$ is greater than a metallicity-dependent critical value. We thus conclude that I-fronts around BHs are unstable only for relatively massive BHs moving trough very dense molecular clouds. We briefly discuss the observational consequences of the X-ray luminosity bursts likely associated with this instability.
AB - In this paper, we focus on understanding the physical processes that lead to stable or unstable ionization fronts (I-fronts) observed in simulations of moving black holes (BHs). The front instability may trigger bursts of gas accretion, rendering the BH significantly more luminous than at steady state. We perform a series of idealized three-dimensional radiation hydrodynamics simulations resolving the I-fronts around BHs of mass MBH and velocity $v$∞ accreting from a medium of density nH. The I-front, with radius RI, transitions from D-Type to R-Type as the BH velocity becomes larger than a critical value $v_\mathrm{R}\sim 40\, \mathrm{km\,s}^{-1}$. The D-Type front is preceded by a bow-shock of thickness ΔRI that decreases as $v$∞ approaches $v$R. We find that both D-Type and R-Type fronts can be unstable given the following two conditions: (i) for D-Type fronts the shell thickness must be ΔRI/RI < 0.05 (i.e. $v_\infty \gtrsim 20\, \mathrm{km\,s}^{-1}$), while no similar restriction holds for R-Type fronts; (ii) the temperature jump across the I-front must be TII/TI > 3. This second condition is satisfied if $T_\mathrm{I}\lt 5000\, \mathrm{K}$ or if $n_\mathrm{H}\, M_\mathrm{BH} \gtrsim 10^6\, M_\odot \, \mathrm{cm^{-3}}$. Due to X-ray pre-heating typically $T_\mathrm{I} \sim 10^4\, \mathrm{K}$, unless the D-Type shell is optically thick to X-rays, which also happens when $n_\mathrm{H}\, M_\mathrm{BH}$ is greater than a metallicity-dependent critical value. We thus conclude that I-fronts around BHs are unstable only for relatively massive BHs moving trough very dense molecular clouds. We briefly discuss the observational consequences of the X-ray luminosity bursts likely associated with this instability.
KW - accretion, accretion discs
KW - black hole physics
KW - hydrodynamics
KW - instabilities
KW - methods: numerical
KW - radiative transfer
UR - http://www.scopus.com/inward/record.url?scp=85089846478&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85089846478&partnerID=8YFLogxK
U2 - 10.1093/mnras/staa1394
DO - 10.1093/mnras/staa1394
M3 - Article
AN - SCOPUS:85089846478
VL - 495
SP - 2966
EP - 2978
JO - Monthly Notices of the Royal Astronomical Society
JF - Monthly Notices of the Royal Astronomical Society
SN - 0035-8711
IS - 3
ER -