Galactic Black Hole Candidates Research Articles

MAXI J0637–430: A Possible Candidate for Bulk Motion Comptonization?

The transient Galactic black hole candidate MAXI J0637–430 went through an outburst in 2019–2020 for the very first time. This outburst was active for almost 6 months from 2019 November to 2020 May. We study the spectral properties of this source during that outburst using archival data from NICER/XTI, Swift/XRT, and NuSTAR/FPM satellites/instruments. We have analyzed the source during six epochs on which simultaneous NICER–NuSTAR and Swift/XRT–NuSTAR data were available. Using both phenomenological and physical model fitting approaches, we analyzed the spectral data in the broad 0.7–70 keV energy band. We first used a combination of disk blackbody with power-law, disk blackbody with broken power-law, and disk blackbody with power-law and bulk motion Comptonization (BMC) models. For a better understanding of the accretion picture, e.g., understanding how the accretion rates change with the changing size of the perceived Compton cloud, we used the two-component advective flow (TCAF) model with the broken power-law, and TCAF with the combined power-law, BMC models. For the last three epochs, the diskbb+power-law and TCAF models were able to spectrally fit the data for acceptable χ 2/degrees of freedom (dof). However, for the first three epochs, we needed an additional component to fit spectra for acceptable χ 2/dof. From our analysis, we reported on the possible presence of another component during these first three epochs when the source was in the high soft state. This additional component in this state is best described by the BMC phenomenon. From the TCAF model fitting, we estimated the average mass of the source as .

Accretion properties and estimation of spin of galactic black hole candidate Swift J1728.9–3613with NuSTAR during its 2019 outburst

ABSTRACT Black hole X-ray binaries (BHXRBs) play a crucial role in understanding the accretion of matter onto a black hole. Here, we focus on exploring the transient BHXRB Swift J1728.9–3613 discovered by Swift/BAT and MAXI/GSC during its January 2019 outburst. We present measurements on its accretion properties, long time-scale variability, and spin. To probe these properties, we make use of several NICER observations and an unexplored data set from NuSTAR, as well as long-term light curves from MAXI/GSC. In our timing analysis, we provide estimates of the cross-correlation functions between light curves in various energy bands. In our spectral analysis, we employ numerous phenomenological models to constrain the parameters of the system, including flavours of the relativistic reflection model Relxill to model the Fe Kα line and the >15 keV reflection hump. Our analysis reveals that: (i) Over the course of the outburst, the total energy released was ∼5.2 × 1044 ergs, corresponding to roughly 90 per cent of the mass of Mars being devoured. (ii) We find a continuum lag of 8.4 ± 1.9 d between light curves in the 2–4 and 10–20 keV bands, which could be related to the viscous inflow time-scale of matter in the standard disc. (iii) Spectral analysis reveals a spin parameter of ∼0.6–0.7 with an inclination angle of ∼45°–70° and an accretion rate during the NuSTAR observation of ${\sim}17\ \hbox{per cent}\ L_{\rm Edd}$.

The Spin Measurement of MAXI J0637-430: a Black Hole Candidate with High Disk Density

The Galactic black hole candidate MAXI J0637-430 was first discovered by MAXI/GSC on 2019 November 2. We study the spectral properties of MAXI J0637-430 by using the archived NuSTAR data and Swift/XRT data. After fitting the eight spectra by using a disk component and a powerlaw component model with absorption, we select the spectra with relatively strong reflection components for detailed X-ray reflection spectroscopy. Using the most state-of-art reflection model, relxillCp, the spectral fitting measures a black hole spin a * > 0.72 and the inclination angle of the accretion disk i = degrees, at a 90% confidence level. In addition, the fitting results show an extreme supersolar iron abundance. Combined with the fitting results of reflection model reflionx_hd, we consider that this unphysical iron abundance may be caused by a very high-density accretion disk (n e > 2.34 × 1021 cm−3) or a strong Fe Kα emission line. The soft excess is found in the soft state spectral fitting results, which may be an extra free–free heating effect caused by high density of the accretion disk. Finally, we discuss the robustness of black hole spin obtained by X-ray reflection spectroscopy. The result of relatively high spin is self-consistent with broadened Fe Kα line. Iron abundance and disk density have no effect on the spin results.

A Noninteracting Galactic Black Hole Candidate in a Binary System with a Main-sequence Star

We describe the discovery of a solar neighborhood (d = 468 pc) binary system with a main-sequence sunlike star and a massive noninteracting black hole candidate. The spectral energy distribution of the visible star is described by a single stellar model. We derive stellar parameters from a high signal-to-noise Magellan/MIKE spectrum, classifying the star as a main-sequence star with T eff = 5972 K, , and M = 0.91 M ⊙. The spectrum shows no indication of a second luminous component. To determine the spectroscopic orbit of the binary, we measured the radial velocities of this system with the Automated Planet Finder, Magellan, and Keck over four months. We show that the velocity data are consistent with the Gaia astrometric orbit and provide independent evidence for a massive dark companion. From a combined fit of our spectroscopic data and the astrometry, we derive a companion mass of M ⊙. We conclude that this binary system harbors a massive black hole on an eccentric (e = 0.46 ± 0.02), 185.4 ± 0.1 day orbit. These conclusions are independent of El-Badry et al., who recently reported the discovery of the same system. A joint fit to all available data yields a comparable period solution but a lower companion mass of . Radial velocity fits to all available data produce a unimodal solution for the period that is not possible with either data set alone. The combination of both data sets yields the most accurate orbit currently available.

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.

Properties of MAXI J1348-630 during Its Second Outburst in 2019

The newly discovered galactic black hole candidate (BHC) MAXI J1348-630 showed two major outbursts in 2019, just after its discovery. Here, we provide a detailed spectral and temporal analysis of the less-studied second outburst using archive data from multiple satellites, namely Swift, MAXI, NICER, NuSTAR and AstroSat. The outburst continued for around two and a half months. Unlike the first outburst from this source, this second outburst was a ‘failed’ one. The source did not transition to soft or intermediate spectral states. During the entire outburst, the source was in the hard state with high dominance of non-thermal photons. The presence of strong shocks are inferred from spectral fitting using a TCAF model. In NuSTAR spectra, weak reflection is observed from spectral fitting. Low-frequency quasi-periodic oscillations are also detected in AstroSat data.

Accretion flow properties of MAXI J1910-057/Swift J1910.2–0546 during its 2012–13 outburst

Galactic black hole candidate MAXI J1910-057/Swift J1910.2–0546 was simultaneously discovered by MAXI/GSC and Swift/BAT satellites during its first outburst in 2012. We study the detailed spectral and temporal properties of the source in a broad energy range using archival data from Swift/XRT, MAXI/GSC, and Swift/BAT satellites/instruments. Low frequency quasi periodic oscillations are observed during the outburst. The combined 1–50 keV spectra are analyzed using the transonic flow solution based Two Component Advective Flow (TCAF) model. Based on the variations of soft and hard X-ray fluxes, their hardness ratios and the variations of the spectral model fitted parameters, we find that the source has evolved through six spectral states. We interpret this spectral state evolution to be a result of the release of the leftover matter from the pile-up radius due to a sudden rise of viscosity causing a rebrightening. We show a possible configuration of the evolution of accretion flow during the outburst. From the spectral analysis with TCAF model, we estimate the probable mass of the black hole to lie in the range 6.31 M⊙ to 13.65 M⊙, and the source distance is estimated to be 1.9-8.3 kpc from transition luminosity considerations.

A 2-hr binary period for the black hole transient MAXI J0637-430

ABSTRACT We revisit various sets of published results from X-ray and optical studies of the Galactic black hole (BH) candidate MAXI J0637-430, which went into outburst in 2019. Combining the previously reported values of peak outburst luminosity, best-fitting radii of inner and outer accretion disc, viewing angle, exponential decay time-scale, and peak-to-peak separation of the He II λ4686 disc emission line, we improve the constraints on the system parameters. We estimate a heliocentric distance d ≈ (8.7 ± 2.3) kpc, a projected Galactocentric distance R ≈ (13.2 ± 1.8) kpc and a height |z| ≈ (3.1 ± 0.8) kpc from the Galactic plane. It is the currently known Milky Way BH candidate located farthest from the Galactic Centre. We infer a BH mass M1 ≈ (5.1 ± 1.6)M⊙, a spin parameter a* ≲ 0.25, a donor star mass M2 ≈ (0.25 ± 0.07)M⊙, a peak Eddington ratio λ ≈ 0.17 ± 0.11 and a binary period $P_{\rm orb} \approx 2.2^{+0.8}_{-0.6}$ hr. This is the shortest period measured or estimated so far for any Galactic BH X-ray binary. If the donor star is a main-sequence dwarf, such a period corresponds to the evolutionary stage where orbital shrinking is driven by gravitational radiation and the star has regained contact with its Roche lobe (low end of the period gap). The three Galactic BHs with the shortest period (≲3 hr) are also those with the highest vertical distance from the Galactic plane (≳2 kpc). This is probably because binaries with higher binding energies can survive faster natal kicks.

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.

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.

Anomalous nature of outbursts of black hole candidate 4U 1630−472

ABSTRACT The Galactic black hole candidate (BHC) 4U 1630−472 has gone through several outbursts (13 to be particular) in the last two and a half decades starting from the RXTE era till date. Like the outbursts of other transient BHCs, the outbursts of this source show variations in duration, peak numbers, highest peak flux, etc. However, unlike other soft X-ray transients, this source showed outbursts of two types, such as normal and super. The normal outbursts of duration ∼100−200 d are observed with an average recurrence or quiescence period of ∼500 d. The super outbursts of duration ∼1.5−2.5 yr, consist of more than one normal outburst and one mega outburst. We make an effort to separate the flux contributions of the normal and mega components from the super outbursts, and try to understand the nature of evolution of both types (normal and mega) of outbursts, based on the quiescent period prior to the outbursts. Archival data of RXTE or ASM from 1996 January to 2011 June, and MAXI or GSC from 2009 August to 2020 July are used for our study. Based on the variations of various properties of both the normal and mega outbursts, we think that this BHC is a special source which may contain two companions. Two companions might be responsible for two types of outbursts.

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.

AstroSat observation of non-resonant type-C QPOs in MAXI J1535-571

Galactic transient black hole candidate (BHC) MAXI J1535-571 was discovered on 2017 September 02 simultaneously by {\it MAXI}/GSC and {\it Swift}/BAT instruments. It has also been observed by India's first multi-wavelength astronomy-mission satellite {\it AstroSat}, during the rising phase of its 2017-18 outburst. We make both the spectral and the temporal analysis of the source during 2017 September 12-17 using data of {\it AstroSat}'s Large Area X-ray Proportional Counter (LAXPC) in the energy range of $3-40$~keV to infer the accretion flow properties of the source. Spectral analysis is done with the physical two-component advective flow (TCAF) solution-based {\it fits} file. From the nature of the variation of the TCAF model fitted physical flow parameters, we conclude and confirm that the source was in the intermediate spectral state during our analysis period. We observe sharp type-C quasi-periodic oscillations (QPOs) in the frequency range of $\sim 1.75-2.81$~Hz. For a better understanding of the nature and evolution of these type-C QPOs, a dynamic study of the power density spectra is done. We also investigate the origin of these QPOs from the shock oscillation model. We find that non-satisfaction of Rankine-Hugoniot conditions for non-dissipative shocks and not their resonance oscillations is the cause of the observed type-C QPOs.

Accretion flow properties of GRS 1716-249 during its 2016–17 ‘failed’ outburst

In $2016-17$, the Galactic transient black hole candidate GRS 1716-249 exhibited an outburst event after a long period of quiescence of almost 23 years. The source remained in the outbursting phase for $\sim 9$ months. We study the spectral and temporal properties of the source during this outburst using archival data from four astronomy satellites, namely MAXI, Swift, NuSTAR and AstroSat. Initial spectral analysis is done using combined disk black body and power-law models. For a better understanding of the accretion flow properties, we studied spectra with the physical two component advective flow (TCAF) model. Accretion flow parameters are extracted directly from the spectral fits with the TCAF model. Low frequency quasi-periodic oscillations are also observed in the Swift/XRT and AstroSat/LAXPC data. From the nature of the variation of the spectral and temporal properties, we find the source remains in hard state during the entire outburst. It never had a transition to other states which makes this event a `failed' outburst. An absence of the softer spectral states is consistent with the class of short orbital period objects, where the source belongs to. From the spectral fit, we also estimate the probable mass of GRS~1716-249 to be in the range of $4.50-5.93 M_\odot$ or $5.02^{+0.91}_{-0.52} M_\odot$.

Using space-VLBI to probe gravity around Sgr A*

Context. The Event Horizon Telescope (EHT) will soon provide the first high-resolution images of the Galactic Centre supermassive black hole candidate Sagittarius A* (Sgr A*), enabling us to probe gravity in the strong-field regime. In addition to studying the accretion process in extreme environments, the obtained data and reconstructed images could be used to investigate the underlying spacetime structure. In its current configuration, EHT is able to distinguish between a rotating Kerr black hole and a horizon-less object such as a boson star. Future developments can increase the ability of EHT to tell different spacetimes apart. Aims. We investigate the capability of an advanced EHT concept, including an orbiting space antenna, to image and distinguish different spacetimes around Sgr A*. Methods. We used general-relativistic magneto-hydrodynamical simulations of accreting compact objects (Kerr and dilaton black holes as well as boson stars) and computed their radiative signatures via general-relativistic radiative transfer. To facilitate a comparison with upcoming and future EHT observations, we produced realistic synthetic data including the source variability, diffractive, and refractive scattering while incorporating the observing array, including a space antenna. From the generated synthetic observations, we dynamically reconstructed black hole shadow images using regularised maximum entropy methods. We employed a genetic algorithm to optimise the orbit of the space antenna with respect to improved imaging capabilities and u − v-plane coverage of the combined array (ground array and space antenna) and developed a new method to probe the source variability in Fourier space. Results. The inclusion of an orbiting space antenna improves the capability of EHT to distinguish the spin of Kerr black holes and dilaton black holes based on reconstructed radio images and complex visibilities.

Properties of Faint X-ray Activity of XTE J1908+094 in 2019

We study the properties of the faint X-ray activity of Galactic transient black hole candidate XTE J1908+094 during its 2019 outburst. Here, we report the results of detailed spectral and temporal analysis during this outburst using observations from Nuclear Spectroscopic Telescope Array (NuSTAR). We have not observed any quasi-periodic-oscillations (QPOs) in the power density spectrum (PDS). The spectral study suggests that the source remained in the softer (more precisely, in the soft–intermediate) spectral state during this short period of X-ray activity. We notice a faint but broad Fe Kα emission line at around 6.5 keV. We also estimate the probable mass of the black hole to be 6.5−0.7+0.5M⊙, with 90% confidence.

Rapid compact jet quenching in the Galactic black hole candidate X-ray binary MAXI J1535−571

ABSTRACTWe present results from six epochs of quasi-simultaneous radio, (sub-)millimetre, infrared, optical, and X-ray observations of the black hole X-ray binary MAXI J1535−571. These observations show that as the source transitioned through the hard–intermediate X-ray state towards the soft–intermediate X-ray state, the jet underwent dramatic and rapid changes. We observed the frequency of the jet spectral break, which corresponds to the most compact region in the jet where particle acceleration begins (higher frequencies indicate closer to the black hole), evolves from the infrared band into the radio band (decreasing by ≈3 orders of magnitude) in less than a day. During one observational epoch, we found evidence of the jet spectral break evolving in frequency through the radio band. Estimating the magnetic field and size of the particle acceleration region shows that the rapid fading of the high-energy jet emission was not consistent with radiative cooling; instead, the particle acceleration region seems to be moving away from the black hole on approximately dynamical time-scales. This result suggests that the compact jet quenching is not caused by local changes to the particle acceleration, rather we are observing the acceleration region of the jet travelling away from the black hole with the jet flow. Spectral analysis of the X-ray emission shows a gradual softening in the few days before the dramatic jet changes, followed by a more rapid softening ∼1–2 d after the onset of the jet quenching.

Discovery of the Black Hole X-Ray Binary Transient MAXI J1348–630

Abstract We report the first half-year monitoring of the new Galactic black hole candidate MAXI J1348–630, discovered on 2019 January 26 with the Gas Slit Camera on board the Monitor of All-sky X-ray Image (MAXI). During the monitoring period, the source exhibited two outburst peaks, where the first peak flux (at T = 14 days from the discovery of T = 0) was ∼4 Crab (2–20 keV) and the second one (at T = 132 days) was ∼0.4 Crab (2–20 keV). The source exhibited distinct spectral transitions between the high/soft and low/hard states and an apparent “q”-shape curve on the hardness-intensity diagram, both of which are well-known characteristics of black hole binaries (BHBs). Compared to other bright black hole transients, MAXI J1348–630 is characterized by its low disk temperature (∼0.75 keV at the maximum) and high peak flux in the high/soft state. The low peak temperature leads to a large innermost radius that is identified as the innermost stable circular orbit, determined by the black hole mass and spin. Assuming the empirical relation between the soft-to-hard transition luminosity (L trans) and the Eddington luminosity (L Edd), L trans/L Edd ≈ 0.02, and a face-on disk around a non-spinning black hole, the source distance and the black hole mass are estimated to be D ≈ 4 kpc and , respectively. The black hole is more massive if the disk is inclined and the black hole is spinning. These results suggest that MAXI J1348–630 may host a relatively massive black hole among the known BHBs in our Galaxy.

A parameter survey of Sgr A* radiative models from GRMHD simulations with self-consistent electron heating

ABSTRACT The Galactic centre black hole candidate Sgr A* is the best target for studies of low-luminosity accretion physics, including with near-infrared (NIR) and submillimetre wavelength long baseline interferometry experiments. Here, we compare images and spectra generated from a parameter survey of general relativistic MHD simulations to a set of radio to NIR observations of Sgr A*. Our models span the limits of weak and strong magnetization and use a range of sub-grid prescriptions for electron heating. We find two classes of scenarios can explain the broad shape of the submillimetre spectral peak and the highly variable NIR flaring emission. Weakly magnetized ‘disc-jet’ models where most of the emission is produced near the jet wall, consistent with past work, as well as strongly magnetized (magnetically arrested disc) models where hot electrons are present everywhere. Disc-jet models are strongly depolarized at submillimetre wavelengths as a result of strong Faraday rotation, inconsistent with observations of Sgr A*. We instead favour the strongly magnetized models, which provide a good description of the median and highly variable linear polarization signal. The same models can also explain the observed mean Faraday rotation measure and potentially the polarization signals seen recently in Sgr A* NIR flares.

Inference on disk-jet connection of MAXI J1836–194 from spectral analysis with the TCAF solution

Galactic transient black hole candidate (BHC) MAXI J1836–194 was discovered on 2011 Aug 30, by MAXI/GSC and Swift/BAT. The source activity during this outburst continued for ∼ 3 months before entering into the quiescent state. It again became active in March 2012 and continued for another ∼ 2 months. In this paper, 3 – 25 keV RXTE/PCA spectra from the 2011 outburst and 0.5 – 10.0 keV Swift/XRT data during its 2012 outburst are analyzed with the two-component advective flow (TCAF) model based fits files in XSPEC. We calculate the X-ray contributions coming from jets/outflow using a newly developed method based on the deviation of the TCAF model normalization. We also study the correlation between observed radio and estimated jet X-ray fluxes. The correlation indices (b) are found to be 1.79 and 0.61, when the 7.45GHz Very Large Array (VLA) radio flux is correlated with the total X-ray and jet X-ray fluxes in 3 – 25 keV range respectively. It has been found that the jet contributes in X-rays up to a maximum of 86% during its 2011 outburst. This makes the BHC MAXI J1836–194 strongly jet dominated during the initial rising phase.