State Of Black Hole Binaries Research Articles

The origin of long soft lags and the nature of the hard-intermediate state in black hole binaries

ABSTRACT Fast variability of the X-ray corona in black hole binaries can produce a soft lag by reverberation, where the reprocessed thermalized disc photons lag behind the illuminating hard X-rays. This lag is small, and systematically decreases with increasing mass accretion rate towards the hard–soft transition, consistent with a decreasing truncation radius between the thin disc and X-ray hot inner flow. However, the soft lag suddenly increases dramatically just before the spectrum becomes disc dominated (hard-intermediate state). Interpreting this as reverberation requires that the X-ray source distance from the disc increases dramatically, potentially consistent with switching to X-rays produced in the radio jet. However, this change in lag behaviour occurs without any clear change in hard X-ray spectrum, and before the plasmoid ejection event that might produce such a source (soft-intermediate state). Instead, we show how the soft lag can be interpreted in the context of propagation lags from mass accretion rate fluctuations. These normally produce hard lags, as the model has radial stratification, with fluctuations from larger radii modulating the harder spectra produced at smaller radii. However, all that is required to switch the sign is that the hottest Comptonized emission has seed photons that allow it to extend down in energy below the softer emission from the slower variable turbulent region from the inner edge of the disc. Our model connects the timing change to the spectral change, and gives a smooth transition of the X-ray source properties from the bright hard state to the disc-dominated states.

Retraction of: Transient jet ejections associated with limit-cycle behaviors in the very high state of black hole binaries

Retraction of: Transient jet ejections associated with limit-cycle behaviors in the very high state of black hole binaries

Retracted: Transient jet ejections associated with limit-cycle behaviors in the very high state of black hole binaries

Abstract The ejection mechanism of transient relativistic jets from black hole binaries is studied. Based on observations of the limit-cycle behaviors of the superluminal jet source GRS 1915+105, we infer that transient jet ejections could happen just after the slim disk emerging from the standard disk at some distance runs over the standard disk and reaches the vicinity of the central black hole. The standard disk releases about half of the gravitational energy in the course of the accretion, but the released radiative energy could be absorbed by the optically thick slim disk covering the standard disk in this situation. Then, since the inward motion of the slim disk is much faster than that of the standard disk, a quantity of energy released by an amount of gas in the standard disk is received by the much smaller amount of gas in the slim disk. As a result, the energy per mass received by the slim disk is expected to be greatly amplified and is estimated to become highly relativistic. Since the energy is much larger than the gravitational energy, the height of the slim disk could increase significantly. Hence, the innermost part of the slim disk from which almost all the angular momentum has been transferred outward could have a much larger height than the black hole size, so they could collide with one another around the central axis of the disk, turning to an outward flow along the axis normal to the disk plane. The flow in this direction can be approximated to be that through the de Laval nozzle and could become supersonic near the distance where the flow has the smallest cross-section.

A full spectral-timing model to map the accretion flow in black hole binaries: the low/hard state of MAXI J1820+070

ABSTRACT The nature and geometry of the accretion flow in the low/hard state of black hole binaries is currently controversial. While most properties are generally explained in the truncated disc/hot inner flow model, the detection of a broad residual around the iron line argues for strong relativistic effects from an untruncated disc. Since spectral fitting alone is somewhat degenerate, we combine it with the additional information in the fast X-ray variability and perform a full spectral-timing analysis for NICER and NuSTAR data on a bright low/hard state of MAXI J1820+070. We model the variability with propagating mass accretion rate fluctuations by combining two separate current insights: that the hot flow is spectrally inhomogeneous, and that there is a discontinuous jump in viscous time-scale between the hot flow and variable disc. Our model naturally gives the double-humped shape of the power spectra, and the increasing high-frequency variability with energy in the second hump. Including reflection and reprocessing from a disc truncated at a few tens of gravitational radii quantitatively reproduces the switch in the lag-frequency spectra, from hard lagging soft at low frequencies (propagation through the variable flow) to the soft lagging hard at the high frequencies (reverberation from the hard X-ray continuum illuminating the disc). The viscous time-scale of the hot flow is derived from the model, and we show how this can be used to observationally test ideas about the origin of the jet.

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 machine learning approach for classification of accretion states of black hole binaries

ABSTRACT In this paper, we employ Machine Learning algorithms on multimission observations for the classification of accretion states of outbursting black hole X-ray binaries for the first time. Archival data from RXTE, Swift, MAXI, and AstroSat observatories are used to generate the hardness intensity diagrams (HIDs) for outbursts of the sources XTE J1859+226 (1999 outburst), GX 339−4 (2002, 2004, 2007, and 2010 outbursts), IGR J17091−3624 (2016 outburst), and MAXI J1535−571 (2017 outburst). Based on variation of X-ray flux, hardness ratios, presence of various types of quasi-periodic oscillations (QPOs), photon indices, and disc temperature, we apply clustering algorithms like K-Means clustering and Hierarchical clustering to classify the accretion states (clusters) of each outburst. As multiple parameters are involved in the classification process, we show that clustering algorithms club together the observations of similar characteristics more efficiently than the ‘standard’ method of classification. We also infer that K-Means clustering provides more reliable results than Hierarchical clustering. We demonstrate the importance of the classification based on machine learning by comparing it with results from ‘standard’ classification.

Comparing X-ray color selection in separating X-ray binary classes using color-color-intensity diagrams

X-ray binaries exhibit a wide range of properties but there are few accepted methods to determine the nature of the compact object. Color-Color-Intensity diagrams have been suggested as a means of distinguishing between systems containing black holes from those containing neutron stars. However, this technique has been verified with data from only one instrument (RXTE/ASM) with a single set of X-ray colors defined using data available only in pre-determined energy bands. We test a selection of X-ray colors with a more sensitive instrument to determine the reliability of this method. We use data from the MAXI/Gas Slit Camera, which allows users to specify energy-bands. We test X-ray colors that have been previously defined in the literature as well as ones that we define specifically in this paper. A representative set of systems are used to construct Color-Color-Intensity diagrams in each set of colors to determine which are best for separating different classes. For studying individual sources certain bands are more effective than others. For a specified energy range, the separation of soft states in black hole binaries was possible only where both soft and hard colors included information from the lowest energy band. We confirm that Color-Color-Intensity diagrams can distinguish between systems containing black holes or neutron stars in all X-ray colors tested; this suggests an universality in the accretion processes governing these different classes. We suggest possible physical processes driving different classes of X-ray binaries to different locations in Color-Color-Intensity diagrams.

Reverberation reveals the truncated disc in the hard state of GX 339-4

The nature and geometry of the hard state in black hole binaries is controversial. The broadband continuum spectrum and fast variability properties can be explained in a model where the inner disc evaporates into a geometrically thick, hot flow. However these models are challenged by the persistent detection of an extremely broad iron line, which requires that the disc extends down to the last stable orbit of a high spin black hole. This line width can be considerably reduced if the Comptonisation continuum is multi-component rather than single temperature, but such models are highly degenerate. Here we show a specific model of a radially stratified continuum coupled to a model of propagating fluctuations, fit to some of the best hard state data from GX 339-4. This full spectral-timing model can fit the time averaged spectrum, the power spectra in different energy bands, and the frequency dependent lags between these bands. For the first time we also include disc reverberation and show that this same spectral-timing model successfully \textit{predicts} the lag-energy spectra on all timescales. This gives a more robust method to determine the inner radius of the disc, which is of order $20~R_g$, i.e. significantly truncated. This opens up the way to use the fast variability spectral-timing data to trace the source geometry of black hole binaries in all states.

Numerical models of VHE emission by magnetic reconnection in X-ray binaries: GRMHD simulations and Monte Carlo cosmic-ray emission

AbstractGalactic microquasars have been detected at very-high-energies (VHE) (> 100 GeV) and the particle acceleration mechanisms that produce this emission are not yet well-understood. Here we investigate a hadronic emission scenario where cosmic-rays (CRs) are accelerated in magnetic reconnection events by the turbulent, advected-dominated accretion flow (ADAF) believed to be present in the hard state of black hole binaries. We present Monte Carlo simulations of CR emission plus γ-γ and inverse Compton cascades, injecting CRs with a total energy consistent with the magnetic energy of the plasma. The background gas density, magnetic, and photon fields where CRs propagate and interact are modelled with general relativistic (GR), magneto-hydrodynamical simulations together with GR radiative transfer calculations. Our approach is applied to the microquasar Cygnus X-1, where we show a model configuration consistent with the VHE upper limits provided by MAGIC collaboration.

A unified accretion-ejection paradigm for black hole X-ray binaries

Context. X-ray binaries display cycles of strong activity during which their luminosity varies across several orders of magnitude. The rising phase is characterized by a hard X-ray spectrum and radio emission due to jets (hard state), whereas the declining phase displays a soft X-ray spectrum and no jet signature (soft state). The origin of these correlated accretion-ejection and spectral hysteresis cycles is still under investigation. Aims. We elaborate on the previously described paradigm, where the increase and decrease in the disk accretion rate is accompanied by a modification of the disk magnetization μ, which in turn determines the dominant torque allowing accretion. For μ greater than some threshold, the accretion flow produces jets that vertically carry away the disk angular momentum (jet-emitting disk, or JED mode), whereas for smaller μ, the turbulence transfers the disk angular momentum outward in the radial direction (standard accretion disk, or SAD mode). The goal of this paper is to investigate the spectral signatures of the JED configurations. Methods. We have developed a two-temperature plasma code that computes the disk local thermal equilibria, taking into account the advection of energy in an iterative way. Our code addresses optically thin/thick transitions, both radiation and gas supported regimes, and computes in a consistent way the emitted spectrum from a steady-state disk. The optically thin emission is obtained using the BELM code, which provides accurate spectra for bremsstrahlung and synchrotron emission processes as well as for their local Comptonization. Results. For a range in radius and accretion rates, JEDs exhibit three thermal equilibria, one thermally unstable and two stable: a cold (optically thick and geometrically thin) and a hot (optically thin and geometrically thick) equilibrium. From the two thermally stable solutions, a hysteresis cycle is naturally obtained. However, standard outbursting X-ray binary cycles cannot be reproduced. Another striking feature of JEDs is their ability to reproduce luminous hard states. At high accretion rates, JEDs become slim, where the main cooling is advection. Conclusions. When the loss of angular momentum and power in jets is consistently taken into account (JED mode), accretion disks have spectral signatures that are consistent with hard states, up to high luminosities. When no jet is present (SAD mode), the spectral signature is consistent with the soft state. These two canonical spectral states of black hole binaries can be explained in terms of two completely different dynamical solutions, namely JED and SAD. The observed spectral cycles can therefore be directly understood in terms of dynamical transitions from one accretion mode to another. These transitions must involve states where some regions emit jets and others do not, however, which argues for hybrid disk configurations.

Systematic analysis of low/hard state RXTE spectra of GX 339–4 to constrain the geometry of the system

One of the popular models for the low/hard state of black hole binaries is that the standard accretion disk is truncated and the hot inner region produces, via Comptonization, hard X-ray flux. This is supported by the value of the high energy photon index, which is often found to be small, ∼ 1.7(< 2), implying that the hot medium is starved of seed photons. On the other hand, the suggestive presence of a broad relativistic Fe line during the hard state would suggest that the accretion disk is not truncated but extends all the way to the innermost stable circular orbit. In such a case, it is a puzzle why the hot medium would remain photon starved. The broad Fe line should be accompanied by a broad smeared reflection hump at ∼ 30 keV and it may be that this additional component makes the spectrum hard and the intrinsic photon index is larger, i.e. >2. This would mean that the medium is not photon deficient, reconciling the presence of a broad Fe line in the observed hard state. To test this hypothesis, we have analyzed the RXTE observations of GX 339–4 from the four outbursts during 2002–2011 and identify observations when the system was in the hard state and showed a broad Fe line. We have then attempted to fit these observationswith models,which include smeared reflection, to understandwhether the intrinsic photon index can indeed be large. We find that, while for some observations the inclusion of reflection does increase the photon index, there are hard state observations with a broad Fe line that have photon indices less than 2.

Modelling the energy dependence of black hole binary flows

We build a full spectral-timing model for the low/hard state of black hole binaries assuming that the spectrum of the X-ray hot flow can be produced by two Comptonisation zones. Slow fluctuations generated at the largest radii/softest spectral region of the flow propagate down to modulate the faster fluctuations produced in the spectrally harder region close to the black hole. The observed spectrum and variability are produced by summing over all regions in the flow, including its emission reflected from the truncated disc. This produces energy-dependent Fourier lags qualitatively similar to those in the data. Given a viscous frequency prescription, the model predicts Fourier power spectral densities and lags for any energy bands. We apply this model to archival RXTE data from Cyg X-1, using the time-averaged energy spectrum together with an assumed emissivity to set the radial bounds of the soft and hard Comptonisation regions. We find that the power spectra cannot be described by any smooth model of generating fluctuations, instead requiring that there are specific radii in the flow where noise is preferentially produced. We also find fluctuation damping between spectrally distinct regions is required to prevent all the variability power generated at large radii being propagated into the inner regions. Even with these additions, we can fit either the power spectra at each energy, or the lags between energy bands, but not both. We conclude that either the spectra are more complex than two zone models, or that other processes are important in forming the variability.

Hard X-ray variability of V404 Cygni during the 2015 outburst

Aims. Hard X-ray spectra of black hole binaries (BHB) are produced by Comptonization of soft seed photons by hot electrons near the black hole. The slope of the resulting ray spectra is governed by two main parameters: the electron temperature and the optical depth of the emitting plasma. Given the extreme brightness of V404~Cyg during the 2015 outburst, we aim to constrain the source spectral properties using an unprecedented time resolution in hard X-rays, and to monitor their evolution over the outburst. Methods. We have extracted and analyzed 602 X-ray spectra of V404 Cyg obtained by the IBIS/ISGRI instrument on-board INTEGRAL during the 2015 June outburst, using effective integration times ranging between 8 and 176000 seconds. We have fit the resulting spectra in the 20-200 keV energy range. Results. We find that while the light curve and soft X-ray spectra of V404 Cyg are remarkably different from those of other BHBs, the spectral evolution of V404~Cyg in hard X-rays and the relations between the spectral parameters are consistent with those observed in other BHBs. We identify a hard branch} where the electron temperature is anti-correlated with the hard X-ray flux, and a soft flaring branch where the relation reverses. In addition, we find that during long X-ray plateaus detected at intermediate fluxes, the thermal Comptonization models fail to describe the spectra. However, the statistics improve if we allow the absorption to vary freely in the fits to these spectra. Conclusions. We conclude that the hard branch in V404 Cyg is analogous to the canonical hard state of BHBs. V404~Cyg never seems to enter the canonical soft state, although the soft flaring branch bears resemblance to the BHB intermediate state and ultra-luminous state. The X-ray plateaus are likely the result of absorption by a Compton-thick outflow which reduces the observed flux by a factor of about 10.[abridged]

Dark jets in the soft X-ray state of black hole binaries?

X-ray binary observations led to the interpretation that powerful compact jets, produced in the hard state, are quenched when the source transitions to its soft state. The aim of this paper is to discuss the possibility that a powerful dark jet is still present in the soft state. Using the black hole X-ray binaries GX339-4 and H1743-322 as test cases, we feed observed X-ray power density spectra in the soft state of these two sources to an internal shock jet model. Remarkably, the predicted radio emission is consistent with current upper-limits. Our results show that, for these two sources, a compact dark jet could persist in the soft state with no major modification of its kinetic power compared to the hard state.

Mass-scaling as a method to constrain outflows and particle acceleration from low-luminosity accreting black holes

The `fundamental plane of black hole accretion' (FP), a relation between the radio luminosities ($L_R$), X-ray luminosities ($L_X$), and masses ($M_{BH}$) of hard/quiescent state black hole binaries and low-luminosity active galactic nuclei, suggests some aspects of black hole accretion may be scale invariant. However, key questions still exist concerning the relationship between the inflow/outflow behaviour in the `classic' hard state and quiescence, which may impact this scaling. We show that the broadband spectra of A0620-00 and~\sgra~(the least luminous stellar mass/supermassive black holes on the FP) can be modelled simultaneously with a physically-motivated outflow-dominated model where the jet power and all distances are scaled by the black hole mass. We find we can explain the data of both A0620-00 and~\sgra~(in its non-thermal flaring state) in the context of two outflow-model scenarios: (1) a synchrotron-self-Compton dominated state in which the jet plasma reaches highly sub-equipartition conditions (for the magnetic field with respect to that of the radiating particles), and (2) a synchrotron dominated state in the fast-cooling regime in which particle acceleration occurs within the inner few gravitational radii of the black hole and plasma is close to equipartition. We show that it may be possible to further discriminate between models (1) and (2) through future monitoring of its submm/IR/X-ray emission, in particular via time lags between the variable emission in these bands.

A HARD X-RAY STUDY OF THE NORMAL STAR-FORMING GALAXY M83 WITH NuSTAR

ABSTRACT We present the results from sensitive, multi-epoch NuSTAR observations of the late-type star-forming galaxy M83 (d = 4.6 Mpc). This is the first investigation to spatially resolve the hard ( E &gt; 10 keV) X-ray emission of this galaxy. The nuclear region and ∼20 off-nuclear point sources, including a previously discovered ultraluminous X-ray source, are detected in our NuSTAR observations. The X-ray hardnesses and luminosities of the majority of the point sources are consistent with hard X-ray sources resolved in the starburst galaxy NGC 253. We infer that the hard X-ray emission is most likely dominated by intermediate accretion state black hole binaries and neutron star low-mass X-ray binaries (Z-sources). We construct the X-ray binary luminosity function (XLF) in the NuSTAR band for an extragalactic environment for the first time. The M83 XLF has a steeper XLF than the X-ray binary XLF in NGC 253, which is consistent with previous measurements by Chandra at softer X-ray energies. The NuSTAR integrated galaxy spectrum of M83 drops quickly above 10 keV, which is also seen in the starburst galaxies NGC 253, NGC 3310, and NGC 3256. The NuSTAR observations constrain any active galactic nucleus (AGN) to be either highly obscured or to have an extremely low luminosity of ≲1038 erg s−1 (10–30 keV), implying that it is emitting at a very low Eddington ratio. An X-ray point source that is consistent with the location of the nuclear star cluster with an X-ray luminosity of a few times 1038 erg s−1 may be a low-luminosity AGN but is more consistent with being an X-ray binary.

A magnetic model for low/hard state of black hole binaries

A magnetic model for the low/hard state (LHS) of two black hole X-ray binaries (BHXBs), H1743–322 and GX 339–4, is proposed based on transport of the magnetic field from a companion into an accretion disk around a black hole (BH). This model consists of a truncated thin disk with an inner advection-dominated accretion flow (ADAF). The spectral profiles of the sources are fitted in agreement with the data observed at four different dates corresponding to the rising phase of the LHS. In addition, the association of the LHS with a quasi-steady jet is modeled based on transport of magnetic field, where the Blandford-Znajek (BZ) and Blandford-Payne (BP) processes are invoked to drive the jets from BH and inner ADAF. It turns out that the steep radio/X-ray correlations observed in H1743–322 and GX 339–4 can be interpreted based on our model.

INVESTIGATING THE CONNECTION BETWEEN QUASI-PERIODIC OSCILLATIONS AND SPECTRAL COMPONENTS WITH NuSTAR DATA OF GRS 1915+105

ABSTRACT Low-frequency quasi-periodic oscillations (QPOs) are commonly observed during the hard states of black hole binaries. Several studies have established various observational/empirical correlations between spectral parameters and QPO properties, indicating a close link between the two. However, the exact mechanism of generation of QPOs is not yet well understood. In this paper, we present our attempts to comprehend the connection between the spectral components and the low-frequency QPO (LFQPO) observed in GRS 1915+105 using the data from NuSTAR. Detailed spectral modeling as well as the presence of the LFQPO and its energy dependence during this observation have been reported by Miller et al. and Zhang et al., respectively. We investigate the compatibility of the spectral model and the energy dependence of the QPO by simulating light curves in various energy bands for small variation of the spectral parameters. The basic concept here is to establish the connection, if any, between the QPO and the variation of either a spectral component or a specific parameter, which in turn can shed some light on the origin of the QPO. We begin with the best-fit spectral model of Miller et al. and simulate the light curve by varying the spectral parameters at frequencies close to the observed QPO frequency in order to generate the simulated QPO. Furthermore we simulate similar light curves in various energy bands in order to reproduce the observed energy dependence of the rms amplitude of the QPO. We find that the observed trend of increasing rms amplitude with energy can be reproduced qualitatively if the spectral index is assumed to be varying with the phases of the QPO. Variation of any other spectral parameter does not reproduce the observed energy dependence.

The effects of thermodynamic stability on wind properties in different low-mass black hole binary states

We present a systematic theory-motivated study of the thermodynamic stability condition as an explanation for the observed accretion disk wind signatures in different states of low mass black hole binaries (BHB). The variability in observed ions is conventionally explained either by variations in the driving mechanisms or the changes in the ionizing flux or due to density effects, whilst thermodynamic stability considerations have been largely ignored. It would appear that the observability of particular ions in different BHB states can be accounted for through simple thermodynamic considerations in the static limit. Our calculations predict that in the disk dominated soft thermal and intermediate states, the wind should be thermodynamically stable and hence observable. On the other hand, in the powerlaw dominated spectrally hard state the wind is found to be thermodynamically unstable for a certain range of 3.55 <= log \xi <= 4.20. In the spectrally hard state, a large number of the He-like and H-like ions (including e.g. Fe XXV, Ar XVIII and S XV) have peak ion fractions in the unstable ionization parameter (\xi) range, making these ions undetectable. Our theoretical predictions have clear corroboration in the literature reporting differences in wind ion observability as the BHBs transition through the accretion states Lee et al. 2002; Miller et al. 2008; Neilsen & Lee 2009; Blum et al. 2010; Ponti et al. 2012; Neilsen & Homan 2012). While this effect may not be the only one responsible for the observed gradient in the wind properties as a function of the accretion state in BHBs, it is clear that its inclusion in the calculations is crucial to understanding the link between the environment of the compact object and its accretion processes.

Inhomogeneous accretion discs and the soft states of black hole X-ray binaries

Abstract Observations of black hole binaries (BHBs) have established a rich phenomenology of X-ray states. The soft states range from the low variability, accretion disc dominated thermal (TD) state to the higher variability, non-thermal steep power law (SPL) state. The disc component in all states is typically modelled with standard thin disc accretion theory. However, this theory is inconsistent with optical/UV spectral, variability and gravitational microlensing observations of active galactic nuclei (AGNs), the supermassive analogues of BHBs. An inhomogeneous disc (ID) model with large (≃0.4 dex) temperature fluctuations in each radial annulus can qualitatively explain all of these AGN observations. The inhomogeneity may be a consequence of instabilities in radiation-dominated discs, and therefore may be present in BHBs as well. We show that ID models can explain many features of the TD and SPL states of BHBs. The observed relationships between spectral hardness, disc fraction and rms variability amplitude in BHBs are reproduced with temperature fluctuations similar to those inferred in AGNs, suggesting a unified picture of luminous accretion discs across orders of magnitude in black hole mass. This picture can be tested with spectral fitting of ID models, X-ray polarization observations and radiation magnetohydrodynamic simulations. If BHB accretion discs are indeed inhomogeneous, only the most disc-dominated states (disc fraction ≳0.95) can be used to robustly infer black hole spin using current continuum fitting methods.