Formation of magnetically supported disks during hard-to-soft transitions in black hole accretion flows

Mami Machida, Kenji E. Nakamura, Ryoji Matsumoto

Research output: Contribution to journalArticle

62 Citations (Scopus)

Abstract

We carried out three-dimensional global resistive magnetohydrodynamic simulations of the cooling instability in optically thin hot black hole accretion flows by assuming bremsstrahlung cooling. General relativistic effects are simulated using the pseudo-Newtonian potential. The cooling instability grows when the density of the accretion disk becomes sufficiently large. We found that as the instability grows the accretion flow changes from an optically thin, hot, gas pressure-supported state (low/hard state) to a cooler, magnetically supported, quasi-steady state. During this transition, the magnetic pressure exceeds the gas pressure because the disk shrinks in the vertical direction while almost conserving the toroidal magnetic flux. Since further vertical contraction of the disk is suppressed by magnetic pressure, the cool disk stays in an optically thin, spectrally hard state. The magnetically supported disk exists for a time scale much longer than the thermal time scale, and comparable to the accretion time scale. We examined the stability of the magnetically supported disk analytically, assuming that the toroidal magnetic flux is conserved, and found it to be thermally and secularly stable. Our findings may explain why black hole candidates stay in luminous, hard state even when their luminosity exceeds the threshold for the onset of the cooling instability.

Original languageEnglish
Pages (from-to)193-202
Number of pages10
JournalPublications of the Astronomical Society of Japan
Volume58
Issue number1
DOIs
Publication statusPublished - Jan 1 2006

Fingerprint

accretion
cooling
timescale
guy wires
gas pressure
magnetic flux
magnetohydrodynamics
gas
magnetohydrodynamic simulation
quasi-steady states
contraction
high temperature gases
relativistic effects
coolers
accretion disks
bremsstrahlung
luminosity
thresholds
simulation

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Formation of magnetically supported disks during hard-to-soft transitions in black hole accretion flows. / Machida, Mami; Nakamura, Kenji E.; Matsumoto, Ryoji.

In: Publications of the Astronomical Society of Japan, Vol. 58, No. 1, 01.01.2006, p. 193-202.

Research output: Contribution to journalArticle

@article{35fe8d1732af442f9356251c49a7689f,
title = "Formation of magnetically supported disks during hard-to-soft transitions in black hole accretion flows",
abstract = "We carried out three-dimensional global resistive magnetohydrodynamic simulations of the cooling instability in optically thin hot black hole accretion flows by assuming bremsstrahlung cooling. General relativistic effects are simulated using the pseudo-Newtonian potential. The cooling instability grows when the density of the accretion disk becomes sufficiently large. We found that as the instability grows the accretion flow changes from an optically thin, hot, gas pressure-supported state (low/hard state) to a cooler, magnetically supported, quasi-steady state. During this transition, the magnetic pressure exceeds the gas pressure because the disk shrinks in the vertical direction while almost conserving the toroidal magnetic flux. Since further vertical contraction of the disk is suppressed by magnetic pressure, the cool disk stays in an optically thin, spectrally hard state. The magnetically supported disk exists for a time scale much longer than the thermal time scale, and comparable to the accretion time scale. We examined the stability of the magnetically supported disk analytically, assuming that the toroidal magnetic flux is conserved, and found it to be thermally and secularly stable. Our findings may explain why black hole candidates stay in luminous, hard state even when their luminosity exceeds the threshold for the onset of the cooling instability.",
author = "Mami Machida and Nakamura, {Kenji E.} and Ryoji Matsumoto",
year = "2006",
month = "1",
day = "1",
doi = "10.1093/pasj/58.1.193",
language = "English",
volume = "58",
pages = "193--202",
journal = "Publication of the Astronomical Society of Japan",
issn = "0004-6264",
publisher = "Astronomical Society of Japan",
number = "1",

}

TY - JOUR

T1 - Formation of magnetically supported disks during hard-to-soft transitions in black hole accretion flows

AU - Machida, Mami

AU - Nakamura, Kenji E.

AU - Matsumoto, Ryoji

PY - 2006/1/1

Y1 - 2006/1/1

N2 - We carried out three-dimensional global resistive magnetohydrodynamic simulations of the cooling instability in optically thin hot black hole accretion flows by assuming bremsstrahlung cooling. General relativistic effects are simulated using the pseudo-Newtonian potential. The cooling instability grows when the density of the accretion disk becomes sufficiently large. We found that as the instability grows the accretion flow changes from an optically thin, hot, gas pressure-supported state (low/hard state) to a cooler, magnetically supported, quasi-steady state. During this transition, the magnetic pressure exceeds the gas pressure because the disk shrinks in the vertical direction while almost conserving the toroidal magnetic flux. Since further vertical contraction of the disk is suppressed by magnetic pressure, the cool disk stays in an optically thin, spectrally hard state. The magnetically supported disk exists for a time scale much longer than the thermal time scale, and comparable to the accretion time scale. We examined the stability of the magnetically supported disk analytically, assuming that the toroidal magnetic flux is conserved, and found it to be thermally and secularly stable. Our findings may explain why black hole candidates stay in luminous, hard state even when their luminosity exceeds the threshold for the onset of the cooling instability.

AB - We carried out three-dimensional global resistive magnetohydrodynamic simulations of the cooling instability in optically thin hot black hole accretion flows by assuming bremsstrahlung cooling. General relativistic effects are simulated using the pseudo-Newtonian potential. The cooling instability grows when the density of the accretion disk becomes sufficiently large. We found that as the instability grows the accretion flow changes from an optically thin, hot, gas pressure-supported state (low/hard state) to a cooler, magnetically supported, quasi-steady state. During this transition, the magnetic pressure exceeds the gas pressure because the disk shrinks in the vertical direction while almost conserving the toroidal magnetic flux. Since further vertical contraction of the disk is suppressed by magnetic pressure, the cool disk stays in an optically thin, spectrally hard state. The magnetically supported disk exists for a time scale much longer than the thermal time scale, and comparable to the accretion time scale. We examined the stability of the magnetically supported disk analytically, assuming that the toroidal magnetic flux is conserved, and found it to be thermally and secularly stable. Our findings may explain why black hole candidates stay in luminous, hard state even when their luminosity exceeds the threshold for the onset of the cooling instability.

UR - http://www.scopus.com/inward/record.url?scp=33645153288&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=33645153288&partnerID=8YFLogxK

U2 - 10.1093/pasj/58.1.193

DO - 10.1093/pasj/58.1.193

M3 - Article

VL - 58

SP - 193

EP - 202

JO - Publication of the Astronomical Society of Japan

JF - Publication of the Astronomical Society of Japan

SN - 0004-6264

IS - 1

ER -