出版社：SISSA, Scuola Internazionale Superiore di Studi Avanzati
摘要：We explore the relationship between the hard X-ray photon index G and the Eddington ratio
(x = LX (0:5¡25 keV)=LEdd) in six X-ray binaries (XRBs) with well-constrained black hole
masses and distances. We find that different XRBs follow different anti-correlations between G
and x when x is less than a critical value, while they follow the same positive correlation when
x is larger than the critical value. This anti-correlation and positive correlation are also found
in low luminosity active galactic nuclei (LLAGNs) and luminous QSOs respectively. The anticorrelation
and positive correlation of different XRBs roughly converge to the same point (logx =
¡2:1§0:2;G = 1:5§0:1), which may correspond to the accretion mode transition, since that the
anti-correlation and positive correlation between G and x are consistent with the prediction of
advection dominated accretion flows (ADAFs) and standard disk/corona system respectively. We
note that traditional low-hard state are divided into two parts by the cross point logx » ¡2:1, i.e.,
faint-hard state in the anti-correlation part and bright-hard state in the positive correlation part.
The accretion process in the bright-hard state may be still the standard accretion disk as that in
the high/soft state, which is consistent with that both the cold disk component and broad Fe K
emission line are observed in some bright-hard state of XRBs (e.g., GX 339-4, Miller et al. 2006).
The ADAF is only important in the faint-hard state XRBs.
Motivated by the similarities of the state transition and timing properties of the ultraluminous Xray
sources (ULXs) to that of XRBs, we then constrain the black hole masses for seven luminous
ULXs assuming that their X-ray spectral evolution is similar to that of XRBs (i.e., photon index is
only depend on the Eddington ratio). We find that the BH masses of these seven ULXs are around
104M¯, which are typical intermediate-mass BHs (IMBHs). Our results are roughly consistent
with the BH masses constrained from the model fitting with a multi-color disk and/or the timing
properties (e.g., QPO and break frequency).
