We present X-ray/γ-ray spectra of Cyg X-1 observed during the transition from the hard to the soft state and in the soft state by ASCA, RXTE and CGRO/OSSE in 1996 May and June. The spectra consist of a dominant soft component below ∼2 keV and a power-law-like continuum extending to at least ∼800 keV. We interpret them as emission from an optically thick, cold accretion disc and from an optically thin, non-thermal corona above the disc. A fraction f≳0.5 of total available power is dissipated in the corona.



We model the soft component by multicolour blackbody disc emission taking into account the torque-free inner-boundary condition. If the disc extends down to the minimum stable orbit, the ASCARXTE data yield the most probable black hole mass of MX≈10 M⊙ and an accretion rate, , locating Cyg X-1 in the soft state in the upper part of the stable, gas-pressure-dominated, accretion-disc solution branch.



The spectrum of the corona is well modelled by repeated Compton scattering of seed photons from the disc off electrons with a hybrid, thermal/non-thermal distribution. The electron distribution can be characterized by a Maxwellian with an equilibrium temperature of kTe∼30–50 keV, a Thomson optical depth of τ∼0.3 and a quasi-power-law tail. The compactness of the corona is 2≲lh≲7, and a presence of a significant population of electron–positron pairs is ruled out.



We find strong signatures of Compton reflection from a cold and ionized medium, presumably an accretion disc, with an apparent reflector solid angle, Ω/2π∼0.5–0.7. The reflected continuum is accompanied by a broad iron Kα line.

/pdf/radiation-mechanisms-and-geometry-of-cygnus-x-1-in-the-soft-f2nokvhzlr.pdf

Radiation mechanisms and geometry of cygnus X-1 in the soft state

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, outer disk that does not penetrate much in to the corona. 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.

Unification of Spectral States of Accreting Black Holes

The origin of dramatically different electron distributions responsible for Comptonization in black hole X-ray binaries (BHBs) in their various states is discussed. We solve the coupled kinetic equations for photons and electrons without approximations on the relevant cross-sections accounting for Compton scattering, synchrotron radiation, and Coulomb collisions. In the absence of external soft photons, the electrons are efficiently thermalized by synchrotron self-absorption and Coulomb scattering even for pure nonthermal electron injection. The resulting quasi-thermal synchrotron self-Compton spectra have very stable slopes and electron temperatures similar to the hard states of BHBs. The observed hard X-ray spectral slopes, the cutoff at 100 keV, and the MeV tail together require low magnetic fields, ruling out the magnetic dissipation mechanism. The motion of the accretion disk toward the black hole results in larger Compton cooling and lower equilibrium electron temperature. Our self-consistent simulations show that in this case both electron and photon distributions attain a power-law-dominated shape similar to what is observed in the soft state. The electron distribution in the Cyg X-1 soft state might require a strong magnetic field, being consistent with the magnetically dominated corona.

http://iopscience.iop.org/article/10.1088/0004-637X/690/2/L97/pdf

On the origin of spectral states in accreting black holes

The INTEGRAL satellite extensively observed the black hole binary Cygnus X-1 from 2002 November to 2004 November during calibration, open time and core program (Galactic Plane Scan) observations. These data provide evidence for significant spectral variations over the period. In the framework of the accreting black hole phenomenology, the source was most of the time in the Hard State and occasionally switched to the so-called "Intermediate State". Using the results of the analysis performed on these data, we present and compare the spectral properties of the source over the whole energy range (5 keV-1 MeV) covered by the high-energy instruments on board INTEGRAL, in both observed spectral states. Fe line and reflection component evolution occurs with spectral changes in the hard and soft components. The observed behaviour of Cygnus X-1 is consistent with the general picture of galactic black holes. Our results give clues to the physical changes that took place in the system (disc and corona) at almost constant luminosity during the spectral transitions and provide new measures of the spectral model parameters. In particular, during the Intermediate State of 2003 June, we observe in the Cygnus X-1 data a high-energy tail at several hundred keV in excess of the thermal Comptonization model which suggests the presence of an additional non-thermal component.

/pdf/the-broad-band-spectrum-of-cygnus-x-1-measured-by-integral-4gtu109d5b.pdf

The broad-band spectrum of Cygnus X-1 measured by INTEGRAL

We present the results of spectral analysis of 42 simultaneous broad-band Ginga--OSSE and RXTE--OSSE observations of Cyg X-1 carried out in 1991 and 1996--1999. The hardest spectra in our sample in the energy range from 3 to \~1000 keV can be well described by thermal Comptonization model with reflection from the cold disc, while the rest of the spectra are more complex and require an additional component below 10 keV. We consider a number of physically realistic models to describe the shape of the E<10 keV excess. The additional soft component can result from thermal Comptonization by electrons with a low Compton parameter, or can be a part of a nonthermal, power-law like emission extending above 1 MeV. We study correlations between parameters obtained from the spectral fits with different models. We confirm a general correlation between the photon index $\Gamma$ and the amplitude of reflection R. We find that simple phenomenological models (like power-law plus Compton reflection) applied to the narrow band (3--20 keV) data overestimated the values of R and $\Gamma$, although the simple models did rank correctly the spectra according to R and $\Gamma$. The dynamic corona model provides a satisfactory description of the observed correlation, while the hot inner disc models have problems in reproducing it quantitatively. We do not find significant correlation between the electron temperature and other spectral parameters, while the optical depth of the hot medium seems to decrease when the spectrum becomes softer. It is also shown that spectral parameters are well correlated with the timing characteristics of the source.

Broad-band spectra of Cyg X-1 and correlations between spectral characteristics

We present the latest update of the 1.809 MeV sky survey obtained with COMPTEL. Based on all observations taken since the launch of CGRO in spring 1991 to early summer this year we obtain 1.809 MeV all sky maps using different imaging methods. The background is modelled on the basis of an adjacent energy approach. We confirm the previously reported characteristics of the galactic 1.809 MeV emission, specifically excesses in regions away from the inner Galaxy. The observed 1.8 MeV gamma-ray line is ascribed to the radioactive decay of 26Al in the interstellar medium. 26Al has been found to be predominantly synthesised in massive stars and their subsequent core-collapse supernovae, which is confirmed in tracer comparisons. Due to this, one anticipates flux enhancements aligned with regions of recent star formation, such as apparently observed in the Cygnus and Vela regions.

https://arxiv.org/pdf/astro-ph/0104047.pdf

The Comptel 1.809 Mev Survey

The COMPTEL 1.809 MeV survey

/pdf/observations-of-cygnus-x-1-comptel-during-1991-34eiw5cvcf.pdf

Observations of Cygnus X-1 COMPTEL during 1991

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The Comptel 1.809 Mev Survey

The COMPTEL 1.809 MeV survey

Observations of Cygnus X-1 COMPTEL during 1991