Abstract

Black hole X-ray binaries (BHXBs) are known to display five distinct spectral states. In order of increasing luminosity these are the quiescent state, low state, intermediate state, high state, and very high state. We present a self-consistent model of accretion flows around black holes that unifies all of these states except the very high state. The model is an extension of the following paradigm, which has been applied successfully to the quiescent state. The accretion flow consists of two zones, an inner advection-dominated accretion flow (ADAF) that extends from the black hole horizon to a transition radius rtr, and an outer thin accretion disk that is present beyond rtr. Above the disk is a hot corona, which is a continuation of the inner ADAF. The model consistently treats the dynamics of the accreting gas, the thermal balance of the ions and electrons in the two-temperature ADAF and corona, and the radiation processes that produce the observed spectrum. At low mass accretion rates, 0.01 (in Eddington units), the inner ADAF zone in the model radiates extremely inefficiently, and the outer thin disk is restricted to large radii (rtr ~ 102-104, in Schwarzschild units). The luminosity therefore is low, and this configuration is identified with the quiescent state. For 0.01 and up to a critical value crit${r crit}$ --> ~0.08, the radiative efficiency of the ADAF increases rapidly and the system becomes fairly luminous. The spectrum is very hard and peaks around 100 keV. This is the low state. The exact value of crit${r crit}$ --> depends on the viscosity parameter α (crit${r crit}$ --> ~1.3α -->2; the paper assumes α = 0.25). For values of > crit${r crit}$ --> and up to a second critical value about 10% higher, the ADAF progressively shrinks in size, the transition radius decreases, and the X-ray spectrum changes continuously from hard to soft. We identify this stage with the intermediate state. Finally, when is sufficiently large, the inner ADAF zone disappears altogether and the thin accretion disk extends down to the marginally stable orbit. The spectrum is dominated by an ultrasoft component with a weak hard tail. This is the high state. Model spectra calculated with this unified scenario agree well with observations of the quiescent, low, intermediate, and high states. Moreover, the model provides a natural explanation for the low state to high state transition in BHXBs. We also make a tentative proposal for the very high state, but this aspect of the model is less secure. An important feature of the model is that it is essentially parameter free. We test the model against observations of the soft X-ray transient Nova Muscae during its 1991 outburst. The model reproduces the observed light curves and spectra surprisingly well and makes a number of predictions that can be tested by observations of other BHXBs.

Highlights

  • Galactic X-ray sources are classified as black hole X-ray binaries (BHXB, see van Paradijs & McClintock 1995; Tanaka & Lewin 1995; Tanaka & Shibazaki 1996 and Liang 1997 for reviews) if the binary mass function is ∼> 3M⊙ (A0620-00, V404 Cyg, Nova Muscae 1991, GRO J1655-40, GS 2000-251, Nova Ophiuchi 1997) or if there is other less direct evidence that the accreting compact object is more massive than 3M⊙ (e.g. Cyg X-1, LMC X-3, GRO J0422+32)

  • Quiescent/Off state: in the last few years it has become possible to measure X-ray spectra of systems in the“off” or “quiescent” state. These observations have been done on a subset of Black hole X-ray binaries (BHXBs), called soft X-ray transients (SXTs), which spend most of their lives in quiescence

  • NMY and NBM proposed that the accretion flow in quiescent SXTs consists of two zones, an inner optically thin advection-dominated accretion flow (ADAF) extending from the black hole horizon to a transition radius at ∼ 104 Schwarzschild radii and an outer thin disk beyond this radius

Read more

Summary

Introduction

Galactic X-ray sources are classified as black hole X-ray binaries (BHXB, see van Paradijs & McClintock 1995; Tanaka & Lewin 1995; Tanaka & Shibazaki 1996 and Liang 1997 for reviews) if the binary mass function is ∼> 3M⊙ (A0620-00, V404 Cyg, Nova Muscae 1991, GRO J1655-40, GS 2000-251, Nova Ophiuchi 1997) or if there is other less direct evidence that the accreting compact object is more massive than 3M⊙ (e.g. Cyg X-1, LMC X-3, GRO J0422+32). NMY and NBM proposed that the accretion flow in quiescent SXTs consists of two zones, an inner optically thin ADAF extending from the black hole horizon to a transition radius at ∼ 104 Schwarzschild radii and an outer thin disk beyond this radius. Using this model they explained the optical and X-ray spectra of two well-studied systems, V404 Cyg and A0620-00, while Hameury et al (1997) found a good fit for GRO J1655-40.

Geometry of the Flow and Model Parameters
Flow Dynamics and Energy Balance
Pair Production
Numerical Solution
Modeling the Spectral States of Black Hole X-Ray Binaries
Quiescent State
Low State
Low State to High State Transition
High State
Very High State
Outburst of Nova Muscae 1991
Optical Observations in Quiescence
X-ray Observations in Outburst
Modeling the X-ray and γ-ray Lightcurves
Modeling the Spectral Evolution of the System
Discussion and Conclusions
Energy Balance Equation for Electrons
Findings
Electron-Positron Pair Production and Annihilation
Full Text
Published version (Free)

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call