arXiv subjects

Antonios Nathanail

Publications and source records attributed to Antonios Nathanail.

46 records · Page 3Linked to original sources

arXiv2018

Cosmological production of black holes: a way to constrain alternative theories of gravity

Primordial black holes are considered to be pair created quantum-mechanically during inflation. In the context of General Relativity (GR), it has been shown that the pair creation rate is exponentially decreasing during inflation. Specifically, tiny black holes are favored in the early universe, but they can grow with the horizon scale, as inflation approaches its end. At the same time, cosmological, and not only, shortcomings of GR have triggered the pursuit for a new, alternative theory of gravity. In this paper, by using probability amplitudes from the No Boundary Proposal (NBP), we argue that any alternative gravity should have a black hole creation rate similar to that of GR; that is, in the early universe the creation of small black holes is in favor, while in the late universe larger black holes are being exponentially suppressed. As an example, we apply this argument in $f(R)$-theories of gravity and derive a general formula for the rate in any $f(R)$-theory with constant curvature. Finally, we consider well known $f(R)$-models and using this formula we put constraints in their free parameters.

gr-qc↗

arXiv2018

Gravitational wave bursts from long gamma-ray bursts

One of the most luminous explosions detected, gamma-ray bursts, especially the so-called long-duration bursts, most probably consist of an intrinsic core-collapse to a black hole inside a super massive star. We point out that this collapse alone will give a generic gravitational wave burst. It has been shown that the strength of this burst depends on the dimensionless spin parameter of the collapsing object. Under descent assumptions the gamma-ray burst's central engine powers the explosion electromagnetically due to the rotation of the newly formed black hole. We argue that the peak luminosity and the isotropic energy of the gamma-ray burst can be associated with the spin of the black hole, due to this mechanism. Since, both gravitational and electromagnetic emission depend on the spin, they can be correlated and thus give a straight estimate for the gravitational wave burst, when we have in hand a gamma-ray burst with known distance. We discuss detectability limits for present and future detectors.

astro-ph.HE↗

arXiv2017

Numerical simulations of the Cosmic Battery in accretion flows around astrophysical black holes

We implement the KORAL code to perform two sets of very long general relativistic radiation magnetohydrodynamic simulations of an axisymmetric optically thin magnetized flow around a non-rotating black hole: one with a new term in the electromagnetic field tensor due to the radiation pressure felt by the plasma electrons on the comoving frame of the electron-proton plasma, and one without. The source of the radiation is the accretion flow itself. Without the new term, the system evolves to a standard accretion flow due to the development of the magneto-rotational instability (MRI). With the new term, however, the system eventually evolves to a magnetically arrested state (MAD) in which a large scale jet-like magnetic field threads the black hole horizon. Our results confirm the secular action of the Cosmic Battery in accretion flows around astrophysical black holes.

astro-ph.HE↗

arXiv2017

Gravitational collapse to a Kerr-Newman black hole

We present the first systematic study of the gravitational collapse of rotating and magnetised neutron stars to charged and rotating (Kerr-Newman) black holes. In particular, we consider the collapse of magnetised and rotating neutron stars assuming that no pair-creation takes place and that the charge density in the magnetosphere is so low that the stellar exterior can be described as an electrovacuum. Under these assumptions, which are rather reasonable for a pulsar that has crossed the 'death line', we show that when the star is rotating, it acquires a net initial electrical charge, which is then trapped inside the apparent horizon of the newly formed back hole. We analyse a number of different quantities to validate that the black hole produced is indeed a Kerr-Newman one and show that, in the absence of rotation or magnetic field, the end result of the collapse is a Schwarzschild or Kerr black hole, respectively.

astro-ph.HE↗

arXiv2015

The rapid decay phase of the afterglow as the signature of the Blandford-Znajek mechanism

Gamma-ray bursts (GRBs) are believed to be powered by the electromagnetic extraction of spin energy from a black hole endowed with a magnetic field supported by electric currents in a surrounding disk (Blandford & Znajek 1977). A generic feature of this mechanism is that, under certain fairly general assumptions, the energy loss rate decays exponentially. In this work, we are looking precisely for such exponential decay in the lightcurves of long duration GRBs observed with the XRT instrument on the Swift satellite. We found out that almost 30 % of XRT lightcurves show such behavior before they reach the afterglow plateau. According to Blandford & Znajek, the duration of the burst depends on the magnetic flux accumulated on the event horizon. This allows us to estimate the surface magnetic field of a possible progenitor. Our estimations are consistent with magnetic fields observed in Wolf-Rayet stars.

astro-ph.HE↗

arXiv2015

Are Ultra Long Gamma Ray Bursts powered by black holes spinning down?

Gamma-ray bursts (GRBs) are violent explosions, coming from cosmological distances. They are detected in gamma-rays (also X-rays, UV, optical, radio) almost every day, and have typical durations of a few seconds to a few minutes. Some GRBs have been reported with extraordinary duration of 10^4 sec, the so-called Ultra Long GRBs. It has been debated whether these form a new distinct class of events or whether they are similar to long GRBs. According to Blandford & Znajek (1977), the spin energy of a rotating black hole can be extracted electromagnetically, should the hole be endowed with a magnetic field supported by electric currents in a surrounding disk. We argue that this can be the case for the central engines of GRBs and we show that the duration of the burst depends on the magnetic flux accumulated on the event horizon of the black hole. We thus estimate the surface magnetic field of a possible progenitor star, and we conclude that an Ultra Long GRB may originate from a progenitor star with a relatively low magnetic field.

astro-ph.HE↗

arXiv2015

The Cosmic Battery in Astrophysical Accretion Disks

The aberrated radiation pressure at the inner edge of the accretion disk around an astrophysical black hole imparts a relative azimuthal velocity on the electrons with respect to the ions which gives rise to a ring electric current that generates large scale poloidal magnetic field loops. This is the Cosmic Battery established by Contopoulos and Kazanas in 1998. In the present work we perform realistic numerical simulations of this important astrophysical mechanism in advection-dominated accretion flows-ADAF. We confirm the original prediction that the inner parts of the loops are continuously advected toward the central black hole and contribute to the growth of the large scale magnetic field, whereas the outer parts of the loops are continuously diffusing outward through the turbulent accretion flow. This process of inward advection of the axial field and outward diffusion of the return field proceeds all the way to equipartition, thus generating astrophysically significant magnetic fields on astrophysically relevant timescales. We confirm that there exists a critical value of the magnetic Prandtl number between unity and 10 in the outer disk above which the Cosmic Battery mechanism is suppressed.

astro-ph.HE↗

arXiv2015

Black hole spin down in GRB observations and Cosmology

According to Blandford & Znajek (1977), energy can be extracted electromagnetically from a rotating black hole, should the latter be endowed with a magnetic field supported by electric currents in a surrounding disk. We show that exact models of black hole magnetospheres produce Poynting flux that decreases almost exponentially with time. We went through the Swift BAT-XRT lightcurves and identified a subclass of GRBs that exhibits a clear exponential decay over more than three orders of magnitude in flux (EDOHS GRBs). We estimate the energy given-off in the X-rays and discuss a possible correlation between the peak brightness of the X-ray prompt emission and its decay time. We investigate a possible application of this result in high redshift Cosmology.

astro-ph.HE↗

arXiv2014

Black Hole Magnetospheres

We investigate the structure of the steady-state force-free magnetosphere around a Kerr black hole in various astrophysical settings. The solution Psi(r,theta) depends on the distributions of the magnetic field line angular velocity omega(Psi) and the poloidal electric current I(Psi). These are obtained self-consistently as eigenfunctions that allow the solution to smoothly cross the two singular surfaces of the problem, the Inner Light Surface (ILS) inside the ergosphere, and the Outer Light Surface (OLS), which is the generalization of the pulsar light cylinder. Magnetic field configurations that cross both singular surfaces (e.g. monopole, paraboloidal) are uniquely determined. Configurations that cross only one light surface e.g. the artificial case of a rotating black hole embedded in a vertical magnetic field) are degenerate. We show that, similarly to pulsars, black hole magnetospheres naturally develop an electric current sheet that potentially plays a very important role in the dissipation of black hole rotational energy and in the emission of high-energy radiation.

astro-ph.HE↗

arXiv2013

The orthogonal gamma-ray burst model

We explore the analogy between a rotating magnetized black hole and an axisymmetric pulsar and derive its electromagnetic spindown after its formation in the core collapse of a supermassive star. The spindown shows two characteristic phases, an early Blandford-Znajek phase that lasts a few hundred seconds, and a late pulsar-like afterglow phase that lasts much longer. During the first phase, the spindown luminosity decreases almost exponentially, whereas during the afterglow phase it decreases as t^{-a} with 1<a<1,5. We associate our findings with long duration gamma-ray bursts (GRB) and compare with observations.

astro-ph.HE↗