Black Hole Candidate Research Articles

The peculiar spectral evolution of the new X-ray transient MAXI J0637–430

ABSTRACT We studied the transient Galactic black hole candidate MAXI J0637−430 with data from Insight-HXMT, Swift, and XMM–Newton. The broad-band X-ray observations from Insight-HXMT help us constrain the power-law component. MAXI J0637–430 is located at unusually high Galactic latitude; if it belongs to the Galactic thick disc, we suggest a most likely distance ≲7 kpc. Compared with other black hole transients, MAXI J0637–430 is also unusual for other reasons: a fast transition to the thermal dominant state at the start of the outburst; a low peak temperature and luminosity (we estimate them at ≈0.7 keV and ≲0.1 times Eddington, respectively); a short decline time-scale; a low soft-to-hard transition luminosity (≲0.01 times Eddington). We argue that such properties are consistent with a small binary separation, short binary period (P ∼ 2 h), and low-mass donor star (M2 ∼ 0.2 M⊙). Moreover, spectral modelling shows that a single disc blackbody component is not a good fit to the thermal emission. Soft spectral residuals, and deviations from the standard $L_{\rm disc} \propto T_{\rm in}^4$ relation, suggest the need for a second thermal component. We propose and discuss various scenarios for such component, in addition to those presented in previous studies of this source. For example, a gap in the accretion disc between a hotter inner ring near the innermost stable orbit, and a cooler outer disc. Another possibility is that the second thermal component is the thermal plasma emission from an ionized outflow.

The Host Galaxy of the Recoiling Black Hole Candidate in 3C 186: An Old Major Merger Remnant at the Center of a z = 1 Cluster

3C 186, a radio-loud quasar at z = 1.0685, was previously reported to have both velocity and spatial offsets from its host galaxy, and has been considered as a promising candidate for a gravitational wave recoiling black hole triggered by a black hole merger. Another possible scenario is that 3C 186 is in an ongoing galaxy merger, exhibiting a temporary displacement. In this study, we present analyses of new deep images from the Hubble Space Telescope WFC3-IR and Advanced Camera for Surveys, aiming to characterize the host galaxy and test this alternative scenario. We carefully measure the light-weighted center of the host and reveal a significant spatial offset from the quasar core (11.1 ± 0.1 kpc). The direction of the confirmed offset aligns almost perpendicularly to the radio jet. We do not find evidence of a recent merger, such as a young starburst in disturbed outskirts, but only marginal light concentration in F160W at ∼30 kpc. The host consists of mature (≳200 Myr) stellar populations and one compact star-forming region. We compare with hydrodynamical simulations and find that those observed features are consistently seen in late-stage merger remnants. Taken together, those pieces of evidence indicate that the system is not an ongoing/young merger remnant, suggesting that the recoiling black hole scenario is still a plausible explanation for the puzzling nature of 3C 186.

Testing Horndeski Gravity from EHT Observational Results for Rotating Black Holes

The Event Horizon Telescope (EHT) collaboration recently unveiled the first image of the supermassive black hole M87*, which exhibited a ring of angular diameter θ d = 42 ± 3 μas, a circularity deviation of ΔC ≤ 0.1, and also inferred a black hole mass of M = (6.5 ± 0.7) × 109 M ⊙. This provides a new window onto tests of theories of gravity in the strong-field regime, including probes of violations of the no-hair theorem. It is widely believed that the Kerr metric describes astrophysical black holes, as encapsulated in the critical but untested no-hair theorem. Modeling Horndeski gravity black holes—with an additional hair parameter h besides the mass M and spin a of the Kerr black hole—as the supermassive black hole M87*, we observe that to be a viable astrophysical black hole candidate, the EHT result constrains the (a, h) parameter space. However, a systematic bias analysis indicates that rotating Horndeski black hole shadows may or may not capture Kerr black hole shadows, depending on the parameter values; the latter is the case over a substantial part of the constrained parameter space, allowing Horndeski gravity and general relativity to be distinguishable in the said space, and opening up the possibility of potential modifications to the Kerr metric.

Insight-HXMT Study of the Inner Accretion Disk in the Black Hole Candidate EXO 1846–031

We study the spectral evolution of the black hole candidate EXO 1846−031 during its 2019 outburst, in the 1–150 keV band, with the Hard X-ray Modulation Telescope. The continuum spectrum is well modeled with an absorbed disk-blackbody plus cutoff power law, in the hard, intermediate, and soft states. In addition, we detect an ≈6.6 keV Fe emission line in the hard intermediate state. Throughout the soft intermediate and soft states, the fitted inner disk radius remains almost constant; we suggest that it has settled at the innermost stable circular orbit (ISCO). However, in the hard and hard intermediate states, the apparent inner radius was unphysically small (smaller than the ISCO), even after accounting for the Compton scattering of some of the disk photons by the corona in the fit. We argue that this is the result of a high hardening factor, f col ≈ 2.0–2.7, in the early phases of the outburst evolution, well above the canonical value of 1.7 suitable for a steady disk. We suggest that the inner disk radius was already close to the ISCO in the low/hard state. Furthermore, we propose that this high value of the hardening factor in the relatively hard state was probably caused by the additional illuminating of the coronal irradiation onto the disk. Additionally, we estimate the spin parameter using the continuum-fitting method, over a range of plausible black hole masses and distances. We compare our results with the spin measured using the reflection-fitting method and find that the inconsistency of the two results is partly caused by different choices of f col.

Supplement: “An Isolated Mass-gap Black Hole or Neutron Star Detected with Astrometric Microlensing” (2022, ApJL, 933, L23)

This supplement provides supporting material for Lam et al. We briefly summarize past gravitational microlensing searches for black holes (BHs) and present details of the observations, analysis, and modeling of five BH candidates observed with both ground-based photometric microlensing surveys and Hubble Space Telescope astrometry and photometry. We present detailed results for four of the five candidates that show no or low probability for the lens to be a BH. In these cases, the lens masses are <2 M ⊙, and two of the four are likely white dwarfs or neutron stars. We also present detailed methods for comparing the full sample of five candidates to theoretical expectations of the number of BHs in the Milky Way (∼108).

Gaia May Detect Hundreds of Well-characterized Stellar Black Holes

Detection of black holes (BHs) with detached luminous companions (LCs) can be instrumental in connecting the BH properties with their progenitors since the latter can be inferred from the observable properties of the LC. Past studies showed the promise of Gaia astrometry in detecting BH–LC binaries. We build on these studies by (1) initializing the zero-age binary properties based on realistic, metallicity-dependent star formation history in the Milky Way (MW); (2) evolving these binaries to current epoch to generate realistic MW populations of BH–LC binaries; (3) distributing these binaries in the MW, preserving the complex age–metallicity-Galactic position correlations; (4) accounting for extinction and reddening using three-dimensional dust maps; and (5) examining the extended Gaia mission’s ability to resolve BH–LC binaries. We restrict ourselves to detached BH–LC binaries with orbital period P orb ≤ 10 yr such that Gaia can observe at least one full orbit. We find that (1) the extended Gaia mission can astrometrically resolve ∼30–300 detached BH–LC binaries depending on our assumptions of supernova physics and astrometric detection threshold; (2) Gaia’s astrometry alone can indicate BH candidates for ∼10–100 BH–LC binaries by constraining the dark primary mass ≥3 M ⊙; and (3) distributions of observables, including orbital periods, eccentricities, and component masses, are sensitive to the adopted binary evolution model and hence can directly inform binary evolution models. Finally, we comment on the potential to further characterize these BH binaries through radial velocity measurements and observation of X-ray counterparts.

A NuSTAR and Swift view of the hard state of MAXI J1813−095

ABSTRACT We present an analysis of the NuSTAR and Swift spectra of the black hole candidate MAXI J1813−095 in a failed-transition outburst in 2018. The NuSTAR observations show evidence of reflected emission from the inner region of the accretion disc. By modelling the reflection component in the spectra, we find a disc inner radius of Rin &amp;lt; 7 rg. This result suggests that either a slightly truncated disc or a non-truncated disc forms at a few per cent of the Eddington limit in MAXI J1813−095. Our best-fitting reflection models indicate that the geometry of the innermost accretion remains consistent during the period of NuSTAR observations. The spectral variability of MAXI J1813−095 from multi-epoch observations is dominated by the variable photon index of the Comptonization emission.

Recurrent X-ray flares of the black hole candidate in the globular cluster RZ 2109 in NGC 4472

We report the systematic analysis of the X-ray observations of the ultraluminous X-ray source XMMU J122939.7+075333 located in the globular cluster RZ 2109 in the Virgo galaxy NGC 4472. The inclusion of observations and time intervals ignored in previous works and the careful selection of extraction regions and energy bands have allowed us to identify new flaring episodes, in addition to those that made it one of the best black hole candidates in globular clusters. Although most observations are too short and sparse to recognize a regular pattern, the spacing of the three most recent X-ray flares is compatible with a recurrence time of ∼34 h. If confirmed by future observations, this behavior together with the soft spectrum of the X-ray flares would be strikingly similar to the quasiperiodic eruptions recently discovered in galactic nuclei. Following one of the possible interpretations of these systems and of a peculiar class of extragalactic X-ray transients, we explore the possibility that XMMU J122939.7+075333 might be powered by the partial disruption of a white dwarf by an intermediate-mass (M ∼ 700 M⊙) black hole.

The evolution of the high-frequency variability in the black hole candidate GRS 1915+105 as seen by RXTE

ABSTRACT GRS 1915+105 can show type-C quasi-periodic oscillations (QPOs) in the power density spectrum. A high-frequency QPO (HFQPO) at 67 Hz has been observed in this source, albeit less often than the type-C QPOs. Besides these features, GRS 1915+105 sometimes shows a broad bump in the power spectrum at around 30–150 Hz. We study the power spectra of GRS 1915+105 with the Rossi X-ray Timing Explorer when the source was in the χ class. We find that the rms amplitude of the bump depends strongly upon both the frequency of the type-C QPO and the hardness ratio, and is correlated with the corona temperature and anticorrelated with the radio flux at 15 GHz. The characteristic frequency of the bump is better correlated with a combination of the frequency of the type-C QPO and the hardness ratio than with the frequency of the type-C QPO alone. The rms amplitude of the bump generally increases with energy from ∼1–2 per cent at ∼3 keV to ∼10–15 per cent at ∼30 keV. We suggest that the bump and the HFQPO may be the same variability component but the properties of the corona affect the coherence of this variability, leading either to a HFQPO when the spectrum is in the relatively soft γ class, or to a bump when the spectrum is in the hard χ class. Finally, we discuss the anticorrelation between the rms amplitude of the bump and the radio flux in the context of the relation between the corona and the jet.

Signatures of supermassive black hole binaries on maser systems

ABSTRACT We illustrate a novel signature of black hole binaries from their effect on the kinematics of water maser emission in their environments. With the help of simulations, we establish the condition for clumps to mase based on their coherence lengths calibrated to those of the known maser galaxy NGC 4258. This is then used to identify masing clumps around a binary black hole system, and quantify the kinematic and spectral differences relative to the single black hole case. For some generic circumstances, blueshifted masers around a binary black hole are found to preferentially follow the Keplerian rotation curve observed in the single black hole case. The redshifted ones, however, are found to visibly deviate from this relation, and also display more scatter with a tendency towards lower absolute values of the velocity along the line of sight. The spectrum of the masers as a function of line-of-sight velocity also shows a double peaked structure, reminiscent of recent observations of systems such as Mrk 1. Our results motivate future prospects for identifying binary black hole candidates with the help of water maser emissions.

Population Properties of Gravitational-wave Neutron Star–Black Hole Mergers

Abstract Over the course of the third observing run of the LIGO–Virgo–KAGRA Collaboration, several gravitational-wave (GW) neutron star–black hole (NSBH) candidates have been announced. By assuming that these candidates are real signals with astrophysical origins, we analyze the population properties of the mass and spin distributions for GW NSBH mergers. We find that the primary BH mass distribution of NSBH systems, whose shape is consistent with that inferred from the GW binary BH (BBH) primaries, can be well described as a power law with an index of α = 4.8 − 2.8 + 4.5 plus a high-mass Gaussian component peaking at ∼ 33 − 9 + 14 M ⊙ . The NS mass spectrum could be shaped as a nearly flat distribution between ∼1.0 and 2.1 M ⊙. The constrained NS maximum mass agrees with that inferred from NSs in our Galaxy. If GW190814 and GW200210 are NSBH mergers, the posterior results of the NS maximum mass would be always larger than ∼2.5 M ⊙ and significantly deviate from that inferred in Galactic NSs. The effective inspiral spin and effective precession spin of GW NSBH mergers are measured to potentially have near-zero distributions. The negligible spins for GW NSBH mergers imply that most events in the universe should be plunging events, which support the standard isolated formation channel of NSBH binaries. More NSBH mergers to be discovered in the fourth observing run would help to more precisely model the population properties of cosmological NSBH mergers.

An Ultraluminous Supersoft Source in a Dwarf Galaxy of A85: An Intermediate-mass Black Hole Candidate

Abstract We study a large sample of dwarf galaxies using archival Chandra X-ray observations, with the aim of detecting accreting intermediate-mass black holes (IMBHs). IMBHs are expected to inhabit dwarf galaxies and to produce specific signatures in terms of luminosity and X-ray spectra. We report the discovery of an X-ray source associated with an A85 dwarf galaxy that fits the IMBH description. The stellar mass of the host galaxy is estimated to be 2 × 108 M ⊙, which makes it one of the least massive galaxies to potentially host an accreting black hole. The source is detected in the soft band, under 1 keV, and is undetected at higher energies. The X-ray luminosity is ≈1041 erg s−1, making it almost three orders of magnitude more luminous than the most luminous stellar-mass supersoft emitters. From the galaxy stellar mass versus black hole mass relation, we estimate the mass to be within the intermediate regime. Another method that resulted in an intermediate mass relies on the fact that supersoft emission is expected to be associated with high accretion rates, approaching the Eddington limit. We suggest that the observed offset of the X-ray source from the galactic center (≈1.8 kpc) is due to galaxy interactions, and we present evidence from the literature that supports the relation between black hole activity and galaxy interactions.

MOMO – V. Effelsberg, Swift, and Fermi study of the blazar and supermassive binary black hole candidate OJ 287 in a period of high activity

ABSTRACT We report results from our ongoing project MOMO (Multiwavelength Observations and Modelling of OJ 287). In this latest publication of a sequence, we combine our Swift UVOT–XRT and Effelsberg radio data (2.6–44 GHz) between 2019 and 2022.04 with public SMA data and gamma-ray data from the Fermi satellite. The observational epoch covers OJ 287 in a high state of activity from radio to X-rays. The epoch also covers two major events predicted by the binary supermassive black hole (SMBH) model of OJ 287. Spectral and timing analyses clearly establish: a new UV–optical minimum state in 2021 December at an epoch where the secondary SMBH is predicted to cross the disc surrounding the primary SMBH; an overall low level of gamma-ray activity in comparison to pre-2017 epochs; the presence of a remarkable, long-lasting UV–optical flare event of intermediate amplitude in 2020–2021; a high level of activity in the radio band with multiple flares; and particularly a bright, ongoing radio flare peaking in 2021 November that may be associated with a gamma-ray flare, the strongest in 6 yr. Several explanations for the UV–optical minimum state are explored, including the possibility that a secondary SMBH launches a temporary jet, but the observations are best explained by variability associated with the main jet.

Discovery and Long-term Broadband X-Ray Monitoring of Galactic Black Hole Candidate MAXI J1803–298

Abstract We report the results from the broadband X-ray monitoring of the new Galactic black hole candidate MAXI J1803−298 with MAXI/GSC and Swift/BAT during its outburst. After the discovery on 2021 May 1, the soft X-ray flux below 10 keV rapidly increased for ∼10 days, then gradually decreased over five months. In the brightest phase, the source exhibited the state transition from the low/hard state to the high/soft state via the intermediate state. The broadband X-ray spectrum during the outburst is well described with a disk blackbody plus its thermal or nonthermal Comptonization. Before the transition, the source spectrum is described by a thermal Comptonization component with a photon index of ∼1.7 and an electron temperature of ∼30 keV, while a strong disk blackbody component is observed after the transition. The spectral properties in these periods are consistent with the low/hard state and the high/soft state, respectively. A sudden flux drop with a duration of a few days, unassociated with a significant change in the hardness ratio, was found in the intermediate state. A possible cause of this variation is that the mass accretion rate rapidly increased at the disk transition, which induced a strong Compton-thick outflow and scattered out the X-ray flux. Assuming a nonspinning black hole, we estimate the black hole mass of MAXI J1803−298 to be 5.8 ± 0.4 ( cos i / cos 70 ° ) − 1 / 2 ( D / 8 kpc ) M ⊙ (where i and D are the inclination angle and the distance, respectively) from the inner disk radius obtained in the high/soft state.

Peculiar Disk Behaviors of the Black Hole Candidate MAXI J1348–630 in the Hard State Observed by Insight-HXMT and Swift

Abstract We present a spectral study of the black hole candidate MAXI J1348−630 during its 2019 outburst, based on monitoring observations with Insight-HXMT and Swift. Throughout the outburst, the spectra are well fitted with power-law plus disk-blackbody components. In the soft-intermediate and soft states, we observed the canonical relation L ∝ T in 4 between disk luminosity L and peak color temperature T in, with a constant inner radius R in (traditionally identified with the innermost stable circular orbit). At other stages of the outburst cycle, the behavior is more unusual, inconsistent with the canonical outburst evolution of black hole transients. In particular, during the hard rise, the apparent inner radius is smaller than in the soft state (and increasing), and the peak color temperature is higher (and decreasing). This anomalous behavior is found even when we model the spectra with self-consistent Comptonization models, which take into account the upscattering of photons from the disk component into the power-law component. To explain both anomalous trends at the same time, we suggest that the hardening factor for the inner-disk emission was larger than the canonical value of ≈1.7 at the beginning of the outburst. A more physical trend of radii and temperature evolution requires a hardening factor evolving from ≈3.5 at the beginning of the hard state to ≈1.7 in the hard-intermediate state. This could be evidence that the inner disk was in the process of condensing from the hot, optically thin medium and had not yet reached a sufficiently high optical depth for its emission spectrum to be described by the standard optically thick disk solution.

AstroSat observation of X-ray dips and state transition in the black hole candidate MAXI J1803–298

ABSTRACT We present the results obtained from broad-band X-ray timing and spectral analysis of black hole (BH) candidate MAXI J1803–298 using an AstroSat observation on 2021 May 11–12. Four periodic absorption dips with a periodicity of 7.02 ± 0.18 h are detected in the light curve. AstroSat observe the source when it was undergoing a transition from hard-intermediate state to soft-intermediate state. Our timing analysis reveals the presence of a sharp Type-C quasi-periodic oscillation (QPO) in the power-density spectra (PDS) with an evolving QPO frequency ranging from 5.31 ± 0.02 to 7.61 ± 0.09 Hz. We investigate the energy dependence of the QPO and do not find this feature in the PDS above 30 keV. The combined 0.7–80 keV SXT and LAXPC spectra are fitted with a model consisting of thermal multi-colour blackbody emission and Comptonized emission components. We perform time-resolved spectroscopy by extracting spectra during the dip and non-dip phases of the observation. A neutral absorber is detected during the dip and non-dip phases though a signature of an ionized absorber is also present in the dip phases. The spectral and temporal parameters are found to evolve during our observation. We estimate the mass function of the system as f(M) = 2.1–7.2 M⊙ and the mass of the BH candidate in the range of MBH ∼ 3.5–12.5 M⊙.

Detailed analysis on the reflection component for the black hole candidate MAXI J1348-630

ABSTRACT The black hole candidate MAXI J1348-630 was discovered in 2019 January 26, with the Gas Slit Camera (GSC) on-board MAXI. We report a detailed spectral analysis of this source by using the archived data of NuSTAR. A total of nine observations covered the complete outburst evolution of MAXI J1348-630 from the hard state to the soft state and finally back to the hard state. Additionally, the intermediate state is found in the transition from the hard state to the soft state. We use the state-of-the-art reflection model relxill family to fit all the nine spectra, and the spectra from two focal plane module detectors of NuSTAR are jointly fitted for each observation. In particular, we concentrate on the results of the black hole spin parameter and the inclination of the accretion disc. Based on the analysis of the inner radius of the accretion disc, we obtain the spin parameter $a_* =0.78_{-0.04}^{+0.04}$, and the inclination angle of the inner disc $i = 29.2_{-0.5}^{+0.3}$°. Furthermore, we also find that when the black hole is in the hard state, the accretion disc would show a significant truncation. The high iron abundance and ionization of the accretion disc obtained in the fitting results can be possibly explained by the high density of the accretion disc.

Estimating the Spin of the Black Hole Candidate MAXI J1659-152 with the X-Ray Continuum-fitting Method

Abstract As a transient X-ray binary, MAXI J1659-152 contains a black hole candidate as its compact star. MAXI J1659-152 was discovered on 2010 September 25 during its only known outburst. Previously published studies of this outburst indicate that MAXI J1659-152 may have an extreme retrograde spin, which, if confirmed, would provide an important clue as to the origin of black hole spin. In this paper, utilizing updated dynamical binary system parameters (i.e., the black hole mass, the orbital inclination, and the source distance) provided by Torres et al., we analyze 65 spectra of MAXI J1659-152 from RXTE/PCA, in order to assess the spin parameter. With a final selection of nine spectra matching our f sc ≲ 25%, soft state criteria, we apply a relativistic thin disk spectroscopic model kerrbb2 over 3.0–45.0 keV. We find that inclination angle correlates inversely with spin, and, considering the possible values for inclination angle, we constrain spin to be −1 &lt; a * ≲ 0.44 at a 90% confidence interval via X-ray continuum fitting. We can only rule out an extreme prograde (positive) spin. We confirm that an extreme retrograde solution is possible and is not ruled out by considering accretion torques given the young age of the system.

Hard-state Optical Wind during the Discovery Outburst of the Black Hole X-Ray Dipper MAXI J1803–298

Abstract We present 12 epochs of optical spectroscopy taken across the discovery outburst of the black hole (BH) candidate MAXI J1803−298 with the Gran Telescopio Canarias and Very Large Telescope. The source followed a standard outburst evolution with hard and soft states. The system displays a triangular shape in the hardness intensity diagram, consistent with that seen in high-inclination BH transients and the previously reported detection of X-ray dips. The two epochs observed during the initial hard state exhibited asymmetric emission-line profiles, including a P-Cygni profile simultaneously detected in Hα and He i 6678, which indicates the presence of an optical wind in the system. The remaining spectra, obtained during the transition to the soft state and the subsequent decay, are instead characterized by narrower, double-peaked emission lines embedded into broad absorption components. One epoch (intermediate state) also includes near-infrared (NIR) coverage, revealing complex line profiles in the Paschen and Bracket series, which suggests that the outflow is still present during the outburst decay through the soft state. The growing list of low-mass X-ray binaries with optical and NIR outflow signatures indicates that these are common features. Furthermore, the lowest luminosity spectrum exhibits an Hα FWHM of 1570 ± 100 km s−1. This, together with previous constraints on the binary parameters, allows us to favor a compact object mass of ∼3–10 M ⊙, further supporting its BH nature.

NGC 2004 #115: a black hole imposter containing three luminous stars

ABSTRACT NGC 2004 #115 is a recently identified black hole (BH) candidate in the Large Magellanic Cloud (LMC) containing a B star orbiting an unseen companion in a 2.9 d orbit and a Be star tertiary. We show that the unseen companion is not a 25 M⊙ BH, but a $(2\!-\!3)\, {\rm M}_{\odot }$ luminous star. Analysing the Optical Gravitational Lensing Experiment (OGLE) and MAssive Compact Halo Object (MACHO) light curves of the system, we detect ellipsoidal variability with amplitude 10 times larger than would be expected if the companion were a 25 M⊙ BH, ruling out the low inclination required for a massive companion. The light curve also shows a clear reflection effect that is well modelled with a $2.5\, {\rm M}_{\odot }$ main-sequence secondary, ruling out a lower mass BH or neutron star companion. We consider and reject models in which the system is a binary containing a stripped star orbiting the Be star: only a triple model with an outer Be star can explain both the observed light curve and radial velocities. Our results imply that the B star, whose slow projected rotation velocity and presumed tidal synchronization were interpreted as evidence for a low inclination (and thus a high companion mass), is far from being tidally synchronized: despite being in a 2.9 d orbit that is fully or nearly circularized (e &amp;lt; 0.04), its surface rotation period appears to be at least 20 d. We offer cautionary notes on the interpretation of dormant BH candidates in binaries.