Black Hole Candidate X-ray Binaries Research Articles

Analysis of the reflection spectra of MAXI J1535-571 in the hard and intermediate states

ABSTRACT We report results on the joint-fit of the NuSTAR and HXMT data for the black hole X-ray binary candidate MAXI J1535-571. The observations were obtained in 2017 when the source evolved through the hard, hard-intermediate, and soft-intermediate states over the rising phase of the outburst. After subtracting continuum components, X-ray reflection signatures are clearly showed in those observations. By modelling the relativistic reflection in detail, we find that the inner radius Rin is relatively stable with Rin ≲ 1.55Rg during the three states, which implies that the inner radius likely extends to the innermost stable circular orbit even in the bright hard state. When adopting Rin = RISCO, the spin parameter is constrained to be $0.985_{-0.004}^{+0.002}$ at 90 per cent confidence (statistical only). The best-fitting results reveal that the inclination of the inner accretion disc is ∼70–74 deg, which notably conflicts with the apparent orientation of the ballistic jet (≤45 deg). In addition, both the photon index and the electron temperature increase during the transition from hard to soft state. It seems that the corona evolves from dense low-temperature in the low/hard state to tenuous high-temperature after the state transition, which indicates that the state transition is accompanied by the evolution of the coronal properties.

A long-lived compact jet in the black hole X-ray binary candidate AT2019wey

AT2019wey is a transient discovered by the Asteroid Terrestrial-impact Last Alert System survey in December of 2019. Follow-up optical, radio, and X-ray observations led to classification of this source as a Galactic black hole X-ray binary candidate. We carried out one-epoch 6.7 GHz European Very Long Baseline Interferometry Network and two-epoch multi-frequency (1.6, 4.5, 6.7 GHz) Very Long Baseline Array observations within a year after its discovery. These observations reveal a fading and flat-spectrum radio source with no discernible motion. These features suggest the detection of a compact jet. The source appears resolved at milliarcsecond scales, and the source angular size versus frequency trend is consistent with scatter broadening. This allows us to constrain the lower limit of the source distance to 6 kpc if the scattering medium is in a Galactic spiral arm. For a source location at greater than 3 kpc, the estimated upper limit of the peculiar velocity suggests the asymmetric natal kick may have occurred during the black hole formation stage.

MeerKAT radio detection of the Galactic black hole candidate Swift J1842.5−1124 during its 2020 outburst

ABSTRACT Swift J1842.5−1124 is a transient Galactic black hole X-ray binary candidate, which underwent a new outburst in 2020 May. We performed multi-epoch MeerKAT radio observations under the ThunderKAT large survey programme, coordinated with quasi-simultaneous Swift/XRT X-ray observations during the outburst, which lasted nearly a month. We were able to make the first-ever radio detection of this black hole binary with the highest flux density of 229 ± 31 $\mu$Jy when the source was in the hard state, after non-detection in the radio band in the soft state which occurred immediately after its emergence during the new X-ray outburst. Therefore, its radio and X-ray properties are consistent with the disc-jet coupling picture established in other black hole X-ray binaries. We place the source’s quasi-simultaneous X-ray and radio measurements on the radio/X-ray luminosity correlation plane; two quasi-simultaneous radio/X-ray measurements separated by 11 d were obtained, which span ∼2 dex in the X-ray luminosity. If the source follows the black hole track in the radio/X-ray correlation plane during the outburst, it would lie at a distance beyond ∼5 kpc.

The MAVERIC Survey: Dynamical Origin of Radio Sources in Galactic Globular Clusters

Abstract We investigate potential correlations between radio source counts (after background corrections) of 22 Galactic globular clusters (GCs) from the MAVERIC survey and the stellar encounter rates (Γ) and masses (M) of the GCs. Applying a radio luminosity limit of , we take a census of radio sources in the core and those within the half-light radius of each cluster. By following a maximum likelihood method and adopting a simplified linear model, we find an unambiguous dependence of core radio source counts on Γ and/or M at 90% confidence, but no clear dependence of source counts within the half-light radius on either Γ or M. Five of the identified radio sources in GC cores above our adopted limit are millisecond pulsars or neutron star X-ray binaries, the dependence of which on Γ is well known, but another is a published black hole (BH) X-ray binary candidate, and 10 others are not identified. Accounting for these verified cluster members increases the significance of the correlation with M and/or Γ (to 99% confidence) for fits to core and half-light region source counts, while excluding a dependence on Γ alone at 90% (core) and 68% (half-light) confidence. This is consistent with published dynamical simulations of GC BH interactions that argue Γ will be a poor predictor of the distribution of accreting BHs in GCs. Future multiwavelength follow-up to verify cluster membership will enable stronger constraints on the dependence of radio source classes on cluster properties, promising a new view on the dynamics of BHs in GCs.

Optical Analysis and Modeling of HD96670, a New Black Hole X-Ray Binary Candidate

Abstract We report on optical observations and modeling of HD96670, a single-line spectroscopic binary in the Carina OB2 association. We collected 10 epochs of optical spectroscopy, and optical photometry on 17 nonconsecutive nights on the source. We construct a radial velocity curve from the spectra, and update the orbital period of the binary to be P = 5.28388 ± 0.00046 days. The spectra show oxygen and helium absorption, consistent with an O-type primary. We see no evidence for spectral lines from the secondary star in the binary. We model the optical light curve and radial velocity curve simultaneously using the Wilson–Devinney code and find a best-fit mass of for the primary, and for the secondary. An object of this mass is consistent with either a B-type star, or a black hole. Given that we see no absorption lines from the secondary, in combination with an observed hard power-law X-ray spectrum with Γ = 2.6 detected past 10 keV that may have been produced by wind accretion onto the secondary, we conclude that the secondary is most likely a black hole. We see asymmetrical helium lines with a shape consistent with the presence of a third star. If the secondary is indeed a black hole, this system would add to the small sample of only four possible black hole high mass X-ray binaries in the galaxy.

The appearance of a compact jet in the soft–intermediate state of 4U 1543−47

ABSTRACT Recent advancements in the understanding of jet–disc coupling in black hole candidate X-ray binaries (BHXBs) have provided close links between radio jet emission and X-ray spectral and variability behaviour. In ‘soft’ X-ray states the jets are suppressed, but the current picture lacks an understanding of the X-ray features associated with the quenching or recovering of these jets. Here, we show that a brief, ∼4 d infrared (IR) brightening during a predominantly soft X-ray state of the BHXB 4U 1543−47 is contemporaneous with a strong X-ray type B quasi-periodic oscillation, a slight spectral hardening and an increase in the rms variability, indicating an excursion to the soft–intermediate state (SIMS). This IR ‘flare’ has a spectral index consistent with optically thin synchrotron emission and most likely originates from the steady, compact jet. This core jet emitting in the IR is usually only associated with the hard state, and its appearance during the SIMS places the ‘jet line’ between the SIMS and the soft state in the hardness–intensity diagram for this source. IR emission is produced in a small region of the jets close to where they are launched (∼0.1 light-seconds), and the time-scale of the IR flare in 4U 1543−47 is far too long to be caused by a single, discrete ejection. We also present a summary of the evolution of the jet and X-ray spectral/variability properties throughout the whole outburst, constraining the jet contribution to the X-ray flux during the decay.

Studying the Reflection Spectra of the New Black Hole X-Ray Binary Candidate MAXI J1631−479 Observed by NuSTAR: A Variable Broad Iron Line Profile

Abstract We present results from the Nuclear Spectroscopic Telescope Array observations of the new black hole X-ray binary candidate MAXI J1631–479 at two epochs during its 2018–2019 outburst, which caught the source in a disk dominant state and a power-law dominant state. Strong relativistic disk reflection features are clearly detected, displaying significant variations in the shape and strength of the broad iron emission line between the two states. Spectral modeling of the reflection spectra reveals that the inner radius of the optically thick accretion disk evolves from <1.9 to 12 ± 1 r g (statistical errors at 90% confidence level) from the disk dominant to the power-law dominant state. Assuming in the former case that the inner disk radius is consistent with being at the innermost stable circular orbit, we estimate a black hole spin of a* > 0.94. Given that the bolometric luminosity is similar in the two states, our results indicate that the disk truncation observed in MAXI J1631–479 in the power-law dominant state is unlikely to be driven by a global variation in the accretion rate. We propose that it may instead arise from local instabilities in the inner edge of the accretion disk at high accretion rates. In addition, we find an absorption feature in the spectra centered at 7.33 ± 0.03 keV during the disk dominant state, which is evidence for the rare case that an extremely fast disk wind ( ) is observed in a low-inclination black hole binary, with the viewing angle of 29° ± 1° as determined by the reflection modeling.

Broadband X-Ray Spectral and Timing Analyses of the Black Hole Binary Candidate Swift J1658.2–4242: Rapid Flux Variation and the Turn-on of a Transient QPO

Abstract We report results from joint Nuclear Spectroscopic Telescope Array, Swift, and XMM-Newton observations of the newly discovered black hole X-ray binary candidate Swift J1658.2–4242 in the intermediate state. We observe a peculiar event in this source, with its X-ray flux rapidly decreasing by ∼45% in ∼40 s, accompanied by only subtle changes in the shape of the broadband X-ray spectrum. In addition, we find a sudden turn-on of a transient quasi-periodic oscillation (QPO) with a frequency of 6–7 Hz around the time of the flux change, and the total fractional rms amplitude of the power spectrum increases from ∼2% to ∼10%. The X-ray spectral and timing analyses indicate that the flux decrease is driven by intrinsic changes in the accretion flow around the black hole, rather than intervening material along the line of sight. In addition, we do not significantly detect any relativistic disk reflection component, indicating it is much weaker than previously observed while the source was in the bright hard state. We propose accretion disk instabilities triggered at a large disk radius as the origin of the fast transition in spectral and timing properties, and discuss possible causes of the unusual properties observed in Swift J1658.2–4242. The prompt flux variation detected along with the emergence of a QPO makes the event an interesting case for investigating QPO mechanisms in black hole X-ray binaries.

Bright Mini-outburst Ends the 12 yr Long Activity of the Black Hole Candidate Swift J1753.5–0127

Abstract We present optical, UV, and X-ray monitoring of the short orbital period black hole X-ray binary candidate Swift J1753.5–0127, focusing on the final stages of its 12 yr long outburst that started in 2005. From 2016 September onward, the source started to fade, and within 3 months, the optical flux almost reached the quiescent level. Soon after that, using a new proposed rebrightening classification method, we recorded a mini-outburst and a reflare in the optical light curves, peaking in 2017 February (V ∼ 17.0) and May (V ∼ 17.9), respectively. Remarkably, the mini-outburst has a peak flux consistent with the extrapolation of the slow decay before the fading phase preceding it. The following reflare was fainter and shorter. We found from optical colors that the temperature of the outer disk was ∼11,000 K when the source started to fade rapidly. According to the disk instability model, this is close to the critical temperature when a cooling wave is expected to form in the disk, shutting down the outburst. The optical color could be a useful tool to predict decay rates in some X-ray transients. We notice that all X-ray binaries that show mini-outbursts following a main outburst are short orbital period systems (<7 hr). In analogy with another class of short-period binaries showing similar mini-outbursts, the cataclysmic variables of the RZ LMi type, we suggest that mini-outbursts could occur if there is a hot inner disk at the end of the outburst decay.

A Wildly Flickering Jet in the Black Hole X-Ray Binary MAXI J1535–571

Abstract We report on the results of optical, near-infrared (NIR), and mid-infrared observations of the black hole X-ray binary candidate (BHB) MAXI J1535–571 during its 2017/2018 outburst. During the first part of the outburst (MJD 58004–58012), the source shows an optical–NIR spectrum that is consistent with an optically thin synchrotron power law from a jet. After MJD 58015, however, the source faded considerably, the drop in flux being much more evident at lower frequencies. Before the fading, we measure a dereddened flux density of ≳100 mJy in the mid-infrared, making MAXI J1535–571 one of the brightest mid-infrared BHBs known so far. A significant softening of the X-ray spectrum is evident contemporaneous with the infrared fade. We interpret it as being due to the suppression of the jet emission, similar to the accretion–ejection coupling seen in other BHBs. However, MAXI J1535–571 did not transition smoothly to the soft state, instead showing X-ray hardness deviations associated with infrared flaring. We also present the first mid-IR variability study of a BHB on minute timescales, with a fractional rms variability of the light curves of ∼15%–22%, which is similar to that expected from the internal shock jet model, and much higher than the optical fractional rms (≲7%). These results represent an excellent case of multiwavelength jet spectral timing and demonstrate how rich, multiwavelength time-resolved data of X-ray binaries over accretion state transitions can help in refining models of the disk–jet connection and jet launching in these systems.

THE DYNAMICAL EVOLUTION OF STELLAR BLACK HOLES IN GLOBULAR CLUSTERS

Our current understanding of the stellar initial mass function and massive star evolution suggests that young globular clusters may have formed hundreds to thousands of stellar-mass black holes, the remnants of stars with initial masses from $\sim 20 - 100\, M_\odot$. Birth kicks from supernova explosions may eject some black holes from their birth clusters, but most should be retained. Using a Monte Carlo method we investigate the long-term dynamical evolution of globular clusters containing large numbers of stellar black holes. We describe numerical results for 42 models, covering a range of realistic initial conditions, including up to $1.6\times10^6$ stars. In almost all models we find that significant numbers of black holes (up to $\sim10^3$) are retained all the way to the present. This is in contrast to previous theoretical expectations that most black holes should be ejected dynamically within a few Gyr. The main reason for this difference is that core collapse driven by black holes (through the Spitzer "mass segregation instability") is easily reverted through three-body processes, and involves only a small number of the most massive black holes, while lower-mass black holes remain well-mixed with ordinary stars far from the central cusp. Thus the rapid segregation of stellar black holes does not lead to a long-term physical separation of most black holes into a dynamically decoupled inner core, as often assumed previously. Combined with the recent detections of several black hole X-ray binary candidates in Galactic globular clusters, our results suggest that stellar black holes could still be present in large numbers in many globular clusters today, and that they may play a significant role in shaping the long-term dynamical evolution and the present-day dynamical structure of many clusters.

THE VARIABLE NEAR-INFRARED COUNTERPART OF THE MICROQUASAR GRS 1758–258

We present a new study of the microquasar system GRS 1758–258 in the near-infrared domain based on archival observations with the Hubble Space Telescope and the NICMOS camera. In addition to confirming the near-infrared counterpart pointed out by Munoz-Arjonilla et al., we show that this object displays significant photometric variability. From its average magnitudes, we also find that GRS 1758–258 fits well within the correlation between the optical/near-infrared and X-ray luminosity known to exist for low-mass, black-hole candidate X-ray binaries in a hard state. Moreover, the spectral energy distribution built using all radio, near-infrared, and X-ray data available closest in time to the NICMOS observations can be reasonably interpreted in terms of a self-absorbed radio jet and an irradiated accretion disk model around a stellar-mass black hole. All these facts match the expected behavior of a compact binary system and strengthen our confidence in the counterpart identification.

Relations between X-ray timing features and spectral parameters of Galactic black hole X-ray binaries

We present a study of correlations between spectral and timing parameters for a sample of black hole X-ray binary candidates. Data are taken from GX 339-4, H 1743-322, and XTE J1650-500, as the Rossi X-ray Timing Explorer (RXTE) observed complete outbursts of these sources. In our study we investigate outbursts that happened before the end of 2009 to make use of the high-energy coverage of the HEXTE detector and select observations that show a certain type of quasi-periodic oscillations (type-C QPOs). The spectral parameters are derived using the empirical convolution model simpl to model the Comptonized component of the emission together with a disc blackbody for the emission of the accretion disc. Additional spectral features, namely a reflection component, a high-energy cut-off, and excess emission at 6.4 keV, are taken into account. Our investigations confirm the known positive correlation between photon index and centroid frequency of the QPOs and reveal an anticorrelation between the fraction of up-scattered photons and the QPO frequency. We show that both correlations behave as expected in the "sombrero" geometry. Furthermore, we find that during outburst decay the correlation between photon index and QPO frequency follow a general track, independent of individual outbursts.

The faint 2011 outburst of the black hole X-ray binary candidate MAXI J1543−564

We report on a spectral-timing analysis of the black hole X-ray binary candidate MAXI J1543−564 during its 2011 outburst. All 99 pointed observations of this outburst obtained with the Rossi X-ray Timing Explorer (RXTE) were included in our study. We computed the fundamental diagrams commonly used to study black hole transients, and fitted power density and energy spectra to study the spectral and timing parameters along the outburst. The determination of timing parameters and hence of exact transitions between different states was hampered by the rather low count rate at which this outburst was observed. We detected two periods of exponential decay, one after the source was brightest, which was interrupted by several flares, and another one during the high soft state. The detection of these decays allowed us to obtain an estimate for the source distance of at least 8.5 kpc. This leaves two possible explanations for the observed low count rate; either the source has a distance similar to that of other black hole X-ray binary candidates and it is intrinsically faint, or it has a similar luminosity, but is located more than 12 kpc away from us. Furthermore, in the high/soft state the source spectrum appears to be completely disc dominated.

A new globular cluster black hole in NGC 4472

We discuss CXOU 1229410+075744, a new black hole candidate in a globular cluster in the elliptical galaxy NGC 4472. By comparing two Chandra observations of the galaxy, we find a source that varies by at least a factor of 4, and has a peak luminosity of at least 2 × 1039 erg s−1. As such, the source varies by significantly more than the Eddington luminosity for a single neutron star, and is a strong candidate for being a globular cluster black hole. The source's X-ray spectrum also evolves in a manner consistent with what would be expected from a single accreting stellar mass black hole. We consider the properties of the host cluster of this source and the six other strong black hole X-ray binary candidates, and find that there is suggestive evidence that black hole X-ray binary formation is favoured in bright and metal-rich clusters, just as is the case for bright X-ray sources in general.

Quenched millimetre emission from Cygnus X-1 in a soft X-ray state

We present millimetre wavelength observations of the black hole candidate X-ray binary Cygnus X-1 which indicate a suppression, or quenching, of the emission as the source switches to a softer X-ray state. Combining the data with those for another black hole candidate, XTE J1118+480, we demonstrate that the millimetre emission shows the same coupling to X-rays as the radio emission, although with a much stronger sensitivity to spectral shape. We therefore confirm the association of the millimetre emission with the jets in low/hard state black hole candidate X-ray binaries.

Jet dominated states in X-ray binaries

Abstract We demonstrate that at relatively low mass accretion rates, black hole candidate (BHC) X-ray binaries (XRBs) should enter ‘jet-dominated’ states, in which the majority of the liberated accretion power is in the form of a (radiatively inefficient) jet and not dissipated as X-rays in the accretion flow. This result follows from the empirically established non-linear relation between radio and X-ray power from low/hard state BHC XRBs, which we assume also to hold for neutron star (NS) XRBs. Conservative estimates of the jet power indicate that all BHC XRBs in ‘quiescence’ should be in this jet-dominated regime. In combination with an additional empirical result, namely that BHC XRBs are more ‘radio loud’ than NS XRBs, we find that in quiescence NS XRBs should be up to two orders of magnitude more luminous in X-rays than BHC XRBs, without requiring any significant advection of energy into a black hole. This ratio is as observed, and such observations should therefore no longer be considered as direct evidence for the existence of black hole event horizons. Furthermore, even if BHCs do contain black holes with event horizons, this work demonstrates that there is no requirement for the advection of significant amounts of accretion energy across the horizon.

A dipping black hole X-ray binary candidate in NGC 55

XMM-Newton EPIC observations have revealed a bright point-like X-ray source in the nearby Magellanic-type galaxy NGC 55. At the distance of NGC 55, the maximum observed X-ray luminosity of the source, designated as XMMU J001528.9-391319, is L(x) ~ 1.6 x 10^{39} erg/s, placing the object in the ultraluminous X-ray source (ULX) regime. The X-ray lightcurve exhibits a variety of features including a significant upward drift over the 60 ks observation. Most notably a series of X-ray dips are apparent with individual dips lasting for typically 100-300 s. Some of these dips reach almost 100 percent diminution of the source flux in the 2.0-4.5 keV band. The EPIC CCD spectra can be modelled with two spectral components, a very soft powerlaw continuum (Gamma ~ 4) dominant below 2 keV, plus a multi-colour disc (MCD) component with an inner-disc temperature kT ~ 0.8 keV. The observed upperward drift in the X-ray flux can be attributed to an increase in the level of the MCD component, whilst the normalisation of the powerlaw continuum remains unchanged. The dipping episodes correspond to a loss of signal from both spectral components, although the blocking factor is at least a factor two higher for the MCD component. XMMU J001528.9-391319 can be considered as a candidate black-hole binary (BHB) system. A plausible explanation of the observed temporal and spectral behaviour is that we view the accretion disc close to edge-on and that, during dips, orbiting clumps of obscuring material enter our line of sight and cause significant blocking or scattering of the hard thermal X-rays emitted from the inner disc. In contrast, the more extended source of the soft powerlaw flux is only partially covered by the obscuring matter during the dips.

X-ray time lags from a pivoting power law in black holes

Most black hole candidate X-ray binaries show Fourier time lags between softer and harder X-rays. The hard photons seem to arrive up to a few ms after the soft for a given Fourier frequency of the perturbation. The energy dependence of the time lags has a roughly logarithmic behavior. Up to now most theories fail to explain the observed magnitude and Fourier frequency dependence of the lags or fail other statistical tests. We show that the time lags can arise from a simple pivoting power law model, which creates the logarithmic dependence on the photon energy at once. A pivoting power law arises naturally from jet/synchrotron models for the X-ray emission, but may also be applicable to corona models. A hint to the coherence features of the light-curves can be obtained from the power spectral density, which can be decomposed into a few broad Lorentzians that could arise from a couple of strongly damped oscillators with low quality factors below one. Using small variations of the power law index for each Lorentzian separately the lags can be derived analytically. They show the correct Fourier frequency dependence of the time lags. If one assumes variations of the power law index by ±0.2 the model can account for the observed magnitude of the time lags in Cyg X-1. The model can also be applied to TeV blazars, where a pivoting power law and hard lags have been observed directly in some cases. As a further test we calculated the cross- and auto-correlation functions for our model, which also show qualitatively the observed behavior. The auto-correlation function for higher energies has a narrower peak than at lower energies and the cross-correlation function is asymmetric but peaks nearly at zero. The coherence function for the model is in agreement with the observed data in the Fourier regime, where the model is valid.

X‐Ray Spectral Properties of Low‐Mass X‐Ray Binaries in Nearby Galaxies

We investigated the X-ray spectral properties of a collection of low-mass X-ray binaries (LMXBs) within a sample of 15 nearby early-type galaxies observed with Chandra. We find that the spectrum of the sum of the sources in a given galaxy is remarkably similar from galaxy to galaxy when only sources with X-ray luminosities less than 10^39 ergs/s (0.3-10 keV) are considered. Fitting these lower luminosity sources in all galaxies simultaneously yielded a best-fit power law exponent of Gamma = 1.56 +/- 0.02 (or kT_brem = 7.3 +/- 0.3 keV). This is the tightest constraint to date on the spectral properties of LMXBs in external galaxies. There is no apparent difference in the spectral properties of LMXBs that reside inside and outside globular clusters. We demonstrate how the uniformity of the spectral properties of LMXBs can lead to more accurate determinations of the temperature and metallicity of the hot gas in galaxies. Although few in number in any given galaxy, sources with luminosities of 1-2 x 10^39 ergs/s are present in 10 of the galaxies. The spectra of these luminous sources are softer than the spectra of the rest of the sources, and are consistent with the spectra of Galactic black hole X-ray binary candidates when they are in their very high state. The spectra of these sources are very different than those of ultraluminous X-ray sources (ULXs) found within spiral galaxies, suggesting that the two populations of X-ray luminous objects have different formation mechanisms. The number of sources with apparent luminosities above 2 x 10^39 ergs/s is equal to the number of expected background AGN and thus appear to not be associated with the galaxy, indicating that very luminous sources are absent or very rare in early-type galaxies. (Abridged)