The Nature of X-ray Bright Optically Normal Galaxies

Recent X-ray surveys by {\it Chandra} and {\it XMM-Newton} have revealed a population of X-ray bright, optically normal galaxies (XBONGs) at moderate redshifts. We propose that many XBONGs are powered by an inner radiatively inefficient accretion flow (RIAF) plus an outer radiatively efficient thin accretion disk. The absence of optical/UV activity in XBONGs is explained by the truncation of the thin disk near the black hole, while the relatively strong X-ray emission is explained as inverse Compton emission from the hot RIAF. As an example, we show that the spectra of two XBONGs can be fit fairly well with such a model. By comparing these two sources to other accreting black holes, we argue that XBONGs are intermediate in their characteristics between distant luminous active galactic nuclei and nearby low-luminosity nuclei.Comment: 15 pages, 3 figures, the final version accepted by ApJ; substantially shortened but new material adde

Thermal Instability and Photoionized X-ray Reflection in Accretion Disks

We study the X-ray illumination of an accretion disk. We relax the simplifying assumption of constant gas density used in most previous studies; instead we determine the density from hydrostatic balance. It is found that the thermal ionization instability prevents the illuminated gas from attaining temperatures at which the gas is unstable. In particular, the uppermost layers of the X-ray illuminated gas are found to be almost completely ionized and at the local Compton temperature ($\sim 10^7 - 10^8$ K); at larger depths, the gas temperature drops abruptly to form a thin layer with $T\sim 10^6$ K, while at yet larger depths it decreases sharply to the disk effective temperature. We find that most of the Fe K$\alpha$ line emission and absorption edge are produced in the coolest, deepest layers, while the Fe atoms in the hottest, uppermost layers are generally almost fully ionized, hence making a negligible contribution to reprocessing features in $\sim 6.4-10$ keV energy range. We provide a summary of how X-ray reprocessing features depend on parameters of the problem. The results of our self-consistent calculations are both quantitatively and qualitatively different from those obtained using the constant density assumption. Therefore, we conclude that X-ray reflection calculations should always utilize hydrostatic balance in order to provide a reliable theoretical interpretation of observed X-ray spectra of AGN and GBHCs.Comment: Submitted to ApJ; 16 pages plus 13 figure

The Structure and X-ray Recombination Emission of a Centrally Illuminated Accretion Disk Atmosphere and Corona

We model an accretion disk atmosphere and corona photoionized by a central X-ray continuum source. We calculate the opacity and radiation transfer for an array of disk radii, to obtain the two-dimensional structure of the disk and its X-ray recombination emission. The atmospheric structure is insensitive to the viscosity alpha. We find a feedback mechanism between the disk structure and the central illumination, which expands the disk and increases the solid angle subtended by the atmosphere. We model the disk of a neutron star X-ray binary. We map the temperature, density, and ionization structure of the disk, and we simulate the high resolution spectra observable with the Chandra and XMM-Newton grating spectrometers. The X-ray emission lines from the disk atmosphere are detectable, especially for high-inclination binary systems. The grating observations of two classes of X-ray binaries already reveal important spectral similarities with our models. The line spectrum is very sensitive to the structure of each atmospheric layer, and it probes the heating mechanisms in the disk. The model spectrum is dominated by double-peaked lines of H-like and He-like ions, plus weak Fe L. Species with a broad range of ionization levels coexist at each radius: from Fe XXVI in the hot corona, to C VI at the base of the atmosphere. The choice of stable solutions affects the spectrum, since a thermal instability is present in the regime where the X-ray recombination emission is most intense.Comment: 32 pages, incl. 26 figures, accepted for publication in Ap

Two-temperature coronal flow above a thin disk

We extended the disk corona model (Meyer & Meyer-Hofmeister 1994; Meyer, Liu, & Meyer-Hofmeister 2000a) to the inner region of galactic nuclei by including different temperatures in ions and electrons as well as Compton cooling. We found that the mass evaporation rate and hence the fraction of accretion energy released in the corona depend strongly on the rate of incoming mass flow from outer edge of the disk, a larger rate leading to more Compton cooling, less efficient evaporation and a weaker corona. We also found a strong dependence on the viscosity, higher viscosity leading to an enhanced mass flow in the corona and therefore more evaporation of gas from the disk below. If we take accretion rates in units of the Eddington rate our results become independent on the mass of the central black hole. The model predicts weaker contributions to the hard X-rays for objects with higher accretion rate like narrow-line Seyfert 1 galaxies (NLS1s), in agreement with observations. For luminous active galactic nuclei (AGN) strong Compton cooling in the innermost corona is so efficient that a large amount of additional heating is required to maintain the corona above the thin disk.Comment: 17 pages, 6 figures. ApJ accepte

On the Properties of Inner Cool Disks in the Hard State of Black Hole X-Ray Transient Systems

The formation of a cool disk in the innermost regions of black hole X-ray transient systems in the low hard state is investigated. Taking into account the combined cooling associated with the Compton and conductive energy transport processes in a corona, the radial structure of a disk is described for a range of mass accretion rates. The mass flow in an optically thick inner region can be maintained by the condensation of matter from a corona with the disk temperature and luminosity varying continuously as a function of the accretion rate. Although such a disk component can be present, the contribution of the optically thick disk component to the total luminosity can be small since the mass flow due to condensation in the optically thick disk underlying the corona can be significantly less than the mass flow rate in the corona. The model is applied to the observations of the low quiescent state of the black hole source GX 339-4 at luminosities of around $0.01 L_{Edd}$ and is able to explain the temperature of the thermal component at the observed luminosities. Since conductive cooling dominates Compton cooling at low mass accretion rates, the luminosity corresponding to the critical mass accretion rate above which a weak thermal disk component can be present in the low hard state is estimated to be as low as $0.001 L_{Edd}$.Comment: 30 pages, 8 figures, accepted for publication in Ap

The Nature of the Emission Components in the Quasar/NLS1 PG1211+143

We present the study of the emission properties of the quasar PG1211+143, which belongs to the class of Narrow Line Seyfert 1 galaxies. On the basis of observational data analyzed by us and collected from the literature, we study the temporal and spectral variability of the source in the optical/UV/X-ray bands and we propose a model that explains the spectrum emitted in this broad energy range. In this model, the intrinsic emission originating in the warm skin of the accretion disk is responsible for the spectral component that is dominant in the softest X-ray range. The shape of reflected spectrum as well as Fe K line detected in hard X-rays require the reflecting medium to be mildly ionized (xi~500). We identify this reflector with the warm skin of the disk and we show that the heating of the skin is consistent with the classical alpha P_{tot} prescription, while alpha P_{gas} option is at least two orders of magnitude too low to provide the required heating. We find that the mass of the central black hole is relatively small (M_BH~10^7- 10^8 Msun, which is consistent with the Broad Line Region mapping results and characteristic for NLS1 class.Comment: 22 pages, 10 figures, accepted to Ap

On the turbulent α\alpha-disks and the intermittent activity in AGN

We consider effects of the MHD turbulence on the viscosity during the evolution of the thermal-viscous ionization instability in the standard $\alpha$-accretion disks. We consider the possibility that the accretion onto a supermassive black hole proceeds through an outer standard accretion disk and inner, radiatively inefficient and advection dominated flow. In this scenario we follow the time evolution of the accretion disk in which the viscosity parameter $\alpha$ is constant throughout the whole instability cycle, as implied by the strength of MHD turbulence. We conclude that the hydrogen ionization instability is a promising mechanism to explain the intermittent activity in AGN.Comment: 13 pages, 9 figures; ApJ accepte

The Contribution of Particle Impact to the Production of Fe K Emission from Accreting Black Holes

The iron K line is perhaps the most important spectral diagnostic available in the study of accreting black holes. The line is thought to result from the reprocessing of external X-rays by the surface of the accretion disk. However, as is observed in the solar corona, illumination by energetic particles may also produce line emission. In principle, such a process may be uncorrelated with the observed X-rays and could explain some of the unexpected variability behavior of the Fe line. This paper compares predictions of iron K flux generated by impacting electrons and protons to that from photoionization. Non-thermal power-laws of electrons are considered as well as thermal distributions of electrons and virialized protons. The electrons are thought to originate in a magnetically dominated accretion disk corona, while the protons are considered in the context of a two phase (hot/cold) accretion scenario. In each case, the Fe K flux from particle impact is found to be < 1% of that produced by photoionization by a hard X-ray power-law (normalized to the same energy flux as the particles). Thus, the electrons or protons must strike the disk with 100--10,000 times more energy flux than radiation for particle impact to be a significant producer of Fe K flux. This situation is difficult to reconcile with the observations of hard X-ray spectra, or the proposed particle acceleration mechanisms in the accretion disk corona. Truncated accretion flows must be externally illuminated by hard X-rays in order to produce the Fe line, as proton impact is very inefficient in generating line emission. In contrast to the Sun, our conclusion is that, with the possible exception for localized regions around magnetic footpoints, particle impact will not be an important contributor to the X-ray emission in accreting black holes.Comment: 27 pages, 6 figures, ApJ accepte

A simple model for magnetic reconnection heated corona

We construct a simple model for a magnetic reconnection heated corona above a thin accretion disk in AGNs and Galactic black hole candidates(GBHCs). The basic assumptions are that (1) the magnetic reconnection heat is cooled down overwhelmingly by Compton scattering in the corona and that (2) thermal conduction is dominantly cooled by evaporation of the chromospheric plasma in the disk-corona interface before Compton cooling sets in. With these two basic equations as well as equi-partition of magnetic energy with gas energy in the disk, we can consistently determine the fraction of accretion energy dissipated in the corona without free parameters, and thus determine the temperature and all other quantities in both the corona and disk for given black hole mass and accretion rate. Then, we calculate the luminosity contributed from the disk and corona and the coronal flux weighted Compton $y$ parameter. It is found that, at a low luminosity (less than 0.1 Eddington luminosity) the spectrum is hard with energy spectral index of around 1, while at a high luminosity (larger than 0.1 Eddington luminosity) the spectrum can be either soft or hard. We also find that the situation is almost same for supermassive and stellar-mass black holes. These features are consistent with observations of AGNs and GBHCs.Comment: 8 pages including 2 figures. accepted by ApJ Lette

Accretion disk models and their X-ray reflection signatures. I. Local spectra

X-ray illumination of accretion disks is an invaluable diagnostic of the structure of these disks because of the associated iron K$\alpha$ emission. Here we point out that the resulting reflected spectra depend very sensitively on the geometry of the X-ray source, and that this fact can be efficiently used to test these models observationally. In particular, we discuss three different accretion disk geometries: the ``lamppost model'', accretion disks with magnetic flares, and the model with a full corona overlying a cold thin disk. We show that in the case of the lamppost model, unless the X-ray luminosity of the central source is larger than that of the cold disk by a factor of 10 or more, a significant fraction of iron in the ionized skin of the disk is in the hydrogen and helium-like ions. Because these ions have large fluorescence yields, the resulting reflected spectra look strongly ionized, with Equivalent Width (EW) of the line {\em increasing} with X-ray luminosity $L_x$ up to the maximum of $\sim 500$ eV. This situation contrasts to the magnetic flare model, where the large X-ray flux near flares completely ionizes the skin of the disk and thus the resulting spectra appear to be that from a neutral material. The line EW in this model {\em anti-correlates} with X-ray luminosity, and becomes arbitrarily small when $L_x$ is a good fraction of the Eddington luminosity. Finally, in the full corona case, due to the additional pressure and weight of the corona, the gas pressure (and its density) below the corona is always large enough to make the gas very cool and effectively neutral. No highly ionized skin forms in such a model. If the corona is Thomson thin, then EW of the line does not depend on the accretion disk or corona luminosities for the full corona model.Comment: submitted to ApJ April 28 2000; 9 pages of text plus 15 figure