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Register a new userThe Emission from Inner Disk and Corona in the Low and Intermediate Spectral States of Black Hole X-ray Binaries
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Recent observations reveal that a cool disk may survive in the innermost stable circular orbit (ISCO) for some black hole X-ray binaries in the canonical low/hard state. The spectrum is characterized by a power law with a photon index $Gamma sim 1.5-2.1$ in the range of 2-10 keV and a weak disk component with temperature of $sim 0.2$ keV. In this work, We revisit the formation of such a cool, optically thick, geometrically thin disk in the most inner region of black hole X-ray binaries at the low/hard state within the context of disk accretion fed by condensation of hot corona. By taking into account the cooling process associated with both Compton and conductive processes in a corona, and the irradiation of the hot corona to the disk, we calculate the structure of the corona. For viscosity parameter $alpha=0.2$, its found that the inner disk can exist for accretion rate ranging from $dot M sim 0.006-0.03 dot M_{rm Edd}$, over which the electron temperatures of the corona are in the range of $1-5times 10^9 rm K$ producing the hard X-ray emission. We calculate the emergent spectra of the inner disk and corona for different mass accretion rates. The effect of viscosity parameter $alpha$ and albedo $a$ ($a$ is defined as the energy ratio of reflected radiation from the surface of the thin disk to incident radiation upon it from the corona) to the emergent spectra are also presented. Our model is used to explain the recent observations of GX 339-4 and Cyg X-1, in which the thin disk may exist at ISCO region in the low/hard state at luminosity around a few percent of $L_{rm Edd}$. Its found that the observed maximal effective temperature of the thermal component and the hard X-ray photon index $Gamma$ can be matched well by our model.
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In this chapter, I present the main X-ray observational characteristics of black-hole binaries and low magnetic field neutron-star binaries, concentrating on what can be considered similarities or differences, with particular emphasis on their fast-timing behaviour.
X-ray flux from the inner hot region around central compact object in a binary system illuminates the upper surface of an accretion disc and it behaves like a corona. This region can be photoionised by the illuminating radiation, thus can emit different emission lines. We study those line spectra in black hole X-ray binaries for different accretion flow parameters including its geometry. The varying range of model parameters captures maximum possible observational features. We also put light on the routinely observed Fe line emission properties based on different model parameters, ionization rate, and Fe abundances. We find that the Fe line equivalent width $W_{rm E}$ decreases with increasing disc accretion rate and increases with the column density of the illuminated gas. Our estimated line properties are in agreement with observational signatures.
Galactic black-hole X-ray binaries (BHBs) emit a compact, optically thick, mildly relativistic radio jet when they are in the hard and hard-intermediate states. In these states, BHBs exhibit a correlation between the time lag of hard photons with respect to softer ones and the photon index of the power law component that characterizes the X-ray spectral continuum above $sim$ 10 keV. The correlation, however, shows large scatter. Our objective is to investigate the role that the inclination of the system plays on the correlation between the time lag and the photon index. We find that the correlation between the time lag and the photon index is tight in low-inclination systems and becomes weaker in high-inclination systems. The amplitude of the lags is also larger at low and intermediate inclination angles than at high inclination. Our jet model that reproduces the process of Comptonization in an extended jet can account for the observations remarkably well.
The broad iron spectral features are often seen in X-ray spectra of Active Galactic Nuclei (AGN) and black-hole binaries (BHB). These features may be explained either by the relativistic disc reflection scenario or the partial covering scenario: It is hardly possible to determine which model is valid from time-averaged spectral analysis. Thus, X-ray spectral variability has been investigated to constrain spectral models. To that end, it is crucial to study iron structure of BHBs in detail at short time-scales, which is, for the first time, made possible with the Parallel-sum clocking (P-sum) mode of XIS detectors on board Suzaku. This observational mode has a time-resolution of 7.8~ms as well as a CCD energy-resolution. We have carried out systematic calibration of the P-sum mode, and investigated spectral variability of the BHB GRS 1915+105. Consequently, we found that the spectral variability of GRS 1915+105 does not show iron features at sub-seconds. This is totally different from variability of AGN such as 1H0707--495, where the variation amplitude significantly drops at the iron K-energy band. This difference can be naturally explained in the framework of the partial covering scenario.
We analyze three prototypical black hole (BH) X-ray binaries (XRBs), 4u1630, gro1655 and h1743, in an effort to systematically understand the intrinsic state transition of the observed accretion-disk winds between windon and windoff states by utilizing state-of-the-art {it Chandra}/HETGS archival data from multi-epoch observations. We apply our magnetically-driven wind models in the context of magnetohydrodynamic (MHD) calculations to constrain their (1) global density slope ($p$), (2) their density ($n_{17}$) at the foot point of the innermost launching radius and (3) the abundances of heavier elements ($A_{rm Fe,S,Si}$). Incorporating the MHD winds into {tt xstar} photoionization calculations in a self-consistent manner, we create a library of synthetic absorption spectra given the observed X-ray continua. Our analysis clearly indicates a characteristic bi-modal transition of multi-ion X-ray winds; i.e. the wind density gradient is found to steepen (from $p sim 1.2-1.4$ to $sim 1.4-1.5$) while its density normalization declines as the source transitions from windon to windoff state. The model implies that the ionized wind {it remains physically present} even in windoff state, despite its absent appearance in the observed spectra. Super-solar abundances for heavier elements are also favored. Our global multi-ion wind models, taking into account soft X-ray ions as well as Fe K absorbers, show that the internal wind condition plays an important role in wind transitions besides photoionization changes. % Simulated {it XRISM}/Resolve and {it Athena}/X-IFU spectra are presented to demonstrate a high fidelity of the multi-ion wind model for better understanding of these powerful ionized winds in the coming decades.
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