104 research outputs found
Observational appearance of rapidly rotating neutron stars: X-ray bursts, cooling tail method, and radius determination
Neutron stars (NSs) in low-mass X-ray binaries rotate at frequencies high
enough to significantly deviate from sphericity ( 200--600 Hz). We
investigate the effects of rapid rotation on the observational appearance of a
NS. We propose analytical formulae relating gravitational mass and equatorial
radius of the rapidly rotating NS to the mass and radius of a
non-rotating NS of the same baryonic mass using accurate fully relativistic
computations. We compute spectra from an oblate rotating NS observed at
different inclination angles using the modified oblate Schwarzschild (MOS)
approximation, where light bending is computed in Schwarzschild metric, but
frame dragging and quadrupole moment of a NS are approximately accounted for in
the photon redshift calculations. We generalize the cooling tail method to the
case of a rapidly rotating NS to obtain the most probable values of and
of the corresponding non-rotating NS with the same baryonic mass. We
approximate the local spectra from the NS surface by a diluted blackbody using
previously computed NS atmosphere models. We show that the NS radius could be
overestimated by 3--3.5 km for face-on stars of km rotating at
700 Hz if the version of the cooling tail method for a non-rotating
NS is used. We apply the method to an X-ray burst observed from the NS rotating
at 532 Hz in SAX J1810.82609. The resulting radius of the
non-rotating NS (assuming ) becomes km if it is
viewed at inclination i=60 deg and km for a face-on view, which
are smaller by 0.6 and 1.2 km than the radius obtained using standard cooling
tail method ignoring rotation. The corresponding equatorial radii of these
rapidly rotating NSs are 12.3 km (for i=60 deg) and 11.6\,km
(for i=0 deg).Comment: 17 pages, 16 figures, accepted for publication in Astronomy and
Astrophysic
On the spreading layer emission in luminous accreting neutron stars
Emission of the neutron star surface potentially contains information about
its size and thus of vital importance for high energy astrophysics. In spite of
the wealth of data on the emission of luminous accreting neutron stars, the
emission of their surfaces is hard to disentangle from their time averaged
spectra. A recent X-ray transient source XTE J1701-462 has provided a unique
dataset covering the largest ever observed luminosity range for a single
source. In this paper, we extract the spectrum of the boundary layer between
the inner part of the accretion disc and the neutron star surface with the help
of maximally spectral model-independent method. We show compelling evidences
that the energy spectrum of the boundary layer stays virtually the same over
factor of 20 variations of the source luminosity. It is rather wide and cannot
be described by a single temperature blackbody spectrum, probably because of
the inhomogeneity of the boundary layer and a spread in the colour temperature.
The observed maximum colour temperature of the boundary/spreading layer
emission of kT~2.4-2.6 keV is very close to the maximum observed colour
temperature in the photospheric radius expansion X-ray bursts, which is set by
the limiting Eddington flux at the neutron star surface. Observed stability of
the boundary layer spectrum and its maximum colour temperature strongly
supports theoretical models of the boundary/spreading layers on surfaces of
luminous accreting neutron stars, which assume the presence of a region
emitting at the local Eddington limit. Variations in the luminosity in that
case lead to changes in the size of this region, but affect less the spectral
shape. Elaboration of this model will provide solid theoretical grounds for
measurements of the neutron star sizes using the emission of the
boundary/spreading layers of luminous accreting neutron stars.Comment: 7 pages, 7 figures, accepted for publication in MNRA
Optically thick envelopes around ULXs powered by accreating neutron stars
Magnetized neutron stars power at least some ultra-luminous X-ray sources.
The accretion flow in these cases is interrupted at the magnetospheric radius
and then reaches the surface of a neutron star following magnetic field lines.
Accreting matter moving along magnetic field lines forms the accretion envelope
around the central object. We show that, in case of high mass accretion rates
the envelope becomes closed and optically
thick, which influences the dynamics of the accretion flow and the
observational manifestation of the neutron star hidden behind the envelope.
Particularly, the optically thick accretion envelope results in a multi-color
black-body spectrum originating from the magnetospheric surface. The spectrum
and photon energy flux vary with the viewing angle, which gives rise to
pulsations characterized by high pulsed fraction and typically smooth pulse
profiles. The reprocessing of radiation due to interaction with the envelope
leads to the disappearance of cyclotron scattering features from the spectrum.
We speculate that the super-orbital variability of ultra-luminous X-ray sources
powered by accreting neutron stars can be attributed to precession of the
neutron star due to interaction of magnetic dipole with the accretion disc.Comment: 8 pages, 6 figures, accepted for publication in MNRA
Expected polarization properties of nonmagnetized CCOs
Central compact objects (CCOs) are neutron stars found close to the center of
some supernova remnants. A certain number of them are presumably covered by
carbon envelopes. Their unpulsed thermal X-ray emission can originate either
from the entire surface covered by a carbon atmosphere or alternatively from a
nonuniformly emitting hydrogen atmosphere. However, the latter scenario appears
unlikely given the available upper limits on the amplitude of pulsations. Here
we explore a possibility to further discriminate between the two scenarios
using X-ray polarimetric observations. We compute the polarization degree (PD)
for nonmagnetized pure-carbon and pure-hydrogen atmospheres with effective
temperatures of between 1 and 6 MK and find that it can reach up to 25% and 40%
for hydrogen and carbon atmospheres, respectively, in the photon energy band
1-10 keV. However, given the available constraints on possible inhomogeneities
of the temperature distribution deduced from models of the X-ray spectrum of
the CCO in HESS J1731-347, the integrated PD appears to be very low for both
carbon (<0.25%) and hydrogen (a few percent) compositions in the energy band of
2-8 keV covered by the recently launched Imaging X-ray Polarimetry Explorer. We
therefore conclude that polarization from CCOs is not expected to be detectable
by current facilities, but future detection would strongly support nonuniform
hydrogen composition models.Comment: 10 pages, 16 figures. accepted to be published by A&A, corrected
after the language Editor remarks, misprints in some numbers were corrected,
a new version of Fig.1
Magnetospheric return-current-heated atmospheres of rotation-powered millisecond pulsars
We computed accurate atmosphere models of rotation-powered millisecond
pulsars in which the polar caps of a neutron star (NS) are externally heated by
magnetospheric return currents. The external ram pressure, energy losses, and
stopping depth of the penetrating charged particles were computed
self-consistently with the atmosphere model, instead of assuming a simplified
deep-heated atmosphere in radiative equilibrium. We used exact Compton
scattering formalism to model the properties of the emergent X-ray radiation.
The deep-heating approximation was found to be valid only if most of the heat
originates from ultra-relativistic bombarding particles with Lorentz factors of
. In the opposite regime, the atmosphere attains a distinct
two-layer structure with an overheated optically thin skin on top of an
optically thick cool plasma. The overheated skin strongly modifies the emergent
radiation: it produces a Compton-upscattered high-energy tail in the spectrum
and alters the radiation beaming pattern from limb darkening to limb
brightening for emitted hard X-rays. This kind of drastic change in the
emission properties can have a significant impact on the inferred NS pulse
profile parameters as performed, for example, by Neutron star Interior
Composition ExploreR. Finally, the connection between the energy distribution
of the return current particles and the atmosphere emission properties offers a
new tool to probe the exact physics of pulsar magnetospheres.Comment: 13 pages, 10 figures, published in A&
Colors and patterns of black hole X-ray binary GX 339-4
Black hole X-ray binaries show signs of non-thermal emission in the
optical/near-infrared range. We analyze the optical/near-infrared SMARTS data
on GX3394 over the 2002--2011 period. Using the soft state data, we estimate
the interstellar extinction towards the source and characteristic color
temperatures of the accretion disk. We show that various spectral states of
regular outbursts occupy similar regions on the color-magnitude diagrams, and
that transitions between the states proceed along the same tracks despite
substantial differences in the observed light curves morphology. We determine
the typical duration of the hard-to-soft and soft-to-hard state transitions and
the hard state at the decaying stage of the outburst to be one, two and four
weeks, respectively. We find that the failed outbursts cannot be easily
distinguished from the regular ones at their early stages, but if the source
reaches 16 mag in -band, it will transit to the soft state. By subtracting
the contribution of the accretion disk, we obtain the spectra of the
non-thermal component, which have constant, nearly flat shape during the
transitions between the hard and soft states. In contrast to the slowly
evolving non-thermal component seen at optical and near-infrared wavelengths,
the mid-infrared spectrum is strongly variable on short timescales and
sometimes shows a prominent excess with a cutoff below Hz. We show
that the radio to optical spectrum can be modeled using three components
corresponding to the jet, hot flow and irradiated accretion disk.Comment: Accepted for publication in Astronomy & Astrophysics, 19 pages, 6
tables, 18 figure
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