Simultaneous X-ray and 7-ray observations of Cyg X-1 in the hard state by Ginga and OSSE

Abstract

We present four X-ray/γ-ray spectra of Cyg X-1 observed in the hard ('low') state simultaneously by Ginga and GRO OSSE on 1991 June 6. The four spectra have almost identical spectral form but vary in the normalisation within a factor of two. The 3-30 keV Ginga spectra are well represented by power laws with an energy spectral index of α ∼ 0.6 and a Compton reflection component including a fluorescent Fe Kα corresponding to the solid angle of the reflector of ∼ 0.3 × 2π. These spectra join smoothly on to the OSSE range (≥ 50 keV) and are then cut off above ∼ 150 keV. The overall spectra can be modelled by repeated Compton scattering in a mildlyrelativistic, thermal plasma with the optical depth of τ ∼ 1. However, the high-energy cutoff is steeper than that due to single-temperature thermal Comptonisation. It can be described by a superposition of dominant τ ∼ 1-2, thermal emission at kT ∼ 100 keV and a Wien-like component from an optically-thick plasma at kT ∼ 50 keV.The X-ray spectra do not show the presence of an anisotropy break required if thermal Compton scattering takes place in a corona above a cold disc. Also, the flat spectral index shows that the plasma is soft-photon starved, i.e., the luminosity in incident soft X-ray seed photons is very much less than that in the hard X-rays. Furthermore, the observed solid angle of the reflector is significantly less than 2π. These facts taken together strongly rule out a disc-corona geometry. Rather, the observed spectra are consistent with a geometry in which the cold accretion disc (which both supplies the seed soft X-rays and reflects hard X-rays) only exists at large radii, while the Comptonising hot plasma is located in an inner region with no cold disc. This hot plasma consists of either pure e ± pairs if the source size is ∼ 5 Schwarzschild radii or it contains also protons if the size is larger.

Spectral analysis of theXMM–Newtondata of GX 339–4 in the low/hard state: disc truncation and reflection

“…Their main finding appears that in order to obtain good fits with an expected Fe abundance, A Fe 2, they require certain complexity of the continuum, namely the local spectra close to the BH (which dominate the reflection due to light bending) to be harder than those farther away (which dominate the direct spectrum, especially at low energies). This spectral hardening with the decreasing radius, i.e., following the accretion flow, is fully consistent with a number of other findings, e.g., the presence of hard lags (Kotov et al 2001), spectral variability requiring multiple Compton components, softer at larger radii (Yamada et al 2013), and the concave shape of the ∼1-10 keV continuum of BHXRBs, e.g., Gierliński et al (1997), Frontera et al (2001), Di Salvo et al (2001).…”

Limits on pair production and multizone Comptonization: the broad-band X/γ-ray spectrum of XTE J1550–564 revisited

“…While this may be justifiable in less luminous sources, like e.g. Cyg X-1 (Gierliński et al 1997), it is certainly not so in the more luminous sources and/or states. Our modelling gives a source luminosity of ∼3.7 × 10 38 erg s −1 (for M = 10 M and D = 5.3 kpc; Orosz et al 2002).…”

Fluorescent iron lines as a probe of astrophysical black hole systems

“…Ginga, similar to EXOSAT, had poor spectral resolution (E/∆E ≈ 10). Marek Gierlinski and collaborators described these observations [273] with models comprised of low reflection fractions (Ω/2π = 0.2 − 0.5), and narrow iron lines with moderate equivalent widths (EW= 90-140 eV). As described in §3.…”