arXiv subjects

Sanjar Shaymatov

Publications and source records attributed to Sanjar Shaymatov.

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arXiv2021

On overspinning of black holes in higher dimensions

It turns out that repulsive effect due to rotation of a rotating black hole dominates over attraction due to mass for large $r$ in dimensions $>5$. This gives rise to a remarkable result that black hole in these higher dimensions in contrast to lower dimensional ones cannot be overspun even under linear test particle accretion. Further if a black hole in dimension $>4$ has one of its rotation parameters zero, it has only one horizon and hence it can never be overspun. Thus rotating black holes in six and higher dimensions, and those with one rotation parameter switched off, would always obey the weak cosmic censorship conjecture (WCCC).

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arXiv2020

Dynamics of test particles around a Bardeen black hole surrounded by perfect fluid dark matter

We study the dynamics of (i) neutral test particles, (ii) magnetically charged test particles, and (iii) test magnetic dipole around a regular Bardeen black hole surrounded by perfect fluid dark matter (PFDM). It has been shown how the magnetic charge of the black hole and the parameter of the surrounding PFDM can influence the innermost stable circular orbit (ISCO) radius of a test particle. We have found that the ISCO radius is strongly affected as a consequence of the combined effect of the magnetic charge parameter and the perfect fluid dark matter. The black hole magnetic charge parameter $g$ and the combined effect of perfect fluid dark matter can mimic the black hole rotation parameter up to $a/M\approx 0.9$. It has been observed that the ISCO for magnetic dipole disappears at the values exceeding the calculated upper value for the magnetic interaction parameter $β$. The upper limit decreases with the increase of both the dark matter and magnetic charge parameters. Finally, as an astrophysical application, we have analyzed degeneracy effects of spin of Kerr black holes and magnetic charge of regular Bardeen black holes for the different values of the dark matter parameter providing exactly the same value for ISCO radius of a magnetic dipole with the same value of the parameter $β=10.2$ of the magnetar called PSR J1745-2900 orbiting around supermassive black hole Sagittarius A*.

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arXiv2020

Testing Cosmic Censorship Conjecture for Extremal and Near-extremal (2+1)-dimensional MTZ Black Holes

We test the validity of the weak cosmic censorship conjecture for the ($2+1$)-dimensional charged anti-de Sitter black hole solution, which was derived by Martinez, Teitelboim, and Zanelli (MTZ). We first construct a thought experiment by throwing test charged particles on an extremal MTZ black hole. We derive that extremal ($2+1$) dimensional black holes can be overcharged by test particles, unlike their analogues in $4$ and higher dimensions. Nearly-extremal black holes can also be overcharged, by a judicious choice of energy and charge for the test particles in the case when we ignore the second order effects. Contrary to this, nearly-extremal black holes cannot be overcharged by the second order perturbation, obeying the weak cosmic censorship conjecture.

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arXiv2020

Can the dynamics of test particles around charged stringy black holes mimic the spin of Kerr black holes?

We study the motion of electrically charged particles, magnetic monopoles, and magnetic dipoles around electrically and magnetically charged stringy black holes. From the analysis of the radius of the innermost stable circular orbit (ISCO) of electrically charged particles, we show that the electric charge $Q$ of stringy black holes can mimic well the spin of Kerr black holes; the black hole magnetic charge $Q_m$ can mimic spins up to $a_* \simeq 0.85$ for magnetic dipoles; the magnetic charge parameter $g$ of a magnetic monopole can mimic spins up to $a_* \simeq 0.8$. This is due to the destructive character of the magnetic field and such a result excludes the existence of an appreciable black hole magnetic charge in astrophysical black holes from the observation of rapidly rotating objects with dimensionless spin up to $a_* \simeq 0.99$. We then consider the magnetar SGR (PSR) J1745-2900 as a magnetic dipole orbiting the supermassive black hole Sagittarius A* (Sgr A*). We show that Sgr A* may be interpreted as a stringy black hole with magnetic charge $Q_{\rm m}/M \leq 0.4118$.

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arXiv2020

Geodesic circular orbits sharing the same orbital frequencies in the black string spacetime

Here we consider isofrequency pairing of geodesic orbits that share the same three orbital frequencies associated with $Ω^{\hat{r}}$, $Ω^{\hatφ}$, and $Ω^{\hatω}$ in a particular region of parameter space around black string spacetime geometry. We study the effect arising from the extra compact spatial dimension on the isofrequency pairing of geodesic orbits and show that such orbits would occur and become in a cylindrical manner in the allowed region as particles move along the black string. We find that the effect due to the extra dimension leads to an increase in the number of the isofrequency pairing of geodesic orbits. We also find that isofrequency pairing of geodesic orbits in the region of parameter space can not be realized beyond the critical value $J_{cr} \approx 0.096$ of the conserved quantity of the motion due to the extra compact spatial dimension for given parameters of the black string.

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arXiv2020

Charged particle and epicyclic motions around $4D$ Einstein-Gauss-Bonnet black hole immersed in an external magnetic field

We investigate particle motion in the vicinity of a $4D$ Einstein-Gauss-Bonnet (EGB) black hole immersed in external asymptotically uniform magnetic field. It is well known that magnetic fields can strongly affect charged particle motion in the black hole vicinity due to the Lorenz force. We find that the presence of the Gauss-Bonnet (GB) coupling gives rise to a similar effect, reducing the radius of the innermost stable circular orbit (ISCO) with respect to the purely relativistic Schwarzschild black hole. Further, we consider particle collisions in the black hole vicinity to determine the center of mass energy and show that this energy increases with respect to the Schwarzschild case due to the effect of the GB term. Finally, we consider epicyclic motion and its frequencies and resonance as a mean to test the predictions of the model against astrophysical observations. In particular we test which values of the parameters of the theory best fit the 3:2 resonance of high-frequency quasi-periodic oscillations in three low-mass X-ray binaries.

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arXiv2020

Five dimensional charged rotating minimally gauged supergravity black hole cannot be over-spun and/or over-charged in non-linear accretion

Generally black hole could be over charged/spun violating the weak cosmic censorship conjecture (WCCC) for linear order accretion while the same is always restored back for non-linear accretion. The only exception however is that of a five dimensional rotating black hole with single rotation that cannot be overspun even at linear order. In this paper we investigate this question for a five dimensional charged rotating minimally gauged supergravity black hole and show that it could not be overspun under non-linear accretion and thereby respecting WCCC. However in the case of single rotation WCCC is however also respected for linear accretion when angular momentum of accreting particle is greater than its charge irrespective of relative dominance of charge and rotation parameters of black hole.

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arXiv2020

Six-dimensional Myers-Perry rotating black hole cannot be overspun

Though under nonlinear accretion, all black holes in four and higher dimensions obey the weak cosmic censorship conjecture (CCC); however, they generally violate it for linear test particle accretion with the exception of five-dimensional rotating black hole with a single rotation. In dimensions greater than five, there exists no extremal condition for black hole with single rotation and hence it can never be overspun. However, the extremal condition does exist for five-dimensional black hole with two rotations and then it could indeed be overspun under linear accretion. In this paper, we study the case of six-dimensional rotating black hole with two rotations and show that unlike the five-dimensional black hole it cannot be overspun under linear accretion. Though for nonlinear accretion, this result is anyway expected to hold good, yet we have verified it with an explicit calculation. Further, we would like to conjecture that so should be the case in all dimensions greater than six. Thus, the weak CCC may always be obeyed even at linear accretion process for rotating black hole in all dimensions greater than five.

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arXiv2019

The higher dimensional Myers-Perry black hole with single rotation always obeys the Cosmic Censorship Conjecture

Even though the Myers-Perry five dimensional rotating black hole with two rotations could be overspun by test particle accretion, yet it turns out as we show in this letter that it cannot do so for a single rotation. On the other hand it is known that there exists no extremal limit for a black hole with single rotation in dimensions greater than equal to six. It has been proven that all higher dimensional ($>4$) rotating black holes with only one single rotation can never be overspun under test particle linear accretion and hence would always obey CCC in the weak form.

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arXiv2014

Destroying a near-extremal Kerr black hole with a charged particle: Can a test magnetic field serve as a cosmic censor?

We investigate effect of a test magnetic field on the process of destroying near-extremal Kerr black hole with a charged test particle. It has been shown that it would be possible to throw a charged test particle into the near extremal rotating black hole and make it go past the extremality i.e. turn Kerr black hole into the Kerr-Newmann naked singularity. Typically in an astrophysical scenario black holes are believed to be surrounded by a magnetic field. Magnetic field although small, affects motion of charged particles drastically due to the large Lorentz force, as the electromagnetic force is much stronger that the gravity. Thus a test magnetic field can affect the process of destroying black holes and restore the cosmic censorship in the astrophysical context. We show that a test magnetic field would act as a cosmic censor beyond a certain threshold value. We try to gauge the magnitude of the magnetic field by comparing its energy density with that of the change in the curvature induced by the test particle. We find that the magnetic field required in only as strong as or slightly stronger as compared to the value for which its effect of the background geometry is comparable to the tiny backreaction as that of the test particle. In such a case however one has to take take into account effect of the magnetic field on the background geometry, which is difficult to implement in the absence of any exact near-extremal rotating magnetized black hole solutions. We argue that magnetic field would still act as a cosmic censor.

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