Galactic Stellar-mass Black Hole Research Articles

Mining for Candidates of Galactic Stellar-mass Black Hole Binaries with LAMOST

Abstract We study the prospects of searching for black hole (BH) binary systems with a stellar-mass BH and a non-compact visible companion, by utilizing the spectroscopic data of the Large Sky Area Multi-object Fiber Spectroscopic Telescope (LAMOST). We simulate the Galactic BH binary population and determine its optical visibility by considering the stellar synthetic population model and the distributions of binary orbital parameters. By convolving the visibility of BH binaries with the LAMOST detection sensitivity, we predict that ≳400 candidate BH binaries can be found by the low-resolution, non-time-domain survey, and ∼50–350 candidates by the LAMOST ongoing medium-resolution, time-domain spectroscopic survey. Most of the candidates are short-period (0.2–2 days) binaries with M-, K-, G-, or F-type companions, in which ∼47% have a mass function (the lower limit of the BH mass) larger than 3 M ⊙. By complementing the LAMOST spectroscopic data with other photometric/spectroscopic surveys or follow-up observations, these candidates could be confirmed. Therefore, by exploring the LAMOST data, we can enlarge the sample of dynamically confirmed BH binaries significantly, which can improve our understanding of the mass distribution of BHs and the stellar evolution model.

Ultraluminous X-Ray Sources

We review observations of ultraluminous X-ray sources (ULXs). X-ray spectroscopic and timing studies of ULXs suggest a new accretion state distinct from those seen in Galactic stellar-mass black hole binaries. The detection of coherent pulsations indicates the presence of neutron-star accretors in three ULXs and therefore apparently super-Eddington luminosities. Optical and X-ray line profiles of ULXs and the properties of associated radio and optical nebulae suggest that ULXs produce powerful outflows, also indicative of super-Eddington accretion. We discuss models of super-Eddington accretion and their relationship to the observed behaviors of ULXs. We review the evidence for intermediate-mass black holes (IMBHs) in ULXs. We consider the implications of ULXs for super-Eddington accretion in active galactic nuclei, heating of the early Universe, and the origin of the black hole binary recently detected via gravitational waves.

THE SLIM-DISK STATE OF THE ULTRALUMINOUS X-RAY SOURCE IN M83

The transient ULX in M83 that went into outburst in or shortly before 2010 is still active. Our new XMM-Newton spectra show that it has a curved spectrum typical of the upper end of the high/soft state or slim-disk state. It appears to be spanning the gap between Galactic stellar-mass black holes and the ultraluminous state, at X-ray luminosities $\approx (1$-$3) \times 10^{39}$ erg s$^{-1}$ (a factor of two lower than in the 2010-2011 Chandra observations). From its broadened disk-like spectral shape at that luminosity, and from the fitted inner-disk radius and temperature, we argue that the accreting object is an ordinary stellar-mass black hole with $M \sim$$10$-$20 M_{\odot}$. We suggest that in the 2010-2011 Chandra observations, the source was seen at a higher accretion rate, resulting in a power-law-dominated spectrum with a soft excess at large radii.

Oscillating shocks in the low angular momentum flows as a source of variability of accreting black holes

We derive the conditions for shock formation in a quasi-spherical, slightly rotating flows. We verify the results of semi-analytical, stationary calculations with the time evolution studied by numerical hydro-simulations, and we study the oscillations of the shock position. We also study the behaviour of flows with varying specific angular momentum, where the 'hysteresis' type of loop is found when passing through the multiple sonic points region. Our results are in agreement with the timescales and shapes of the luminosity flares observed in Sgr A*. These models may also be applicable for the Galactic stellar mass black holes, like GX 339-4 or GRS 1915+105, where periodic oscillations of X-ray luminosity are detected.

A mass of less than 15 solar masses for the black hole in an ultraluminous X-ray source.

Most ultraluminous X-ray sources have a typical set of properties not seen in Galactic stellar-mass black holes. They have luminosities of more than 3×10(39)ergs per second, unusually soft X-ray components (with a typical temperature of less than about 0.3kiloelectronvolts) and a characteristic downturn in their spectra above about 5kiloelectronvolts. Such puzzling properties have been interpreted either as evidence of intermediate-mass black holes or as emission from stellar-mass black holes accreting above their Eddington limit, analogous to some Galactic black holes at peak luminosity. Recently, a very soft X-ray spectrum was observed in a rare and transient stellar-mass black hole. Here we report that the X-ray source P13 in the galaxy NGC7793 is in a binary system with a period of about 64 days and exhibits all three canonical properties of ultraluminous sources. By modelling the strong optical and ultraviolet modulations arising from X-ray heating of the B9Ia donor star, we constrain the black hole mass to be less than 15 solar masses. Our results demonstrate that in P13, soft thermal emission and spectral curvature are indeed signatures of supercritical accretion. By analogy, ultraluminous X-ray sources with similar X-ray spectra and luminosities of up to a few times 10(40)ergs per second can be explained by supercritical accretion onto massive stellar-mass black holes.

A self-consistent hybrid Comptonization model for broad-band spectra of accreting supermassive black holes

The nature of the broad-band spectra of supermassive accreting black holes in active galactic nuclei (AGNs) is still unknown. The hard X-ray spectra of Seyferts as well as of Galactic stellar-mass black holes (GBHs) are well represented by thermal Comptonization, but the origin of the seed photons is less certain. The MeV tails observed in GBHs provide evidence in favour of non-thermal electron tails and it is possible that such electrons are also present in the X-ray emitting regions of AGNs. Using simulations with the kinetic code that self-consistently models electron and photon distributions, we find that the power-law-like X-ray spectra in AGNs can be explained in terms of the synchrotron self-Compton radiation of hybrid thermal/non-thermal electrons, similarly to the hard/low state of GBHs. Under a very broad range of parameters the model predicts a rather narrow distribution of photon spectral slopes consistent with that observed from LINERs and Seyferts at luminosities less than 3 per cent of the Eddington luminosity. The entire infrared to X-ray spectrum of these objects can be described in terms of our model, suggesting a tight correlation between the two energy bands. We show that the recently found correlation between slope and the Eddington ratio at higher luminosities can be described by the increasing fraction of disc photons in the emitting region, which may be associated with the decreasing inner radius of the optically thick accretion disc. The increasing flux of soft photons is also responsible for the transformation of the electron distribution from nearly thermal to almost completely non-thermal. The softer X-ray spectra observed in narrow-line Seyfert galaxies may correspond to non-thermal Comptonization of the disc photons, predicting that no cutoff should be observed up to MeV energies in these sources, similarly to the soft-state GBHs.

Exploring the nature of the brightest hyper‐luminous X‐ray source

AbstractThe small subset of hyper‐luminous X‐ray sources with luminosities in excess of ∼1041 erg s–1 are hard to explain without the presence of an intermediate mass black hole, as significantly super‐Eddington accretion and/or very small beaming angles are required. The recent discovery of HLX‐1, the most luminous object in this class with a record breaking luminosity of ∼1042 erg s–1 in the galaxy ESO 243‐49, therefore currently provides some of the strongest evidence for the existence of intermediate mass black holes. HLX‐1 is almost an order of magnitude brighter than the other hyperluminous sources, and appears to exhibit X‐ray spectral and flux variability similar to Galactic stellar mass black hole X‐ray binaries. In this paper we review the current state of knowledge on this intriguing source and outline the results of multi‐wavelength studies from radio to ultra‐violet wavelengths, including imaging and spectroscopy of the recently identified optical counterpart obtained with the Very Large Telescope. These results continue to support an intermediate mass black hole in excess of 500 M⊙ (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

HIGH-ENERGY EMISSION AND ABSORPTION IN CYGNUS X-1

The high-mass microquasar Cygnus X-1 has been detected during a flaring state at very high energies, E > 200 GeV . The observation was performed by the Atmospheric Cherenkov Telescope MAGIC. It constitutes the first experimental evidence of very-high energy (VHE) emission produced by a Galactic stellar-mass black hole. The observed signal was detected in coincidence with an X–ray flare. The gamma-ray flare occurred when the compact object was located behind the companion star with respect to the line of sight (superior conjunction of the compact object). In this configuration the absorption of VHE photons by annihilation with the stellar photons is expected to be maximum. This suggests that the emission has been originated far above the compact object. The energy spectrum is well fitted by a relatively soft power-law. We present a model for the absorption and the emission of VHE gamma-rays in Cyg X-1. Detailed calculations of the gamma-ray opacity due to pair creation are provided and used to establish constraints on the emitting region. We propose that the high energy flare was the result of the interaction between the jet and a very dense clump from the stellar wind.

THE FIRST ACCURATE PARALLAX DISTANCE TO A BLACK HOLE

Using astrometric VLBI observations, we have determined the parallax of the black hole X-ray binary V404 Cyg to be 0.418 +/- 0.024 milliarcseconds, corresponding to a distance of 2.39 +/- 0.14 kpc, significantly lower than the previously accepted value. This model-independent estimate is the most accurate distance to a Galactic stellar-mass black hole measured to date. With this new distance, we confirm that the source was not super-Eddington during its 1989 outburst. The fitted distance and proper motion imply that the black hole in this system likely formed in a supernova, with the peculiar velocity being consistent with a recoil (Blaauw) kick. The size of the quiescent jets inferred to exist in this system is less than 1.4 AU at 22 GHz. Astrometric observations of a larger sample of such systems would provide useful insights into the formation and properties of accreting stellar-mass black holes.

Supermassive black holes in elliptical galaxies: switching from very bright to very dim

Relativistic outflows (mainly observed in the radio) are a characteristic feature of both Galactic stellar-mass black holes and supermassive black holes (SMBHs). Simultaneous radio and X-ray observations of Galactic sources have shown that the outflow is strong at low accretion rates, but it weakens dramatically or disappears completely at high accretion rates, manifesting structural changes in the accretion flow. It is reasonable to assume that SMBHs follow the same trend. For low-luminosity SMBHs in nearby elliptical galaxies and clusters, recent observations strongly suggest that the outflows play the central role in keeping the gas hot (mechanical feedback). If the outflow is quenched in SMBHs at high accretion rates similarly to the behaviour of Galactic sources, then the straightforward consequence is a relatively weak feedback of rapidly accreting SMBHs. We argue that elliptical galaxies and their central engines should then evolve through two stages. Early on, the central SMBH rapidly grows by accreting cooling gas at a near-Eddington rate with high radiative efficiency but with weak feedback on the infalling gas. This stage terminates when the black hole has grown to a sufficiently large mass that its feedback (radiative and/or mechanical), despite the low gas heating efficiency, is able to suppress gas cooling. After that the system switches to a stable state corresponding to passively evolving ellipticals, when the accretion rate and radiative efficiency are very low, but the gas heating efficiency is high and energy input from the relativistic outflow keeps the gas hot.