State Of Black Hole Candidates Research Articles

UNVEILING THE HIGH ENERGY TAIL OF 1E 1740.7-2942 WITHINTEGRAL

The microquasar 1E 1740.7-2942 is observed with Integral since Spring 2003. Here, we report on the source high energy behaviour by using the first three years of data collected with SPI and IBIS telescopes, taking advantage of the instruments complementarity. Light curves analysis showed two main states for 1E 1740.7-2942: the canonical low/hard state of black-hole candidates and a ``dim'' state, characterised by a ~ 20 times fainter emission, detected only below 50 keV and when summing more than 1Ms of data. For the first time the continuum of the low/hard state has been measured up to ~ 600 keV with a spectrum that is well represented by a thermal Comptonization plus an additional component necessary to fit the data above 200 keV. This high energy component could be related to non-thermal processes as already observed in other black-hole candidates. Alternatively, we show that a model composed by two thermal Comptonizations provides an equally representative description of the data: the temperature of the first population of electrons results as (kTe)_1 ~ 30 keV while the second, (kTe)_2, is fixed at 100 keV. Finally, searching for 511 keV line showed no feature, either narrow or broad, transient or persistent.

Simultaneous Multiwavelength Observations of the Low/Hard State of the X‐Ray Transient Source SWIFT J1753.5−0127

We report the results of simultaneous multiwavelength observations of the X-ray transient source SWIFT J1753.5-0127 performed with INTEGRAL, RXTE, NTT, REM, and VLA on 2005 August 10-12. The source, which underwent an X-ray outburst since 2005 May 30, was observed during the INTEGRAL Target of Opportunity program dedicated to new X-ray novae located in the Galactic halo. Broadband spectra and fast timing variability properties of SWIFT J1753.5-0127 are analyzed together with the optical, near-infrared, and radio data. We show that the source was significantly detected up to 600 keV with Comptonization parameters and timing properties typical of the so-called low/hard state of black hole candidates. We build a spectral energy distribution and show that SWIFT J1753.5-0127 does not follow the usual radio/X-ray correlation of X-ray binaries in the low/hard state. We give estimates of distance and mass. We conclude that SWIFT J1753.5-0127 belongs to the X-ray nova class and that it is likely a black hole candidate transient source of the Galactic halo that remained in the low/hard state during its main outburst. We discuss our results in the context of Comptonization and jet models.

Low-frequency, one-armed oscillations in black hole accretion flows obtained from direct 3D MHD simulations

AbstractWe present the results of global 3D MHD simulations of optically thin black hole accretion flows. The initial disk is embedded in a low density, spherical, isothermal halo and threaded by weak (β ≡ Pgas/Pmag = 100) toroidal magnetic field. General relativistic effects are simulated by using the pseudo-Newtonian potential. When the Maxwell stress in the innermost region of the disk is reduced due to the loss of magnetic flux or by decrease of disk temperature, inner torus is created around 4 – 10rs. We found that in such an inner torus, one-armed (m = 1) density enhancement grows and that the inner torus oscillates quasi-periodically. The oscillation period is about 0.1s when we assume a 10M⊙ black hole. This frequency agrees with the low-frequency QPOs observed in low/hard state of black hole candidates. The disk ejects winds whose opening angle is about 30 degree. The maximum velocity of the wind is about 0.05c.

X-ray Spectral Behavior in the Low/Hard States of Black Hole Candidates

We carried out systematic spectral analysis for 10 black hole candidates (BHCs) in the low/hard states, using archival RXTE and Beppo-SAX data which cover a broad energy range of 2–250 keV. For in total 156 observations, the simple analytic model, a power law with an exponentiall cutoff modified with a smeared edge model, were adopted. Consequently, we found that the photon index was distributed over 1.2–1.8 and the cutoff energy was more than 40 keV.We also found that there is the different behaviour between the photon index/cutoff energy and the luminosity at lower ahd higher luminosities with its value of 2×1037 erg s-1. For example, a clear anti-correlation is found between the cutoff energy and the luminosity when its value is more than 2×1037 erg s-1. In addition, we performed the same analysis for 1 neutron star (NS) and 1 active galactic nuclei (AGN) for comparison. No clear differences between BHCs, NS and AGN were found, which might suggest that the accretion mechanisms in the low/hard states are independent of the mass of the central object.

X-Ray Spectral Behavior in the Low/Hard States of BHCs

We carried out systematic analysis of 8 black hole candidates (BHCs) and 1 neutron star (NS) in the low/hard states. using archival RXTE data which cover an energy range of 2-250 keV. As a result, we found that the photonindex is distributed in the range of 1.2-1.8, and the cutoff energy is more than 40 keV. As the X-ray luminosity increases, both the photon index and the cutoff energy tend to become smaller. This may suggest that high energy electrons are cooled by inverse Compton scattering from soft photons. No clear differences between BHC and NS were found.

Radio/X-ray correlation in the low/hard state of GX 339–4

We present the results of a long-term study of the black hole candidate GX 339-4 using simultaneous radio (from the Australia Telescope Compact Array) and X-ray (from the Rossi X-ray Timing Explorer and BeppoSAX) observations performed between 1997 and 2000. We find strong evidence for a correlation between these two emission regimes that extends over more than three decades in X-ray flux, down to the quiescence level of GX 339-4. This is the strongest evidence to date for such strong coupling between radio and X-ray emission. We discuss these results in light of a jet model that can explain the radio/X-ray correlation. This could indicate that a significant fraction of the X-ray flux that is observed in the low-hard state of black hole candidates may be due to optically thin synchrotron emission from the compact jet.

The X-Ray Globular Cluster Population in NGC 1399

We report on the Chandra observations of the elliptical galaxy NGC 1399, concentrating on the X-ray sources identified with globular clusters (GCs). A large fraction of the 2-10 keV X-ray emission in the 8' × 8' Chandra image is resolved into point sources with luminosities ≥5 × 1037 ergs s-1. These sources are most likely low-mass X-ray binaries (LMXBs). In a region imaged by the Hubble Space Telescope, about 70% of the X-ray sources are located within GCs. This association suggests that in giant elliptical galaxies luminous X-ray binaries preferentially form in GCs. Many of the GC sources have super-Eddington luminosities (for an accreting neutron star), and their average luminosity is higher than the non-GC sources. The X-ray spectral properties of both GC and non-GC sources are similar to those of LMXBs in our Galaxy. Two of the brightest sources, one of which is in a GC, have an ultrasoft spectrum, similar to that seen in the high state of black hole candidates. The "apparent" super-Eddington luminosity in many cases may be due to multiple LMXB systems within individual GCs but with some of the most extremely luminous systems containing massive black holes.

Magnetic flares in accretion disc coronae and the spectral states of black hole candidates: the case of GX339-4

We examine the constraints that the observations of different spectral states displayed by Galactic black hole candidates impose on the properties of magnetic flares resulting from the reconnection of flux tubes that rise from the accretion disc into a corona because of the magnetic buoyancy (Parker) instability. Using observations of one of the best-studied examples, GX339-4, we identify the geometry and physical conditions characterizing each of these states. We find that, if in the soft state flaring occurs at small scaleheights above the accretion disc, a soft thermal-like spectrum, characteristic of this state, can result from the heating and consequent reradiation of the hard X-rays produced by such flares. The hard tail can then be produced by Comptonization of the soft radiation. Conversely, the hard state may result from a phase in which flares are triggered high above the underlying accretion disc and produce X-rays via Comptonization of either internal cyclo-synchrotron radiation or soft disc photons. The spectral characteristics of the different states are naturally accounted for by the choice of geometry: when flares are triggered high above the disc the system is photon-starved, hence the hard Comptonized spectrum of the hard state. Intense flaring close to the disc greatly enhances the local soft-photon field with the result that the spectrum softens. We interpret these two states as being related to two different phases of magnetic energy dissipation. We speculate that, in the soft state, Parker instability in the disc may favour the emergence of large numbers of relatively low-magnetic-field flux tubes. In the hard state, only intense magnetic fields become buoyant and magnetic loops are able to rise and expand in the coronal atmosphere. This possibility can also qualitatively account for the observed short time-scale variability and the characteristics of the X-ray-reflected component of the hard state.

Spectral Properties of Accretion Disks around Black Holes. II. Sub‐Keplerian Flows with and without Shocks

Close to a black hole, the density of the sub-Keplerian accreting matter becomes higher compared to a spherical flow due to the presence of a centrifugal barrier independent of whether or not a standing shock actually forms. This hot dense flow intercepts soft photons from a cold Keplerian disk and reprocesses them to form high energy X-rays and gamma rays. We study the spectral properties of various models of accretion disks where a Keplerian disk on the equatorial plane may or may not be flanked by a sub-Keplerian disk and the sub-Keplerian flow may or may not possess standing shocks. From comparison with the spectra, we believe that the observed properties could be explained better when both the components (Keplerian and sub-Keplerian) are simultaneously present close to a black hole, even though the sub-Keplerian halo component may have been produced out of the Keplerian disk itself at larger radii. We are able to understand soft and hard states of black hole candidates, properties of X-ray novae outbursts, and quasi-periodic oscillations of black hole candidates using these two component models. We fit spectra of X-ray novae GS1124-68 and GS2000+25 and satisfactorily reproduce the light curves of these objects.

Spectral states of Galactic black hole candidates: Results of observations with ART-P/Granat

Observations of Galactic black hole candidates performed with the ART-P telescope aboard Granat in 1990–1992 focused our attention on the problem of studying spectral variability of these sources as a function of the accretion rate. We show that in contrast with the current models of disk accretion onto a black hole the hard spectral state of the sources in which they exhibit a Cyg X-1 type hard X-ray spectrum always occurs at smaller accretion rate than the soft state. The arguments that hard X-rays detected during the “very high” and “low” states of black hole candidates have a distinct origin are presented.

Evidence for an Anomalous State in the Black Hole Candidate 4U 1630−47

We have reanalyzed two EXOSAT medium-energy observations during part of the 1984 X-ray outburst of the black hole candidate transient system 4U 1630-47. One observation (May 10) shows fast-timing properties (on timescales shorter than ~250 s) typical of those of other black hole candidates in their high states. The power spectrum shows a power law with index ~0.7 and a fractional rms amplitude (0.001-1 Hz) of ~2%. Another observation, obtained 1 month earlier (April 11), shows different fast-timing behavior. The power spectrum of this observation can be described by a power law with index ~1.5 and a fractional rms amplitude (0.001-1 Hz) of ~7%. This is inconsistent with the behavior during low, high, or very high states known to occur in most of the black hole candidates. Instead, the timing properties resemble those recently reported from the Advanced Satellite for Cosmology and Astrophysics observations of the superluminal black hole candidates GRS 1915+105 and GRO J1655-40 (X-ray Nova Sco 1994). This timing behavior may represent a new state of black hole candidates. In addition, the X-ray spectra of the three sources show similar properties. Since GRS 1915+105 and GRO J1655-40 exhibit superluminal jets, we propose that 4U 1630-47 is also a relativistic jet source.

Phase lags and coherence of X-ray variability in black hole candidates

The ‘low’ (hard or ‘non-thermal’) state of black hole candidates is sometimes modelled via an optically thick, hot Compton cloud that obscures a softer input source such as an accretion disc. In these models the observed output spectra consist entirely of photons reprocessed by the cloud, making it difficult to extract information about the input spectra. Recently, Miller has argued that the Fourier phase (or time) lag between hard and soft X-ray photons actually represents the phase lags intrinsic to the input source, modulo a multiplicative factor. The phase lags thus would be a probe of the input photon source. In this paper we examine this claim and find that, though true for the limited parameter space considered by Miller, the intrinsic phase lag disappears whenever the output photon energy is much greater than the input photon energy. The remaining time lags represent a ‘shelf’ due to differences between mean diffusion times across the cloud. As pointed out by Miller, the amplitude of this shelf — which is present even when the intrinsic time lags remain — is indicative of the size and temperature of the Compton cloud, and is a function of the two energies being compared. However, we find that, with previous instruments such as Ginga, the shelf, if present, was probably obscured by counting noise. A more sensitive measure of Compton cloud parameters may be obtainable from the coherence function, which is derived from the amplitude of the Fourier cross power spectral density. This function has been seen to decrease exponentially at high Fourier frequencies in Cygnus X-1. Coherence loss is characteristic of Compton clouds that undergo large variations of size and/or temperature on time-scales longer than about 10 s. We argue that observing phase lags and coherence at a variety of energies and Fourier frequencies is more likely to reveal information about the nature of Compton clouds than about their soft input sources.

On the Spectral Slopes of Hard X-Ray Emission from Black Hole Candidates

Abstract In the high state of black hole candidates, in which 2-10 keV luminosity is relatively high, the energy index of the power-law like hard X-ray emission above ˜ 10 keV is typically ˜ 1.5. On the other hand, in the low state, the hard X-ray energy index is 0.3–0.9. In this paper, we suggest that this difference of the hard X-ray spectral slopes may be due to two different Comptonization mechanisms. We propose that, in the high state, the hard component is governed by the Comptonization due to the bulk motion of the almost freely falling (convergent accretion) flow close to the black hole, rather than thermal Comptonization. The spectral slope of the hard component is insensitive to the disk accretion rate governing the soft component, hence is nearly invariant in spite of the soft component variations. The power-law component due to the bulk motion Comptonization has a sharp cut-off at around the electron rest mass energy, which is consistent with high energy observations of the high state. In the low state, the spectrum is formed due to thermal Comptonization of the low-frequency disk radiation by a sub-Keplerian component (possibly undergoing a centrifugally-supported shock) which is originated from the Keplerian disk. In the limit of low disk accretion rate, the power law index is uniquely determined by the mass accretion rate of the sub-Keplerian component.

Accretion disc turbulence and the X-ray power spectra of black hole high states

The high state of black hole candidates is characterized by a quasi- thermal emission component at $kT \sim 1$ keV. In addition, this state tends to have very low variability which indicates that it is relatively stable, at least on {\it short} time scales. Most models of the high state imply that the bulk of the emission comes from an optically thick accretion disc; therefore, this state may be an excellent laboratory for testing our ideas about the physics of accretion discs. In this work we consider the implications of assuming that accretion disc viscosity arises from some form of turbulence. Specifically, we consider the simple case of three dimensional hydrodynamic turbulence. It is found that the coupling of such turbulence to acoustic modes in the disc can alter the disc emission. We calculate the amplitude and frequencies of this modulation, and we express our results in terms of the X-ray power spectral density. We compare our calculations with observations of the black hole candidate GS 1124-683, and show that for certain parameters we can reproduce some of the high frequency power. We then briefly explore mechanisms for producing the low frequency power, and note the difficulty that a single variability mechanism has in reproducing the full range of observed variability. In addition, we outline ways in which future spacecraft missions -- such as USA and XTE -- can further constrain our model, especially at frequencies above $\sim 10^2$ Hz.

A kinematic model for black hole X-ray power spectra

view Abstract Citations (21) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Kinematic Model for Black Hole X-Ray Power Spectra Nowak, Michael A. Abstract The X-ray spectra of black hole candidates typically fall into one of three states: an 'off state,' characterized by low intensity and power-law spectrum; a 'high state,' characterized by little variability and a thermal-like spectrum peaking in the soft X-rays; and a 'low-state,' characterized by rapid X-ray variability and a spectrum with a power-law slope that extends to large X-ray energies. Recent observations of black hole candidates have revealed an additional 'very high state' characterized by rapid variability and a thermal-like spectrum with a hard tail. In this paper a simple kinematic model for the observed X-ray power spectra during the very high state of black hole candidates is presented. The model is based on viscous and thermal instabilities present in accretion disks. In this picture, the very high state is a transition phase between the high and low states that is characterized by a quasi-stable disk oscillating on the local instability timescales. The disk can be stabilized by a hot wind that in turn would feed a Compton cloud. The cloud would produce the hard tail in the observed X-ray spectra. The model is able to reproduce both the overall shape and the amplitude of the power spectral density and the observed frequency dependent lags between the hard and soft X-rays. In addition, the required energetics of a hot wind is consistent with the energy required to feed a Compton cloud. Specific comparisons between the model and data taken during the very high state of GX 339-4 are made. Publication: The Astrophysical Journal Pub Date: February 1994 DOI: 10.1086/173762 Bibcode: 1994ApJ...422..688N Keywords: Astronomical Models; Black Holes (Astronomy); Kinematics; Mathematical Models; Power Spectra; X Ray Astronomy; X Ray Spectra; X Ray Stars; Accretion Disks; Thermal Instability; Viscous Flow; Astrophysics; BLACK HOLE PHYSICS; X-RAYS: STARS full text sources ADS | data products SIMBAD (3)