Multistate observations of the Galactic black hole XTE J1752−223: evidence for an intermediate black hole spin

We have studied the properties of thin accretion disks around swirling-Kerr black holes, which own an extra swirling parameter describing the rotation of the immersed universe. Our results show that the swirling parameter leaves distinct imprints on the energy flux, temperature distribution and emission spectra of the disk and gives rise to some new effects that differ from those induced by the black hole's spin. With the increasing of the swirling parameter, both the energy flux and radiated temperature in the disk increase in the inner region where circular orbital radii are smaller and decrease in the outer region where circular orbital radii are larger. In contrast, these quantities consistently increase with the black hole's spin. Although the swirling parameter and the black hole's spin parameter lead to higher cut-off frequencies, the background swirling reduces the observed luminosity of the disk at lower frequencies and enhances it only at higher frequencies, which is quite distinct from that of the black hole's spin. Furthermore, the conversion efficiency increases with the black hole's spin parameter, but decreases with the swirling parameters. Additionally, the effects of the swirling parameter are found to be suppressed by the black hole's spin parameter. These results could help us further understand the properties of thin accretion disks and the swirling of the universe background.

We investigate the gamma-ray spectrum emitted from an ADAF and its dependence on the spin parameter of a central Kerr black hole, in order to examine whether the spectrum can be used to probe the spin parameter of black holes. We consider that the gamma-rays are produced through the decay of neutral pions created by proton-proton collisions in the vicinity of the central black hole. Since the energy distribution of the ion particles in an ADAF is not known, we consider two types of proton energy distributions: a thermal distribution and a power-law distribution. In the thermal model, we find that changes in the spin parameter from –0.95 to 0.95 can enhance the gamma-ray intensity by orders of magnitude. Thus, if the proton gas in an ADAF has a thermal distribution, the gamma-ray spectrum can be used as a probe to investigate the spin parameter of the central black hole. In the nonthermal model, on the other hand, the gamma-ray intensity is much less sensitive to the changes in the spin parameter than in the thermal model, and it would be difficult to estimate the spin parameter from the gamma-ray spectrum. We apply our model to the Galactic Center, Sgr A*. The unidentified gamma-ray source 3EG J1746–2851 is observed towards Sgr A* by EGRET. Our results show that the gamma-ray intensities predicted from our models are much lower than observations and we cannot find the spin parameter. We, however, consider that this is not a serious problem against our model since it is unclear whether the observed gammarays are from a point or a diffuse source at the Galactic Center. In order to investigate the spin parameter via the gamma-rays from the Galactic Center instruments with higher angular resolution is needed such as GLAST.

The choked accretion model consists of a purely hydrodynamical mechanism in which, by setting an equatorial to polar density contrast, a spherically symmetric accretion flow transitions to an inflow-outflow configuration. This scenario has been studied in the case of a (non-rotating) Schwarzschild black hole as central accretor, as well as in the non-relativistic limit. In this article, we generalize these previous works by studying the accretion of a perfect fluid onto a (rotating) Kerr black hole. We first describe the mechanism by using a steady-state, irrotational analytic solution of an ultrarelativistic perfect fluid, obeying a stiff equation of state. We then use hydrodynamical numerical simulations in order to explore a more general equation of state. Analyzing the effects of the black hole's rotation on the flow, we find in particular that the choked accretion inflow-outflow morphology prevails for all possible values of the black hole's spin parameter, showing the robustness of the model.

Accretion disk reflection spectra, including broad iron emission lines, bear the imprints of the strong Doppler shifts and gravitational red-shifts close to black holes. The extremity of these shifts depends on the proximity of the innermost stable circular orbit to the black hole, and that orbit is determined by the black hole spin parameter. Modeling relativistic spectral features, then, gives a means of estimating black hole spin. We report on the results of fits made to archival X-ray spectra of stellar-mass black holes and black hole candidates, selected for strong disk reflection features. Following recent work, these spectra were fit with reflection models and disk continuum emission models (where required) in which black hole spin is a free parameter. Although our results must be regarded as preliminary, we find evidence for a broad range of black hole spin parameters in our sample. The black holes with the most relativistic radio jets are found to have high spin parameters, though jets are observed in a black hole with a low spin parameter. For those sources with constrained binary system parameters, we examine the distribution of spin parameters versus black hole mass, binary mass ratio, and orbital period. We discuss the results within the context of black hole creation events, relativistic jet production, and efforts to probe the innermost relativistic regime around black holes.

We present our analysis of X-ray spectral properties observed from the Seyfert 1 galactic nucleus NGC 7469 using theRossiX-ray Timing Explorer (RXTE) and Advanced Satellite for Cosmology and Astrophysics mission (ASCA) observations. We demonstrate strong observational evidence that NGC 7469 undergoes spectral transitions from the low hard state (LHS) to the intermediate state (IS) during these observations. The RXTE observations (1996–2009) show that the source was in the IS ∼75% of the time only ∼25% of the time in the LHS. The spectra of NGC 7469 are well fitted by the so-called bulk motion Comptonization (BMC) model for all spectral states. We have established the photon index (Γ) saturation level, Γsat= 2.1 ± 0.1, in the Γ versus mass accretion rate,Ṁcorrelation. This Γ –Ṁcorrelation allows us to estimate the black hole (BH) mass in NGC 7469 to beMBH≥ 3 × 106M⊙assuming the distance to NGC 7469 of 70 Mpc. For this BH mass estimate, we use the scaling method taking Galactic BHs, GRO J1655–40, Cyg X–1, and an extragalactic BH source, NGC 4051 as reference sources. The Γ versusṀcorrelation revealed in NGC 7469 is similar to those in a number of Galactic and extragalactic BHs and it clearly shows the correlation along with the strong Γ saturation at ≈2.1. This is robust observational evidence for the presence of a BH in NGC 7469. We also find that the seed (disk) photon temperatures are quite low, of the order of 140–200 eV, which are consistent with a high BH mass in NGC 7469 that is more than 3 × 106solar masses.

Accretion disk wind during the outburst of the stellar-mass black hole MAXI J1348-630

We investigate the dynamical stability of circular orbits around a Kerr black hole embedded in a Dehnen-type dark matter halo. The effective spacetime metric of the combined system is constructed using the Newman–Janis algorithm, and the effective potential for test-particle motion in the equatorial plane is derived. The stability of circular orbits is analyzed through the Hessian matrix of the effective potential, while the stability strength and restoring-force distribution are employed to quantify the orbital response to small perturbations. Our results show that the presence of the dark matter halo significantly alters the spatial structure of stable circular orbits, leading to non-continuous stable regions whose location and extent depend sensitively on the halo’s characteristic density, scale radius, and the black hole spin. The innermost stable circular orbit (ISCO) is shifted relative to the vacuum Kerr case, with its position determined by the combined effects of the spin and halo parameters. Two-dimensional heatmaps, parameter scans, and three-dimensional visualizations systematically illustrate how the black hole spin and dark matter halo properties influence the ISCO and the distribution of stable orbits. Finally, we analyze the influence of the dark matter halo on the structure of the black hole event horizon. These results provide a detailed theoretical investigation of orbital dynamics around rotating black holes in dark-matter-rich environments.

We determine the spin of a supermassive black hole in the context of disc-seismology by comparing newly detected quasi-periodic oscillations (QPOs) of radio emission in the Galactic centre, Sagittarius A* (Sgr A*), as well as infrared and X-ray emissions with those of the Galactic black holes. We find that the spin parameters of black holes in Sgr A* and in Galactic X-ray sources have a unique value of ≈0.44 which is smaller than the generally accepted value for supermassive black holes, suggesting evidence for the angular momentum extraction of black holes during the growth of supermassive black holes. Our results demonstrate that the spin parameter approaches the equilibrium value where spin-up via accretion is balanced by spin-down via the Blandford–Znajek mechanism regardless of its initial spin. We anticipate that measuring the spin of black holes by using QPOs will open a new window for exploring the evolution of black holes in the Universe.

We report on radio, near-infrared, optical and X-ray observations of the black hole candidate (BHC) XTE J1550−564 performed during its 2000 X-ray outburst. These observations have allowed us to sample the behavior of XTE J1550−564 in the X-ray Low Hard and Intermediate/Very High states. The radio emission in the Low Hard state most likely originates from a compact jet and the synchrotron emission from this jet may extend up to the optical range or beyond, therefore indicating that the total power of the compact jet is a significant fraction of the total luminosity of the system. In the Intermediate/Very High state the radio emission is quenched, implying a suppression of the outflow. We discuss the properties of radio emission in the X-ray states of BHCs.

Purpose of Study: The study's objective is to investigate the function of entropy in black holes, with a particular emphasis on the ways in which entropy aids in the comprehension of the properties of various varieties of black holes, such as Schwarzschild, Kerr, and charged black holes (Reissner-Nordström and Kerr-Newman). The objective of the investigation is to examine the unique entropy characteristics that are associated with each form of black hole within the context of black hole thermodynamics. Methodology: The entropy of black holes is examined through a theoretical approach that utilizes the principles of thermodynamics and information theory. The analysis entails a comparison of the entropy properties of Schwarzschild, Kerr, and charged black holes, taking into account their distinctive characteristics and the implications for black hole thermodynamics. Results: The analysis demonstrates that the inherent properties of each form of black hole are directly correlated with their distinctive entropy characteristics. Schwarzschild black holes, Kerr black holes, and charged black holes exhibit unique entropy patterns, which contribute to a more exhaustive comprehension of black hole thermodynamics and provide more profound insights into their thermodynamic behavior. Applications: The results have substantial implications for the advancement of theoretical physics, particularly in the field of black hole thermodynamics. The development of more precise models and predictions regarding black hole behavior can be facilitated by an understanding of the entropy characteristics of various varieties of black holes. This knowledge has the potential to inform future research in quantum gravity and cosmology.

We have carried out observations of the X-ray transient GX339-4 during its high-soft and low-hard X-ray spectral states. Our high-resolution spectroscopic observation in 1999 April suggests that the H-alpha line has a single-peaked profile in the low-hard state as speculated in our previous paper. The HeII 4686 line, however, has a double-peaked profile in both the high-soft and low-hard states. This suggests that the line-emission mechanism is different in the two states. Our interpretation is that double-peaked lines are emitted from a temperature-inversion layer on the accretion-disk surface when it is irradiatively heated by soft X-rays. Single-peaked lines may be emitted from outflow/wind matter driven by hard X-ray heating. We have constructed a simple plane-parallel model and we use it to illustrate that a temperature-inversion layer can be formed at the disk surface under X-ray illumination. We also discuss the conditions required for the formation of temperature inversion and line emission. Based on the velocity separations measured for the double-peaked lines in the high-soft state, we propose that GX339-4 is a low-inclination binary system. The orbital inclination is about 15 deg if the orbital period is 14.8 hours.

If the linear polarization of the optical emission of active galactic nuclei (AGNs) arises in magnetized accretion disk (the Milne problem), the degree of polarization should depend strongly on the spin of the central black hole. For the same black hole luminosities and masses, the polarization is substantially higher for rotating Kerr than for non-rotating Schwarzschild black holes. Statistically, this means that the majority of AGNs displaying appreciable linear polarization should have Kerr black holes. The spin dependence of the polarization is due to the fact that the radius of the innermost stable circular orbit risco depends on the spin—this radius is three gravitational radii for a Schwarzschild black hole, and a factor of six smaller for a rapidly rotating black hole. This means that the magnetic field in the region of emergence of the optical emission, which decreases with distance from risco, is higher for a non-rotating than for a rapidly rotating black hole. This higher magnetic field gives rise to strong Faraday depolarization, explaining the effect considered here.

Can we determine a spin parameter of a black hole by observation of a black hole shadow in an accretion disk? In order to answer this question, we make a qualitative analysis and a quantitative analysis of a shape and a position of a black hole shadow casted by a rotating black hole on an optically thick accretion disk and its dependence on an angular momentum of a black hole. We have found black hole shadows with a quite similar size and a shape for largely different black hole spin parameters and a same black hole mass. Thus, it is practically difficult to determine a spin parameter of a black hole from a size and a shape of a black hole shadow in an accretion disk. We newly introduce a bisector axis of a black hole shadow named a shadow axis. For a rotating black hole a shape and a position of a black hole shadow are not symmetric with respect to a rotation axis of a black hole shadow. So, in this case the minimum interval between a mass center of a black hole and a shadow axis is finite. An extent of this minimum interval is roughly proportional to a spin parameter of a black hole for a fixed inclination angle between a rotation axis of a black hole and a direction of an observer. In order to measure a spin parameter of a black hole, if a shadow axis is determined observationally, it is crucially important to determine a position of a mass center of a black hole in a region of a black hole shadow.

Shadows and quasinormal modes of rotating black holes in Horndeski theory: Parameter constraints using EHT observations of M87* and Sgr A*

In this paper we have investigated the dynamics of neutral, electrically charged and magnetized particles around renormalized group improved (RGI) Schwarzschild black hole in the presence of external asymptotically uniform magnetic field. We have analyzed the spacetime structure around RGI black hole by investigating Ricci, the square of Ricci tensor and Kretschmann curvature scalars and shown that only in the case when the parameter $\ensuremath{\gamma}=0$ the curvature becomes infinite at the center of the black hole, while for nonzero values of $\ensuremath{\gamma}$ parameter the black hole curvature reflects the properties of regular black hole. Analyzing the innermost stable circular orbits of test neutral particles around RGI black hole and comparing with the results for rotating Kerr black hole we have shown that RGI black hole parameters can mimic the rotation parameter of Kerr black hole up to $a/M\ensuremath{\lesssim}0.31$ providing the same innermost stable circular orbit (ISCO) radius. Since according to the astronomical observations of the accretion disks confirm that the astrophysical black holes are rapidly rotating with the spin parameter up to $a/M\ensuremath{\sim}0.99$ one may conclude that the effects of parameters of RGI Schwarzschild black hole on the circular orbits of the neutral particles cannot mimic the Kerr black hole. Then the Hamilton-Jacobi equation has been used to analyze the charged and magnetized particles motion near the RGI black hole in the presence of the strong interaction between external asymptotically uniform magnetic (electromagnetic) field and magnetized (electrically charged) particle. We have shown that RGI black hole parameters quantitatively change the dynamics of the charged and magnetized particles, in particular ISCO radius of the particles decreases with increasing the parameter $\ensuremath{\lambda}$, while the increase of the parameter $\ensuremath{\gamma}$ causes to increase of it. The stability analysis of the circular orbits of the magnetized particles has shown that the population of magnetars with the strong surface magnetic field up to ${10}^{15}$ Gauss is excluded from the very close environment of a supermassive black hole due to destructive properties of the magnetic field.