Putative Black Hole Research Articles

Chaotic LLM billiards

We study null geodesics of the ten-dimensional LLM geometries. In particular, we show that there are a subset of these null geodesics that are confined to the LLM plane. The effective dynamics of these in-plane geodesics is that of a Hamiltonian system with two degrees of freedom (a phase space of dimension 4). We show that these are chaotic. In the two-coloring of the LLM plane, if they start in the empty region, they cannot penetrate the filled region and viceversa. The dynamical problem is therefore very similar to that of a billiards problem with fixed obstacles. We study to what extent LLM geometries with many droplets may be treated as an incipient black hole and draw analogies with the fuzzball proposal. We argue that for in-plane null geodesics deep in the interior of a region with a lot of droplets, in order to exit towards the AdS boundary they will need to undergo a process that resembles diffusion. This mechanism can account for signals getting lost in the putative black hole for a very long time.

The central black hole in the dwarf spheroidal galaxy Leo I: Not supermassive, at most an intermediate-mass candidate

It has recently been claimed that a surprisingly massive black hole (BH) is present in the core of the dwarf spheroidal galaxy (dSph) Leo I. This finding, based on integral field spectroscopy, challenges the typical expectation that dSphs host intermediate-mass BHs since such a BH would be classified as supermassive. Indeed, the analysis points toward Leo I harboring a BH with a lower mass limit exceeding a few $10^6 at $1 and the no-BH case excluded at 95<!PCT!> significance. Such a mass, which is comparable to the entire stellar mass of the galaxy, makes Leo I a unique system that warrants further investigation. Using equilibrium models based on distribution functions that depend on actions fJ coupled with the same integral field spectroscopy data and an extensive exploration of a very large parameter space, we demonstrate, within a comprehensive Bayesian model--data comparison framework, that the posterior on the BH mass is flat toward the low-mass end and, thus, that the kinematics of the central galaxy region only imposes an upper limit on the BH mass of few $10^5 (at $3 Such an upper limit indicates that the putative BH of Leo I is indeed an intermediate-mass BH, and it is also in line with formation scenarios and expectations from scaling relations at the mass regime of dwarf galaxies.

MWC 656 is unlikely to contain a black hole

Contact. MWC 656 was reported to be the first known Be star with a black-hole (BH) companion on a 60 d period. The mass of the proposed BH companion is estimated to be between 4 and 7 M⊙. This estimate is based on radial velocity (RV) measurements derived from the Fe IIλ4583 emission line of the Be star disc and from the He IIλ4686 emission line, assumed to be formed in a disc around the putative BH. Aims. Using new high-resolution spectroscopic data, we investigate whether MWC 656 truly contains a BH. Methods. We used the cross-correlation method to calculate the RVs of both the Be star and the He IIλ4686 emission line to derive a new orbital solution. We also used a spectral disentangling method to look for the spectral signature of a companion. Results. We derived an orbital period of 59.028 ± 0.011 d and a mass ratio of q = MHe II/MBe = 0.12 ± 0.03, much lower than the previously reported value of q = 0.41 ± 0.07. By adopting a mass for the Be star of MBe = 7.8 ± 2.0 M⊙, the mass of the companion is 0.94 ± 0.34 M⊙. For the upper limit of MBe = 16 M⊙ and q = 0.15, the mass of the companion is 2.4 M⊙. Performing the disentangling on mock spectra shows that the spectral signature of a non-degenerate stellar companion with such a low mass cannot be retrieved using our data. Conclusions. Our measurements do not support the presence of a BH companion in MWC 656. Rather, the derived upper limit on the mass of the companion indicates that it is either a neutron star, a white dwarf, or a hot helium star. Far-UV data will aid in rejecting or confirming a hot helium-star companion.

The B-type Binaries Characterisation Programme – II. VFTS 291: a stripped star from a recent mass transfer phase

ABSTRACT Recent studies of massive binaries with putative black hole companions have uncovered a phase of binary evolution that has not been observed before, featuring a bloated stripped star that very recently ceased transferring mass to a main-sequence companion. In this study, we focus on the candidate system VFTS 291, a binary with an orbital period of 108 d and a high semi-amplitude velocity (K1 = 93.7 ± 0.2 km s−1). Through our analysis of the disentangled spectra of the two components, together with dynamical and evolutionary arguments, we identify a narrow-lined star of ∼1.5–2.5 $\, \mathrm{M}_\odot$ dominating the spectrum, and an early B-type main-sequence companion of 13.2 ± 1.5 $\, \mathrm{M}_\odot$. The low mass of the narrow-lined star, and the high mass ratio, suggest that VFTS 291 is a post-mass-transfer system, with the narrow-lined star being bloated and stripped of its hydrogen-rich envelope, sharing many similarities with other recently discovered stripped stars. Our finding is supported by our detailed binary evolution models, which indicate that the system can be well explained by an initial configuration consisting of an 8.1 $\, \mathrm{M}_\odot$ primary with an 8 $\, \mathrm{M}_\odot$ companion in a 7 d orbital period. While some open questions remain, particularly concerning the surface helium enrichment of the stripped star and the rotational velocity of the companion, we expect that high-resolution spectroscopy may help reconcile our estimates with theory. Our study highlights the importance of multi-epoch spectroscopic surveys to identify and characterize binary interaction products, and provides important insights into the evolution of massive binary stars.

Weighing the Darkness. II. Astrometric Measurement of Partial Orbits with Gaia

Over the course of several years, stars trace helical trajectories as they traverse across the sky due to the combined effects of proper motion and parallax. It is well known that the gravitational pull of an unseen companion can cause deviations to these tracks. Several studies have pointed out that the astrometric mission Gaia will be able to identify a slew of new exoplanets, stellar binaries, and compact object companions with orbital periods as short as tens of days to as long as Gaia's lifetime. Here, we use mock astrometric observations to demonstrate that Gaia can identify and characterize black hole companions to luminous stars with orbital periods longer than Gaia's lifetime. Such astrometric binaries have orbital periods too long to exhibit complete orbits, and instead are identified through curvature in their characteristic helical paths. By simultaneously measuring the radius of this curvature and the orbital velocity, constraints can be placed on the underlying orbit. We quantify the precision with which Gaia can measure orbital accelerations and apply that to model predictions for the population of black holes orbiting stars in the stellar neighborhood. Although orbital degeneracies imply that many of the accelerations induced by hidden black holes could also be explained by faint low-mass stars, we discuss how the nature of certain putative black hole companions can be confirmed with high confidence using Gaia data alone.

First direct dynamical detection of a dual supermassive black hole system at sub-kiloparsec separation

We investigated whether the two recently discovered nuclei in NGC 7727 both host a super-massive black hole (SMBH). We used the high spatial resolution mode of the integral-field spectrograph MUSE on the VLT in adaptive optics mode to resolve the stellar kinematics within the sphere of influence of both putative black holes. We combined the kinematic data with an HST-based mass model and used Jeans models to measure their SMBH mass. We report the discovery of a dual SMBH system in NGC 7727. We detect a SMBH in the photometric center of the galaxy in Nucleus 1, with a mass of MSMBH = 1.54−0.15+0.18 × 108 M⊙. In the second nucleus, which is 500 pc offset from the main nucleus, we also find a clear signal for a SMBH with a mass of MBH = 6.33−1.40+3.32 × 106 M⊙. Both SMBHs are detected at high significance. The off-axis nature of Nucleus 2 makes modeling the system challenging; however, a number of robustness tests suggest that a black hole is required to explain the observed kinematics. The SMBH in the offset Nucleus 2 makes up 3.0% of its total mass, which means its SMBH is over-massive compared to the MBH − MBulge scaling relation. This confirms it as the surviving nuclear star cluster of a galaxy that has merged with NGC 7727. This discovery is the first dynamically confirmed dual SMBH system with a projected separation of less than a kiloparsec and the nearest dynamically confirmed dual SMBH at a distance of 27.4 Mpc. The second Nucleus is in an advanced state of inspiral, and it will eventually result in a 1:24 mass ratio SMBH merger. Optical emission lines suggest Nucleus 2 is a Seyfert galaxy, making it a low-luminosity Active Galactic Nuclei. There are likely many more quiescent SMBHs as well as dual SMBH pairs in the local Universe that have been missed by surveys that focus on bright accretion signatures.

Detection of a 100,000 M ⊙ black hole in M31's Most Massive Globular Cluster: A Tidally Stripped Nucleus

Abstract We investigate the presence of a central black hole (BH) in B023-G078, M31's most massive globular cluster. We present high-resolution, adaptive-optics assisted, integral-field spectroscopic kinematics from Gemini/NIFS that show a strong rotation (∼20 km s−1) and a velocity dispersion rise toward the center (37 km s−1). We combine the kinematic data with a mass model based on a two-component fit to HST ACS/HRC data of the cluster to estimate the mass of a putative BH. Our dynamical modeling suggests a >3σ detection of a BH component of 9.1 − 2.8 + 2.6 × 10 4 M ⊙ (1σ uncertainties). The inferred stellar mass of the cluster is 6.22 − 0.05 + 0.03 × 10 6 M ⊙ , consistent with previous estimates, thus the BH makes up 1.5% of its mass. We examine whether the observed kinematics are caused by a collection of stellar mass BHs by modeling an extended dark mass as a Plummer profile. The upper limit on the size scale of the extended mass is 0.56 pc (95% confidence), which does not rule out an extended mass. There is compelling evidence that B023-G078 is the tidally stripped nucleus of a galaxy with a stellar mass >109 M ⊙, including its high-mass, two-component luminosity profile, color, metallicity gradient, and spread in metallicity. Given the emerging evidence that the central BH occupation fraction of >109 M ⊙ galaxies is high, the most plausible interpretation of the kinematic data is that B023-G078 hosts a central BH. This makes it the strongest BH detection in a lower-mass (<107 M ⊙) stripped nucleus, and one of the few dynamically detected intermediate-mass BHs.

Modeling and detecting resonant tides of exotic compact objects

The event horizon of a black hole in general relativity absorbs all infalling radiation. Any observation of the contrary would immediately challenge the expectation that astrophysical black holes are described by the vacuum Kerr geometry. If a putative black hole does reflect part of the ingoing radiation, its quasinormal mode structure is drastically altered. Low frequency modes can be introduced that are resonantly excited during the inspiral of a binary system. We study the resulting phase shift of the gravitational wave signal. Building on neutron star results, we obtain a model-independent expression for the phase shift that depends only on quasinormal modes and Love numbers of the compact object. We find that the phase shift might be detectable with Einstein Telescope for asymmetric binaries in high signal-to-noise events ($\ensuremath{\sim}{10}^{3}$), but by far cannot explore the Planck scale.

A stripped-companion origin for Be stars: clues from the putative black holes HR 6819 and LB-1

ABSTRACT HR 6819 is a bright (V = 5.36), blue star recently proposed to be a triple containing a detached black hole (BH). We show that the system is a binary and does not contain a BH. Using spectral decomposition, we disentangle the observed composite spectra into two components: a rapidly rotating Be star and a slowly rotating B star with low surface gravity (log g ≈ 2.75). Both stars show periodic radial velocity (RV) variability, but the RV semi-amplitude of the B star’s orbit is $K_{\rm B}= (62.7 \pm 1)\, \rm km\, s^{-1}$, while that of the Be star is only $K_{\rm Be} = (4.5\pm 2)\, \rm km\, s^{-1}$. This implies that the B star is less massive by at least a factor of 10. The surface abundances of the B star bear imprints of CNO burning. We argue that the B star is a bloated, recently stripped helium star with mass ${\approx}0.5\, \mathrm{ M}_{\odot }$ that is currently contracting to become a hot subdwarf. The orbital motion of the Be star obviates the need for a BH to explain the B star’s motion. A stripped-star model reproduces the observed luminosity of the system, while a normal star with the B star’s temperature and gravity would be more than 10 times too luminous. HR 6819 and the binary LB-1 probably formed through similar channels. We use MESA (Modules for Experiments in Stellar Astrophysics) models to investigate their evolutionary history, finding that they likely formed from intermediate-mass ($3\!-\!7\, \mathrm{ M}_{\odot }$) primaries stripped by slightly lower-mass secondaries and are progenitors to Be + sdOB binaries such as ϕ Persei. The lifetime of their current evolutionary phase is on average 2 × 105 yr, of the order of half a per cent of the total lifetime of the Be phase. This implies that many Be stars have hot subdwarf and white dwarf companions, and that a substantial fraction ($20\!-\!100{{\ \rm per\ cent}}$) of field Be stars form through accretion of material from a binary companion.

On gravitational echoes from ultracompact exotic stars

At the dawn of a golden age for gravitational wave astronomy, we must leave no stone unturned in our quest for new phenomena beyond our current understanding of General Relativity (GR), particle physics and nuclear physics. In this paper we discuss gravitational echoes from ultracompact stars. We restrict our analysis to exact solutions of Einstein field equations in GR that are supported by an isotropic fluid with a physically motivated equations of state (EoS), and in particular we impose the constraint of causality. Our main conclusion is that ultracompact objects belonging to this category are not able to generate gravitational echoes like those that characterize the relaxation phase of a putative black hole mimicker. Nevertheless, we identify a class of physical exotic objects that are compact enough to accommodate the presence of an external unstable light ring, thus opening the possibility of trapping gravitational radiation and affecting the ringdown phase of a merger event. Most importantly, we show that once rotation is included these stars—contrary to what usually expected for ultracompact objects—are not plagued by any ergoregion instability. We extend our analysis for arbitrary values of angular velocity up to the Keplerian limit, and we comment about potential signals relevant for gravitational wave interferometers.

Mass and spin of a Kerr black hole in modified gravity and a test of the Kerr black hole hypothesis

In this paper we compute the Arnowitt-Deser-Misner (ADM) mass, the angular momentum and the charge of the Kerr black hole solution in the scalar-tensor-vector gravity theory [known as the Kerr-MOG (modified-gravity) black hole configuration]; we study in detail as well several properties of this solution such as the stationary limit surface, the event horizon, and the ergosphere, and conclude that the new deformation parameter $\alpha$ affects the geometry of the Kerr-MOG black hole significantly in addition to the ADM mass and spin parameters. Moreover, the ADM mass and black hole event horizon definitions allow us to set a novel upper bound on the deformation parameter and to reveal the correct upper bound on the black hole spin. We further find the geodesics of motion of stars and photons around the Kerr-MOG black hole. By using them we reveal the expressions for the mass and the rotation parameter of the Kerr-MOG black hole in terms of the red- and blueshifts of photons emitted by geodesic particles, i.e., by stars. These calculations supply a new and simple method to further test the general theory of relativity in its strong field limit: If the measured red- and blueshifts of photons exceed the bounds imposed by the general theory of relativity, then the black hole is not of Kerr type. It could also happen that the measurements are allowed by the Kerr-MOG metric, implying that the correct description of the dynamics of stars around a given black hole should be performed using MOG or another modified theory of gravity that correctly predicts the observations. In particular, this method can be applied to test the nature of the putative black hole hosted at the center of the Milky Way in the near future.

Angular momentum of dark matter black holes

The putative black holes which may constitute all the dark matter are described by a Kerr metric with only two parameters, mass M and angular momentum J. There has been little discussion of J since it plays no role in the upcoming attempt at detection by microlensing. Nevertheless J does play a central role in understanding the previous lack of detection, especially of CMB distortion. We explain why bounds previously derived from lack of CMB distortion are too strong for primordial black holes with J non-vanishing. Almost none of the dark matter black holes can be from stellar collapse, and nearly all are primordial, to avoid excessive CMB distortion.

An intermediate-mass black hole in the centre of the globular cluster 47 Tucanae.

Intermediate-mass black holes should help us to understand the evolutionary connection between stellar-mass and super-massive black holes. However, the existence of intermediate-mass black holes is still uncertain, and their formation process is therefore unknown. It has long been suspected that black holes with masses 100 to 10,000 times that of the Sun should form and reside in dense stellar systems. Therefore, dedicated observational campaigns have targeted globular clusters for many decades, searching for signatures of these elusive objects. All candidate signatures appear radio-dim and do not have the X-ray to radio flux ratios required for accreting black holes. Based on the lack of an electromagnetic counterpart, upper limits of 2,060 and 470 solar masses have been placed on the mass of a putative black hole in 47 Tucanae (NGC 104) from radio and X-ray observations, respectively. Here we show there is evidence for a central black hole in 47 Tucanae with a mass of solar masses when the dynamical state of the globular cluster is probed with pulsars. The existence of an intermediate-mass black hole in the centre of one of the densest clusters with no detectable electromagnetic counterpart suggests that the black hole is not accreting at a sufficient rate to make it electromagnetically bright and therefore, contrary to expectations, is gas-starved. This intermediate-mass black hole might be a member of an electromagnetically invisible population of black holes that grow into supermassive black holes in galaxies.

Probing spacetime around Sagittarius A* using modeled VLBI closure phases

The emission region and black hole shadow of Sagittarius A*, the supermassive black hole at the Galactic Center, can be probed with millimeter Very Long Baseline Interferometry. Our goal is to probe the geometry of the emitting plasma around Sgr A* by using modeled mm-VLBI closure phase calculations at 1.3 mm and to constrain the observer's inclination angle and position angle of the black hole spin axis. We have simulated images for three different models of the emission of Sgr A*: an orbiting spot, a disk model, and a jet model. The orbiting spot model was used as a test case scenario, while the disk and jet models are physically driven scenarios based on standard three-dimensional general relativistic magnetohydrodynamic simulations of hot accretion flows. Our results are compared to currently available closure phase observational limits. Our results indicate that more models with closer to edge-on viewing angles are consistent with observational limits. In general, jet and disk geometries can reproduce similar closure phases for different sets of viewing and position angles. Consequently, the favored black hole spin orientation and its magnitude are strongly model dependent. We find that both the jet and the disk models can explain current VLBI limits. We conclude that new observations at 1.3 mm and possibly at longer wavelengths including other triangles of VLBI baselines are necessary to interpret Sgr A* emission and the putative black hole spin parameters.

Revisiting the dynamical case for a massive black hole in IC10 X-1

Abstract The relative phasing of the X-ray eclipse ephemeris and optical radial velocity (RV) curve for the X-ray binary IC10 X-1 suggests that the He [λ4686] emission line originates in a shadowed sector of the stellar wind that avoids ionization by X-rays from the compact object. The line attains maximum blueshift when the wind is directly towards us at mid X-ray eclipse, as is also seen in Cygnus X-3. If the RV curve is unrelated to stellar motion, evidence for a massive black hole (BH) evaporates because the mass function of the binary is unknown. The reported X-ray luminosity, spectrum, slow QPO and broad eclipses caused by absorption/scattering in the Wolf–Rayet (WR) wind are all consistent with either a low-stellar-mass BH or a neutron star (NS). For an NS, the centre of mass lies inside the WR envelope whose motion is then far below the observed 370 km s−1 RV amplitude, while the velocity of the compact object is as high as 600 km s−1. The resulting 0.4 per cent Doppler variation of X-ray spectral lines could be confirmed by missions in development. These arguments also apply to other putative BH binaries whose RV and eclipse curves are not yet phase-connected. Theories of BH formation and predicted rates of gravitational wave sources may need revision.

A XMM-Newton observation of a sample of four close dwarf spheroidal galaxies

We present the results of the analysis of deep archival \sat\ observations towards the dwarf spheroidal galaxies Draco, Leo I, Ursa Major II and Ursa Minor in the Milky Way neighbourhood. The X-ray source population is characterized and cross-correlated with available databases with the aim to infer their nature. We also investigate if intermediate-mass black holes are hosted in the center of these galaxies. In the case of Draco, we detect 96 high-energy sources, two of them being possibly local stars, while no evidence for any X-ray emitting central compact object is found. Towards the Leo I and UMa II field of view we reveal 116 and 49 X-ray sources, respectively. None of them correlates with the putative central black holes and only one is likely associated with a UMa II local source. The study of the UMi dwarf galaxy shows 54 high-energy sources and a possible association {with a source at the dSph center}. We put an upper limit to the central compact object luminosity of 4.02$\times$10$^{33}$ erg/s. Furthermore, via the correlation with a radio source near the galactic center, we get that the putative black hole should have a mass of $\left(2.76^{+32.00}_{-2.54}\right)\times10^6 M_{\odot}$ and be radiatively inefficient. This confirms a previous result obtained by using Chandra data alone.

THE EVENT HORIZON OF M87

The 6 billion solar mass supermassive black hole at the center of the giant elliptical galaxy M87 powers a relativistic jet. Observations at millimeter wavelengths with the Event Horizon Telescope have localized the emission from the base of this jet to angular scales comparable to the putative black hole horizon. The jet might be powered directly by an accretion disk or by electromagnetic extraction of the rotational energy of the black hole. However, even the latter mechanism requires a confining thick accretion disk to maintain the required magnetic flux near the black hole. Therefore, regardless of the jet mechanism, the observed jet power in M87 implies a certain minimum mass accretion rate. If the central compact object in M87 were not a black hole but had a surface, this accretion would result in considerable thermal near-infrared and optical emission from the surface. Current flux limits on the nucleus of M87 strongly constrain any such surface emission. This rules out the presence of a surface and thereby provides indirect evidence for an event horizon.

SDSS1133: an unusually persistent transient in a nearby dwarf galaxy

While performing a survey to detect recoiling supermassive black holes, we have identified an unusual source having a projected offset of 800 pc from a nearby dwarf galaxy. The object, SDSS J113323.97+550415.8, exhibits broad emission lines and strong variability. While originally classified as a supernova (SN) because of its non-detection in 2005, we detect it in recent and past observations over 63 yr and find over a magnitude of rebrightening in the last 2 yr. Using high-resolution adaptive optics observations, we constrain the source emission region to be ≲12 pc and find a disturbed host-galaxy morphology indicative of recent merger activity. Observations taken over more than a decade show narrow [O iii] lines, constant ultraviolet emission, broad Balmer lines, a constant putative black hole mass over a decade of observations despite changes in the continuum, and optical emission-line diagnostics consistent with an active galactic nucleus (AGN). However, the optical spectra exhibit blueshifted absorption, and eventually narrow Fe ii and [Ca ii] emission, each of which is rarely found in AGN spectra. While this peculiar source displays many of the observational properties expected of a potential black hole recoil candidate, some of the properties could also be explained by a luminous blue variable star (LBV) erupting for decades since 1950, followed by a Type IIn SN in 2001. Interpreted as an LBV followed by an SN analogous to SN 2009ip, the multidecade LBV eruptions would be the longest ever observed, and the broad Hα emission would be the most luminous ever observed at late times (>10 yr), larger than that of unusually luminous SNe such as SN 1988Z, suggesting one of the most extreme episodes of pre-SN mass-loss ever discovered.

Advanced LIGO’s ability to detect apparent violations of the cosmic censorship conjecture and the no-hair theorem through compact binary coalescence detections

We study the ability of the advanced Laser Interferometer Gravitational-wave Observatory (aLIGO) to detect apparent violations of the cosmic censorship conjecture and the no-hair theorem. The cosmic censorship conjecture, which is believed to be true in the theory of general relativity, limits the spin-to-mass-squared ratio of a Kerr black hole. The no-hair theorem, which is also believed to be true in the theory of general relativity, suggests a particular value for the tidal Love number of a non-rotating black hole. Using the Fisher matrix formalism, we examine the measurability of the spin and tidal deformability of compact binary systems involving at least one putative black hole. Using parameter measurement errors and correlations obtained from the Fisher matrix, we determine the smallest detectable violation of bounds implied by the cosmic censorship conjecture and the no-hair theorem. We examine the effect of excluding unphysical areas of parameter space when determining the smallest detectable apparent violations, and we examine the effect of different post-Newtonian corrections to the amplitude of the compact binary coalescence gravitational waveform. In addition, we perform a brief study of how the recently calculated 3.0 pN and 3.5 pN spin-orbit corrections to the phase affect spin and mass parameter measurability. We find that physical priors on the symmetric mass ratio and higher harmonics in the gravitational waveform could significantly affect the ability of aLIGO to investigate cosmic censorship and the no-hair theorem for certain systems.

AnXMM-Newtonsearch for X-ray sources in the Fornax dwarf galaxy

We report the results of a deep archive XMM-Newton observation of the Fornax spheroidal galaxy that we analyzed with the aim of fully characterizing the X-ray source population (in most of the cases likely to be background active galactic nuclei) detected towards the target. A cross correlation with the available databases allowed us to find a source that may be associated with a variable star belonging to the galaxy. We also searched for X-ray sources in the vicinity of the Fornax globular clusters GC 3 and GC 4 and found two sources probably associated with the respective clusters. The deep X-ray observation was also suitable for the search of the intermediate-mass black hole (of mass $\simeq 10^{4}$ M$_{\odot}$) expected to be hosted in the center of the galaxy. In the case of Fornax, this search is extremely difficult since the galaxy centroid of gravity is poorly constrained because of the large asymmetry observed in the optical surface brightness. Since we cannot firmly establish the existence of an X-ray counterpart of the putative black hole, we put constraints only on the accretion parameters. In particular, we found that the corresponding upper limit on the accretion efficiency, with respect to the Eddington luminosity, is as low as a few $10^{-5}$.