Unification of Spectral States of Accreting Black Holes

Abstract

Several galactic black holes show transitions between spectral states. The nature of these transitions is not fully understood yet. None of the dynamical accretion disk models can fully describe spectral transitions. In this paper we present a unifying radiation transfer model that can fit the spectral data in both states. Since Cyg X-1 has the best available data, we focus here on modeling this object. We fit individual broad-band (from 1 keV up to 4 MeV) spectral data for the "hard" and "soft" states of Cyg X-1 using an emission model where a central Comptonizing corona/cloud is illuminated by the soft photon emission from a cold disk. We assume that the energy is injected to the corona by two channels: a non-thermal one that injects energetic (> MeV) electrons into the coronal region, and a thermal one that heats injected and ambient electrons once they cool sufficiently to form a Maxwellian distribution, i.e., we consider a hybrid thermal/non-thermal model. The process of photon-photon pair production is included in the model, and the number of pairs produced in the coronal region can be substantial.Using simple scaling laws for the luminosity of the cold disk, the thermal dissipation/heating rate in the corona, and the rate of energy injection from a non-thermal source, all as functions of radius of the corona, we explain the hard-to-soft transition as the result of a decrease in the size of the corona and the inner radius of the cold disk by a factor ∼ 5. For the case of Cyg X-1, we show that the bolometric luminosity of the source (mass accretion rate) does not change significantly during the transition, and thus the transition is probably the result of a disk instability. Observations of Galactic Black HolesGalactic black holes radiate X-rays and γ-rays in one of several spectral states. Recently, it has become clear that in the soft state, the power-law spectrum (with typical energy spectral index α ∼ 1.5) extends to photon energies ∼ m e c 2 without any obvious break [1][2][3]. The black hole spectra in the hard state instead show a cutoff at ∼ 100 keV [3][4][5] and can be fit quite well with thermal Comptonization models up to energies ∼ 300 keV. However, there is evidence in the COMPTEL and BATSE data that the hard state spectrum of Cyg X-1 show high energy excess at > ∼ 500 keV [6,7]. This excess can be interpreted as a signature of non-thermal electrons in the X/γ-ray source [8,9]. This excess can also be explained in terms of a multi-zone models where thermal electrons have significantly different temperatures [7,10,11].While the data in any one state can be fit quite successfully by one of the models mentioned above, none of the models proposed thus far can fit data from both states. Esin et al. [12] explain the spectral transitions in terms of the advection dominated disks. Their prediction that electron temperature in the soft state should be smaller than that in the hard state (because of the significant increase in the number of ambient soft photons) is in perfect agreement with our f...

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