Herman J. Mosquera Cuesta

Warping and precession in galactic and extragalactic accretion disks

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 (tidal, irradiative, magnetic, and Bardeen-Petterson) 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 to be 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.

Confronting the Hubble diagram of gamma-ray bursts with Cardassian cosmology

We construct the Hubble diagram of gamma-ray bursts (GRBs) with redshifts reaching up to z~6, by using five luminosity versus luminosity indicator relations calibrated with the Cardassian cosmology. This model has a major interesting feature: despite being matter dominated and flat, it can explain the present accelerated expansion of the universe. This is the first study of this class of models using high redshift GRBs. We have performed a χ squared 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 that χ2 analysis are in agreement with those obtained from previous investigations. The influence of these best fit parameters on the redshift at which the universe would start to follow the Cardassian expansion, i.e., zcard, and on both the redshift at which the universe supposedly had started to accelerate, i.e., zacc, and the age–redshift relation, H0t0, are also discussed. Our results also show that the universe, from the point of view of GRBs, had undergone a transition to acceleration at a redshift z≈0.2–0.7, which agrees with the type Ia supernovae results. One important point that we notice is that despite the statistical analysis being performed with a model that does not need any vacuum energy, we found that the results attained using this cosmological model are compatible with those obtained with the concordance cosmology (Λ-CDM; CDM: cold dark matter), as far as GRBs are concerned. Hence, after confronting the Cardassian scenario with the GRB Hubble diagram, our main conclusion is that GRBs should indeed be considered a tool complementary to several other observational studies for doing precision cosmology.

Observational Evidence of Spin-induced Precession in Active Galactic Nuclei

We show that it is possible to explain the physical origin of jet precession in active galactic nuclei 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 on the physical properties of the accretion disk as well as on the black hole spin in those systems.

Non-linear electrodynamics and the gravitational redshift of highly magnetized neutron stars

We show that nonlinear electrodynamics (NLED) modifies in a f undamental basis the concept of gravitational red-shift (GRS) as it was introduced by Einstein’s general relativity. The effect becomes apparent when light propagation from super strongly magnetized compact objects, as pulsars, is under focus. The analysis, here based on the (exact) nonlinear Lagrangean of Born & Infeld (1934), proves that alike from general relativity (GR); where the GRS independs on any background magnetic (B) field, when NLED is taken into the photon dynamics an effective GRS appears, which happens to decidedly depend on the B-field pervading the pulsar. The resulting GRS tends to infinity as the B-field grows larger, as opposed to the Einstein prediction. As in astrophysics the GRS is admittedly used to infer the mass-radius relation, and thus the equation of state of a compact star, e. g. a neutron star (Cottam, Paerels & Mendez 2002), this unexpected GRS critical change may misguide observers to that fundamental property. Hence, a correct procedure to estimate those valuous physical properties demands a neat separation of the NLED effects from the pure gravitational ones in the light emitted by ultra magnetised pulsars.

Bardeen-Petterson Effect and the Disk Structure of the Seyfert Galaxy NGC 1068

VLBA high spatial resolution observations of the disk structure of the active galactic nucleus (AGN) NGC 1068 have 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 ~107 M☉ 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 galaxys 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 timescale for both angular momenta to get aligned as a function of 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.

Primordial magnetic fields and gravitational baryogenesis in nonlinear electrodynamics

The amplification of the primordial magnetic fields and the gravitational baryogenesis, a mechanism that allows one 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 ~ 10 -37 and η B ≤ 9 × 10 -11 , respectively.

Charge asymmetry in the brane world and the formation of charged black holes

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. Extra dimensions could exist in nature and yet remain hidden to current experiments. Motivated by the old ideas of Kaluza and Klein and the modern string theory one has usually believed that such extra dimensions should be compact. Recently, however, it has been suggested the possibility that such extra dimensions can be rather infinite [1–3] and still be hidden to our eyes. The explanation of why it is so comes from the concept of localization of particles [3–6] in a higher dimensional space (the bulk) around a fixed point that defines a four dimensional hypersurface (the brane). Thus, at low energy, observed particles would behave just as four dimensional fields. A simple realization of this scenario appears on five dimensional theories where the background space is given by the Randall-Sundrum (RS) metric [1]

NEUTRINO MASS SPECTRUM FROM GRAVITATIONAL WAVES GENERATED BY DOUBLE NEUTRINO SPIN-FLIP IN SUPERNOVAE

The supernova (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 nearby their neutrinosphere. The latest research on the SN dynamics suggests that a significant part of these νe can convert into right-handed neutrinos by virtue of the interaction of the electrons and the protons flowing with the SN outgoing plasma, whenever the Dirac neutrino magnetic moment is of strength μν < 10−11μB, with μB being the Bohr magneton. In the SN envelope, some 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 (see the work by Kuznetsov & Mikheev; Kuznetsov, Mikheev, & Okrugin; Akhmedov & Khlopov; Itoh & Tsuneto; and Itoh et al.), a region where the field strength is currently accepted to be B 1013 G. This type of ν oscillation was shown to generate powerful gravitational wave (GW) bursts (see the work by Mosquera Cuesta; Mosquera Cuesta & Fiuza; and Loveridge). If such a double spin-flip mechanism does run into action inside the SN core, then the release of both the oscillation-produced νμ and ντ particles and the GW pulse generated by the coherent ν spin-flips provides a unique emission offset Δ TemiGW ↔ ν = 0 for measuring the ν travel time to Earth. As massive ν particles get noticeably delayed on their 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 ν mass spectrum.

Einstein's gravitational lensing and nonlinear electrodynamics

Einstein (1936) 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 Einsteins 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 Einsteins prediction. This may prove very crucial in determining physical properties of high B-field stars from their X-ray emission.

Nonlinear electrodynamics and CMB polarization

Recently WMAP and BOOMERanG experiments have set stringent constraints on the polarization angle of photons propagating in an expanding universe: