Power spectral properties of the soft spectral states in four black hole transients

The X-ray variability in the soft X-ray spectral state of black hole binaries is primarily characterized by a power-law noise(PLN), which is thought to originate from the propagation of the modulation in the mass accretion rate of a standard accretion disk flow. Such a PLN has also been revealed in the disk spectral component in the hard and the intermediate states in several black hole binaries. Here we present an investigation of the Rossi X-ray Timing Explorer(RXTE) observations of four black hole transients in which soft spectral states were observed twenty times or more. We show that in the soft spectral state, the PLN index varied in a large range between –1.64 and –0.62, and the fractional rms variability calculated in the 0.01 – 20 Hz frequency range reached as large as 7.67% and as low as 0.83%. Remarkably,we have found evidence of an inclination dependence of the maximal fractional rms variability, the averaged fractional rms variability and the fractional rms variability of the median in the sample based on current knowledge of inclination of black hole binaries. An inclination dependence has only been predicted in early magnetohydrodynamic simulations of isothermal disks limited to a high-frequency regime. In theory,the noise index is related to the physics of inward propagation of disk fluctuations, while the fractional rms amplitude reflects the intrinsic properties of the magnetohydrodynamic nature of the accretion flow.Our results therefore suggest that X-ray variability in the soft state can be used to put constraints on the properties of the accretion flow as well as the inclination of the accretion disk.

A magnetic reconnection model for quasi-periodic oscillations in black hole systems

The quasi-periodic oscillations （QPOs） in black hole （BH） systems with different scales are interpreted based on the magnetic reconnection of large-scale mag- netic fields generated by toroidal electric currents flowing in the inner region of the accretion disk, where the current density is assumed to be proportional to the mass density of the accreting plasma. The magnetic connection （MC） is taken into account in resolving dynamic equations describing the accretion disk, in which the MC be- tween the inner and outer disk regions, between the plunging region and the disk, and between the BH horizon and the disk are involved. It turns out that a single QPO frequency associated with several BH systems with different scales can be fitted by in- voking the magnetic reconnection due to the MC between the inner and outer regions of the disk, including the BH binaries XTE J1859＋226, XTE J1650-500 and GRS 1915＋105 and the massive BHs in NGC 5408 X-1 and RE J1034＋396. In addition, the X-ray spectra corresponding to the QPOs for these sources are fitted based on the typical disk-corona model.