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We examine the dynamics of particles around a rotating regular black hole. In particular we focus on the effects of the characteristic length parameter of the spinning black hole on the motion of the particles by solving the equation of…

The present paper is devoted to a new black bounce solution that regularize the well-known rotating black string in $3+1$ dimensions. To do so, the procedure pointed out by Simpson-Visser is followed, which has been already applied…

A black string is a solution of the four-dimensional general relativity in cylindrical symmetric anti-de Sitter (AdS) spacetime pioneered by Lemos [6]. We obtain an exact rotating black string surrounded by quintessence matter in anti-de…

We find new classes of {\it exact} string solutions in a variety of curved backgrounds. They include stationary and dynamical (open, closed, straight, finitely and infinitely long) strings as well as {\it multi-string} solutions, in terms…

The motion of a string in curved spacetime is discussed in detail. The basic formalism for string motion in Minkowski spacetime and in curved spacetime is presented. The description applies to cosmic strings as well as to fundamental…

The traditional black hole has a horizon, with a singularity inside the horizon. But actual microstates of black holes are `fuzzballs', with no horizon and a complex internal structure. We take the simplest hole in string theory -- the…

We perform the first numerical simulations modeling the inflow and outflow of magnetized plasma in the Kerr-Sen spacetime, which describes classical spinning black holes (BHs) in string theory. We find that the Blandford-Znajek (BZ)…

This paper explores the dynamics of both neutral as well as charged particles orbiting near a rotating black hole in scalar-tensor-vector gravity. We study the conditions for the particle to escape at the innermost stable circular orbit. We…

Negative energy objects generally lead to instabilities and a number of other disturbing behaviors. In particular, negative energy fluxes lead to a breakdown of the classical area theorem for black hole horizons, which can lead to…

In this work, we track the evolution of an ultrarelativistic fluid onto a Kerr black hole, on the equatorial plane. In this treatment, we consider the limit where the rest mass density is neglected, that is, the approximation is valid in…

By performing two-dimensional axisymmetric general relativistic radiation magnetohydrodynamics simulations with spin parameter $a^*$ varying from -0.9 to 0.9, we investigate the dependence on the black hole spin of the energy flow from…

The energy condition inequalities for the matter stress energy comprised out of the dilaton and Maxwell fields in the dilaton-Maxwell gravity theories emerging out of string theory are examined in detail. In the simplistic 1+1 dimensional…

The interpretation of so-called cosmic string in black hole spacetimes has settled down to an unsatisfactory state. In this article we try to provide a different model for these cosmic strings by explicit construction of these spacetimes…

We investigate the black hole - string transition in the two-dimensional Lorentzian black hole system from the exact boundary states that describe the rolling D-brane falling down into the two-dimensional black hole. The black hole - string…

Stationary axisymmetric magnetic fields are expelled from outer horizons of black holes as they become extremal. Extreme black holes exhibit Meissner effect also within exact Einstein--Maxwell theory and in string theories in higher…

In this Chapter we would like to review a "~phenomenological~" approach taking into account the most fundamental feature of string theory or, more in general, of quantum gravity, whatever its origin, which is the existence of a minimal…

A new approach to constraining or determining the spin of a massive black hole is proposed. A key parameter in the analysis is the dimensionless ratio, r, of the energy released to the mass of the hole. It is shown that the black hole spin,…

There is strong observational evidence that AGN, Galactic X-ray transients and (probably) $\gamma$-ray bursts are associated with black holes, and that these sources are able to form collimated, ultrarelativistic outflows. There is much…

We study the gravitational lensing by a Kerr-Sen Black Hole arising in heterotic string theory. A closed form expression for the bending angle of light in equatorial plane of Kerr-Sen Black Hole is derived as a function of impact parameter,…

We review recent progress in our understanding of the physics of black holes. In particular, we discuss the ideas from string theory that explain the entropy of black holes from a counting of microstates of the hole, and the related…