30 research outputs found
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 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 region of the extreme 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
Slow stable hybrid stars: a new class of compact stars that fulfills all current observational constraints
We study hybrid stars considering the effects on stellar stability of the
hadron-quark conversion speed at the sharp interface. The equation of state is
constructed by combining a model-agnostic hadronic description with a constant
speed of sound model for quark matter. We show that current LIGO/Virgo, NICER,
low-density nuclear and high-density perturbative QCD constraints can be
satisfied in two scenarios with low and high transition pressures. If the
conversion speed is slow, a new class of hybrid objects is possible and very
stiff hadronic equations of state cannot be discarded.Comment: 7 pages, 4 figure
Three approaches for the classification of protoneutron star oscillation modes
The future detection of gravitational waves (GWs) from a galactic
core-collapse supernova will provide information on the physics inside
protoneutron stars (PNS). In this work, we apply three different classification
methods for the PNS non-radial oscillation modes: Cowling classification,
Generalized Cowling Nomenclature (GCN), and a Classification Based on Modal
Properties (CBMP). Using PNS models from D simulations of core-collapse
supernovae, we find that in the early stages of the PNS evolution, typically
before seconds after the bounce, the Cowling classification is
inconsistent, but the GCN and the CBMP provide complementary information that
helps to understand the evolution of the modes. In the GCN, we note several
avoided crossings as the mode frequencies evolve at early times, while the CBMP
tracks the modes across the avoided crossings. We verify that the strongest
emission of GWs by the PNS corresponds to the -mode in the GCN, indicating
that the mode trapping region alternates between the core and the envelope at
each avoided crossing. At later times, approximately seconds after the
bounce, the three classification methods present a similar description of the
mode spectrum. We use our results to test universal relations for the PNS modes
according to their classification and find that the behaviour of the universal
relations for - and -modes is remarkably simple in the CBMP.Comment: 11 pages, 8 figures. Matches the version accepted on MNRA
Properties of Magnetized Quark-Hybrid Stars
The structure of a magnetized quark-hybrid stars (QHS) is modeled using a
standard relativistic mean-field equation of state (EoS) for the description of
hadronic matter. For quark matter we consider a bag model EoS which is modified
perturbatively to account for the presence of a uniform magnetic field. The
mass-radius (M-R) relationship, gravitational redshift and rotational Kepler
periods of such stars are compared with those of standard neutron stars (NS).Comment: 5 pages, 2 figures, prepared for the 2nd International Symposium on
Strong Electromagnetic Fields and Neutron Stars (SMFNS2011), Varadero, Cuba,
5-7 May 201
Instabilities of naked singularities and black hole interiors in general relativity
Metrics representing black holes in General Relativity may exhibit naked singularities for certain values of their parameters. This is the case for super-extremal (J2 \u3e M \u3e 0) Kerr and super-extremal (|Q| \u3e M \u3e 0) Reissner-Nördstrom spacetimes, and also for the negative mass Schwarzschild spacetime. We review our recent work where we show that these nakedly singular spacetimes are unstable under linear gravitational perturbations, a result that supports the cosmic censorship conjecture, and also that the inner stationary region beyond the inner horizon of a Kerr black hole (J2 \u3c M) is linearly unstable. © 2009 World Scientific Publishing Company