2,796 research outputs found
Black hole solutions of gravity theories with nonminimal coupling between matter and curvature
We study black hole solutions in an extension of General Relativity (GR) with
an explicit non-minimal coupling between matter and curvature. General black
hole solutions satisfying the known energy conditions are derived including the
ones with anti-de Sitter background. These solutions differ from those of GR
just by a coupling function dependent rescaling of the mass and charge of the
black hole and by a "dressing" of the cosmological constant. The existence of
black hole solutions of the nonminimally coupled theory as well as the
conditions for a suitable weak field limit are considered as a constraint on
the coupling function responsible for the nonminimal coupling between matter
and curvature. The "dressing" of the cosmological constant is then used to
address the cosmological constant problem.Comment: 24 pages, no figure
Relation between the properties of the kilohertz quasi-periodic oscillations and spectral parameters in 4U 163653
We investigate the relation between the parameters of the energy spectrum and
the frequency and amplitude of the kilohertz quasi-periodic oscillations (kHz
QPOs) in the low-mass X-ray binary 4U 163653. We fit the -keV
spectrum of this source with a model that includes a thermal Comptonisation
component. We show that the frequencies of both kHz QPOs follow the same
relation as a function of the parameters of this spectral component, except for
a systematic frequency shift, whereas the rms fractional amplitude of each QPO
follows a different relation with respect to those same parameters. This
implies that, while the dynamical mechanism that sets the frequencies of the
QPO can be the same for both kHz QPOs, the radiative mechanisms that set the
amplitudes of the lower and the upper kHz QPO are likely different. We discuss
the implications of these results to the modelling of the kHz QPOs and the
possibility that the lower kHz QPO reflects a resonance between the
Comptonising medium and the photons from the accretion disc and/or the neutron
star surface.Comment: 7 pages, 4 figures, 2 tables. Accepted for publication in MNRA
Effective Fluid Description of the Dark Universe
We propose an effective anisotropic fluid description for a generic
infrared-modified theory of gravity. In our framework, the additional component
of the acceleration, commonly attributed to dark matter, is explained as a
radial pressure generated by the reaction of the dark energy fluid to the
presence of baryonic matter. Using quite general assumptions, and a microscopic
description of the fluid in terms of a Bose-Einstein condensate of gravitons,
we find the static, spherically symmetric solution for the metric in terms of
the Misner-Sharp mass function and the fluid pressure. At galactic scales, we
correctly reproduce the leading MOND-like and subleading
terms in the weak-field expansion of the potential. Our
description also predicts a tiny (of order for a typical spiral
galaxy) Machian modification of the Newtonian potential at galactic scales,
which is controlled by the cosmological acceleration.Comment: 13 pages, no figures. Replaced version: major revisions in the
introduction, microscopic derivation of Tully-Fisher relation using
Bose-Einstein condensate of gravitons. Some typos correcte
The reflection spectrum of the low-mass X-ray binary 4U 1636-53
We present 3-79 keV NuSTAR observations of the neutron star low-mass X-ray
binary 4U 1636-53 in the soft, transitional and hard state. The spectra display
a broad emission line at 5-10 keV. We applied several models to fit this line:
A GAUSSIAN line, a relativistically broadened emission line model, KYRLINE, and
two models including relativistically smeared and ionized reflection off the
accretion disc with different coronal heights, RELXILL and RELXILLLP. All
models fit the spectra well, however, the KYRLINE and RELXILL models yield an
inclination of the accretion disc of with respect to the line
of sight, which is at odds with the fact that this source shows no dips or
eclipses. The RELXILLLP model, on the other hand, gives a reasonable
inclination of . We discuss our results for these models in this
source and the possible primary source of the hard X-rays.Comment: 9 pages, 8 figure
Iron-line and continuum variations in the XMM-Newton and Suzaku spectra of the neutron-star low-mass X-ray binary 4U 1636-53
We used six simultaneous XMM-Newton and Rossi X-ray Timing Explorer plus five
Suzaku observations to study the continuum spectrum and the iron emission line
in the neutron-star low-mass X-ray binary 4U 1636-53. We modelled the spectra
with two thermal components (representing the accretion disc and boundary
layer), a Comptonised component (representing a hot corona), and either a
Gaussian or a relativistic line component to model an iron emission line at
about 6.5 keV. For the relativistic line component we used either the diskline,
laor or kyrline model, the latter for three different values of the spin
parameter. The fitting results for the continuum are consistent with the
standard truncated disc scenario. We also find that the flux and equivalent
width of the iron line first increase and then decrease as the flux of the
Comptonised component increases. This could be explained either by changes in
the ionisation state of the accretion disc where the line is produced by
reflection, or by light bending of the emission from the Comptonised component
if the height at which this component is produced changes with mass accretion
rate.Comment: Accepted for publication in MNRAS, 16 pages, 10 figures, 5 table
Relation between spectral changes and the presence of the lower kHz QPO in the neutron-star low-mass X-ray binary 4U 1636-53
We fitted the -keV spectrum of all the observations of the
neutron-star low-mass X-ray binary 4U 163653 taken with the {\it Rossi X-ray
Timing Explorer} using a model that includes a thermal Comptonisation
component. We found that in the low-hard state the power-law index of this
component, , gradually increases as the source moves in the
colour-colour diagram. When the source undergoes a transition from the hard to
the soft state drops abruptly; once the source is in the soft state
increases again and then decreases gradually as the source spectrum
softens further. The changes in , together with changes of the electron
temperature, reflect changes of the optical depth in the corona. The lower
kilohertz quasi-periodic oscillation (kHz QPO) in this source appears only in
observations during the transition from the hard to the soft state, when the
optical depth of the corona is high and changes depends strongly upon the
position of the source in the colour-colour diagram. Our results are consistent
with a scenario in which the lower kHz QPO reflects a global mode in the system
that results from the resonance between, the disc and/or the neutron-star
surface, and the Comptonising corona.Comment: 9 pages, 6 figures. Accepted for publication in MNRA
Broad iron emission line and kilohertz quasi-periodic oscillations in the neutron star system 4U 1636-53
Both the broad iron (Fe) line and the frequency of the kilohertz
quasi-periodic oscillations (kHz QPOs) in neutron star low-mass X-ray binaries
(LMXBs) can potentially provide independent measures of the inner radius of the
accretion disc. We use XMM-Newton and simultaneous Rossi X-ray Timing Explorer
observations of the LMXB 4U 1636-53 to test this hypothesis. We study the
properties of the Fe-K emission line as a function of the spectral state of the
source and the frequency of the kHz QPOs. We find that the inner radius of the
accretion disc deduced from the frequency of the upper kHz QPO varies as a
function of the position of the source in the colour-colour diagram, in
accordance with previous work and with the standard scenario of accretion disc
geometry. On the contrary, the inner disc radius deduced from the profile of
the Fe line is not correlated with the spectral state of the source. The values
of the inner radius inferred from kHz QPOs and Fe lines, in four observations,
do not lead to a consistent value of the neutron star mass, regardless of the
model used to fit the Fe line. Our results suggest that either the kHz QPO or
the standard relativistic Fe line interpretation does not apply for this
system. Furthermore, the simultaneous detection of kHz QPOs and broad Fe lines
is difficult to reconcile with models in which the broadening of the Fe line is
due to the reprocessing of photons in an outflowing wind.Comment: 12 pages, 6 figures, Accepted for publication in MNRA
- …