Thermal X-Ray Iron Line Emission From the Advection-Dominated Accretion Flow in the Galactic Binary Gx 339–4

Abstract

We explore thermal X-ray iron line emission from the galactic X-ray binary GX 339-4 in the off state, using the models of the advection dominated accretion flow (ADAF) without or with outflows. The equivalent widths of hydrogen-like and helium-like thermal iron lines are calculated with different model parameters including viscosity parameter α, mass accretion rate at the outer radius of the ADAFṁ out and outflow strength parameter p. Our calculations show that the equivalent widths of thermal iron lines emitted from the pure ADAF, i.e., the ADAF without outflows, should be very small, assuming a solar metallicity for the accreting gas in the accretion flow. Strong thermal iron lines are expected to be emitted from the ADAF with relatively strong outflows. For reasonable choice of parameters, the total equivalent width of the He-like and H-like thermal iron lines reaches to 500 eV for accreting gas with solar metallicity. The observation of strong thermal X-ray lines from GX 339-4 at the off state may give a clue to the accretion mode of the source and provide evidence for the presence of outflows/winds in the accretion flow around the black hole in GX 339-4. It is found that the values ofṁ out and p are degenerate, i.e., the observed X-ray continuum spectrum can be fairly well reproduced with different sets of the parametersṁ out and p. Such degeneracy can be broken when the thermal X-ray line emission data is available. We also compare our results with those in the previous similar work.

Can the slow-rotation approximation be used in electromagnetic observations of black holes?

“…The Novikov-Thorne model (and models that are adapted from it) is the most commonly used framework when fitting for continuum spectrum observations. However, there are other models that incorporate different assumptions or different physics [13], such as the advection-dominated accretion flow (ADAF) model, which could potentially produce a significantly different continuum spectrum [74]. It is possible that the agreement in the continuum spectrum between the slowly rotating metric and the Kerr metric disappears in other accretion disk models and this is worth investigating in the future.…”

A Model for the Correlation of Hard X-Ray Index With Eddington Ratio in Black Hole X-Ray Binaries

“…Recently, observations show that there is a positive correlation between Eddington ratio λ and hard X-ray index Γ for λ > 0.01 (Sobolewska et al 2011;Kalemci et al 2006), and there is an anti-correlation between λ and Γ for λ 0.01 (λ = L bol /L Edd , with L bol being the bolometric luminosity, L Edd = 1.26 × 10 38 m erg s −1 being the Eddington luminosity, m being the central black hole mass scaled with solar mass M ⊙ ) (Wu & Gu 2008). It's interesting that this correlation is also found in active galactic nuclei, i.e., when λ 0.01, the hard X-ray spectrum softens with Eddington ratio (Porquet et al 2004;Shemmer et al 2006;Saez et al 2008;Sobolewska & Papadakis 2009;Cao 2009;Liu et al 2012;Zhou et al 2010;Veledina et al 2011), but when λ 0.01 the hard X-ray spectrum hardens with the Eddington ratio (Constantin et al 2009;Gu & Cao 2009;Younes et al 2011Younes et al , 2012Xu 2011). For the Comptonization models, it's argued that the above correlations mean the ratio of Compton luminosity to seed photon luminosity, ℓ h /ℓ s , changes with Eddington ratio, which is consistent with the scenario that the seed photons are from the synchrotron radiation at lower Eddington ratio and from the truncated thin disk at higher Eddington ratio (Sobolewska et al 2011).…”

“…The different spectral states imply the different accretion model, and the transition of the spectral state may mean the change of the accretion patterns. It is proposed that the different spectral states are primarily governed by the mass accretion rate (Esin et al 1997(Esin et al , 1998Gilfanov et al 2010;Liu et al 1999Liu et al , 2002Qiao & Liu 2009, 2010, 2011Maccarone 2003;Meyer-Hofmeister et al 2012). Homan et al (2001) analyzed the data of the black hole X-ray transient XTE J1550-564, taken with the Rossi X-ray Timing Explorer between 1998 November 22 and 1999 May 20, during which the source went through different spectral state.…”

The quiescent X-ray spectrum of accreting black holes

“…Constraining the X-ray spectral shape at energies in excess of 10 keV would clearly be of benefit here, and with the successful launch of NuSTAR such constraints may be forthcoming in the near future. (ii) Numerous ADAF models predict the presence of narrow emission lines (EW 30 eV) from highly ionized iron, e.g., Narayan & Raymond (1999); Perna et al (2000); Xu (2011). Imprinted in these lines will lie information on the geometry and ionization structure of the accretion flow, e.g., Wang et al (2013) recently used the detection of highly ionized Fe XXV from Sgr A * , which they successfully reproduced with an ADAF model.…”

“…As line emission is predicted from the ADAF (Narayan & Raymond 1999;Perna et al 2000;Xu 2011), we additionally tested for the presence of an emission lines in the iron K region of the spectrum. To do this, narrow Gaussians (σ = 0) were added to the best fit power-law model above consistent with an origin from Fe Kα (6.4 keV), Fe XXV (6.7 keV), and Fe XXVI (6.97 keV).…”