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Figure on the left: The time-averaged flux radiated by an accretion disk around a BMD black hole for different values of s: s = 5 (long dashed line), s = 10 (short dashed line), s = 15 (dotted line) and s = 20, respectively. The flux for a Schwarzschild black hole is plotted with a solid line. The mass accretion rate is 2×10 −6 M ⊙ yr −1 . Figure on the right: The time-averaged flux radiated by the accretion disk around a BMD black hole with s = 20 for different values of the mass accretion rate ˙ M 0 : 1.0 × 10 −6 M ⊙ yr −1 (long dashed line), 1.5 × 10 −6 M ⊙ yr −1 (short dashed line), 2.0 × 10 −6 M ⊙ yr −1 (solid line), 2.5 × 10 −6 M ⊙ yr −1 (dotted line), and 3.0 × 10 −6 M ⊙ yr −1 (dot-dashed line), respectively. The total mass of the black hole is 2.5 × 10 6 M ⊙ .
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The braneworld description of our universe entails a large extra dimension and a fundamental scale of gravity that might be lower by several orders of magnitude as compared to the Planck scale. An interesting consequence of the braneworld scenario is in the nature of the vacuum solutions of the brane gravitational field equations, with properties q...
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Citations
... The black hole solutions in the brane-world model and the corresponding observable effects have also been widely studied [31][32][33][34][35][36][37][38][39][40]. The properties of a thin accretion disks around a brane world black hole have been investigated [41], and it is shown that the particular signatures appeared in the electromagnetic spectrum could offer a the possibility to directly test physical models with extra dimension by using astrophysical observations from accretion disks. We here will consider a spherically symmetric solution in the brane world obtained by Casadio, Fabbri and Mazzacurati [42]. ...
... We here will consider a spherically symmetric solution in the brane world obtained by Casadio, Fabbri and Mazzacurati [42]. The properties of a thin accretion disks around the CFM compact object have also been studied in [41]. However, the thick accretion disk configurations around the CFM compact object remain open. ...
We have studied equipotential surfaces of a thick accretion disk around a Casadio-Fabbri-Mazzacurati (CFM) compact object in the brane-world scenario, which owns a mass parameter together with a parameterized post-newtonian (PPN) parameter. With the increase of the PPN parameter, the size of the thick accretion disk decreases, but the corresponding Roche lobe size increases. Thus, the larger PPN parameter yields the larger region of existing bound disk structures where the fluid is not accreted into the central wormhole. Moreover, with the increase of the PPN parameter, the position of the Roche lobe gradually moves away from the central compact object, the thickness of the region enclosed by the Roche lobe decreases near the compact object, but increases in the region far from the compact object. Our results also show that the pressure gradient in the disk decreases with the PPN parameter. These effects of the PPN parameter on thick accretion disk could help to further understand compact objects in brane-world scenario.
... Additionally, the flux energy and thermal spectrum from accretion disks around various classes of static and rotating braneworld black holes have been investigated in Refs. [58,59]. The weak-field deflection angle and bending of light around massive braneworld black holes have also been studied [60,61]. ...
The paper explores the thermodynamic properties of primordial black holes (PBHs) in the braneworld. Specifically, the researchers examined Hawking radiation and the lifetime of PBHs. Through their analysis, an exact analytical expression for the Bekenstein–Hawking entropy, temperature, and heat capacity was derived. Their findings suggest that the lifetime of PBHs in the early universe is reduced by at least one order of magnitude, ultimately leading to their evaporation. This could explain why we have not observed the final rapid evaporation of PBHs in the recent epoch of the universe.
... The steady-state thin accretion disk is the simplest theoretical model in which the disk has negligible thickness [13][14][15]. The physical properties of matter forming a thin accretion disk in a variety of background spacetimes have been discussed extensively, see for example [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31]. The signatures appeared in the energy flux and the spectrum emitted by the disk can provide us not only the information about BHs, but also the verification of modified gravity theories [32]. ...
We consider the accretion process in the thin disk around a supermassive black hole in Einstein-aether-scalar theory. We probe the effects of the model parameter on the physical properties of the disk. The results show that with increasing value of the parameter, the energy flux, the radiation temperature, the spectra cut-off frequency, the spectra luminosity, and the conversion efficiency of the disk decrease. The disk is hotter and more luminous than that in general relativity for negative parameter, while it is cooler and less luminous for positive parameter. We also find some values of the parameter allowed by the theory are excluded by the physical properties of the disk.
... The modified theory of gravity received lots of attention when considering the solution of accretion disks. Accretion disk properties and their dynamical evolutions were studied in different modified gravity models, such as the innermost circular orbits of spinning and massive particles [23,24], f (R) gravity [25,26], the Einstein-Maxwell dilation theory [27,28], scalar-tensor-vector gravity [29], Einstein-scalar-Gauss-Bonnet gravity [30], 4-D EGB gravity for a non-rotating black hole [31], the effect of EGB coupling constant on the collapsing phenomena and its end state [32], the observational constraint of the EGB coupling constant α [33,34], and the rotating black hole [35]. Ref. Heydari-Fard et al. [35] studied the thin accretion disk around a rapidly rotating black hole. ...
Understanding the physical structures of the accreted matter very close to a black hole in quasars and active galactic nucleus (AGN) is an important milestone to constrain the activities occurring in their centers. In this paper, we numerically investigate the effects of the asymptotic velocities on the physical structures of the accretion disk around the Kerr and Einstein–Gauss–Bonnet (EGB) rapidly rotating black holes. The Bondi–Hoyle accretion is considered with a falling gas towards the black hole in an upstream region of the computational domain. Shock cones are naturally formed in the downstream part of the flow around both black holes. The structure of the cones and the amount of the accreted matter depend on asymptotic velocity V∞ (Mach number) and the types of the gravities (Kerr or EGB). Increasing the Mach number of the in-flowing matter in the supersonic region reduces the shock opening angle and the accretion rates, because of the gas rapidly falling towards the black hole. The EGB gravity leads to an increase in the shock opening angle of the shock cones while the mass-accretion rates dM/dt decrease in EGB gravity with a Gauss–Bonnet (GB) coupling constant α. It is also confirmed that accretion rates and drag forces are significantly altered in the EGB gravity. Our numerical simulation results could be used in identifying the accretion mechanism and physical properties of the accretion disk and black hole in the observed X-rays such as NGC 1313 X-1 and 1313 X-2 and MAXI J1803-298.
... The steady-state thin accretion disk model is the simplest theoretical model in which the disk has negligible thickness [13][14][15]. The physical properties of matter forming a thin accretion disk in a variety of background spacetimes have been discussed extensively, see for example [16][17][18][19][20][21][22][23][24][25][26][27][28][29]. The signatures appeared in the energy flux and the spectrum emitted by the disk can provide us not only the information about BHs, but also the verification of modified gravity theories [30]. ...
We consider the accretion process in the thin disk around a black hole in Einstein-aether-scalar theory. We probe the effects of the model parameter on the physical properties of the disk. The results show that with increasing value of the parameter, the energy flux, the radiation temperature, the spectra cut-off frequency, the spectra luminosity, and the conversion efficiency of the disk decrease. The disk is hotter and more luminous than that in general relativity for negative parameter, while they are cooler and less luminous for positive parameter. We also find some values of the parameter allowed by the theory are excluded by the physical properties of the disk.
... The modified theory of gravity received lots of attention when considering the solution of accretion disks. The accretion disk properties and their dynamical evolutions were studied in different modified gravity models such as the innermost circular orbits of spinning and massive particles (Zhang et al. 2020;Guo & Li 2020), f (R) gravity (Pun et al. 2008;Staykov et al. 2016), Einstein-Maxwell-dilation theory (Karimov et al. 2018;Heydari-Fard et al. 2020), scalar-tensor-vector gravity (Pérez et al. 2017), Einstein-scalar-Gauss-Bonnet gravity (Heydari-Fard & Sepangi 2021), 4D EGB gravity for non-rotating black hole (Liu et al. 2021), the observational constrain the GB coupling constant α (Feng et al. 2020;Clifton et al. 2020), and for rotating black hole (Heydari-Fard et al. 2021). Heydari-Fard et al. (2021) had studied the thin accretion disk around rapidly rotating black hole. ...
Understanding the physical structures of the accreated matter very close to the black hole in quasars and active galactic nucleus (AGNs) is an important milestone to constrain the activities occurring in their centers. In this paper, we numerically investigate the effects of the asymptotic velocities on the physical structures of the accretion disk around the Kerr and Einstein-Gauss-Bonnet (EGB) rapidly rotating black holes. The Bondi-Hoyle accretion is considered with a falling gas towards the black hole in upstream region of the computational domain. The shock cones are naturally produced in the downstream part of the flow around both black holes. It is found that the structure of the cones and the amount of the accreated matter depend on asymptotic velocity $V_{\infty}$ (Mach number) and the types of the gravities (Kerr or EGB). Increasing the Mach number of the inflowing matter in the supersonic region causes the shock opening angle and accretion rates getting smaller because of the rapidly falling gas towards the black hole. The EGB gravity leads to an increase in the shock opening angle of the shock cones while the mass accretion rates $\dot{M}$ are decreasing in EGB gravity with a Gauss-Bonnet (GB) coupling constant $\alpha$. It is also confirmed that accretion rates and drag forces are significantly altered in the EGB gravity. Our numerical simulation results could be used to identify the accreation mechanism and physical properties of the accretion disk and black hole in the observed $X-$ rays such as NGC $1313$ $X-1$ and $1313$ $X-2$ and MAXI $J1803-298$.
... He calculated thermal spectra of accretion disks around static boson stars and compared the theoretical predictions with that of a Schwarzschild black hole. Thermal spectra of accretion disks around wormholes, brane world black holes, gravastars, etc. were later calculated by other authors [34,35,53,54]. The first observational constraints on the Kerr metric with the continuum-fitting method were reported by my group at Fudan University in Ref. [42] using the model described in [8,12], where light bending was taken into account. ...
Einstein’s theory of General Relativity is one of the pillars of modern physics. For decades, the theory has been mainly tested in the weak field regime with experiments in the Solar System and observations of binary pulsars. Thanks to a new generation of observational facilities, the past 5 years have seen remarkable changes in this field and there are now numerous efforts for testing General Relativity in the strong field regime with black holes and neutron stars using different techniques. Here I will review the work of my group at Fudan University devoted to test General Relativity with black hole X-ray data.
... Indeed, it has been recently shown that a negative tidal charge increases the efficiency of energy extraction from the black hole via the Penrose process [11]. Still concerning about rotating black holes, the tidal charge has also important implications to astrophysics, as it may alters the radii size of ISCO [10,12,13] and of the shadow cast by the black hole [6,[14][15][16]. ...
The changes a (negative) tidal charge causes at the phenomenon of superradiance which occurs around rotating black holes are investigated. This is made by computing the amplification factors of massless scalar waves being scattered by the black hole. It is shown that the increase of the tidal-charge intensity leads to a considerable enhancement of energy extraction from near-extreme black holes. Such improvement results from the fact that extreme black holes with more negative tidal charges spin faster. Maximum amplification decreases with the increase of the tidal charge intensity if the angular momentum of the black hole per unit mass is fixed. The tidal charge may also change crucially the superradiance phenomenon of massless scalar waves causing maximum amplification to occur for $$m > 1$$ m > 1 differently from the case of Kerr black holes.
... Since their appearance, rotating black holes with tidal charge have been subject of many studies. Apart from the previously cited works in this context, which evolve superradiance, the Penrose process, and the constraining of the tidal charge via the shadow of the galaxy M87 core, other works concerning this system address to the determination of ISCOs [38,42,43], shadows [44,36,35,45], light deflections and frame dragging [46], and orbital resonances [47]. ...
The absorption cross section of rotating black holes in the Randall-Sundrum brane-world scenario for massless scalar waves is obtained via the partial-wave method. A variety of incidence directions with respect to the black hole rotation axis is considered. Special attention is given to near-extreme black holes, in which cases the absorption cross section increases as the tidal charge becomes more negative. It is shown that black holes with different configurations can present capture cross sections/shadows of same size. However, in such cases, the absorption spectra are different mainly for small frequencies. Numeric results are compared with the capture cross sections in the high-frequency limit, and with the black hole area in the low-frequency limit showing good agreement.
... Indeed, it has been recently shown that a negative tidal charge increases the efficiency of energy extraction from the black hole via the Penrose process [7]. Still concerning about rotating black holes, the tidal charge has also important implications to astrophysics, as it may alters the radii size of ISCO [6,8,9] and of the shadow cast by the black hole [10][11][12][13]. ...
The changes a (negative) tidal charge causes at the phenomenon of superradiance which occurs around rotating black holes are investigated. This is made by computing the amplification factors of massless scalar waves being scattered by the black hole. It is shown that the increase of the tidal charge intensity leads to a considerable enhancement of energy extraction from near-extreme black holes. Such improvement results from the fact that extreme black holes with more negative tidal charges spin faster. Maximum amplification decreases with the increase of the tidal charge intensity if the angular momentum of the black hole per unit mass is kept fix. The tidal charge may also change crucially the superradiance phenomenon of massless scalar waves causing maximum amplification to occur for $m > 1$ differently from the case of Kerr black holes.
