Carlos H. Coimbra-Araújo

Casadio-Fabbri-Mazzacurati Black Strings and Braneworld-induced Quasars Luminosity Corrections

This paper aims to evince the corrections on the black string warped horizon in the braneworld paradigm, and their drastic physical consequences, as well as to provide subsequent applications in astrophysics. Our analysis concerning black holes on the brane departs from the Schwarzschild case, where the black string is unstable to large-scale perturbation. The cognizable measurability of the black string horizon corrections due to braneworld effects is investigated, as well as their applications in the variation of quasars luminosity. We delve into the case wherein two solutions of Einstein’s equations proposed by Casadio, Fabbri and Mazzacurati, regarding black hole metrics presented a post-Newtonian parameter measured on the brane. In this scenario, it is possible to analyze purely the braneworld corrected variation in quasars luminosity, by an appropriate choice of the post-Newtonian parameter that precludes Hawking radiation on the brane: the variation in quasars luminosity is uniquely provided by pure braneworld effects, as the Hawking radiation on the brane is suppressed.

ANTI-DE SITTER CURVATURE RADIUS CONSTRAINED BY QUASARS IN BRANE-WORLD SCENARIOS

This paper is intended to investigate the luminosity due to the accretion of gas in supermassive black holes (SMBHs) in the center of quasars, using a brane-world scenario naturally endowed with extra dimensions, whereon theories formulated introduce corrections in the field equations at high energies. SMBHs possess the necessary highly energetic environment for the introduction of these corrections, which are shown to produce small deviations in all SMBH properties and, consequently, corrections in the accretion theory that supports quasars radiative processes. The radiative flux observed from quasars indicates these deviations, from which the magnitude of the AdS5 bulk curvature radius, and consequently the extra dimension compactification radius is estimated.

Ultra high energy cosmic rays and possible signature of black strings

Ultra high energy cosmic rays (UHECRs) probably originate in extreme conditions in which extra dimension effects might be important. In this paper we calculate the correction in black hole accretion mechanisms due to extra dimension effects in the static and rotating cases. A parametrization of the external Kerr horizons in both cases is presented and analysed. We use previous calculations of upper limits on the UHECR flux to set limits on the UHECR production efficiency of nine sources. The upper limit on the UHECR luminosity calculation is based on GeV-TeV gamma-ray measurements. The total luminosity due to the accretion mechanism is compared to the upper limit on UHECRs. The dependence of the UHECR production efficiency upper limit on black hole mass is also presented and discussed

Gravity with Extra Dimensions and Dark Matter Interpretation: Phenomenological Example via Miyamoto–Nagai Galaxy

Any connection between dark matter and extra dimensions is revealed by the effective energy-momentum tensor associated with the theory. In order to investigate and test such a relationship, we examine a higher-dimensional space–time endowed with a factorizable general metric with a configuration such that its density profile coincides with the Newtonian potential for disk galaxies. An appropriate solution representing stratifications of mass in the central-bulge and disk part of galaxies, e.g., the Miyamoto–Nagai ansatz, is used to solve the Einstein equations. We compute the stable rotation curves of such systems and, with no adjustable parameters, accurately recover the observational data for flat or not asymptotically flat galaxy rotation curves. We present examples of density profiles and compare them to the profile obtained from purely Newtonian potentials.

Luminosity of ultrahigh energy cosmic rays and bounds on magnetic luminosity of radio-loud active galactic nuclei

We investigate the production of magnetic flux from rotating black holes in active galactic nuclei (AGNs) and compare it with the upper limit of ultrahigh energy cosmic ray (UHECR) luminosities, calculated from observed integral flux of GeV-TeV gamma rays for nine UHECR AGN sources. We find that, for the expected range of black hole rotations (0.44<a<0.80), the corresponding bounds of theoretical magnetic luminosities from AGNs coincides with the calculated UHECR luminosity. We argue that such result possibly can contribute to constrain AGN magnetic and dynamic properties as phenomenological tools to explain the requisite conditions to proper accelerate the highest energy cosmic rays.

Central accumulation of magnetic flux in massive Seyfert galaxies as a possible engine to trigger ultrahigh energy cosmic rays

In the present paper we investigate the production of ultrahigh energy cosmic rays (UHECRs) from Seyferts. We discuss the UHECR luminosities obtained by two possible engine trigger models: pure radiative transfer and the energy extraction from poloidal magnetic flux. The first case is modeled by Kerr slim disk or Bondi accretion mechanisms. Since it is assumed that the broadband spectra of Seyferts indicate that at least the outer portions of their accretion disks are cold and geometrically thin, and since our results point that the consequent radiative energy transfer is inefficient, we build the second approach based on massive Seyferts with sufficient central poloidal magnetic field to trigger an outflow of magnetically driven charged particles capable to explain the observed UHECRs and gamma rays in Earth experiments from a given Seyfert source.

Luminosity of ultrahigh energy cosmic rays as a probe of black strings

Ultrahigh energy cosmic rays (UHECRs) can originate from extragalactic sources as Active Galactic Nuclei. We propose a mechanism to calculate bounds on the upper limits of the AGN luminosity fraction that can be converted into UHECRs. This result comes from the mechanism powered by central black holes to produce the AGN luminosity and observation of UHECRs and gamma-rays from experiments to reconstruct proton and iron luminosities of a given AGN source.

Estimative of conversion fractions of AGN magnetic luminosity to produce ultra high energy cosmic rays from the observation of Fermi-LAT gamma rays

A fraction of the magnetic luminosity (LB) produced by Kerr black holes in some active galactic nuclei (AGNs) can produce the necessary energy to accelerate ultra high energy cosmic rays (UHECRs) beyond the GZK limit, observed, e.g., by the Pierre Auger experiment. Nevertheless, the direct detection of those UHECRs has a lack of information about the direction of the source from where those cosmic rays are coming, since charged particles are deflected by the intergalactic magnetic field. This problem arises the needing of alternative methods to evaluate the luminosity of UHECRs (LCR) from a given source. Methods proposed in literature range from the observation of upper limits in gamma rays to the observation of upper limits in neutrinos produced by cascade effects during the propagation of UHECRs. In this aspect, the present work proposes a method to calculate limits of the main possible conversion fractions ηCR = LCR/LB for nine UHECR AGN Seyfert sources based on the respective observation of gamma ray ...

Stability of perturbed geodesics in nD axisymmetric spacetimes

The eect of self-gravity of a disk matter is evaluated by the simplest modes of oscillation frequencies for perturbed circular geodesics. It is plotted the radial profiles of free oscillations of an equatorial circular geodesic perturbed within the orbital plane or in the vertical direction. The calculation is carried out to geodesics of an axisymmetric n-dimensional spacetime. The profiles are computed by examples of disks embeded in five-dimensional or six-dimensional spacetime, where it is studied the motion of free test particles for three axisymmetric cases: (i) the Newtonian limit of a general proposed 5D and 6D axisymmetric spacetime; (ii) a simple Randall-Sundrum 5D spacetime; (iii) general 5D and 6D Randall-Sundrum spacetime. The equation of motion of such particles is derived and the stability study is computed for both horizontal and vertical directions, to see how extra dimensions could aect the system. In particular, we investigate a disk constructed from Miyamoto-Nagai and Chazy-Curzon with a cut parameter to generate a disk potential. Those solutions have a simple extension for extra dimensions in the case (i), and by solving vacuum Einstein field equations for a kind of Randall-SundrumWeyl metric in cases (ii) and (iii). We find that it is possible to compute a range of possible solutions where such perturbed geodesics are stable. Basicaly, the stable solutions appear, for the radial direction, in special cases when the system has 5D and in all cases when the system has 6D; and, for the axial direction, in all cases when the system has both 5D or 6D.

Brownian motion of black holes in stellar systems with non-Maxwellian distribution of the field stars

A massive black hole at the center of a dense stellar system, such as a globular cluster or a galactic nucleus, is subject to a random walk due gravitational encounters with nearby stars. It behaves as a Brownian particle, since it is much more massive than the surrounding stars and moves much more slowly than they do. If the distribution function for the stellar velocities is Maxwellian, there is a exact equipartition of kinetic energy between the black hole and the stars in the stationary state. However, if the distribution function deviates from a Maxwellian form, the strict equipartition cannot be achieved. The deviation from equipartition is quantified in this work by applying the Tsallis q-distribution for the stellar velocities in a q-isothermal stellar system and in a generalized King model.