We report results from X-ray and optical observations of the Galactic black hole candidate MAXI J1828-249, performed with Suzaku and the Kanata telescope around the X-ray flux peak in the 2013 outburst. The time-averaged X-ray spectrum covering 0.6--168 keV was approximately characterized by a strong multi-color disk blackbody component with an inner disk temperature of ~0.6 keV, and a power-law tail with a photon index of ~2.0. We detected an additional structure at 5-10 keV, which can be modelled neither with X-ray reflection on the disk, nor relativistic broadening of the disk emission. Instead, it was successfully reproduced with a Comptonization of disk photons by thermal electrons with a relatively low temperature (<~10 keV). We infer that the source was in the intermediate state, considering its long-term trend in the hardness intensity diagram, the strength of the spectral power-law tail, and its variability properties. The low-temperature Comptonization component could be produced in a boundary region between the truncated standard disk and the hot inner flow, or a Comptonizing region that somehow developed above the disk surface. The multi-wavelength spectral energy distribution suggests that the optical and UV fluxes were dominated by irradiated outer disk emission.
The black hole candidate Swift J1753.5-0127 went into outburst in 2005 June. Rather than fade into quiescence as most black-hole-candidate transients do, it has remained in a low hard spectral state for most of the $\sim$ 9 years after outburst. The persistent emission while in a hard state is reminiscent of the black hole Cyg X-1 and the black hole candidates 1E 1740.7-2942 and GRS 1758-258. Thus far hard X-ray/soft gamma-ray results have focused mainly on the 2005 flare, with a few additional observations during 2007. Here, we present results from INTEGRAL/SPI observations from 2005 - 2010 s…
The theoretical concept of Chakrabarti-Titarchuk two Component Advective Flow (TCAF) model was introduced around two decade ago in mid-90s. Recently after the inclusion of TCAF model into XSPEC as an additive table model, we find that it is quite capable to fit spectra from different phases of few transient black hole candidates (TBHCs) during their outbursts. This quite agrees with our theoretical understanding. Here, a brief summary of our recent studies of spectral and temporal properties of few TBHCs during their X-ray outbursts with TCAF will be discussed.
We use arguments developed in previous work to identify a second black hole candidate associated with a M31 globular cluster, Bo 144, on the basis of X-ray spectral and timing properties. The 2002 XMM-Newton observation of the associated X-ray source (hereafter XBo 144) revealed behaviour that is common to all low-mass X-ray binaries (LMXBs) in the low-hard state. Studies have shown that neutron star LMXBs exhibit this behaviour at 0.01-1000 keV luminosities <=10% of the Eddington limit (L_Edd). However, the unabsorbed 0.3-10 keV XBo 144 luminosity was ~0.30 L_Edd for a 1.4 M_sun neutron star,…
In our study of the timing properties of few Galactic black hole candidates evolutions of the low and intermediate frequency quasi-periodic oscillations (LIFQPOs) are observed. In 2005, for explaining evolution of QPO frequency during rising phase of 2005 GRO J1655-40 outburst, Chakrabarti and his students introduced a new model, namely propagating oscillatory shock (POS) model. Here we present the results obtained from the same POS model fitted QPO evolutions during both the rising and declining phases of the outbursts of 2005 GRO J165540, 2010-11 GX 339-4, and 2010 & 2011 H 1743-322.
We present a systematic search for periodically varying quasar and supermassive black hole binary (SMBHB) candidates in the Pan-STARRS1 Medium Deep Survey. From $\sim9,000$ color-selected quasars in a $\sim50$ deg$^{2}$ sky area, we initially identify $26$ candidates with more than $1.5$ cycles of variation. We extend the baseline of observations via our imaging campaign with the Discovery Channel Telescope and the Las Cumbres Observatory network and reevaluate the candidates using a more rigorous, maximum likelihood method. Using a range of statistical criteria and assuming the Damped Random …
MAXI J1803-298, a newly-discovered Galactic transient and black hole candidate, was first detected by \emph{MAXI}/GSC on May 1st, 2021. In this paper, we present a detailed spectral analysis of MAXI J1803-298. Utilizing the X-ray reflection fitting method, we perform a joint fit to the spectra of MAXI J1803-298, respectively, observed by \emph{NuSTAR} and \emph{NICER}/XTI on the same day over the energy range between 0.7-79.0 keV, and found its spin (and the inclination angle i) can be constrained to be close to an extreme value, 0.991 ($i\sim$ $70 ^{\circ}$), at 68\% confidence interval. The …
AT2019wey is a Galactic low mass X-ray binary with a candidate black hole accretor first discovered as an optical transient by ATLAS in December 2019. It was then associated with an X-ray source discovered by SRG in March 2020. After observing a brightening in X-rays in August 2020, VLA observations of the source revealed an optically thin spectrum that subsequently shifted to optically thick, as the source continued to brighten in radio. This motivated observations of the source with the VLBA. We found a resolved source that we interpret to be a steady compact jet, a feature associated with b…
We present our monitoring campaign of the outburst of the black-hole candidate Swift J1753.5-0127, observed with the Rossi X-ray Timing Explorer and the Swift satellites. After ~4.5 years since its discovery, the source had a transition to the hard intermediate state. We performed spectral and timing studies of the transition showing that, unlike the majority of the transient black holes, the system did not go to the soft states but it returned to the hard state after a few months. During this transition Swift J1753.5-0127 features properties which are similar to those displayed by the black h…
Galactic black hole candidates GRS 1915+105 and IGR J17091-3624 have many similarities in their light curves and spectral properties. However, very little is known about the orbital elements of their companions. In case the orbits are eccentric, tidal forces by the black hole on the companion can cause modulations of accretion rates in orbital time scale. We look for these modulations in the light curves of these two objects and find that their periodicities are around 28.3 d (29.0 d) and 32.2 d respectively. Eccentricities are at the most 0.071 and 0.46 respectively. We conclude that both the…
In this paper we present a comprehensive study of the mass of the intermediate mass black hole candidate HLX-1 in the galaxy ESO 243-49. We analyse the continuum X-ray spectra collected by Swift, XMM-Newton, and Chandra with the slim disc model, SLIMBH, and estimate the black hole mass for the full range of inclination (inc = 0° - 85°) and spin (a* = 0 - 0.998). The relativistic SLIMBH model is particularly suited to study high luminosity disc spectra as it incorporates the effects of advection, such as the shift of the inner disc edge towards smaller radii and the increasing height of the dis…
I obtained time-series photometry of the compact binary candidate for the Fermi source 4FGL J0935.3+0901. Superposed on the 2.44 hr orbital modulation are day-to-day variations and frequent flaring as seen in several redback and black widow millisecond pulsars (MSPs). The short orbital period favors a black widow. While the modulation of $\leq 1$ mag is smaller than that of most black widows, it could indicate a low orbital inclination. Although a published optical spectrum shows strong emission lines, the light curve evinces pulsar heating of the companion star rather than accretion-disk emis…
We present the results of a broadband (0.5-78 keV) X-ray spectral study of the persistent Galactic black hole X-ray binary GRS 1758-258 observed simultaneously by Swift and NuSTAR. Fitting with an absorbed power-law model revealed a broad Fe line and reflection hump in the spectrum. We used different flavours of the relativistic reflection model for the spectral analysis. All models indicate the spin of the black hole in GRS 1758-258 is >0.92. The source was in the low hard state during the observation, with the hot electron temperature of the corona estimated to be kT$_e$ ~ 140 keV. The black…
Isolated black holes and neutron stars can be revealed through the observation of long duration gravitational microlensing events. A few candidates have been found in surveys of stars in the direction of the Galactic bulge. Recently, thanks to the addition of astrometric information at milliarcsecond level, it has been possible to reduce the uncertainties in the masses and distances for some of these "dark" gravitational lenses and select the most promising candidates. These isolated compact objects might emit X-rays powered by accretion from the interstellar medium. Using data of the Chandra,…
We present here the results of broadband spectral analysis of low-mass X-ray binary and a black hole candidate 4U 1957+11. The source was observed nine times with the Nuclear Spectroscopic Telescope Array (NuSTAR) between 2018 September and 2019 November. During these observations, the spectral state of 4U 1957+11 evolved marginally. The disc dominant spectra are well described with a hot, multicolour disc blackbody with disc temperature varying in the range $kT_{\rm in} \sim$ 1.35-1.86 keV and a non-thermal component having a steep slope ($Γ=$ 2-3). A broad Fe emission line feature (5-8 keV) …
The gravitational-wave event S190521g -- a likely binary black hole merger in the accretion disk of an active galactic nucleus -- was accompanied by an optical counterpart. Such dense environments around luminous energy release regions are favourable for high energy $γ$-ray production. We report on a search for high energy $γ$-rays from the direction of the candidate electromagnetic counterpart to the S190521g event using publicly-available data of the Fermi-LAT space $γ$-ray telescope. No significant signal was found. We present upper limits on the spectral energy distribution of the source i…
We report on the long-term monitoring campaign of the black hole candidate IGR J17091-3624 performed with INTEGRAL and Swift during the peculiar outburst started on January 2011. We have studied the two month spectral evolution of the source in detail. Unlike the previous outbursts, the initial transition from the hard to the soft state in 2011 was not followed by the standard spectral evolution expected for a transient black hole binary. IGR J17091-3624 showed pseudo periodic flare-like events in the light curve, closely resembling those observed from GRS 1915+105. We find evidence that these…
The radio-loud quasar SDSS J013127.34-032100.1at a redshift z=5.18 is one of the most distant radio-loud objects. The radio to optical flux ratio (i.e. the radio-loudness) of the source is large, making it a promising blazar candidate. Its overall spectral energy distribution, completed by the X-ray flux and spectral slope derived through Target of Opportunity Swift/XRT observations, is interpreted by a non-thermal jet plus an accretion disc and molecular torus model. We estimate that its black hole mass is (1.1+-0.2)1e10 Msun. for an accretion efficiency eta=0.08, scaling roughly linearly wit…
We report on the analysis of all 65 pointed Rossi X-ray Timing Explorer observations of the recently discovered soft X-ray transient MAXI J1659-152 (initially referred to as GRB 100925A). The source was studied in terms of its evolution through the hardness-intensity diagram (HID) as well as its X-ray variability properties. MAXI J1659-152 traced out an anti-clockwise loop in the HID, which is commonly seen in transient low-mass X-ray binaries. The variability properties of the source, in particular the detection of type-B and type-C low-frequency quasi-periodic oscillations, and the way they …
In recent years, fundamental relationships for the black hole X-ray binaries have been established between their X-ray luminosity $L_X$ and the photon index $Γ$ of their X-ray spectrum. For the moderate-luminosity regime, an anti-correlation between $Γ$ and $L_X$ has been observed. In this article, aimed to verify if the moderate luminous X-ray binary system LSI +61303 is a black hole, we analyse $Swift$ observations of LSI +61303. We compare the derived $L_X$ vs $Γ$ distribution, first with the statistical trend for black hole X-ray binaries, then with the trend of the pulsar PSR B1259-63, an…
Accreting black holes are known to power relativistic jets, both in stellar-mass binary systems and at the centres of galaxies. The power carried away by the jets, and hence the feedback they provide to their surroundings, depends strongly on their composition. Jets containing a baryonic component should carry significantly more energy than electron-positron jets. While energetic considerations and circular polarisation measurements have provided conflicting circumstantial evidence for the presence or absence of baryons, the only system in which baryons have been unequivocally detected in the …
Astrophysical black hole candidates are thought to be the Kerr black holes predicted by General Relativity, but the actual nature of these objects has still to be proven. The analysis of the electromagnetic radiation emitted by a geometrically thin and optically thick accretion disk around a black hole candidate can provide information about the geometry of the space-time around the compact object and it can thus test the Kerr black hole hypothesis. In this paper, I present a code based on a ray-tracing approach and capable of computing some basic properties of thin accretion disks in space-ti…
We have previously identified 10 M31 black hole candidates (BHCs) in M31, from their X-ray properties alone. They exhibit "hard state" emission spectra that are seen at luminosities < ~10% Eddington in X-ray binaries (XBs) containing a neutron star (NS) or black hole (BH), at luminosities that significantly exceed the NS threshold. Nine of these are associated with globular clusters (GCs); hence, these are most likely low mass X-ray binaries (LMXBs); eight are included in this survey. We have recently discovered that analysis of the long term 0.5--4.5 keV variability of XBs via structure funct…
We have selected SDSS J222032.50+002537.5 and SDSS J142048.01+120545.9 as best blazar candidates out of a complete sample of extremely radio-loud quasars at z>4, with highly massive black holes. We observed them and a third serendipitous candidate with similar features (PMN J2134-0419) in the X-rays with the Swift/XRT telescope, to confirm their blazar nature. We observed strong and hard X-ray fluxes (i.e. $α_X<0.6$, where $F(ν)\proptoν^{-α_X}$ in the 0.3-10keV observed energy range, ~1-40keV rest frame) in all three cases. This allowed us to classify our candidates as real blazars, being char…
We observed the new X-ray transient and black-hole candidate XTE J1652-453 simultaneously with XMM-Newton and the Rossi X-ray Timing Explorer (RXTE). The observation was done during the decay of the 2009 outburst, when XTE J1652-453 was in the hard-intermediate state. The spectrum shows a strong and broad iron emission line with an equivalent width of ~ 450 eV. The profile is consistent with that of a line being produced by reflection off the accretion disk, broadened by relativistic effects close to the black hole. The best-fitting inner radius of the accretion disk is ~ 4 gravitational radii…
We perform a search for stellar-mass black hole candidates in the spatial regions with increased probability of their occurrence, isolated based on the evolutionary scenarios for compact objects originating in disrupted binaries. We analyze the sources located in these regions with available spectral or photometric data, as well as measured proper motions and distances. Nine objects that exhibit characteristics corresponding to theoretical predictions for isolated black holes are marked for further study as black hole candidates.
Swift J1842.5$-$1124 is a transient Galactic black hole X-ray binary candidate, which underwent a new outburst in May 2020. We performed multi-epoch MeerKAT radio observations under the ThunderKAT Large Survey Programme, coordinated with quasi-simultaneous Swift/XRT X-ray observations during the outburst, which lasted nearly a month. We were able to make the first-ever radio detection of this black hole binary with the highest flux density of 229$\pm$31 $μ$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 emer…
The RXTE and Swift observations on the 2009 outburst from a newly discovered transient and black-hole candidate XTE J1652$-$453 are analyzed. The source was observed by RXTE to behave a sequence of spectral states that are typical to the black hole XRBs. During the first 7 observations, the source is diagnosed to stay in a high/soft state, from the spectrum dominated by soft thermal component which contributes an average of $\sim$ 85% to the X-ray flux at 2-20keV, and from the hardness $\sim$ 0.1 showing up in the hardness-intensity diagram (HID). For the last 20 observations the spectral stat…
In general, it is very difficult to test the Kerr-nature of an astrophysical black hole candidate, because it is not possible to have independent measurements of both the spin parameter $a_*$ and possible deviations from the Kerr solution. Non-Kerr objects may indeed look like Kerr black holes with different spin. However, it is much more difficult to mimic an extremal Kerr black hole. The black hole candidate in Cygnus X-1 has the features of a near extremal Kerr black hole, and it is therefore a good object to test the Kerr black hole paradigm. The 3$σ$-bounds $a_* > 0.95$ and $a_* > 0.983$ …
We present the results of a weekly monitoring of the new black hole candidate X-ray binary MAXI J1631-472 carried out with the MeerKAT radio interferometer, the Neil Gehrels Swift Observatory, and the Monitor of All-sky X-ray Image (MAXI) instrument, during its 2018-2019 outburst. The source exhibits a number of X-ray states, in particular both high- and low-luminosity hard states bracketed by extended soft states. Radio flaring is observed shortly after a transition from hard/intermediate states to the soft state. This is broadly in agreement with existing empirical models, but its extended d…
Owing to the frequency and reproducibility of its outbursts, the black-hole candidate GX 339-4 has become the standard against which the outbursts of other black-hole candidate are matched up. Here we present the first systematic study of the evolution of the X-ray lags of the broad-band variability component (0.008-5 Hz) in GX 339-4 as a function of the position of the source in the hardness-intensity diagram. The hard photons always lag the soft ones, consistent with previous results. In the low-hard state the lags correlate with X-ray intensity, and as the source starts the transition to th…
We present the results of X-ray variability and spectral analysis of a sample of 15 new candidates for active galactic nuclei with relatively low-mass black holes (BHs). They are selected from the Second XMM-Newton Serendipitous Source Catalogue based on strong variability quantified by normalized excess variances. Their BH masses are estimated to be 1.1-6.6x10^6 M_solar by using a correlation between excess variance and BH mass. Seven sources have estimated BH masses smaller than 2x10^6 M_solar, which are in the range for intermediate-mass black holes. Eddington ratios of sources with known r…
RZ2109 is the first of several extragalactic globular clusters shown to host an ultraluminous X-ray source. RZ2109 is particularly notable because optical spectroscopy shows it has broad, luminous [OIII] 4959,5007 emission, while also having no detectable hydrogen emission. The X-ray and optical characteristics of the source in RZ2109 make it a good candidate for being a stellar mass black hole accreting from a white dwarf donor (i.e. an ultracompact black hole X-ray binary). In this paper we present optical spectroscopic monitoring of the [OIII]5007 emission line from 2007 to 2018. We find th…
We present a spectral analysis of the black hole candidate and X-ray transient source Swift J1753.5 0127 making use of simultaneous observations of XMM-Newton and Rossi X-ray Timing Explorer (RXTE) in 2006, when the source was in outburst. The aim of this paper is to test whether a thermal component due to the accretion disc is present in the X-ray spectrum. We fit the data with a range of spectral models, and we find that for all of these models the fits to the X-ray energy spectra significantly require the addition of the disc black-body component. We also find a broad iron emission line at …
We report on a Suzaku observation of the newly discovered X-ray binary MAXI J1836-194. The source is found to be in the hard/intermediate spectral state and displays a clear and strong relativistically broadened iron emission line. We fit the spectra with a variety of phenomenological, as well as physically motivated disk reflection models, and find that the breadth and strength of the iron line is always characteristic of emission within a few gravitational radii around a black hole. This result is independent of the continuum used and strongly points toward the central object in MAXI J1836-1…
Quasi-periodic oscillations (QPOs) are a common feature in the X-ray flux of stellar-mass black hole candidates, but their exact origin is not yet known. Recently, some authors have pointed out that data of GRO J1655-40 simultaneously show three QPOs that nicely fit in the relativistic precession model. However, they find an estimate of the spin parameter that disagrees with the measurement of the disk's thermal spectrum. In the present work, I explore the possibility of using the relativistic precession model to test the nature of the black hole candidate in GRO J1655-40. If properly understo…
Astrophysical black hole candidates are thought to be the Kerr black holes of general relativity, but there is not yet direct observational evidence that the spacetime geometry around these objects is described by the Kerr solution. The study of the properties of the electromagnetic radiation emitted by gas or stars orbiting these objects can potentially test the Kerr black hole hypothesis. In this paper, I review the state of the art of this research field, describing the possible approaches to test the Kerr metric with current and future observational facilities and discussing current constr…
We report on the discovery by the Swift Gamma-Ray Burst Explorer of the black hole candidate Swift J1539.2-6227 and the subsequent course of an outburst beginning in November 2008 and lasting at least seven months. The source was discovered during normal observations with the Swift Burst Alert Telescope (BAT) on 2008 November 25. An extended observing campaign with the Rossi X-Ray Timing Explorer (RXTE) and Swift provided near-daily coverage over 176 days, giving us a good opportunity to track the evolution of spectral and timing parameters with fine temporal resolution through a series of spe…
The XMM-Newton spectrum of the black hole candidate XTE J1652-453 shows a broad and strong Fe K-alpha emission line, generally believed to originate from reflection of the inner accretion disc. These data have been analysed by Hiemstra et al. (2011) using a variety of phenomenological models. We re-examine the spectrum with a self-consistent relativistic reflection model. A narrow absorption line near 7.2 keV may be present, which if real is likely the Fe XXVI absorption line arising from highly ionised, rapidly outflowing disc wind. The blue shift of this feature corresponds to a velocity of …
Astrophysical black hole candidates are thought to be the Kerr black holes predicted by General Relativity, as these objects cannot be explained otherwise without introducing new physics. However, there is no observational evidence that the space-time around them is really described by the Kerr solution. The Kerr black hole hypothesis can be tested with the already available X-ray data by extending the continuum-fitting method, a technique currently used by astronomers to estimate the spins of stellar-mass black hole candidates. In general, we cannot put a constraint on possible deviations fro…
In this letter, we note that the observed in the LIGO / Virgo experiment ratio of the detection rate of black holes to the rate of detection of binary neutron stars requires the assumption of a "conservative" collapse of massive stars into a black hole: almost all the mass of the collapsing star goes under the horizon. This is consistent with the large masses of black holes detected by LIGO/Virgo. On the other hand, the assumption of a small loss of matter during the collapse into a black hole is in good agreement with the small eccentricity of Single-lined Binaries. At the same time, the abse…
The LIGO/Virgo Consortium (LVC) released a preliminary announcement of a candidate gravitational wave signal, S190426c, that could have arisen from a black hole-neutron star merger. As the first such candidate system, it's properties such as masses and spin are of great interest. Although LVC policy prohibits disclosure of these properties in preliminary announcements, LVC does release the estimated probabilities that this system is in specific categories, such as binary neutron star, binary black hole and black hole-neutron star. LVC also releases information concerning relative signal streng…
Astrophysical black hole candidates are thought to be the Kerr black holes predicted by General Relativity, but the actual nature of these objects has still to be proven. The Kerr black hole hypothesis can be tested by observing strong gravity features and check if they are in agreement with the predictions of General Relativity. In particular, the study of the properties of the electromagnetic radiation emitted by the gas of the accretion disk can provide information on the geometry of the space-time around these objects and constrain possible deviations from the Kerr background.
H1743-322 is one of the few black hole candidates (BHCs) in low-mass X-ray binaries that shows mHz quasi-periodic oscillations (QPOs) that are not associated with the more common type A, B and C oscillations seen in the X-ray light curves of typical BHCs systems. To better understand the physical origin of the mHz oscillations, we carried out a phase-resolved spectroscopic study of two RXTE observations of this source. As previously reported, the averaged energy spectra of H1743-322 shows a strong iron line at $\sim6.5$ keV. Here we found evidence that the line flux appears to be modulated at …
We report the most recent results of our analysis of X-ray monitoring of new Galactic Black Hole (BH) candidates XTE J1752-225 and MAXI J1659-152 performed by Rossi X-ray Timing Explorer (RXTE). We investigate various aspects of the RXTE data including energy and power spectra, variability energy distribution and phase lags between soft and hard energy bands. The sources generally exhibit the spectral states and evolution expected from an accreting stellar mass BH. The energy distribution of different variability components show that the aperiodic noise has a spectrum consistently softer with …
Black holes have the peculiar and intriguing property of having an event horizon, a one-way membrane causally separating their internal region from the rest of the Universe. Today astrophysical observations provide some evidence for the existence of event horizons in astrophysical black hole candidates. In this short paper, I compare the constraint we can infer from the non-observation of electromagnetic radiation from the putative surface of these objects with the bound coming from the ergoregion instability, pointing out the respective assumptions and limitations.
We present the results obtained from broadband X-ray timing and spectral analysis of black hole candidate MAXI J1803-298 using an AstroSat observation on May 11-12, 2021. Four periodic absorption dips with a periodicity of $7.02 \pm 0.18$ hour 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 $5.31 \pm 0.02$ Hz to $7.61\pm …
Black holes in General Relativity are known as Kerr black holes and are characterized solely by two parameters, the mass $M$ and the spin $J$. All the higher multipole moments of the gravitational field are functions of these two parameters. For instance, the quadrupole moment is $Q=-J^2/M$, which implies that a measurement of $M$, $J$, and $Q$ for black hole candidates would allow one to test whether these objects are really black holes as described by General Relativity. While future gravitational-wave experiments will be able to test the Kerr nature of these objects with very high accuracy,…
We present detailed timing and spectral studies of the black hole candidate MAXI J0637$-$430 during its 2019-2020 outburst using observations with the {\em Neutron Star Interior Composition Explorer (NICER)} and the {\em Neil Gehrels Swift Observatory}. We find that the source evolves through the soft-intermediate, high-soft, hard-intermediate and low-hard states during the outburst. No evidence of quasi-periodic oscillations is found in the power density spectra of the source. Weak variability with fractional rms amplitude $<5\%$ is found in the softer spectral states. In the hard-intermediat…
We report on the detection and subsequent X-ray monitoring of the new Galactic black hole candidate MAXI J1535-571 with the MAXI/GSC. Afterthe discovery on 2017 September 2 made independently with MAXI and the Swift/BAT, the source brightened gradually, and in a few weeks, reached the peak intensity of ~5 Crab, or ~1.6 x 10^{-7} erg cm^{-2} s^{-1} in terms of the 2--20 keV flux. On the initial outburst rise, the X-ray spectrum was described by a power-law model with a photon index of ~2, while after a hard-to-soft transition which occurred on September 18, the spectrum required a disk blackbod…
The mass function of super-massive black holes in our cosmic neighborhood is required to understand the statistics of their activity and consequently the origin of the ultra high energy particles. We determine a mass function of black hole candidates from the entire sky except for the Galactic plane. Using the 2MASS catalogue as a starting point, and the well established correlation between black hole mass and the bulge of old population of stars, we derive a list of nearby black hole candidates within the redshift range z < 0.025, then do a further selection based on the Hubble-type, and give…
The rotational energy of a black hole can be extracted via the Blandford-Znajek mechanism and numerical simulations suggest a strong dependence of the power of the produced jet on the black hole spin. A recent study has found no evidence for a correlation between the spin and the power of steady jets. If the measurements of the spin and of the jet power are correct, it leads one to conclude that steady jets are not powered by the black hole spin. In this paper, I explore another possibility: I assume that steady jets are powered by the spin and I check if current observations can be explained …
We discuss a parametrization to describe possible deviations from the Kerr metric and test astrophysical black hole candidates with electromagnetic radiation. Our metric is a very simple generalization of the Kerr solution with two main properties: $i)$ the phenomenology is quite rich and, for example, it can describe black holes with high Novikov-Thorne radiative efficiency or black holes of very small size; $ii)$ it is suitable for the numerical calculations required to study the spectrum of thin disks. The latter point is our principal motivation to study such a kind of parametrization, bec…
The two high frequency quasi periodic oscillations (HFQPOs) recently reported in the black hole candidate IGR J17091-3624 by Altamirano and Belloni (2012) are in a 5:2 frequency ratio (164 Hz to 66 Hz). This ratio is strongly suggestive of period doubling and nonlinear resonance analogous to phenomena known in RV Tauri-type pulsating stars (and recently discovered also in oscillations of RR Lyrae-type and of BL Herculis-type variables). An interpretation of the frequency ratio in terms of nonlinear interactions and a comparison with the HFQPOs reported in GRS 1915+105 may imply a mass of about…
The super-massive objects in galactic nuclei are thought to be the Kerr black holes predicted by General Relativity, although a definite proof of their actual nature is still lacking. The most massive objects in AGN ($M \sim 10^9 M_\odot$) seem to have a high radiative efficiency ($η\sim 0.4$) and a moderate mass accretion rate ($L_{\rm bol}/L_{\rm Edd} \sim 0.3$). The high radiative efficiency could suggest they are very rapidly-rotating black holes. The moderate luminosity could indicate that their accretion disk is geometrically thin. If so, these objects could be excellent candidates to te…
We address the question of regular primordial black holes with de Sitter interior, their remnants and gravitational vacuum solitons G-lumps as heavy dark matter candidates providing signatures for inhomogeneity of early universe, which is severely constrained by the condition that the contribution of these objects in the modern density does not exceed the total density of dark matter. Primordial black holes and their remnants seem to be most elusive among dark matter candidates. However, we reveal a nontrivial property of compact objects with de Sitter interior to induce proton decay or decay …
We report the discovery of a candidate stellar-mass black hole in the Milky Way globular cluster M62. We detected the black hole candidate, which we term M62-VLA1, in the core of the cluster using deep radio continuum imaging from the Karl G. Jansky Very Large Array. M62-VLA1 is a faint source, with a flux density of 18.7 +/- 1.9 microJy at 6.2 GHz and a flat radio spectrum (alpha=-0.24 +/- 0.42, for S_nu = nu^alpha). M62 is the second Milky Way cluster with a candidate stellar-mass black hole; unlike the two candidate black holes previously found in the cluster M22, M62-VLA1 is associated wit…
Astrophysical black hole candidates are thought to be the Kerr black holes predicted by General Relativity, but there is not yet clear evidence that the geometry of the space-time around these objects is really described by the Kerr metric. In order to confirm the Kerr black hole hypothesis, we have to observe strong gravity features and check that they are in agreement with the ones predicted by General Relativity. In this paper, I study the broad K$α$ iron line, which is often seen in the X-ray spectrum of both stellar-mass and super-massive black hole candidates and whose shape is supposed …
The specific angular momentum of a Kerr black hole must not be larger than its mass. The observational confirmation of this bound which we call a Kerr bound directly suggests the existence of a black hole. In order to investigate observational testability of this bound by using the X-ray energy spectrum of black hole candidates, we calculate energy spectra for a super-spinning object (or a naked singularity) which is described by a Kerr metric but whose specific angular momentum is larger than its mass, and then compare the spectra of this object with those of a black hole. We assume an optica…
Most stellar remnants so far have been found in binary systems, where they interact with matter from their companions. Isolated neutron stars and black holes are difficult to find as they are dark, yet they are predicted to exist in our Galaxy in vast numbers. We explored the OGLE-III database of 150 million objects observed in years 2001-2009 and found 59 microlensing events exhibiting a parallax effect due to the Earth's motion around the Sun. Combining parallax and brightness measurements from microlensing light curves with expected proper motions in the Milky Way, we identified 13 microlen…
