Black Hole Candidate Research Articles

Timing analysis of the new black hole candidate MAXI J1803−298 with Insight–HXMT and NICER

ABSTRACT We present the timing analysis results of MAXI J1803−298, a black hole candidate, during its 2021 outburst using data obtained from the Insight–Hard X-ray Modulation Telescope (Insight–HXMT) and the Neutron star Interior Composition Explorer (NICER). Our analysis reveals that the source undergoes a state transition from the low hard state to the hard intermediate state, followed by the soft intermediate state, ultimately reaching the high soft state. We searched for the quasi-periodic oscillations (QPOs) and studied the characteristics of the outburst. At the beginning of the outburst, the source was in the hard state, many type-C QPOs were seen in the Insight–HXMT data, and the frequency of these QPOs increased from ∼0.16 to 2.6 Hz. Our analysis of the rms–frequency relationship of the type-C QPOs indicates a turning point in the frequency. We also analysed the phase lag versus frequency and energy relationship and deduced that the source likely has a high inclination angle, consistent with previous research. The observed rms and phase-lag features in type-C QPOs could be explained by the Lense–Thirring precession model, although alternatives would be still viable. The lag spectrum of type-B QPOs exhibits a U-shaped pattern similar to that of many other sources, and the type-B QPOs’ rms increases as the energy increases. This phenomenon can be explained by the dual-corona model.

Seven Reflares, a Mini Outburst, and an Outburst: High-amplitude Optical Variations in the Black Hole X-Ray Binary Swift J1910.2–0546

We present long-term (2012–2022) optical monitoring of the candidate black hole X-ray binary Swift J1910.2–0546 with the Faulkes Telescopes and Las Cumbres Observatory network. Following its initial bright 2012 outburst, we find that the source displayed a series of at least seven quasi-periodic, high-amplitude (~3 mag) optical reflares in 2013, with a recurrence time increasing from ∼42 to ∼49 days. In 2014, the source experienced a mini outburst with two peaks in the optical. We also study the recent 2022 outburst of the source at optical wavelengths, and perform a comparative analysis with the earlier rebrightenings. A single X-ray detection and only two radio detections were obtained during the 2013 reflaring period, and only optical detections were acquired in 2014. During the reflaring in both 2013 and 2014, the source showed bluer-when-brighter behavior, having optical colors consistent with blackbody heating and cooling between 4500 and 9500 K, i.e., the temperature range in which hydrogen starts to ionize. Finally, we compare the flaring behavior of the source to rebrightening events in other X-ray binaries. We show that the repeated reflarings of Swift J1910.2–0546 are highly unusual, and propose that they arise from a sequence of repetitive heating and cooling front reflections traveling through the accretion disk.

Investigating effects of dark matter on photon orbits and black hole shadows

We consider Kerr black holes (BHs) surrounded by perfect dark fluid matter (PFDM), with an additional parameter (k) because of PFDM, apart from mass (M) and rotation parameter (a) — the rotating PFDM BHs. We analyse the photon orbits around PFDM BHs and naked singularities (NSs) and emphasize the effect of PFDM on photon boomerangs. Interestingly, the azimuthal oscillations first increase and then decrease for retrograde orbits, whereas they first decrease and then increase for prograde orbits, with increasing k. Unlike in the Kerr NSs, photon boomerangs can form around rotating PFDM NSs. We use the Event Horizon Telescope (EHT) observational results for Schwarzschild shadow deviations of M87* and Sgr A*, δM87∗=−0.01±0.17 and δSgrA∗=−0.08−0.09+0.09(VLTI),−0.04−0.10+0.09(Keck), to report the upper bounds on the PFDM parameter: 0≤k≤0.0792M and kmax∈[0.0507M,0.0611M] respectively. Together with the EHT bounds on the shadows of Sgr A∗ and M87∗, our analysis concludes that a substantial part of the rotating PFDM BH parameter space agrees with the EHT observations. Thus, one must consider the possibility of the rotating PFDM BHs being strong candidates for the astrophysical BHs.

A Detailed View of Low-frequency Quasi-periodic Oscillation in the Broadband 0.2–200 keV with Insight-HXMT and NICER

We report the X-ray timing results of the black hole candidate MAXI J1820+070 during its 2018 outburst using the Hard X-ray Modulation Telescope (Insight-HXMT) and Neutron Star Interior Composition Explorer Mission (NICER) observations. Low-frequency quasi-periodic oscillations (LFQPOs) are detected in the low/hard state and the hard intermediate state, which lasted for ∼90 days. Thanks to the large effective area of Insight-HXMT at high energies and NICER at low energies, we are able to present the energy dependence of the LFQPO characteristics and phase lags from 0.2 to 200 keV, which has never been explored by previous missions. We find that the centroid frequency of the LFQPOs does not change significantly with energy, while the full width at half maximum and fractional rms show a complex evolution with energy. The LFQPO phase lags at high energies and low energies show consistent energy-dependence relations taking the ∼2 keV as reference. Our results suggest that the LFQPOs from high energy come from the LT precession of the relativistic jet, while the low-energy radiation is mainly from the perpendicular innermost regions of the accretion disk.

Constraints on the Cosmological Coupling of Black Holes from the Globular Cluster NGC 3201

Globular clusters are among the oldest stellar populations in the Milky Way; consequently, they also host some of the oldest known stellar-mass black holes, providing insight into black hole formation and evolution in the early (z ≳ 2) universe. Recent observations of supermassive black holes in elliptical galaxies have been invoked to suggest the possibility of a cosmological coupling between astrophysical black holes and the surrounding expanding universe, offering a mechanism for black holes to grow over cosmic time and potentially explaining the origin of dark energy. In this paper, I show that the mass functions of the two radial velocity black hole candidates in NGC 3201 place strong constraints on the cosmologically coupled growth of black holes. In particular, the amount of coupling required to explain the origin of dark energy would either require both NGC 3201 black holes to be nearly face on (a configuration with probability of at most 10−4) or one of the BHs would need to have formed with a mass below that of the most massive neutron stars (2.2 M ⊙). This emphasizes that these and other detached black hole–star binaries can serve not only as laboratories for compact object and binary astrophysics but as constraints on the long-term evolution of astrophysical black holes.

Time-dependent visibility modelling of a relativistic jet in the X-ray binary MAXI J1803−298

ABSTRACT Tracking the motions of transient jets launched by low-mass X-ray binaries (LMXBs) is critical for determining the moment of jet ejection, and identifying any corresponding signatures in the accretion flow. However, these jets are often highly variable and can travel across the resolution element of an image within a single observation, violating a fundamental assumption of aperture synthesis. We present a novel approach in which we directly fit a single time-dependent model to the full set of interferometer visibilities, where we explicitly parametrize the motion and flux density variability of the emission components, to minimize the number of free parameters in the fit, while leveraging information from the full observation. This technique allows us to detect and characterize faint, fast-moving sources, for which the standard time binning technique is inadequate. We validate our technique with synthetic observations, before applying it to three Very Long Baseline Array (VLBA) observations of the black hole candidate LMXB MAXI J1803−298 during its 2021 outburst. We measured the proper motion of a discrete jet component to be 1.37 ± 0.14 mas h−1, and thus we infer an ejection date of MJD $59348.08_{-0.06}^{+0.05}$, which occurs just after the peak of a radio flare observed by the Australia Telescope Compact Array (ATCA) and the Atacama Large Millimeter/Sub-Millimeter Array (ALMA), while MAXI J1803−298 was in the intermediate state. Further development of these new VLBI analysis techniques will lead to more precise measurements of jet ejection dates, which, combined with dense, simultaneous multiwavelength monitoring, will allow for clearer identification of jet ejection signatures in the accretion flow.

A red giant orbiting a black hole

ABSTRACT We report spectroscopic and photometric follow-up of a dormant black hole (BH) candidate from Gaia DR3. The system, which we call Gaia BH2, contains a ∼1 M⊙ red giant and a dark companion with mass $M_2 = 8.9\pm 0.3\, {\rm M}_{\odot }$ that is very likely a BH. The orbital period, Porb = 1277 d, is much longer than that of any previously studied BH binary. Our radial velocity (RV) follow-up over a 7-month period spans >90 per cent of the orbit’s RV range and is in excellent agreement with the Gaia solution. UV imaging and high-resolution optical spectra rule out plausible luminous companions that could explain the orbit. The star is a bright (G = 12.3), slightly metal-poor ($\rm [Fe/H]=-0.22$) low-luminosity giant ($T_{\rm eff}=4600\, \rm K$; $R = 7.8\, R_{\odot }$; $\log \left[g/\left({\rm cm\, s^{-2}}\right)\right] = 2.6$). The binary’s orbit is moderately eccentric (e = 0.52). The giant is enhanced in α-elements, with $\rm [\alpha /Fe] = +0.26$, but the system’s Galactocentric orbit is typical of the thin disc. We obtained X-ray and radio non-detections of the source near periastron, which support BH accretion models in which the net accretion rate at the horizon is much lower than the Bondi–Hoyle–Lyttleton rate. At a distance of 1.16 kpc, Gaia BH2 is the second-nearest known BH, after Gaia BH1. Its orbit – like that of Gaia BH1 – seems too wide to have formed through common envelope evolution. Gaia BH1 and BH2 have orbital periods at opposite edges of the Gaia DR3 sensitivity curve, perhaps hinting at a bimodal intrinsic period distribution for wide BH binaries. Dormant BH binaries like Gaia BH1 and Gaia BH2 significantly outnumber their close, X-ray bright cousins, but their formation pathways remain uncertain.

Updated radial velocities and new constraints on the nature of the unseen source in NGC1850 BH1

ABSTRACT A black hole candidate orbiting a luminous star in the Large Magellanic Cloud young cluster NGC 1850 (∼100 Myr) has recently been reported based on radial velocity and light-curve modelling. Subsequently, an alternative explanation has been suggested for the system: a bloated post-mass transfer secondary star (Minitial ∼ 4–5 M⊙ and Mcurrent ∼ 1–2 M⊙) with a more massive, yet luminous companion (the primary). Upon reanalysis of the MUSE spectra, we found that the radial velocity variations originally reported were underestimated (K2, revised = 176 ± 3 km s−1 versus K2, original = 140 ± 3 km s−1) because of the weighting scheme adopted in the full-spectrum fitting analysis. The increased radial velocity semi-amplitude translates into a system mass function larger than previously deduced (frevised = 2.83 M⊙versus foriginal = 1.42 M⊙). By exploiting the spectral disentangling technique, we place an upper limit of 10 per cent of a luminous primary source to the observed optical light in NGC1850 BH1, assuming that the primary and secondary are the only components contributing to the system. Furthermore, by analysing archival near-infrared data, we find clues to the presence of an accretion disc in the system. These constraints support a low-mass post-mass transfer star but do not provide a definitive answer whether the unseen component in NGC1850 BH1 is indeed a black hole. These results predict a scenario where, if a primary luminous source of mass M ≥ 4.7 M⊙ is present in the system (given the inclination and secondary mass constraints), it must be hidden in a optically thick disc to be undetected in the MUSE spectra.

OJ 287: A new BH mass estimate of the secondary

We present a study of outburst activity in the BL Lacertae source OJ 287, which has been observed extensively with the X-ray telescope (XRT) on board the Neil Gehrels Swift Observatory. We demonstrate that the results of our analysis of X-ray flaring activity using the Swift/XRT data allow for the key characteristics of the OJ 287 secondary to be refined (its nature and mass). We discover that the energy spectra in all spectral states can be fitted using the XSPEC bulk motion Comptonization (BMC) model. As a result we find that the X-ray photon index of the BMC model, Γ, correlates with the mass accretion rate, Ṁ. We establish that Γ increases monotonically with Ṁ from the low-hard state, Γ ∼ 1.5, to the high-soft state, Γ ∼ 2.8, and finally saturates. The index behavior is similar to that in a number of black hole (BH) candidates in which we show that its saturation is observational evidence of the presence of a BH. Based on this correlation, we applied a scaling method and determine that a secondary BH mass in OJ 287 is about ∼1.25 × 108 solar masses, using the well-studied X-ray BH binaries XTE 1550–564, H 1743–322, 4U 1630–47, and GRS 1915+105 as well as extragalactic BHs ESO 243–49 and M101 ULX–1, as reference sources. Also, using the power spectrum analysis, we infer the size of the Compton cloud LCC ∼ 1013 cm where X-ray spectra were formed. Using this value of LCC, we confirm that a BH mass of the secondary in OJ 287 was on the order of 108 solar masses as we derived using the index, Γ–correlation (the scaling method) with respect of the mass accretion rate.

Meet the Parents: The Progenitor Binary for the Supermassive Black Hole Candidate in E1821+643

The remnants of binary black hole mergers can be given recoil kick velocities up to 5000 km s−1 due to anisotropic emission of gravitational waves. E1821+643 is a recoiling supermassive black hole candidate with spectroscopically offset, broad emission lines, consistent with motion of the black hole at ∼2100 km s−1 along the line of sight relative to its host galaxy. This suggests a recoil kick of ∼2200 km s−1. Such a kick is powerful enough to eject E1821+643 from its M gal ∼ 2 × 1012 M ⊙ host galaxy. In this work, we address the question: assuming that E1821+643 is a recoiling black hole, what are the likely properties of the progenitor binary that formed E1821+643? Using astrophysically motivated priors, we infer that E1821+643 was likely formed from a binary black hole system with masses of , (90% credible intervals). Given our model, the black holes in this binary were likely to be spinning rapidly with dimensionless spin magnitudes of , . Such a high recoil velocity is impossible for spins aligned to the orbital angular momentum axis. This suggests that the progenitor for E1821+643 merged in hot gas, which is thought to provide an environment where spin alignment from accretion proceeds slowly relative to the merger timescale. We infer that E1821+643, if it is a recoiling black hole, is likely to be a rapidly rotating black hole with a dimensionless spin of χ = 0.92 ± 0.04. A 2.6 × 109 M ⊙ black hole, recoiling from a gas-rich environment at v ∼ 2200 km s−1, is likely to persist as an active galactic nucleus for ∼107 yr, in which time it traverses ∼25 kpc.

Detection of single-degenerate massive binaries with Gaia: The impact of blue supergiants, triples, mass precision, and high-precision parallax requirements

Context. X-ray-quiet single-degenerate massive binaries are notoriously difficult to detect, and only a few have been identified so far. Yet, recent investigations have shown that hundreds of black holes (BHs) with massive main-sequence (MS) companions (OB+BHs) might be identifiable from Gaia astrometry by using the astrometric mass-ratio function (AMRF). Aims. We aim to investigate a number of biases that can have an impact on the astrometric identification of OB+BH binaries, namely the presence of blue supergiant (BSG) companions instead of dwarfs and the presence of additional companions in the system that are not resolved by Gaia. We also explore the accuracy with which the primary mass needs to be constrained. Moreover, we assess the impact of high-precision constraints on the detection of binaries by using the conservative constraints imposed to release astrometric orbits in the latest Gaia data release, DR3. We then investigate how much less stringent these constraints need to be in order to obtain information on the BH-formation scenario. Methods. We established a mass-magnitude relation of BSGs and from this computed BSG AMRF curves. A mock population of OB/BSG+BH binaries, non-degenerate binaries with an OB or BSG primary (OB/BSG+MS), and triples with OB and BSG primaries is used to determine the fraction of false-positive identifications and the effect of the BSG mass-magnitude relation. We compare the number of sources with astrometric DR3 orbits in the second Alma Luminous Star catalogue with new predictions regarding the detection of OB+BHs using the conservative selection criterion used for publishing astrometric orbits in DR3. Results. We show that the addition of systems with BSG primaries does not significantly impact the fraction of false-positive identifications. Only for triple systems where the outer star is more luminous and a BSG will the usage of the previously established MS curves potentially result in a high amount of false-positive identifications. However, such systems are expected to be rare. We also demonstrate that the mass of the primary does not need to be accurately known to benefit from both the high identification fraction of OB/BSG+BHs and the low fraction of false positives. We find that 11 sources have an astrometric binary orbit available in DR3. None of these sources are OB/BSG+BH candidates. This is in line with the new predictions that use the Gaia DR3 selection criterion. Conclusions. If the evolutionary stages of the primary stars are unknown, the usage of the BSG curves is recommended over the MS curves to avoid high contamination from BSG+MS systems or triples with a more luminous outer star. This way, the false-positive fractions are decreased by an order of magnitude, reaching values ≪1%. However, the fraction of identifiable OB+BHs is also significantly reduced (from 68% to 29%). If the mass of the primary star is not known, it is possible to use a fixed estimate. The non-detection of astrometric OB/BSG+BH systems cannot be attributed to the underlying BH-formation scenario, but rather to the stringent selection criterion imposed on the parallax relative uncertainty in DR3. A relaxation of this condition would be needed to find the bulk of the OB+BH population with Gaia. If possible, we propose that the constraint on the relative parallax precision in DR4 be improved to n × (ϖ/σϖ)DR3,single > n × 1000/Pday, with (ϖ/σϖ)DR3,single the relative parallax precision for the single source solution in DR3 and n the DR4 improvement of the relative precision compared to the DR3 single star solutions. In DR3, this means a 95% decrease in the original criterion.

NICER/NuSTAR Characterization of 4U 1957+11: A Near Maximally Spinning Black Hole Potentially in the Mass Gap

4U 1957+11 is a black hole candidate system that has been in a soft X-ray spectral state since its discovery. We present analyses of recent joint NICER and NuSTAR spectra, which are extremely well described by a highly inclined disk accreting into a near maximally spinning black hole. Owing to the broad X-ray coverage of NuSTAR, the fitted spin and inclination are strongly constrained for our hypothesized disk models. The faintest spectra are observed out to 20 keV, even though their hard tail components are almost absent when described with a simple corona. The hard tail increases with luminosity, but shows clear two-track behavior with one track having appreciably stronger tails. The disk spectrum color-correction factor is anticorrelated with the strength of the hard tail (e.g., as measured by the Compton y parameter). Although the spin and inclination parameters are strongly constrained for our chosen model, the mass and distance are degenerate parameters. We use our spectral fits, along with a theoretical prior on color-correction, an observational prior on likely fractional Eddington luminosity, and an observational prior on distance obtained from Gaia studies, to present mass and distance contours for this system. The most likely parameters, given our presumed disk model, suggest a 4.6 M ⊙ black hole at 7.8 kpc observed at luminosities ranging from ≈1.7% to 9% of Eddington. This would place 4U 1957+11 as one of the few actively accreting sources within the mass gap of ≈2–5 M ⊙ where there are few known massive neutron stars or low-mass black holes. Higher mass and distance, however, remain viable.

Event Horizon and Environs (ETHER): A Curated Database for EHT and ngEHT Targets and Science

The next generation Event Horizon Telescope (ngEHT) will observe multiple supermassive black hole (SMBH) candidates down to a few tens of mJy, and profoundly transform our understanding of the local SMBH population. Given the impossibility of large-area high-resolution millimeter surveys, multi-frequency spectral energy densities (SEDs), and models are required to both identify source samples tailored to specific science goals, and to predict the feasibility of detection of individual interesting sources. Here, we present the Event Horizon and Environs (ETHER) source and SED model database whose primary use is to enable the selection and optimization of targets for EHT and ngEHT science. The living ETHER database currently consolidates 1.6 million black hole mass estimates, ∼15,500 milliarcsec-scale radio fluxes, ∼14,000 hard X-ray fluxes (expected to grow by factor ≳40 with the eROSITA data release) and SED information as obtained from catalogs and database queries, the literature, and our own new observations. Jet and accretion flow models are fit to individual SEDs in an automated way in order to predict the ngEHT observable fluxes from the jet base and accretion inflow. The database can be filtered by parameters or cross matched to a user source list, with the automated SED fitting models optionally fine tuned by the user. We have identified an initial ngEHT ‘gold sample’ for jet base studies and potentially black hole shadows; this sample will grow significantly in the coming years. While the ngEHT requires and will best exploit the ETHER database, six (eleven) ETHER sources have already been observed (scheduled) with the EHT in 2022 (2023), and the database has wide ranging applications in galaxy and black hole mass evolution studies.

The comptonizing medium of the black hole X-ray binary MAXI J1535−571 through type-C quasi-periodic oscillations

ABSTRACT We present a detailed spectral and temporal analysis of the black hole candidate MAXI J1535−571 using NICER observations in 2017 September and October. We focus specifically on observations in the hard-intermediate state when the source shows type-C quasi-periodic oscillations (QPOs). We fitted the time-averaged spectrum of the source and the rms and phase-lag spectra of the QPO with a one-component time-dependent Comptonization model. We found that the corona contracts from ∼104–3× 10 3 km as the QPO frequency increases from ∼ 1.8–9.0 Hz. The fits suggest that the system would consist of two coronas, a small one that dominates the time-averaged spectrum and a larger one, possibly the jet, that dominates the rms and lag spectra of the QPO. We found a significant break in the relation between the spectral parameters of the source and the properties of the QPO, including its lag spectra, with QPO frequency. The change in the relations happens when the QPO frequency crosses a critical frequency ν c ≈ 3.0 Hz. Interestingly, the QPO reaches this critical frequency simultaneously as the radio emission from the jet in this source is quenched.

MAXI J1348–630: Estimating the black hole mass and binary inclination using a scaling technique

The multi-wavelength outburst activity in the recently discovered X-ray binary transient MAXI J1348–630 has sparked a great deal of controversy about the characteristics of this binary and questions around whether the source contains a black hole (BH). Here, we present the results of our analysis of the outburst of MAXI J1348–630 using Swift/XRT data. We find that energy spectra in all spectral states can be modeled using a combination of Comptonization and Gaussian iron-line components. In addition, we show that the X-ray photon index, Γ, is correlated with the mass accretion rate, Ṁ. We find that Γ increases monotonically with Ṁ from the low-hard state to the high-soft state, and then becomes saturated at Γ∼ 3. This index behavior is similar to that exhibited by a number of other BH candidates. This result represents observational evidence of the presence of a BH in MAXI J1348–630. We also show that the value of Γ is correlated with the quasi periodic oscillation frequency, νL. Based on this correlation, we applied a scaling method to estimate a BH mass of 14.8 ± 0.9 M⊙, using the well-studied BH binary XTE J1550–564 as a reference source. The recent discovery of a giant dust scattering ring around MAXI J1348–630 by SRG/eROSITA has refined distance estimates to this X-ray source. With this distance, we were able to estimate the disk inclination i = (65 ± 7)° using the scaling technique for the correlation between Γ and normalization proportional to Ṁ. We detected a specific behavior of the disk seed photon temperature, kTs, immediately before the outburst: kTs initially decreases from 0.4 to 0.2 keV and increases only after the source transits to the outburst rise-maximum phase. An initial decrease in kTs occurred simultaneously with an increase in the illumination fraction, f. We interpreted this effect in terms of the bulk motion Comptonization model. At the start of the outburst, the Compton cloud (or “corona”) is very extended and, thus, the seed photons injected to the corona from the relatively far-away disk region, where kTs is about 0.2–0.4 keV. While Ṁ increases (or luminosity increases), the corona contracts, thus increasing the seed photon temperature, kTs. It is possible that such a decrease in kTs occurring simultaneously with an increase in the illumination fraction, f, can be considered a signature of the readiness of a BH object to go into an outburst phase.

Relating peak optical luminosity and orbital period of stellar-mass black holes in X-ray binaries

ABSTRACT We compare the peak optical luminosity with the orbital period for a sample of 22 stellar-mass black hole candidates with good measurements of both quantities. We find that the peak absolute magnitude for the outbursts follows a linear relation with MV, peak = 3.48(± 0.85) − 3.89(± 0.91) logPorb, which corresponds to a $L_V \propto P_{orb}^{1.56\pm 0.36}$ power-law relation. Excluding V4641 Sgr which is a strong outlier and not likely to have outbursts produced by the standard disc instability model, in addition to BW Cir and V821 Ara – both of which have highly uncertain distances; the new correlation for the 19 sources is found to be MV, peak = 3.01 (± 0.93) − 3.21 (± 1.04) logPorb, which corresponds to $L_V \propto P_{orb}^{1.28 \pm 0.42}$. This is an analogous relationship to the ‘Warner relation’ between orbital period and peak luminosity found for cataclysmic variables. We discuss the implications of these results for finding black hole X-ray binaries in other galaxies and in our own Galaxy with the Large Synoptic Survey Telescope and other future large time domain surveys.

Properties of 2017–18 ‘failed’ outburst of GX 339-4

The Galactic transient black hole candidate GX 339-4 is a very interesting object to study as it showed both complete and failed types of outbursts. We studied both spectral and temporal properties of the 2017–18 outburst of the source using archival data of NICER, AstroSat, MAXI and NuSTAR satellite instruments. This 2017–18 outburst is found to be failed in nature, as during the entire period of the outburst, the source was only in the hard spectral state. Source spectra were highly dominated with the non-thermal fluxes. When we tried to fit spectra with phenomenological models, most of the spectra were fitted with only the powerlaw model, and only six spectra required disk black body plus powerlaw models. While fitting spectra with the physical two-component advective flow (TCAF) model, we observed that the flow was highly dominated by the sub-Keplerian halo rate. The presence of stronger shock at a larger radius from the black hole was also observed during the rising and declining phases of the outburst. A prominent signature of 0.31 Hz QPO is observed with possibly three harmonics.

Radio Observations of Four Active Galactic Nuclei Hosting Intermediate-mass Black Hole Candidates: Studying the Outflow Activity and Evolution

Observational searches for intermediate-mass black holes (IMBHs; 102–106 M ⊙) include relatively isolated dwarf galaxies. For those that host active galactic nuclei (AGNs), the IMBH nature may be discerned through the accretion–jet activity. We present radio observations of four AGN-hosting dwarf galaxies, which potentially harbor IMBHs. Very large array (VLA) observations indicate steep spectra (indices of −0.63 to −1.05) between 1.4 and 9 GHz. However, a comparison with the 9 GHz in-band spectral index shows a steepening for GH047 and GH158 (implying older/relic emission) and flattening for GH106 and GH163 (implying recent activity). Overlapping emission regions in the VLA 1.4 GHz and our very long baseline array (VLBA) 1.5 GHz observations, and possibly symmetric pc-scale extensions, are consistent with recent activity in the latter two. Using the compact VLBA radio luminosity, X-ray luminosity (probing the accretion activity), and the black hole masses, all AGNs are found to lie on the empirical fundamental plane relation. The four AGNs are radio-quiet with relatively higher Eddington ratios (0.04–0.32) and resemble X-ray binaries during spectral state transitions that entail an outflow ejection. Furthermore, the radio to X-ray luminosity ratio of −3.9 to −5.6 in these four sources support the scenarios that include corona mass ejection from the accretion disk and wind activity. The growth to kpc-scales likely proceeds along a similar trajectory to young AGNs and peaked spectrum sources. These complex clues can thus aid in the detection and monitoring of IMBHs in the nearby universe.

Four Eccentric Mergers Increase the Evidence that LIGO–Virgo–KAGRA’s Binary Black Holes Form Dynamically

The growing population of compact binary mergers detected with gravitational waves contains multiple events that are challenging to explain through isolated binary evolution. Such events have higher masses than are expected in isolated binaries, component spin tilt angles that are misaligned, and/or nonnegligible orbital eccentricities. We investigate the orbital eccentricities of 62 binary black hole candidates from the third gravitational-wave transient catalog of the LIGO–Virgo–KAGRA Collaboration with an aligned-spin, moderate-eccentricity waveform model. Within this framework, we find that at least four of these events show significant support for eccentricity e 10 ≥ 0.1 at a gravitational-wave frequency of 10 Hz (>60% credibility, under a log-uniform eccentricity prior that spans the range 10−4 < e 10 < 0.2). Two of these events are new additions to the population: GW191109 and GW200208_22. If the four eccentric candidates are truly eccentric, our results suggest that densely populated star clusters may produce 100% of the observed mergers. However, it remains likely that other formation environments with higher yields of eccentric mergers—for example, active galactic nuclei—also contribute. We estimate that we will be able to confidently distinguish which formation channel dominates the eccentric merger rate after ≳80 detections of events with e 10 ≥ 0.05 at LIGO–Virgo sensitivity, with only ∼5 detectably eccentric events required to distinguish formation channels with third-generation gravitational-wave detectors.

Optical and X-ray follow-up to a globular cluster ultraluminous X-ray source in NGC 4472

ABSTRACT NGC 4472 is home to five ultraluminous X-ray sources hosted by globular clusters. These sources have been suggested as good black hole candidates in extragalactic globular clusters—a highly sought after population that may provide observational information regarding the progenitors of merging black hole binaries. In this body of work, we present X-ray and optical follow-up to one of these sources, CXOUJ1229410+075744 (GCU1). We find no evidence of [OIII] optical emission in GCU1 which indicates a lack of significant evidence for super-Eddington outflows, unlike what is seen in a handful of ULXs in extragalactic GCs. X-ray monitoring from 2019 to 2021 shows no detected X-ray emission above a few × 1038 erg/s. Comparisons of the multiwavelength properties to disc-dominated, near Eddington Galactic black hole low-mass X-ray binaries (GRS 1915+105 and XTEJ1817-330) suggests that GCU1 may show similar behaviour to GRS 1915+105 in terms of X-ray variability and similar relationships between LX and kT, with GCU1 showing maximum X-ray luminosities on order of higher magnitude.