Search

We remind that the ring down features observed in the LIGO GWs resulted from trembling of photon spheres (Rp=3M) of newly formed compact objects and not from the trembling of their event horizons (R=2M). Further, the tentative evidences for late time echoes in GWs might be signatures of horizonless compact objects rather than vacuum black holes (BHs). Similarly, even for an ideal BH, the radius of its shadow is R_shad = \sqrt{3}Rp is actually the gravitationally lensed shadow of its photon sphere. Accordingly any compact object having R \geq R = 3M would generate similar shadow. Thus, no observation has ever detected any event horizon or any exact BH. Also note that the magnetic field embedded in the accreting plasma close to the compact object is expected to have a radial pattern of B \sim 1/r while the stronger BHM dipole magnetic field should fall off as B \sim 1/r3. Accordingly it has been suggested that one may try to infer the true nature of the so-called astrophysical BHs by studying the radial pattern of the magnetic field in their vicinity. But here we highlight that close to the surface of BHMs, the magnetic field pattern differs significantly from the same for non-relativistic dipoles. In particular, we point out that for ultra-compact BHMs, the polar field is weaker than the equatorial field by an extremely large factor of \sim z_s/lnz_s, where z_s>>1 is the surface gravitational redshift. We suggest that by studying the of radial variation as well as significant angular asymmetry of magnetic field structure near the compact object, future observations might differentiate a theoretical black hole from a astrophysical BH mimicker. This study also shows that even if some BHMs would be hypothesized to possess magnetic fields even stronger than that of magnetars, in certain cases, they may effectively behave as atoll type neutron stars possessing extremely low magnetic fields., 28 pages, 2 figures, final version matching the one published in Astrophysics and Space Science

Huge electromagnetic fields are known to be present during the late stages of the dynamics of supernovae. Thus, when dealing with electrodynamics in this context, the possibility may arise to probe nonlinear theories (generalizations of the Maxwellian electromagnetism). We firstly solve Einstein field equations minimally coupled to an arbitrary (current-free) nonlinear Lagrangian of electrodynamics (NLED) in the slow rotation regime $a\ll M$ (black hole's mass), up to first order in $a/M$. We then make use of the robust and self-contained Born-Infeld Lagrangian in order to compare and contrast the physical properties of such NLED spacetime with its Maxwellian counterpart (a slowly rotating Kerr-Newman spacetime), especially focusing on the astrophysics of both neutrino flavor oscillations ($\nu_e \rightarrow \nu_\mu, \nu_\tau$) and spin-flip ($\nu_l \rightarrow \nu_r$, "$l$" stands for "left" and "$r$" stands for "right", change of neutrino handedness) mass level-crossings, the equivalent to gyroscopic precessions. Such analysis proves that in the spacetime of a slowly rotating nonlinear charged black hole (RNCBH), intrinsically associated with the assumption the electromagnetism is nonlinear, the neutrino dynamics in core-collapse supernovae could be significantly changed. In such astrophysical environment a positive enhancement (reduction of the electron fraction $Y_e, Comment: 17 pages, 9 figures, accepted for publication in Phys. Rev. D

Jet superluminal components are recurrently ejected from active galactic nuclei, micro-quasars, T-Tauri star, and several other astrophysical systems. The mechanism driving this powerful phenomenon is not properly settled down yet. In this article we suggest that ejection of ultrarelativistic components may be associated to the superposition of two actions: precession of the accretion disk induced by the Kerr black hole (KBH) spin, and fragmentation of tilted disk; this last being an astrophysical phenomenon driven by the general relativistic Bardeen-Petterson (B-P) effect. As fragmentation of the accretion disk takes place at the B-P transition radius, a suspended accretion state can develop amid this boundary and the innermost stable orbit around the KBH, which drives a turbulent flow along the inner accretion disk. The torus distortion caused by both hydrodynamic coupling of the inner face to the outer face, and Maxwell stresses from the magnetic field makes it to generate gravitational waves from its turbulent flow in the suspended accretion state. This magneto-centrifugal barrier also precludes incoming matter to penetrate the inner disk, creating ``en passant'' a sort of force-free region. The incoming material trapped in this sort of Lagrange internal point will forcibly precess becoming a source of continuous, frequency-modulated gravitational waves. Eventually, a condition of orbital resonance (beating) can activate between the precession pace and the frequency of the warps travelling along the torus due to its differential rotation. As this transition radius behaves like a sort of Lagrange internal point in a force-free magnetosphere, at resonance blobs can be expelled at ultrarelativistic velocities from the B-P radius in virtue of the power provided by either the vertical (to the disk) linear momentum carried by the surface acoustic wave on the torus warps, or the Aschenbach effect in a nearly maximal KBH, or some other orbital resonance like the well known resonance 3:1. The launching of superluminal components of jets should produce powerful gravitational wave (GW) bursts during its early acceleration phase, which can be catched on the fly by current GW observatories. Here we compute the characteristic amplitude and frequency of such signals and show that they are potentially detectable by the GW observatory LISA.

Jet superluminal components are recurrently ejected from active galactic nuclei, micro-quasars, T-Tauri star, and several other astrophysical systems, including gamma-ray burst sources. The mechanism driving this powerful phenomenon is not properly settled down yet. In this article we suggest that ejection of ultrarelativistic components may be associated to the superposition of two actions: precession of the accretion disk induced by the Kerr black hole (KBH) spin, and fragmentation of the tilted disk; this last being an astrophysical phenomenon driven by the general relativistic Bardeen-Petterson (B-P) effect. As fragmentation of the accretion disk takes place at the B-P transition radius, the incoming material that get trapped in this sort of Lagrange internal point will forcibly precess becoming a source of continuous, frequency-modulated gravitational waves. At resonance blobs can be expelled at ultrarelativistic velocities from the B-P radius. The launching of superluminal components of jets should produce powerful gravitational wave (GW) bursts during its early acceleration phase, which can be catched on the fly by current GW observatories. Here we compute the characteristic amplitude and frequency of such signals and show that they are potentially detectable by the GW observatory LISA.

We propose a way to remove black hole singularities by using a particular nonlinear electrodynamics Lagrangian that has been recently used in various astrophysics and cosmological frameworks. In particular, we adapt the cosmological analysis discussed in a previous work to the black hole physics. Such analysis will be improved by applying the Oppenheimer-Volkoff equation to the black hole case. At the end, fixed the radius of the star, the final density depends only on the introduced quintessential density term $\rho_{\gamma}$ and on the mass., Comment: In this last updated version we correct two typos which were present in Eqs. (21) and (22) in the version of this letter which has been published in Mod. Phys. Lett. A 25, 2423-2429 (2010). In the present version, both of Eqs. (21) and (22) are dimensionally and analytically correct

A Bose–Einstein condensate of a neutral vector boson bearing an anomalous magnetic moment is suggested as a model for ferromagnetic origin of magnetic fields in neutron stars. The vector particles are assumed to arise from parallel spin-paired neutrons. A negative pressure perpendicular to the external field B is acting on this condensate, which for large densities, compress the system, and may produce a collapse. An upper bound of the magnetic fields observable in neutron stars is given. In the the non-relativistic limit, the analogy with the behavior of exploding Bose–Einstein condensates (BECs) for critical values of the magnetic field is briefly discussed.

A scenario for SGRs is introduced in which gravitational radiation reaction effects drive the dynamics of an ultrashort orbital period X-ray binary embracing a high-mass donor white dwarf (WD) to a rapidly rotating low magnetised massive neutron star (NS) surrounded by a thick, dense and massive accretion torus. Driven by GR reaction, sparsely, the binary separation reduces, the WD overflows its Roche lobe and the mass transfer drives unstable the accretion disk around the NS. As the binary circular orbital period is a multiple integer number ($m$) of the period of the WD fundamental mode (Pons et al. 2002), the WD is since long pulsating at its fundamental mode; and most of its harmonics, due to the tidal interaction with its NS orbital companion. Hence, when the powerful irradiation glows onto the WD; from the fireball ejected as part of the disk matter slumps onto the NS, it is partially absorbed. This huge energy excites other WD radial ($p$-mode) pulsations (Podsiadlowski 1991,1995). After each mass-transfer episode the binary separation (and orbital period) is augmented significantly (Deloye & Bildsten 2003; Al��cyan & Morsink 2004) due to the binary's angular momentum redistribution. Thus a new adiabatic inspiral phase driven by GR reaction starts which brings the binary close again, and the process repeats. This model allows to explain most of SGRs observational features: their recurrent activity, energetics of giant superoutbursts and quiescent stages, and particularly the intriguing subpulses discovered by BeppoSAX (Feroci et al. 1999), which are suggested here to be {\it overtones} of the WD radial fundamental mode (see the accompanying paper: Mosquera Cuesta 2004b)., This paper was submitted as a "Letter to the Editor" of MNRAS in July 17/2004. Since that time no answer or referee report was provided to the Author [MNRAS publication policy limits reviewal process no longer than one month (+/- half more) for the reviewal of this kind of submission). I hope this contribution is not receiving a similar "peer-reviewing" as given to the A. Dar and A. De Rujula's "Cannonball model for gamma-ray bursts", or to the R.K. Williams' "Penrose process for energy extraction from rotating black holes". The author welcomes criticisms and suggestions on this paper

The idea that the nonlinear electromagnetic interaction, i. e., light propagation in vacuum, can be geometrized was developed by Novello et al. (2000) and Novello & Salim (2001). Since then a number of physical consequences for the dynamics of a variety of systems have been explored. In a recent paper Mosquera Cuesta & Salim (2003) presented the first astrophysical study where such nonlinear electrodynamics (NLEDs) effects were accounted for in the case of a highly magnetized neutron star or pulsar. In that paper the NLEDs was invoked {\it a l��} Euler-Heisenberg, which is an infinite series expansion of which only the first term was used for the analisys. The immediate consequence of that study was an overall modification of the space-time geometry around the pulsar, which is ``perceived'', in principle, only by light propagating out of the star. This translates into an significant change in the surface redshift, as inferred from absorption (emission) lines observed from a super magnetized pulsar. The result proves to be even more dramatic for the so-called magnetars, pulsars endowed with magnetic ($B$) fields higher then the Schafroth quantum electrodynamics critical $B$-field. Here we demonstrate that the same effect still appears if one calls for the NLEDs in the form of the one rigorously derived by Born & Infeld (1934) based on the special relativistic limit for the velocity of approaching of an elementary particle to a pointlike electron [From the mathematical point of view, the Born & Infeld (1934) NLEDs is described by an exact Lagrangean, whose dynamics has been successfully studied in a wide set of physical systems.]., Accepted for publication in Month. Not. Roy. Ast. Soc. latex file, mn-1.4.sty, 5 pages, 2 figures

We show that it is possible to explain the physical origin of jet precession in active galactic nuclei (AGNs) through the misalignment between the rotation axes of the accretion disk and of the Kerr black hole. We apply this scenario to quasars, Seyfert galaxies and also to the Galactic Center black hole Sgr A*, for which signatures of either jet or disk precession have been found. The formalism adopted is parameterized by the ratio of the precession period to the black hole mass and can be used to put constraints to the physical properties of the accretion disk as well as to the black hole spin in those systems., Comment: 10 pages, 1 figure, accepted for publication in ApJ Letters

We discuss the Bose–Einstein condensation of relativistic vector charged particles in a strong external magnetic field in very dense matter, as may be paired spin-up electrons. We show that for electrons such systems may maintain self-consistently magnetic fields of order in between the interval 1010–1013 Gauss. This could be the origin of large magnetic fields in some white dwarfs, but may also impose bounds due to the arising of strong anisotropy in the pressures, which may produce a transverse collapse of the star.

In the weak field limit of nonlinear Lagrangians for electrodynamics, i.e. theories in which the electric fields are much smaller than the scale (threshold) fields introduced by the nonlinearities, a generalization of the Christodoulou–Ruffini mass formula for charged black holes is presented. It proves that the black hole outer horizon never decreases. It is also demonstrated that reversible transformations are, indeed, fully equivalent to constant horizon solutions for nonlinear theories encompassing asymptotically flat black hole solutions. This result is used to decompose, in an analytical and alternative way, the total mass-energy of nonlinear charged black holes, circumventing the difficulties faced to obtain it via the standard differential approach. It is also proven that the known first law of black hole thermodynamics is the direct consequence of the mass decomposition for general black hole transformations. From all the above we finally show a most important corollary: for relevant astrophysical scenarios nonlinear electrodynamics decreases the extractable energy from a black hole with respect to the Einstein–Maxwell theory. Physical interpretations for these results are also discussed.

Active-to-sterile neutrino oscillations and nonspherical distortion of the resonance surface may trigger asymmetric emission of sterile neutrinos during the core bounce of a supernova collapse. The huge binding energy released and the proto-neutron star rapid rotation may power bursts of gravitational waves by the time the neutrino flavor conversions ensue. These bursts would be detectable up to distances of ~2.2 Mpc. The relativistic requirement of an ellipsoidal axisymmetric core at maximum gravitational wave emission may be explained by the neutrinosphere geometry at the oscillations' onset. Thus, neutrino oscillations might naturally be the underlying mechanism producing asymmetric structures, neutron star kicks, and bipolar jet ejecta in supernovae.

Historically, the modifications to standard electrodynamics were introduced for preventing the appearance of infinite physical quantities in theoretical analysis involving electromagnetic interactions. For instance, Born-Infeld [1] proposed a model in which the infinite self energy of point particles (typical of linear electrodynamics) are removed by introducing an upper limit on the electric field strength and by considering the electron an electrically charged particle of finite radius. Along this line, other Lagrangians for nonlinear electrodynamics were proposed by Plebanski, who also showed that Born-Infeld model satisfy physically acceptable requirements [20], due to its feature of being inspired on the special relativity principles. Applications and consequences of nonlinear electrodynamics have been extensively studied in literature, ranging from cosmological and astrophysical models [22] to nonlinear optics, high power laser technology and plasma physics [25].

A few observational and/or experimental results have dramatically pushed forward the research program on gravity as those from the radio-metric Doppler tracking received from the Pioneer 10 and 11 spacecrafts when the space vehicles were at heliocentric distances between 20 and 70 Astronomical Units (AU). These data have conclusively demonstrated the presence of an anomalous, tiny and blue-shifted frequency drift that changes smoothly at a rate of $ \sim 6 \times 10^{-9}$ Hz s$^{-1}$. Those signals, if interpreted as a gravitational pull of the Sun on each Pioneer vehicle, translates into a deceleration of $a_P = (8.74\pm 1.33) \times 10^{-10}$ m s$^{-2}$. This Sunward acceleration appears to be a violation of Newton's inverse-square law of gravitation, and is referred to as the Pioneer anomaly, the nature of which remains still elusive to unveil. Within the theoretical framework of nonlinear electrodynamics (NLED) in what follows we will address this astrodynamics puzzle, which over the last fifteen years has challenged in a fundamental basis our understanding of gravitational physics. To this goal we will first, and briefly, review the history of the Pioneers 10 and 11 missions. Then a synopsis of currently available Lagrangian formulations of NLED is given. And finally, we present our solution of this enigma by invoking a special class of NLED theories featuring a proper description of electromagnetic phenomena taking place in environments where the strength of the (electro)magnetic fields in the background is decidedly low.

Recently WMAP and BOOMERanG experiments have set stringent constraints on the polarization angle of photons propagating in an expanding universe: $\Delta \alpha = (-2.4 \pm 1.9)^\circ$. The polarization of the Cosmic Microwave Background radiation (CMB) is reviewed in the context of nonlinear electrodynamics (NLED). We compute the polarization angle of photons propagating in a cosmological background with planar symmetry. For this purpose, we use the Pagels-Tomboulis (PT) Lagrangian density describing NLED, which has the form $L\sim (X/\Lambda^4)^{\delta - 1}\; X $, where $X=1/4 F_{\alpha\beta} F^{\alpha \beta}$, and $\delta$ the parameter featuring the non-Maxwellian character of the PT nonlinear description of the electromagnetic interaction. After looking at the polarization components in the plane orthogonal to the ($x$)-direction of propagation of the CMB photons, the polarization angle is defined in terms of the eccentricity of the universe, a geometrical property whose evolution on cosmic time (from the last scattering surface to the present) is constrained by the strength of magnetic fields over extragalactic distances., Comment: 17 pages, 2 figures, minor changes, references added

The polarization of Cosmic Microwave Background radiation (CMB) is reviewed in the context of nonlinear electrodynamics (NLED). We compute the polarization angle of photons propagating in a cosmological background with planar symmetry. The Lagrangian density describing the Pagels‐Tomboulis (PT) nonlinear electrodynamics has the form L∼Xδ, where X = 14FαβFαβ, and δ the parameter featuring the non‐Maxwellian character of the PT nonlinear description of the electromagnetic interaction. After looking at the polarization components in the plane orthogonal to the (x)‐direction of propagation of the CMB photons, the polarization angle is defined in terms of the eccentricity of the universe, a geometrical property whose evolution on cosmic time (from the last scattering surface to the present) is constrained by the strength of magnetic fields over extragalactic distances. Then, by making use of recent data on CMB polarization from WMAP and BOOMERanG, we infer the constraints on the parameter δ characterizing the ...

The current understanding of our universe is built upon the information that we extract from light coming from cosmological sources like far‐away galaxies, quasars, supernovae, gamma‐ray bursts, etc, including the emission in radio wavelengths of those sources. The particular analysis of supernovae type Ia (SNIa) observations has led to the idea that the universe is undergoing a late‐time accelerate phase which started when it was at redshift z∼1. The redshift z is a cosmological parameter inferred from observations of emission (or absorption) lines from the expanding SNIa debris or from the supernova host galaxy, presuming the light properties and interactions, as described by Maxwell’s theory, do not change while it travels through the intervening intergalactic magnetic fields. In this paper we demonstrate that the nonlinear electrodynamics (NLED) description of photon propagation through the weak background intergalactic magnetic fields introduces a fundamental modification of the cosmological redshift...

The amplification of the primordial magnetic fields and the gravitational baryogenesis, a mechanism that allows to generate the baryon asymmetry in the Universe by means of the coupling between the Ricci scalar curvature and the baryon current, are reviewed in the framework of the nonlinear electrodynamics. To study the amplification of the primordial magnetic field strength, we write down the gauge invariant wave equations and then solve them (in the long wavelength approximation) for three different eras of the Universe: de Sitter, the reheating and the radiation dominated era. Constraints on parameters entering the nonlinear electrodynamics are obtained by using the amplitude of the observed galactic magnetic fields and the baryon asymmetry, which are characterized by the dimensionless parameters $r\sim 10^{-37}$ and $\eta_B\lesssim 9\times 10^{-11}$, respectively., Comment: 10 pages, 3 tables, 6 figures. Accepted for publication in Physical Review D

Superluminal jet components are recurrently ejected from the core of active galactic nuclei (AGNs). The mechanism driving this powerful phenomenon is not properly understood yet. Here we suggest that the components ejection from AGNs may be related to the astrophysical process known as Bardeen‐Petterson (B‐P) effect, a general relativistic effect which forces a tilt (precessing) accretion disk orbiting a Kerr black hole (KBH) to break apart at the B‐P radius. This transition region hereby builds up a magneto‐centrifugal barrier which precludes incoming matter to penetrate the AGN inner disk or torus, and creates a sort of force‐free bridge or Lagrange internal point in a force‐free magnetosphere. (Inwardly pointing forces are counterbalanced by outwardly pointing forces). The material trapped in such a region will eventually find a condition of orbital resonance (beating) with the warps traveling along the torus due to its differential rotation. At resonance the mass blobs can be expelled from the B‐P radius in virtue of either the vertical (to the disk) linear momentum carried by the torus warps, or the Aschenbach effect in a nearly maximal KBH, or some other orbital resonances like the well known resonance 3:1. The launching of such superluminal components should produce powerful gravitational wave (GW) bursts during its early acceleration phase. These are GW signals that the LISA space‐borne GW observatory can detect for distances upto nearly the Hubble radius.

Neutrino (ν) oscillations during the core collapse and bounce of a supernova (SN) are shown to generate the most powerful detectable gravitational wave (GW) bursts. The SN neutronization phase produces mainly electron (νe) neutrinos, the oscillations of which must take place within a few mean-free paths of their resonance surface located near their neutrinosphere. Here we characterize the GW signals produced by spin-flip oscillations inside the fast-rotating protoneutron star in the SN core. In this novel mechanism, the release of both the oscillation-produced νμ's, ντ's and the spin-flip-driven GW pulse provides a unique emission offset [Formula: see text] for measuring the ν travel time to Earth. As massive ν's get noticeably delayed on its journey to Earth with respect to the GW, they generate over the oscillation transient, the accurate measurement of this time-of-flight delay by SNEWS + LIGO, VIRGO, BBO, DECIGO, etc. can assess the absolute ν mass spectrum straightforwardly.

The supernova (SN) neutronization phase produces mainly electron ($\nu_e$) neutrinos, the oscillations of which must take place within a few mean-free-paths of their resonance surface located nearby their neutrinosphere. The state-of-the-art on the SN dynamics suggests that a significant part of these $\nu_e$ can convert into right-handed neutrinos in virtue of the interaction of the electrons and the protons flowing with the SN outgoing plasma, whenever the Dirac neutrino magnetic moment be of strength $\mu_\nu < 10^{-11} \mu_{\rm B}$, with $\mu_{\rm B}$ being the Bohr magneton. In the supernova envelope, part of these neutrinos can flip back to the left-handed flavors due to the interaction of the neutrino magnetic moment with the magnetic field in the SN expanding plasma (Kuznetsov & Mikheev 2007; Kuznetsov, Mikheev & Okrugin 2008), a region where the field strength is currently accepted to be $B \gtrsim 10^{13}$ ~G. This type of $\nu$ oscillations were shown to generate powerful gravitational wave (GW) bursts (Mosquera Cuesta 2000, Mosquera Cuesta 2002, Mosquera Cuesta & Fiuza 2004, Loveridge 2004). If such double spin-flip mechanism does run into action inside the SN core, then the release of both the oscillation-produced $\nu_\mu$s, $\nu_\tau$s and the GW pulse generated by the coherent $\nu$ spin-flips provides a unique emission offset $\Delta T^{emission}_{\rm GW} \leftrightarrow \nu = 0$ for measuring the $\nu$ travel time to Earth. As massive $\nu$s get noticeably delayed on its journey to Earth with respect to the Einstein GW they generated during the reconversion transient, then the accurate measurement of this time-of-flight delay by SNEWS + LIGO, VIRGO, BBO, DECIGO, etc., might readily assess the absolute $\nu$ mass spectrum., Comment: 14 pages, 2 figures, ApJ aastex.cls style. In press by The Astrophysical Journal

Determination of pulsar parallaxes and proper motions addresses fundamental astrophysical open issues. Here, the ATNF Catalog is scrutinized searching for pulsar distances and proper motions. For a sample of 212 run away pulsars (RAPs), which currently run across the Galaxy at very high speed and undergo large displacements, some gravitational-wave (GW) signals produced by such present accelerations appear to be detectable after calibration against the Advanced LIGO (LIGO II). Motivated by this insight, we address the issue of the pulsar kick at birth. We show that during the short rise fling each run away pulsar (RAP) generates a GW signal with characteristic amplitude and frequency that makes it detectable by current GW interferometers. For a realistic analysis, an efficiency parameter is introduced to quantify the expenditure of the rise fling kinetic energy, which is estimated from the linear momentum conservation law applied to the supernova explosion that kicks out the pulsar. The remaining energy is supposed to be used to make the star to spin. Thus, a comparison with the spin of ATNF pulsars having velocity in the interval 400-500 km s$^{-1}$ is performed. The resulting difference suggests that other mechanisms should dissipate part of that energy to produce the observed pulsar spin periods. Meanwhile, the kick phenomenon may also occur in globular and open star clusters at the formation or disruption of very short period compact binary systems wherein abrupt velocity and acceleration similar to those given to RAPs during the short rise fling can be imparted to each orbital partner. Thus pulsar astrometry from micro- to nano-arsec scales might be of much help. In case of a supernova, the RAP GW signal could be a benchmark for the GW signal from the core collapse., 22 pages, 24 figures, 5 tables, revtex4 style. Accepted for Publication in Journ. Cosm. Astropart. Phys. (JCAP)

It has been claimed that during the late time history of our universe, the fine structure constant of electromagnetism, $\alpha$, has been increasing (Webb et al. 2001; Murphy et al. 2003). The conclusion is achieved after looking at the separation between lines of ions like CIV, MgII, SiII, FeII, among others in the absorption spectra of very distant quasars, and comparing them with their counterparts obtained in the laboratory. However, in the meantime, other teams has claimed either a null result or a decreasing $\alpha$ with respect to the cosmic time (Chand et al. 2004; Levshakov et al. 2004). Also, the current precision of laboratory tests does not allow one to either comfort or reject any of these astronomical observations. Here we suggest that as photons are the sidereal messengers, a nonlinear electrodynamics (NLED) description of the interaction of photons with the weak local background magnetic fields of a gas cloud absorber around the emitting quasar can reconcile the Chand et al. (2004) and Levshakov et al. (2004) findings with the negative variation found by Murphy et al. (2001a, 2001b, 2001c, 2001d) and Webb et al. (2001), and also to find a bridge with the positive variation argued more recently by Levshakov et al. (2006a, 2007). We also show that nonlinear electrodynamics photon propagation in a vacuum permeated by a background magnetic field presents a full agreement with constraints from Oklo natural reactor data. Finally, we show that NLED may render a null result only in a narrow range of the local background magnetic field which should be the case of both the claims by Chand et al.(2004) and by Srianand et al. (2004)., Comment: 6 revtex4 pages, no figures. In press by Monthly Notices of Royal Astronomical Society

We construct the Hubble diagram (HD) of Gamma-Ray Bursts (GRBs) with redshifts reaching up to $z \sim 6$, by using five luminosity vs. luminosity indicator relations calibrated with the Cardassian cosmology. This model has a major interesting feature: despite of being matter-dominated and flat, it can explain the present accelerate expansion of the universe. This is the first study of this class of models using high redshift GRBs. We have performed a $\chi$-square statistical analysis of the GRBs calibrated with the Cardassian model, and also combined them with both the current Cosmic Microwave Background and Baryonic Acoustic Oscillation data. Our results show consistency between the current observational data and the model predictions. In particular, the best-fit parameters obtained from the $\chi^2$-analysis are in agreement with those obtained from the Concordance Cosmology ($\Lambda$-CDM). We determine the redshift at which the universe would start to follow the Cardassian expansion, i. e., \zc, and both the redshift at which the universe had started to accelerate, i. e., \zac, and the age-redshift relation $H_0t_0$. Our results also show that the universe, from the point of view of GRBs, had undergo a transition to acceleration at a redshift $z \approx 0.2-0.7$, which agrees with the SNIa results. Hence, after confronting the Cardassian scenario with the GRBs HD and proving its consistency with it, we conclude that GRBs should indeed be considered a complementary tool to several other astronomical observations for studies of high accuracy in cosmology., Comment: Revtex4 format, 17 pages, 13 figures, 5 tables

The Bardeen-Petterson general relativistic effect has been suggested as the mechanism responsible for precession in some accretion disk systems. Here we examine separately four mechanisms (tidally-induced, irradiation-induced, magnetically-induced and Bardeen-Petterson-induced) that can lead to warping and precession. We use a sample of eight X-ray binaries and four Active Galactic Nuclei (AGNs) that present signatures of warping and/or precession in their accretion disks to explore the viability of the different mechanisms. For the X-ray binaries SMC X-1 and 4U 1907+09 all four mechanisms provide precession periods compatible with those observed, while for Cyg X-1 and the active galaxies Arp 102B and NGC 1068, only two mechanisms are in agreement with the observations. The irradiation-driven instability seems incapable of producing the inferred precession of the active galaxies in our sample, and the tidally-induced precession can probably be ruled out in the case of Arp 102B. Perhaps the best case for a Bardeen-Petterson precession can be achieved for NGC 1068. Our results show that given the many observational uncertainties that still exist, it is extremely difficult to confirm unambiguously that the Bardeen-Petterson effect has been observed in any of the other sources of our sample., 35 pages, 8 figures, accepted for publication in The Astrophysical Journal

The early Universe might have undergone phase transitions at energy scales much higher than the one corresponding to the Grand Unified Theories (GUT) scales. At these higher energy scales, the transition at which gravity separated from all other interactions; the so-called Planck era, more massive strings called supermassive cosmic strings, could have been produced, with energy of about 10^{19}GeV. The dynamics of strings formed with this energy scale cannot be described by means of the weak-field approximation, as in the standard procedure for ordinary GUT cosmic strings. As suggested by string theories, at this extreme energies, gravity may be transmitted by some kind of scalar field (usually called the {\it dilaton}) in addition to the tensor field of Einstein's theory of gravity. It is then permissible to tackle the issue regarding the dynamics of supermassive cosmic strings within this framework. With this aim we obtain the gravitational field of a supermassive screwed cosmic string in a scalar-tensor theory of gravity. We show that for the supermassive configuration exact solutions of scalar-tensor screwed cosmic strings can be found in connection with the Bogomol'nyi limit. We show that the generalization of Bogomol'nyi arguments to the Brans-Dicke theory is possible when torsion is present and we obtain an exact solution in this supermassive regime, with the dilaton solution obtained by consistency with internal constraints., 15 pages, Latex, 4 figures

The occurrence of the phenomenon known as photon acceleration is a natural prediction of nonlinear electrodynamics (NLED). This would appear as an anomalous frequency shift in any modeling of the electromagnetic field that only takes into account the classical Maxwell theory. Thus, it is tempting to address the unresolved anomalous, steady; but time-dependent, blueshift of the Pioneer 10/11 spacecrafts within the framework of NLED. Here we show that astrophysical data on the strength of the magnetic field in both the Galaxy and the local (super)cluster of galaxies support the view on the major Pioneer anomaly as a consequence of the phenomenon of photon acceleration. If confirmed, through further observations or lab experiments, the reality of this phenomenon should prompt to take it into account in any forthcoming research on both cosmological evolution and origin and dynamical effects of primordial magnetic fields, whose seeds are estimated to be very weak., Comment: Final version accepted for publication in Europhysics Letters, uses EPL style, 7 pages

Although we do not observe directly black holes in the Universe, their presence has been inferred in several astrophysical systems, from galactic to extragalactic scales. In this work, we will restrict our focus in the possible signatures of the presence of Kerr black holes in active galactic nuclei, one of the most powerful sources in the Universe. Particularly, we will discuss how jet/accretion disc precession and their directional stability can be used to trace the accretion disc properties as well as the black hole spin in the Seyfert galaxy NGC 1068.

VLBA high spatial resolution observations of the disk structure of the active galactic nucleus NGC 1068 has recently revealed that the kinematics and geometry of this AGN is well characterized by an outer disk of H2O maser emission having a compact milliarcsecond (parsec) scale structure, which is encircling a thin rotating inner disk surrounding a ~10^7 M$_\sun$ compact mass, likely a black hole. A curious feature in this source is the occurrence of a misalignment between the inner and outer parts of the disk, with the galaxy's radio jet being orthogonal to the inner disk. We interpret this peculiar configuration as due to the Bardeen-Petterson effect, a general relativistic effect that warps an initially inclined (to the black hole equator) viscous disk, and drives the angular momentum vector of its inner part into alignment with the rotating black hole spin. We estimate the time-scale for both angular momenta to get aligned as a function the spin parameter of the Kerr black hole. We also reproduce the shape of the parsec and kiloparsec scale jets, assuming a model in which the jet is precessing with a period and aperture angle that decrease exponentially with time, as expected from the Bardeen-Petterson effect., Comment: 12 pages, 3 figures, accepted for publication in The Astrophysical Journal

We proposed in previous papers that variations in the kinematics of the parsec‐scale jet of some active galactic nuclei, as well as their long‐term periodic variability at optical wavelengths, are possibly related to the precession of their jet inlets. In this work, we study the feasibility of explaining its physical origin through rigid‐body precession of the accretion disc due to the misalignment between the angular momenta of the accretion disc and the Kerr black hole, as well as magnetic torques generated from the interaction between a large‐scale magnetic field and the induced electric currents in the disc.

In 1936 Einstein predicted the phenomenon presently known as gravitational lensing (GL). A prime feature of GL is the magnification, because of the gravitational field, of the star visible surface as seen from a distant observer. We show here that nonlinear electrodynamics (NLED) modifies in a fundamental basis Einstein's general relativistic (GR) original derivation. The effect becomes apparent by studying the light propagation from a strongly magnetic ($B$) pulsar (SMP). Unlike its GR counterpart, the photon dynamics in NLED leads to a new effective GL, which depends also on the $B$-field permeating the pulsar. The apparent radius of a SMP appears then unexpectedly diminished, by a large factor, as compared to the classical Einstein's prediction. This may prove very crucial in determining physical properties of high $B$-field stars from their X-ray emission., 12(ws-ijmpa.cls) pages, 3 figures. Accepted for publication in IJMPA

During the core bounce of a supernova collapse resonant active-to-active ($\nu_a \to \nu_a$), as well as active-to-sterile ($\nu_a \to \nu_s$) neutrino ($\nu$) oscillations can take place. Besides, over this phase weak magnetism increases antineutrino ($\bar{\nu}$) mean free paths, and thus its luminosity. Because the oscillation feeds mass-energy into the target $\nu$ species, the large mass-squared difference between species ($\nu_a \to \nu_s$) implies a huge amount of power to be given off as gravitational waves ($L_{\textrm{GWs}} \sim 10^{49}erg s$^{-1}$), due to anisotropic but coherent $\nu$ flow over the oscillation length. This anisotropy in the $\nu$-flux is driven by both the {\it universal spin-rotation} and the spin-magnetic coupling. The new spacetime strain estimated this way is still several orders of magnitude larger than those from $\nu$ diffusion (convection and cooling) or quadrupole moments of the neutron star matter. This new feature turns these bursts the more promising supernova gravitational-wave signal that may be detected by observatories as LIGO, VIRGO, etc., for distances far out to the VIRGO cluster of galaxies., Comment: 10 pages, 1 figure, Proceedings of International Workshop on Astronomy and Relativistic Astrophysics, Olinda (Brazil), October 12-16 (2003), to be published in Int. J. Mod. Phys. D

During supernova core collapse and bounce resonant active-to-active, as well as active-to-sterile, neutrino ($\nu$) oscillations can take place. Over this phase weak magnetism increases antineutrino mean free paths, and thus its luminosity. Because oscillations feed mass-energy into the target $\nu$ species, the large mass-squared difference between $\nu$ states implies a huge amount of power to be given off as gravitational waves (GWs) due to the universal {\it spin-rotation} and the spin-magnetic coupling driven $\nu$ anisotropic flow, which is coherent over the oscillation length. The spacetime strain produced is about two orders of magnitude larger than those from $\nu$ difussion or neutron star matter anisotropies. GWs observatories as LIGO, VIRGO, GEO-600, TAMA-300, etc., can search for these bursts far out to the VIRGO cluster of galaxies., Comment: 3 pages, 1 figure, Proceedings of the "Hadron Physics - RANP 2004", Angra dos Reis - Rio de Janeiro - Brazil, March 28 to April 03

The crucial observation on the occurrence of subpulses (overtones) in the Power Spectral Density of the August 27 (1998) event from SGR1900+14, as discovered by BeppoSAX (Feroci et al. 1999), has received no consistent explanation in the context of the competing theories to explain the SGRs phenomenology: the magnetar and accretion-driven models. Based on the ultra-relativistic, ultracompact X-ray binary model introduced in the accompanying paper (Mosquera Cuesta 2004a), I present here a self-consistent explanation for such an striking feature. I suggest that both the fundamental mode and the overtones observed in SGR1900+14 stem from pulsations of a massive white dwarf (WD). The fundamental mode (and likely some of its harmonics) is excited because of the mutual gravitational interaction with its orbital companion (a NS, envisioned here as point mass object) whenever the binary Keplerian orbital frequency is a multiple integer number ($m$) of that mode frequency (Pons et al. 2002). Besides, a large part of the powerful irradiation from the fireball-like explosion occurring on the NS (after partial accretion of disk material) is absorbed in different regions of the star driving the excitation of other multipoles (Podsiadlowski 1991,1995), i.e., the overtones (fluid modes of higher frequency). Part of this energy is then reemitted into space from the WD surface or atmosphere. This way, the WD lightcurve carries with it the signature of these pulsations inasmuch the way as it happens with the Sun pulsations in Helioseismology. It is shown that our theoretical prediction on the pulsation spectrum agrees quite well with the one found by BeppoSAX (Feroci et al. 1999). A feature confirmed by numerical simulations (Montgomery & Winget 2000)., This paper was submitted as a "Letter to the Editor" of MNRAS in July 17/2004. Since that time no answer or referee report was provided to the Author [MNRAS publication policy limits reviewal process no longer than one month (+/- half more) for the reviewal of this kind of submission]. I hope this contribution is not receiving a similar "peer-reviewing" as given to the A. Dar and A. De Rujula's "Cannonball model for gamma-ray bursts", or to the R.K. Williams' "Penrose process for energy extraction from rotating black holes". The author welcomes criticisms and suggestions on this paper

In general relativity all forms of energy contribute to gravity and not only just ordinary matter as in Newtonian Physics. This fact can be seen in the modified hydrostatic equilibrium equation for relativistic stars pervaded by magnetic (B) fields. It has an additional term coupled to the matter part as well as an anisotropic term which is purely of magnetic origin. That additional term coming from the pressure changed by the radial component of the diagonal electromagnetic field tensor, weakens the gravitational force when B is strong enough and can even produce an unexpected change in the attractive nature of the force by reversing its sign. In an extreme case, this new general relativistic (GR) effect can even trigger an instability in the star as a consequence of the sudden reversal of the hydrostatic pressure gradient. We suggest here that this GR effect may be the possible central engine driving the transient giant outbursts observed in Soft Gamma-ray Repeaters (SGRs). In small regions of the neutron star (NS), strong magnetic condensation can take place. Beyond a critical limit, these highly magnetised bubbles may explode releasing the trapped energy as a burst of gamma-rays of ~ 10^{36-40} erg., Comment: 6 pages, 2 figures

Currently is argued that the best method of determining the neutron star (NS) fundamental properties is by measuring the {\it gravitational redshift} ($z$) of spectral lines produced in the star photosphere. Measurement of $z$ at the star surface provides a unique insight on the NS mass-to-radius relation and thus on its equation of state (EoS), which reflects the physics of the strong interaction between particles making up the star. Evidence for such a measurement has been provided quite recently by Cottam, Paerels & Mendez (2002), and also by Sanwal et {\it al.} (2002). Here we argue that although the quoted observations are undisputed for canonical pulsars, they could be misidentified if the NS is endowed with a super strong $B$ as in the so-called magnetars (Duncan & Thompson 1992) and strange quark magnetars (Zhang 2002), as in the spectral line discovered by Ibrahim et {\it al.} (2002;2003). The source of this new "confusion" redshift is related to nonlinear electrodynamics (NLEDs) effects., 5 pages, emulateapj.sty, accepted for publication in Astrophysical Journal

In braneworld-like solutions of the hierarchy problem gravitons and right-handed (sterile) neutrinos are in principle the unique non-standard model fields allowed to propagate into the bulk, thus their coupling is naturally expected. Since active- to-sterile neutrino oscillations can take place during the core bounce of a supernova collapse, then gravitational waves must be produced over the oscillation length through anisotropic neutrino flow. Because the oscillation feeds mass-energy up into (or takes it out of) the target species, the large mass-squared difference between species makes a huge amount of energy to be given off as gravity waves, which is larger than from neutrino convection and cooling, or quadrupole moments of neutron star matter. The strengthness of these bursts would turn them the more sure supernova gravitational- wave signal detectable by interferometers, for distances out to the VIRGO cluster of galaxies.

In theories with an infinite extra dimension, free particles localized on the brane can leak out to the extra space. We argue that if there were color confinement in the bulk, electrons would be more able to escape than quarks and than protons (which are composed states). Thus, this process generates an electric charge asymmetry on brane matter densities. A primordial charge asymmetry during Big Bang Nucleosynthesis era is predicted. We use current bounds on this and on electron disappearance to constrain the parameter space of these models. Although the generated asymmetry is generically small, it could be particularly enhanced on large densities as in astrophysical objects, like massive stars. We suggest the possibility that such accumulation of charge may be linked, upon supernova collapse, to the formation of a charged Black Hole and the generation of Gamma-Ray Bursts., Comment: Four pages, one figure. Minor changes, conclusions remain

Berezinsky, Hnatyk and Vilenkin showed that superconducting cosmic strings could be central engines for cosmological gamma-ray bursts and for producing the neutrino component of ultra-high energy cosmic rays. A consequence of this mechanism would be that a detectable cusp-triggered gravitational wave burst should be released simultaneously with the $\gamma$-ray surge. If contemporary measurements of both $\gamma$ and $\nu$ radiation could be made for any particular source, then the cosmological time-delay between them might be useful for putting unprecedently tight bounds on the neutrino mass spectrum. Such measurements could consistently verify or rule out the model, since strictly correlated behaviour is expected for the duration of the event and for the time variability of the spectra., Comment: 7 pages, 2 figures, to appear in Proceedings of COSMO-2000: Particle physics and the early universe, Cheju-Do, Korea. Editor J. E. Kim, World Scientific Co

The annihilation of pairs of very weakly interacting particles in the neibourghood of gamma-ray sources is introduced here as a plausible mechanism to overcome the baryon load problem. This way we can explain how these very high energy gamma-ray bursts can be powered at the onset of very energetic events like supernovae (collapsars) explosions or coalescences of binary neutron stars. Our approach uses the weak-scale hidden sector models in which the Higgs sector of the standard model is extended to include a gauge singlet that only interacts with the Higgs particle. These particles would be produced either during the implosion of the red supergiant star core or at the aftermath of a neutron star binary merger. The whole energetics and timescales of the relativistic blast wave, the fireball, are reproduced., 4 pp, 1 ps fig, text revised and improved

In this work, we study the feasibility of four physical mechanisms for driving jet/disk precession in the galaxies NGC 4258, NGC 1068, 3C 120, OJ 287 and Arp 102B. Given the many observational uncertainties that still exist in the parameters of those AGN, we find that it is difficult to discriminate unambiguously what mechanism is actually responsible for precession.

It is shown that a weak modification of general relativity, in the linearized approach, renders a spherically symmetric and stationary model of the universe. This is due to the presence of a third mode of polarization in the linearized gravity in which a "curvature" energy term is present. Such an energy can, in principle, be identified as the Dark Energy. The model can also help to a better understanding of the framework of the Einstein-Vlasov system., Accepted for publication by Europhysics Letters