Black Hole Sgr Research Articles

Discovery of a Dense Association of Stars in the Vicinity of the Supermassive Black Hole Sgr A*

We focus on a sample of 42 sources in the vicinity of the bow-shock source IRS 1W (N-source region), located at a distance of 6.″05 northeast of the supermassive black hole Sagittarius A* (Sgr A*), within the radius of 1.″35. We present the first proper-motion measurements of N sources and find that a larger subset of N sources (28 sources) exhibit a north–west (NW) flying angle. These sources can be bound by an intermediate-mass black hole (IMBH) or the concentration that we observe is due to a disk-like distribution projection along the line of sight. We detect the N sources in the H, K s , and L′ bands. The NW-ward flying sources could be a bound collection of stars. We discuss the tentative existence of an IMBH or an inclined disk distribution to explain the significant overdensity of stars. The first scenario of having an IMBH implies the lower limit of ∼104 M ⊙ for the putative IMBH. Our measurements for the first time reveal that the dense association of stars containing IRS 1W is a comoving group of massive, young stars. This stellar association might be the remnant core of a massive stellar cluster that is currently being tidally stripped as it inspirals toward Sgr A*. The second scenario suggests that the appearance of the N sources might be influenced by the projection of a disk-like distribution of younger He-stars and/or dust-enshrouded stars.

Multistructured Accretion Flow of Sgr A*. I. Examination of a Radiatively Inefficient Accretion Flow Model

The extreme low-luminosity supermassive black hole Sgr A* provides a unique laboratory in which to test models of radiatively inefficient accretion flows (RIAFs). Previous fits to the quiescent Chandra ACIS-S spectrum found that a RIAF model with an equal inflow–outflow balance works well. In this work, we apply the RIAF model to the Chandra HETG-S spectrum obtained through the Chandra X-ray Visionary Program, which displays features suggestive of temperature and velocity structures within the plasma. A comprehensive forward model analysis accounting for the accretion flow geometry and HETG-S instrumental effects is required for a full interpretation of the quiescent Chandra HETG-S spectrum. We present a RIAF model that takes these effects into account. Our fits to the high-resolution grating spectrum indicate an inflow balanced by an outflow (s ∼ 1) alongside a temperature profile that appears shallower than what would be expected from a gravitational potential following 1/r. The data require that the abundance of iron relative to solar is Z Fe < 0.32 Z ⊙ (90% credible interval), much lower than the 2 Z ⊙ metallicity measured in nearby late-type giants. While future missions like NewAthena will provide higher spectral resolution, source separation will continue to be a problem. Leveraging Chandra’s unparalleled spatial resolution, which is not expected to be surpassed for decades, remains essential for detailed investigations of the densely populated Galactic center in X-rays.

Multistructured Accretion Flow of Sgr A*. II. Signatures of a Cool Accretion Disk in Hydrodynamic Simulations of Stellar Winds

Hydrodynamic simulations of the stellar winds from Wolf–Rayet stars within the Galactic center can provide predictions for the X-ray spectrum of the supermassive black hole Sgr A*. Herein, we present results from updated smooth particle hydrodynamics simulations, building on the architecture of Cuadra et al. and Russell et al., and find that a “cold” (104 K) gas disk forms around Sgr A* with a simulation runtime of 3500 yr. This result is consistent with previous grid-based simulations, demonstrating that a cold disk can form regardless of numerical method. We examine the plasma scenarios arising from an environment with and without this cold disk, by generating synthetic spectra for comparison to the quiescent Fe Kα Sgr A* spectrum from Chandra HETGS, taken through the Chandra X-ray Visionary Program. We find that current and future X-ray missions are unlikely to distinguish between the kinematic signatures in the plasma in these two scenarios. Nonetheless, the stellar wind plasma model presents a good fit to the dispersed Chandra spectra within 1.″5 of Sgr A*. We compare our results to the radiatively inefficient accretion flow (RIAF) model fit to the HETGS spectrum presented in Paper I and find that the Bayesian model evidence does not strongly favor either model. With 9″ angular resolution and high spectral resolution of the X-IFU, NewAthena will offer a clearer differentiation between the RIAF plasma model and hydrodynamic simulations, but only a future X-ray mission with arcsecond resolution will significantly advance our understanding of Sgr A*’s accretion flow in X-rays.

Constraining a disformal Schwarzschild black hole in DHOST theories with the orbit of the S2 star

With the observed data of the S2 orbit around the black hole Sgr A∗\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^*$$\\end{document} and the Markov Chain Monte Carlo method, we make a constraint on parameters of a disformal Schwarzschild black hole in quadratic degenerate higher-order scalar-tensor (DHOST) theories. This black hole belongs to a class of non-stealth solutions and owns an extra disformal parameter described the deviation from general relativity. Our results show that the best fit value of the disformal parameter is positive. However, in the range of 1σ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$1\\sigma $$\\end{document}, we also find that general relativity remains to be consistent with the observation of the S2 orbit.

Spiral magnetic fields and their role on accretion dynamics in the circumnuclear disk of Sagittarius A*: Insight from λ = 850 μm polarization imaging

Abstract We showcase a study on the physical properties of the circumnuclear disk (CND) surrounding the supermassive black hole (SMBH) Sgr A* of the Galactic Center, emphasizing the role of magnetic field ($\boldsymbol {B}$ field) with $0.50\,$pc spatial resolution. Based on the sensitive $\lambda = 850\, \mu$m polarization data taken with the JCMT SCUBA2/POL2 (James Clerk Maxwell Telescope Submillimetre Common-User Bolometer Array 2), we analyzed ancillary datasets: CS $J = 2$–1 emission taken with ALMA (Atacama Large Millimeter/submillimeter Array), continuum emissions taken at $\lambda = 6\,$cm and at $\lambda = 37\, \mu$m taken with the VLA (Very Large Array) and SOFIA (the Stratospheric Observatory for Infrared Astronomy telescope). The $\boldsymbol {B}$ field within the CND exhibits a coherent spiral pattern. Applying the model described by Wardle and Königl (1990, ApJ, 362, 12; the WK model) to the observed $\boldsymbol {B}$ field pattern, it favors gas-pressure-dominant models without dismissing a gas-and-$\boldsymbol {B}$ field comparable model, leading us to estimate the $\boldsymbol {B}$-field strength in the ionized cavity around Sgr A* as $0.24^{+0.06}_{-0.04}\,$mG. Analysis based on the WK model further allows us to derive representative $\boldsymbol {B}$-field strengths for the radial, azimuthal, and vertical components as $(B_r, B_\phi , B_z) = (0.4 \pm 0.1, -0.7 \pm 0.2, 0.2 \pm 0.05)\,$mG, respectively. A key finding is that the $|B_\phi |$ component is dominant over $B_r$ and $B_z$ components, consistent with the spiral morphology, indicating that the CND’s $\boldsymbol {B}$-field is predominantly toroidal, possibly shaped by accretion dynamics. Considering the turbulent pressure, estimated plasma $\beta$ values indicate that the effective gas pressure should surpass the magnetic pressure. Assessing the CND of our MWG in the toroidal-and-vertical stability parameter space, we propose that such an “effective” magneto-rotational instability (MRI) may likely be active. The estimated maximum unstable wavelength, $\lambda _{\rm max} = 0.1 \pm 0.1\,$pc, is smaller than the CND’s scale height ($0.2 \pm 0.1\,$pc), which indicates the potential for the effective MRI intermittent cycles of $\sim 10^6\,$yr, which should profoundly affect the CND’s evolution, considering the estimated mass accretion rate of $10^{-2} M_{\odot }\,$yr$^{-1}$ to the SMBH.

Low-frequency Absorption and Radio Recombination Line Features of the Galactic Center Lobe

The Galactic center lobe (GCL) is a ∼1° object located north of the Galactic center. In the mid-infrared, the GCL appears as two 8.0 μm filaments that roughly define an ellipse. There is strong 24 μm and radio continuum emission in the interior of the ellipse. Due to its morphology and location in the sky, previous authors have argued that the GCL is created by outflows from star formation in the central molecular zone or by activity of the central black hole Sgr A*. We present images of the GCL from the GaLactic and Extragalactic All-sky Murchison Widefield Array survey in radio continuum that show thermal absorption against the Galactic center, incompatible with an interpretation of synchrotron self-absorption. Estimates of the cosmic-ray emissivity in this direction allow us to place a distance constraint on the GCL. To be consistent with standard emissivity assumptions, the GCL would be located 2 kpc away. At a distance of 8 kpc, the synchrotron background emissivity is enhanced by ∼75% in the direction of the GCL. We also present radio recombination line data from the Green Bank Telescope that constrain the electron temperature and line widths in this region, which are also more explicable if the GCL lies relatively close.

Dynamical Properties of Magnetized Low-angular-momentum Accretion Flows around a Kerr Black Hole

An essential factor for determining the characteristics of an accretion flow is its angular momentum. According to the angular momentum of the flow, semi-analytical analysis suggests various types of accretion solutions. It is critical to test these with numerical simulations, using the most advanced framework available (general relativistic magnetohydrodynamics), to understand how the flow changes with different angular momentum. By changing the initial condition of the accretion torus minimally, we can simulate a steady, low-angular-momentum accretion flow around a Kerr black hole. We focus primarily on the lower limits of angular momentum and find that an accretion flow with an intermediate range of angular momentum differs significantly from high- or very-low-angular-momentum flows. The intermediate-angular-momentum accretion flow has the highest density, pressure, and temperature near the black hole, making it easier to observe. We find that the density and pressure have power-law scalings ρ ∝ r n−3/2 and p g ∝ r n−5/2, which only hold for very-low-angular-momentum cases. With the increase in flow angular momentum, it develops a nonaxisymmetric nature. In this case, simple self-similarity does not hold. We also find that the sonic surface moves away from the innermost stable circular orbit as the angular momentum decreases. Finally, we emphasize that an intermediate-angular-momentum flow could provide a possible solution to explaining the complex observation features of the supermassive black hole Sgr A* at our galactic center.

Signatures of the accelerating black holes with a cosmological constant from the Sgr [formula omitted] and [formula omitted] shadow prospects

Recently, the Event Horizon Telescope (EHT) achieved the realization of an image of the supermassive black hole Sgr A⋆ showing an angular shadow diameter D=48.7±7μas and the fractional deviation δ=−0.08−0.09+0.09(VLTI),−0.04−0.10+0.09(Keck), alongside the earlier image of M87⋆ with angular diameter D=42±3μas, deviation δ=−0.01−0.17+0.17 and deviations from circularity estimated to be ΔC≲10%. In addition, the shadow radii are assessed within the ranges 3.38≤rsM≤6.91 for M87⋆ and 3.85≤rsM≤5.72 as well as 3.95≤rsM≤5.92 for Sgr A⋆ using the Very Large Telescope Interferometer (VLTI) and Keck observatories, respectively. These values are provided with 1-σ and 2-σ measurements. Such realizations can unveil a better comprehension of gravitational physics at the horizon scale. In this paper, we use the EHT observational results for M87⋆ and Sgr A⋆ to elaborate the constraints on parameters of accelerating black holes with a cosmological constant. Concretely, we utilize the mass and distance of both black holes to derive the observables associated with the accelerating black hole shadow.First, we compare our findings with observed quantities such as angular diameter, circularity, shadow radius, and the fractional deviation from the M87⋆ data. This comparison reveals constraints within the acceleration parameter and the cosmological constant. In fact, the acceleration parameter A is found to be in the interval [0,0.475−0.0125+0.0125], the cosmological constant spans the range of Λ∈[−14.5801−1.1051×10−6+1.1051×10−6,(1.0086×10−6)−1.5396×10−4+9.2121×10−5]. Then by considering the Sgr A⋆ data, the acceleration scheme persists with the same interval range, while, the cosmological constant shifts very slightly to the following domain [−14.7266−1.1051×10−6+1.1051×10−6,(4.6200×10−5)−1.5888×10−4+6.7890×10−5] for the VLTI observatory. Moreover, within the Keck measurements, one can find Λ∈[−14.7266−1.1051×10−6+1.1051×10−6,(2.4149×10−5)−1.5094×10−4+7.2836×10−5]. Besides, for the others parameters, both black holes reveal the range of the spin parameter a in [0,1] and electrical/magnetic charges are {e,g}∈[0,0.09].Further, only the cosmological constant intervals are altered by rsM measurements, precisely within the M87⋆ data, the upper bound shifts to (Λ1-σ=1.1361×10−4,Λ2-σ=1.7822×10−4), the VLTI gives (Λ1-σ=1.5457×10−4,Λ2-σ=1.8208×10−4), while the Keck results uncover Λ2-σ=1.8015×10−4.Lastly, one cannot rule out the possibility of the negative values for the cosmological constant on the emergence of accelerated black hole solutions within the context of minimal gauged supergravity. This intriguing possibility raises the prospect of such solutions being plausible candidates for astrophysical black holes.

Constraining a Companion of the Galactic Center Black Hole Sgr A*

We use 23 yr of astrometric and radial velocity data on the orbit of the star S0-2 to constrain a hypothetical intermediate-mass black hole orbiting the massive black hole Sgr A* at the Galactic center. The data place upper limits on variations of the orientation of the stellar orbit at levels between 0.°02 and 0.°07 per year. We use a combination of analytic estimates and full numerical integrations of the orbit of S0-2 in the presence of a black hole binary. For a companion intermediate-mass black hole outside the orbit of S0-2 (1020 au), we find that a companion black hole with mass m c between 103 and 105 M ⊙ is excluded, with a boundary behaving as . For a companion with a c < 1020 au, a black hole with mass between 103 and 105 M ⊙ is excluded, with . These bounds arise from quadrupolar perturbations of the orbit of S0-2. Significantly stronger bounds on an inner companion arise from the fact that the location of S0-2 is measured relative to the bright emission of Sgr A* and that separation is perturbed by the “wobble” of Sgr A* about the center of mass between it and the companion. The result is a set of bounds as small as 400 M ⊙ at 200 au; the numerical simulations suggest a bound from these effects varying as . We compare and contrast our results with those from a recent analysis by the GRAVITY collaboration.

Multi-TeV dark matter density in the inner Milky Way halo: spectral and dynamical constraints

Abstract We develop a comprehensive study of the gamma-ray flux observed by the High Energy Stereoscopic System (H.E.S.S.) in 5 regions of the Galactic Center (GC). Motivated by previous works on a possible Dark Matter (DM) explanation for the TeV cut-off observed by H.E.S.S. in the innermost ∼ 15 pc of the Galaxy, we aim to constrain the DM distribution up to a radius of ∼ 450 pc from the GC. In this region, the benchmark approach (e.g. cosmological simulations and Galactic dynamics studies) fails to produce a strong prediction of the DM profile. Within our proof-of-concept analysis, we use DRAGON to model the diffuse background emission and determine upper limits on the density distribution of thermal multi-TeV Weakly Interactive Massive Particles (WIMPs), compatible with the observed gamma-ray flux. The results are in agreement with the hypothesis of an enhancement of the DM density in the GC with respect to the benchmark Navarro-Frenk-White (NFW) profile (γ = 1) and allow us to exclude profiles with an inner slope cuspier than γ ≳ 1.3. We also investigate the possibility that such an enhancement could be related to the existence of a DM spike associated with the supermassive black hole Sgr A* at the GC. We find out that the existence of an adiabatic DM spike smoothed by the scattering off of WIMPs by the bulge stars may be consistent with the observed gamma-ray flux if the spike forms on an underlying generalized NFW profile with γ ≲ 0.8, corresponding to a spike slope of γsp-star = 1.5 and spike radius of R sp-stars ∼ 25 30 pc. Instead, in the extreme case of the instantaneous growth of the black hole, the underlying profile could have up to γ ∼ 1.2, a corresponding γsp-inst = 1.4 and R sp-inst ∼ 15–25 pc. Finally, the results of our analysis of the total DM mass enclosed within the S2 orbit (updated with new GRAVITY data) are less stringent than the spectral analysis. Our work aims to guide future studies of the GC region, with both current and next generation of telescopes. In particular, the next Cherenkov Telescope Array will be able to scan the GC region with improved flux sensitivity and angular resolution.

The Evaporating Massive Embedded Stellar Cluster IRS 13 Close to Sgr A*. I. Detection of a Rich Population of Dusty Objects in the IRS 13 Cluster

A detailed analysis of the nuclear star cluster not only concedes the existence of the S cluster, with its fast-moving stars and the supermassive black hole Sgr A*. It also reveals an embedded region of gas and dust with an exceptionally high stellar density called IRS 13. The IRS 13 cluster can be divided into the northern and eastern counterparts, called IRS 13N and IRS 13E, respectively. This work will focus on both regions and study their most prominent members using rich infrared and radio/submillimeter data baselines. Applying a multiwavelength analysis enables us to determine a comprehensive photometric footprint of the investigated cluster sample. Using the ray-tracing-based radiative transfer model HYPERION, the spectral energy distribution of the IRS 13 members suggests a stellar nature of the dusty sources. These putative young stellar objects (YSOs) have a comparable spectroscopic identification to the D and G sources in or near the S cluster. Furthermore, we report the existence of a population of dusty sources in IRS 13 that can be mostly identified in the H, K, and L band. We propose that, together with the objects reported in the literature, this population is the outcome of a recent star formation process. Furthermore, we report that these presumably young objects are arranged in a disk structure. Although it cannot be excluded that the intrinsic arrangement of IRS 13 does show a disk structure, we find indications that the investigated cluster sample might be related to the counterclockwise disk.

A Stability Timescale for Nonhierarchical Three-body Systems

The gravitational three-body problem is a fundamental problem in physics and has significant applications to astronomy. Three-body configurations are often considered stable as long the system is hierarchical; that is, the two orbital distances are well-separated. However, instability, which is often associated with significant energy exchange between orbits, takes time to develop. Assuming two massive objects in a circular orbit and a test particle in an eccentric orbit, we develop an analytical formula estimating the time it takes for the test particle’s orbital energy to change by an order of itself. We show its consistency with results from N-body simulations. For eccentric orbits in particular, the instability is primarily driven not by close encounters of the test particle with one of the other bodies, but by the fundamental susceptibility of eccentric orbits to exchange energy at their periapsis. Motivated by recent suggestions that the galactic center may host an intermediate-mass black hole (IMBH) as a companion to the massive black hole Sgr A*, we use our timescale to explore the parameter space that could harbor an IMBH for the lifetime of the S-cluster of stars surrounding Sgr A*. Furthermore, we show that the orbit of an S-star can be stable for long timescales in the presence of other orbital crossing stars, thus suggesting that the S-cluster may be stable for the lifetimes of its member stars.

Misaligned Jets from Sgr A* and the Origin of Fermi/eROSITA Bubbles

One of the leading explanations for the origin of Fermi Bubbles is past jet activity in the Galactic center supermassive black hole Sgr A*. The claimed jets are often assumed to be perpendicular to the Galactic plane. Motivated by the orientation of pc-scale nuclear stellar disk and gas streams, as well as a low inclination of the accretion disk around Sgr A* inferred by the Event Horizon Telescope, we perform hydrodynamical simulations of nuclear jets significantly tilted relative to the Galactic rotation axis. The observed axisymmetry and hemisymmetry (north–south symmetry) of Fermi/eROSITA bubbles (FEBs) due to quasi-steady jets in Sgr A* could be produced if the jet had a super-Eddington power (≳5 × 1044 erg s−1) for a short time (jet active period ≲6 kyr) for a reasonable jet opening angle (≲10°). Such powerful explosions are, however, incompatible with the observed O viii/O vii line ratio toward the bubbles, even after considering electron–proton temperature nonequilibrium. We argue that the only remaining options for producing FEBs are (i) a low-luminosity (≈1040.5−41 erg s−1) magnetically dominated jet or accretion wind from the Sgr A*, or (ii) a supernovae or tidal disruption event driven wind of a similar luminosity from the Galactic center.

A new method to constrain annihilating dark matter

ABSTRACT Recent indirect searches of dark matter using gamma-ray, radio, and cosmic ray data have provided some stringent constraints on annihilating dark matter. In this article, we propose a new indirect method to constrain annihilating dark matter. By using the data of the G2 cloud near the Galactic supermassive black hole Sgr A*, we can get stringent constraints on the parameter space of dark matter mass and the annihilation cross-section, especially for the non-leptophilic annihilation channels $b\bar{b}$ and W±. For the thermal annihilation cross-section, the lower bounds of dark matter mass can be constrained up to TeV order for the non-leptophilic channels with the standard spike index γsp = 7/3.

An Upper Limit on the Charge of the Black Hole Sgr A* from EHT Observations

The Event Horizon Telescope (EHT) recently released an image of the supermassive black hole Sgr A* showing an angular shadow diameter d sh = 48.7 ± 7 μas and Schwarzschild shadow deviation , using a black hole mass . The EHT image of Sgr A* is consistent with a Kerr black hole’s expected appearance, and the results directly prove the existence of a supermassive black hole at the center of the Milky Way. Here, we use the EHT observational results for Sgr A* to investigate the constraints on its charge with the aid of Kerr-like black holes, paying attention to three leading rotating models, namely Kerr–Newman, Horndeski, and hairy black holes. Modeling the supermassive black hole Sgr A* as these Kerr-like black holes, we observe that the EHT results for Sgr A* place stricter upper limits on the parameter space of Kerr–Newman and Horndeski black holes than those placed by the EHT results for M87*. A systematic bias analysis reveals that observational results from future EHT experiments will place more precise limits on the charge of the black hole Sgr A*. Thus, the Kerr-like black holes and Kerr black holes are indiscernible in a substantial region of the EHT-constrained parameter space; the claim is substantiated by our bias analysis.

Tests of Loop Quantum Gravity from the Event Horizon Telescope Results of Sgr A*

The Event Horizon Telescope (EHT) collaboration’s image of the compact object at the Galactic center is the first direct evidence of the supermassive black hole (BH) Sgr A*. The shadow of Sgr A* has an angular diameter d sh = 48.7 ± 7 μas with fractional deviation from the Schwarzschild BH shadow diameter (for the VLTI and Keck mass-to-distance ratios). Sgr A*'s shadow size is within 10% of Kerr predictions, equipping us with yet another tool to analyze gravity in the strong-field regime, including testing loop quantum gravity (LQG). We use Sgr A*'s shadow to constrain the metrics of two well-motivated LQG-inspired rotating BH (LIRBH) models characterized by an additional deviation parameter L q , which recover the Kerr spacetime in the absence of quantum effects (L q → 0). When increasing the quantum effects through L q , the shadow size increases monotonically, while the shape gets more distorted, allowing us to constrain the fundamental parameter L q . We use the astrophysical observables shadow area A and oblateness D to estimate the BH parameters. It may be useful in extracting additional information about LIRBHs. While the EHT observational results completely rule out the wormhole region in LIRBH-2, a substantial parameter region of the generic BHs in both models agrees with the EHT results. We find that the upper bounds on L q obtained from the shadow of Sgr A*—L q ≲ 0.0423 and L q ≲ 0.0821 for the two LIRBHs, respectively—are more stringent than those obtained from the EHT image of M87*.

Hot Spots in Sgr A* Accretion Disk: Hydrodynamic Insights

The recent image of our galaxy’s supermassive black hole Sgr A* derived from the 7 April 2017 data of the Event Horizon Telescope Collaboration shows multiple hot spots in its accretion disk. Using the analytical framework, we demonstrate that the observed hot spots may not be disjoint elements but causally linked components (“petals”) of one rotating quasi-stationary macro-structure formed in the thermo-vorticial field within the accretion disk.

The impact of gravitational lensing in the reconstruction of stellar orbits around Sgr A*

After the amazing discoveries by the GRAVITY collaboration in the last few years on the star S2 orbiting the black hole Sgr A* in the center of the Milky Way, we present a detailed investigation of the impact of gravitational lensing on the reconstruction of stellar orbits around this massive black hole.We evaluate the lensing astrometric effects on the stars S2, S38 and S55 and how these systematically affect the derived orbital parameters. The effect is below current uncertainties, but not negligible. With the addition of more observations on these stars, it will be possible to let the astrometric shift by lensing emerge from the statistical noise and be finally detected.By repeating the analysis on a smaller semimajor axis a and various inclinations i, we are able to quantify the lensing effects on a broader range of parameters. As expected, for smaller semimajor axes and for nearly edge-on orbits lensing effects increase by about an order of magnitude.

Testing Rotating Regular Metrics with EHT Results of Sgr A*

The Event Horizon Telescope (EHT) observation unveiled the first image of supermassive black hole Sgr A* showing a shadow of diameter θ sh = 48.7 ± 7 μas with fractional deviation from the Schwarzschild black hole shadow diameter . The Sgr A* shadow size is within 10% of the Kerr predictions, providing us with another tool to investigate the nature of strong-field gravity. We use the Sgr A* shadow observables to constrain metrics of four independent and well-motivated, parametrically different from Kerr spacetime, rotating regular spacetimes and the corresponding no-horizon spacetimes. We present constraints on the deviation parameter g of rotating regular black holes. The shadow angular diameter θ sh within the 1σ region places bounds on the parameters a and g. Together with EHT bounds on the θ sh and δ of Sgr A*, our analysis concludes that the three rotating regular black holes, i.e., Bardeen, Hayward, and Simpson–Visser black holes, and corresponding no-horizon spacetimes agree with the EHT results of Sgr A*. Thus, these three rotating regular spacetimes and Kerr black holes are indiscernible in some parameter space, and one cannot rule out the possibility of the former being strong candidates to be astrophysical black holes.

Generalized Extended Uncertainty Principle Black Holes: Shadow and Lensing in the Macro- and Microscopic Realms

Motivated by the recent study about the extended uncertainty principle (EUP) black holes, we present in this study its extension called the generalized extended uncertainty principle (GEUP) black holes. In particular, we investigated the GEUP effects on astrophysical and quantum black holes. First, we derive the expression for the shadow radius to investigate its behavior as perceived by a static observer located near and far from the black hole. Constraints to the large fundamental length scale, L*, up to two standard deviations level were also found using the Event Horizont Telescope (EHT) data: for black hole Sgr. A*, L*=5.716×1010 m, while for M87* black hole, L*=3.264×1013 m. Under the GEUP effect, the value of the shadow radius behaves the same way as in the Schwarzschild case due to a static observer, and the effect only emerges if the mass, M, of the black hole is around the order of magnitude of L* (or the Planck length, lPl). In addition, the GEUP effect increases the shadow radius for astrophysical black holes, but the reverse happens for quantum black holes. We also explored GEUP effects to the weak and strong deflection angles as an alternative analysis. For both realms, a time-like particle gives a higher value for the weak deflection angle. Similar to the shadow, the deviation is seen when the values of L* and M are close. The strong deflection angle gives more sensitivity to GEUP deviation at smaller masses in the astrophysical scenario. However, the weak deflection angle is a better probe in the micro world.