Gustavo E. Romero

Neutrinos from accreting neutron stars

The magnetospheres of accreting neutron stars develop electrostatic gaps with huge potential drops. Protons and ions, accelerated in these gaps along the dipolar magnetic field lines to energies greater than 100 TeV, can impact onto the surrounding accretion disk. A proton-induced cascade develops, and charged pion decays produce ν emission. With extensive disk shower simulations using DPMJET and GEANT4, we have calculated the resulting ν spectrum. We show that the spectrum produced out of the proton beam is a power law. We use this result to propose accretion-powered X-ray binaries (with highly magnetized neutron stars) as a new population of pointlike ν sources for kilometer-scale detectors such as ICECUBE. As a particular example, we discuss the case of A0535+26. We show that ICECUBE should find A0535+26 to be a periodic ν source, one for which the formation and loss of its accretion disk can be fully detected. Finally, we comment briefly on the possibility that smaller telescopes such as AMANDA could also detect A0535+26 by folding observations with the orbital period.

THE MYSTERIOUS ULTRAHIGH ENERGY COSMIC RAY CLUSTERING

We examine the correlation between compact radio quasars (redshifts in the range z=0.3–2.2) and the arrival direction of ultrahigh energy cosmic rays forming clusters. Our Monte Carlo simulation reveals a statistically significant correlation on the AGASA sample: the chance probability of this effect being less than 1%. The implications of this result on the origin of ultrahigh energy cosmic rays are discussed.

AN ANALYSIS OF A REGULAR BLACK HOLE INTERIOR

In this work we address the issue of the thermodynamic behavior of a non-singular spherically-symmetric black hole model introduced by Mbonye and Kanzanas (2005). This spacetime is described by Schwarschilds solution at large radius and by a de Sitter-like solution at small radius. The interior of the black hole consists of matter fields with sound speed bounded by the speed of light. The matter transits smoothly between normal matter and a core of an exotic fluid with an equation of state that approaches p = -ρ when r → 0. We derive the general equations of the thermodynamic quantities for an arbitrary matter density profile, and adjust the results to the specific regular black hole. We also calculate the Weyl and Kretschmann scalars and analyse the behaviour of the gravitational field in connection with the thermodynamics of the matter fields. Finally, we study whether the regular behaviour at the center affects the thermodynamics of the black hole, and we discuss a possible physical interpretation of the state of regular black hole interiors.

ACCRETION DISKS AROUND KERR BLACK HOLES IN MODIFIED GRAVITY

Fil: Perez, Daniela. Provincia de Buenos Aires. Gobernacion. Comision de Investigaciones Cientificas. Instituto Argentino de Radioastronomia. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - La Plata. Instituto Argentino de Radioastronomia; Argentina

Wormholes and Exotic Objects

Wormholes in space-time are presented and their possible astrophysical manifestations are discussed. Problems arising in the possible formation of closed time-like curves, common to both wormhole space-times and rotating black hole interiors, are addressed. Other exotic objects are briefly discussed.

Accretion onto Black Holes

This chapter presents a detailed account of all forms of accretion onto black holes. The basic expressions for spherical, cylindrical, and disk accretion are developed. Advection-dominated accretion is also discussed, as well as different accretion regimes in binary systems.

Space-Time and Gravitation

This chapter introduces the concept of a space-time and outlines the General Theory of Relativity and some of its extensions.

Evidence for Black Holes

The main astrophysical manifestations of black holes are discussed. This includes an account of X-ray binaries and microquasars, active galactic nuclei, gamma-ray bursts, and ultra-luminous X-ray sources. Evidence for the existence of black holes in these sources is reviewed. Recent observational findings on stellar-mass, intermediate-mass, and supermassive black holes are presented and open observational challenges are described.

Neutron production in black hole coronae and proton loading of jets

We study the production of neutrons in the corona of an accreting black hole through the interaction of locally accelerated protons with matter and radiation. A fraction of these neutrons may escape and penetrate into the base of the jet, later decaying into protons. This is a possible mechanism for loading Poynting-dominated outflows with baryons. We characterize the spatial and energy distribution of neutrons in the corona and that of the protons injected in the jet by neutron decay. We assess the contribution of these protons to the radiative spectrum of the jet. We also investigate the fate of the neutrons that escape the corona into the external medium.

SUPERMASSIVE BLACK HOLE BINARIES AT HIGH ENERGIES

An accretion disk around a supermassive black hole may be strongly perturbed by the presence of a secondary black hole. Recent simulations have shown that, under certain conditions, the tidal torques exerted by the secondary black hole may open an annular gap in the disk. In this regime, matter overflows across the secondarys orbit to accrete onto the primary and may feed a pair of relativistic jets. In this work we study the radiative properties of a binary system of supermassive black holes, assuming that a relativistic jet is launched from the primary and the migration of the secondary across the disk proceeds in the overflowing regime. The modified radiative spectrum of the disk is calculated accounting for strong gravitational effects in the innermost region. The jet emits non-thermal radiation all along the electromagnetic spectrum by interaction of locally accelerated electrons with the jets magnetic field and internal and external radiation. In particular, we investigate whether the interaction of the relativistic electrons with the photons emitted by the accretion disk induces any signature in the spectral energy distribution of the jet that may reveal the presence of the secondary black hole.