48 research outputs found
SGR 0418+5729, Swift J1822.3-1606, and 1E 2259+586 as massive fast rotating highly magnetized white dwarfs
Following Malheiro et al. (2012) we describe the so-called low magnetic field
magnetars, SGR 0418+5729, Swift J1822.3--1606, as well as the AXP prototype 1E
2259+586 as massive fast rotating highly magnetized white dwarfs. We give
bounds for the mass, radius, moment of inertia, and magnetic field for these
sources by requesting the stability of realistic general relativistic uniformly
rotating configurations. Based on these parameters, we improve the theoretical
prediction of the lower limit of the spindown rate of SGR 0418+5729; for a
white dwarf close to its maximum stable we obtain the very stringent interval
for the spindown rate of 4.1E-16< dP/dt < 6E-15, where the upper value is the
known observational limit. A lower limit has been also set for Swift
J1822.3-1606 for which a fully observationally accepted spin-down rate is still
lacking. The white dwarf model provides for this source dP/dt> 2.13E-15, if the
star is close to its maximum stable mass. We also present the theoretical
expectation of the infrared, optical and ultraviolet emission of these objects
and show their consistency with the current available observational data. We
give in addition the frequencies at which absorption features could be present
in the spectrum of these sources as the result of the scattering of photons
with the quantized electrons by the surface magnetic field.Comment: to appear in Astronomy & Astrophysic
Fundamental Frequencies in the Schwarzschild Spacetime
We consider the Keplerian, radial and vertical fundamental frequencies in the
Schwarzschild spacetime to study the so-called kilohertz quasi-periodic
oscillations from low-mass X-ray binary systems. We show that, within the
Relativistic Precession Model, the interpretation of observed kilohertz
quasi-periodic oscillations in terms of the fundamental frequencies of test
particles in the Schwarzschild spacetime, allows one to infer the total mass
of the central object, the internal and external radii of
accretion disks, and innermost stable circular orbits for test
particles in a low-mass X-ray binary system. By constructing the relation
between the upper and lower frequencies and exploiting the quasi-periodic
oscillation data of the Z and Atoll sources we perform the non-linear model fit
analysis and estimate the mass of the central object. Knowing the value of the
mass we calculate the internal and external radii of
accretion disks and show that they are larger than , what was
expected.Comment: 7 pages, 6 figures, 1 tabl
Quasinormal modes in the field of a dyon-like dilatonic black hole
Quasinormal modes of massless test scalar field in the background of
gravitational field for a non-extremal dilatonic dyonic black hole are
explored. The dyon-like black hole solution is considered in the gravitational
model involving two scalar fields and two 2-forms. It is governed by two
2-dimensional dilatonic coupling vectors obeying
, . The first
law of black hole thermodynamics is given and the Smarr relation is verified.
Quasinormal modes for a massless scalar (test) field in the eikonal
approximation are obtained and analysed. These modes depend upon a
dimensionless parameter () which is a function of
. For limiting strong () and weak () coupling
cases, they coincide with the well-known results for the Schwarzschild and
Reissner-Nordstr\"om solutions. It is shown that the Hod conjecture, connecting
the damping rate and the Hawking temperature, is satisfied for
and all allowed values of parameters.Comment: 13 pages, 5 (double) figures, LaTex. Subsection 3.3 is extended; two
typos in formulas (22) and (30) are eliminated; 2 figures and 3 references
are added; a paragraph in Sec. 1 and Remark on page 10 are added; several
phrases are modified. To be published in EPJ
The Erez-Rosen solution versus the Hartle-Thorne solution
In this work, we investigate the correspondence between the Erez-Rosen and
Hartle-Thorne solutions. We explicitly show how to establish the relationship
and find the coordinate transformations between the two metrics. For this
purpose the two metrics must have the same approximation and describe the
gravitational field of static objects. Since both the Erez-Rosen and the
Hartle-Thorne solutions are particular solutions of a more general solution,
the Zipoy-Voorhees transformation is applied to the exact Erez-Rosen metric in
order to obtain a generalized solution in terms of the Zipoy-Voorhees parameter
. The Geroch-Hansen multipole moments of the generalized
Erez-Rosen metric are calculated to find the definition of the total mass and
quadrupole moment in terms of the mass , quadrupole and Zipoy-Voorhees
parameters. The coordinate transformations between the metrics are
found in the approximation of q. It is shown that the Zipoy-Voorhees
parameter is equal to with . This result is in agreement
with previous results in the literature.Comment: Accepted for publication. 8 page
A white dwarf merger as progenitor of the anomalous X-ray pulsar 4U 0142+61?
It has been recently proposed that massive fast-rotating highly-magnetized
white dwarfs could describe the observational properties of some of Soft
Gamma-Ray Repeaters (SGRs) and Anomalous X-Ray Pulsars (AXPs). Moreover, it has
also been shown that high-field magnetic (HFMWDs) can be the outcome of white
dwarf binary mergers. The products of these mergers consist of a hot central
white dwarf surrounded by a rapidly rotating disk. Here we show that the merger
of a double degenerate system can explain the characteristics of the peculiar
AXP 4U 0142+61. This scenario accounts for the observed infrared excess. We
also show that the observed properties of 4U 0142+6 are consistent with an
approximately 1.2 M_{\sun} white dwarf, remnant of the coalescence of an
original system made of two white dwarfs of masses 0.6\, M_{\sun} and 1.0\,
M_{\sun}. Finally, we infer a post-merging age kyr,
and a magnetic field G. Evidence for such a magnetic
field may come from the possible detection of the electron cyclotron absorption
feature observed between the and bands at Hz in the
spectrum of 4U 0142+61.Comment: to appear in ApJ Letter
Accretion Disk Luminosity for Black Holes Surrounded by Dark Matter with Anisotropic Pressure
We investigate the luminosity of the accretion disk of a static black hole surrounded by dark matter with anisotropic pressure. We calculate all basic orbital parameters of test particles in the accretion disk, such as angular velocity, angular momentum, energy, and radius of the innermost circular stable orbit as functions of the dark matter density, radial pressure, and anisotropic parameter, which establishes the relationship between the radial and tangential pressures. We show that the presence of dark matter with anisotropic pressure makes a noticeable difference in the geometry around a Schwarzschild black hole, affecting the radiative flux, differential luminosity, and spectral luminosity of the accretion disk
Tracing dark energy history with gamma ray bursts
Observations of gamma-ray bursts up to are best suited to study the
possible evolution of the Universe equation of state at intermediate redshifts.
We apply the Combo-relation to a sample of 174 gamma ray bursts to investigate
possible evidence of evolving dark energy parameter . We first build a
gamma ray burst Hubble's diagram and then we estimate the set (,
) in the framework of flat and non-flat CDM
paradigm. We then get bounds over the CDM model, where is thought to
evolve with redshift, adopting two priors over the Hubble constant in tension
at -, i.e. km/s/Mpc and
km/s/Mpc. We show our new sample provides tighter constraints on
since at we see that agrees within 1 with the
standard value . The situation is the opposite at larger , where gamma
ray bursts better fix that seems to deviate from at -
and - level, depending on the redshift bins. In particular, we
investigate the evolution through a piecewise formulation over seven
redshift intervals. From our fitting procedure we show that at the
case cannot be fully excluded, indicating that dark energy's influence
is not negligible at larger . We confirm the Combo relation as a powerful
tool to investigate cosmological evolution of dark energy. Future space
missions will significantly enrich the gamma ray burst database even at smaller
redshifts, improving de facto the results discussed in this paper.Comment: 10 pages, 7 figures, ApJ submitte