Scalar Resonances in Axially Symmetric Spacetimes

We study properties of resonant solutions to the scalar wave equation in several axially symmetric spacetimes. We prove that non-axial resonant modes do not exist neither in the Lanczos dust cylinder, the $(2+1)$ extreme BTZ spacetime nor in a class of simple rotating wormhole solutions. Moreover, we find unstable solutions to the wave equation in the Lanczos dust cylinder and in the $r^2 <0$ region of the extreme $(2+1)$ BTZ spacetime, two solutions that possess closed timelike curves. Similarities with previous results obtained for the Kerr spacetime are explored.Comment: International Journal of Modern Physics D, in pres

An internal mechanism for the anti-glitch observed in AXP 1E 2259+586

Magnetars are fascinating objects that are thought to be neutron stars powered by their strong internal magnetic fields. Clear evidence of a sudden spin-down was detected in the Anomalous X-ray Pulsar AXP 1E 2259+586, an object cataloged as a magnetar. This event received the name "anti-glitch". In this work we present a simple internal mechanism which could account for the observed sudden spin-down of the star.Comment: Presented in the "Compact Stars in the QCD Phase Diagram IV" conference. September 26-30, 2014, Prerow, German

Magnetised Accretion Discs in Kerr Spacetimes II: Hot Spots

Context. Quasi-periodic variability has been observed in a number of X-ray binaries harboring black hole candidates. In general relativity, black holes are uniquely described by the Kerr metric and, according to the cosmic censorship conjecture, curvature singularities always have to be clothed by an event horizon. Aims. In this paper, we study the effect of an external magnetic field on the observed light curves of orbiting hot spots in thin accretion discs around Kerr black holes and naked singularities. Methods. We employ a ray-tracing algorithm to calculate the light curves and power spectra of such hot spots as seen by a distant observer for uniform and dipolar magnetic field configurations assuming a weak coupling between the magnetic field and the disc matter. Results. We show that the presence of an external dipolar magnetic field leads to potentially observable modifications of these signals for both Kerr black holes and naked singularities, while an external uniform magnetic field has practically no effect. In particular, we demonstrate that the emission from a hot spot orbiting near the innermost stable circular orbit of a naked singularity in a dipolar magnetic field can be significantly harder than the emission of the same hot spot in the absence of such a magnetic field. Conclusions. The comparison of our model with observational data may allow us study the geometry of magnetic fields around compact objects and to test the cosmic censorship conjecture in conjunction with other observables such as thermal continuum spectra and iron line profiles.Comment: 7 pages, 3 figures, submitted to Astronomy and Astrophysic

A simple mechanism for the anti-glitch observed in AXP 1E 2259+586

In this letter, we develop a simple internal mechanism that can account for the recent ?anti-glitch? observed for AXP 1E 2259+586 (|Δν/ν| ≳ 10− 7). We propose that the cumulative decay of the internal toroidal magnetic field component will eventually become large enough to turn an originally stable prolate stellar configuration into an unstable one. The subsequent rearrangement of the stellar structure will result in a sudden spin-down of the whole star. We present order-of-magnitude calculations to give confidence to this scenario, using a simple, but physically reasonable, analytical stellar model. We estimate the energy released by the proposed mechanism and show that it is in agreement with the observations. Based on this model, we predict that to achieve the observed sudden spin-down, a minimum magnetar-like value for the magnetic field strength is needed. Consequently, we do not expect this kind of anti-glitch activity to occur in normal pulsars.Fil: García, Federico. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto Argentino de Radioastronomia (i); Argentina. Universidad Nacional de la Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaFil: Ranea Sandoval, Ignacio Francisco. Universidad Nacional de la Plata. Facultad de Cs.astronomicas y Geofisicas. Area Fisica Teorica; Argentin

Scalar resonances in axially symmetric spacetimes

We study properties of resonant solutions to the scalar wave equation in several axially symmetric spacetimes. We prove that nonaxial resonant modes do not exist neither in the Lanczos dust cylinder, the extreme (2 + 1) dimensional Bañados-Taitelboim-Zanelli (BTZ) spacetime nor in a class of simple rotating wormhole solutions. Moreover, we find unstable solutions to the wave equation in the Lanczos dust cylinder and in the r2 < 0 region of the extreme (2 + 1) dimensional BTZ spacetime, two solutions that possess closed timelike curves. Similarities with previous results obtained for the Kerr spacetime are explored.Facultad de Ciencias Astronómicas y GeofísicasConsejo Nacional de Investigaciones Científicas y Técnica

A simple mechanism for the anti-glitch observed in AXP 1E 2259+586

In this letter, we develop a simple internal mechanism that can account for the recent 'antiglitch' observed for AXP 1E 2259+586 (Δν/ν ≳ 10-7). We propose that the cumulative decay of the internal toroidal magnetic field component will eventually become large enough to turn an originally stable prolate stellar configuration into an unstable one. The subsequent rearrangement of the stellar structure will result in a sudden spin-down of the whole star. We present order-of-magnitude calculations to give confidence to this scenario, using a simple, but physically reasonable, analytical stellar model. We estimate the energy released by the proposed mechanism and show that it is in agreement with the observations. Based on this model, we predict that to achieve the observed sudden spin-down, a minimum magnetar-like value for the magnetic field strength is needed. Consequently, we do not expect this kind of anti-glitch activity to occur in normal pulsars.Facultad de Ciencias Astronómicas y Geofísica

Magnetised accretion discs in Kerr spacetimes

Context. Observational data from X-ray binary systems provide strong evidence of astronomical objects that are too massive and compact to be explained as neutron or hybrid stars. When these systems are in the thermal (high/soft) state, they emit mainly in the 0. 1-5 keV energy range. This emission can be explained by thin accretion discs that formed around compact objects like black holes. The profile of the fluorescent iron line is useful to obtain insight into the nature of the compact object. General relativity does not ensure that a black hole must form after the complete gravitational collapse of very massive stars, and other theoretical models such as naked singularities cannot be discarded. The cosmic censorship conjecture was proposed by Penrose to avoid these possibilities and is yet to be proven. Aims. We study the effect caused by external magnetic fields on the observed thermal spectra and iron line profiles of thin accretion discs formed around Kerr black holes and naked singularities. We aim to provide a tool that can be used to estimate the presence of magnetic fields in the neighbourhood of a compact object and to probe the cosmic censorship conjecture in these particular astrophysical environments. Methods. We developed a numerical scheme able to calculate thermal spectra of magnetised Page-Thorne accretion discs formed around rotating black holes and naked singularities as seen by an arbitrary distant observer. We incorporated two diff erent magnetic field configurations: uniform and dipolar, using a perturbative scheme in the coupling constant between matter and magnetic field strength. Under the same assumptions, we obtained observed synthetic line profiles of the 6.4 keV fluorescent iron line. Results. We show that an external magnetic field produces potentially observable modifications on the thermal energy spectrum and the fluorescent iron line profile. Thermal energy spectra of naked singularities are harder and brighter than those from black holes, and in addition, peak and cut-off energies are affected by the external magnetic field. Moreover, iron line profiles of slowly rotating black holes suffer more changes by a uniform magnetic field, while nearly extremal black holes and naked singularities are more altered in the dipolar case. Based on our calculations, we discard the possibility of modelling the archetypal black-hole candidate in Cygnus X-1 as a naked singularity. Conclusions. Comparison of our models with observational data can be used to probe the cosmic censorship conjecture and to estimate the existence and global geometry of magnetic fields around compact objects by fitting the thermal energy spectra and iron line profiles of X-ray binaries.Facultad de Ciencias Astronómicas y Geofísica