800 research outputs found
Pulse profiles from thermally emitting neutron stars
The problem of computing the pulse profiles from thermally emitting spots on
the surface of a neutron star in general relativity is reconsidered. We show
that it is possible to extend Beloborodov (2002) approach to include (multiple)
spots of finite size in different positions on the star surface. Results for
the pulse profiles are expressed by comparatively simple analytical formulas
which involve only elementary functions.Comment: 8 pages, 6 figures, accepted for publication in Ap
An efficient algorithm for two-dimensional radiative transfer in axisymmetric circumstellar envelopes and disks
We present an algorithm for two-dimensional radiative transfer in
axisymmetric, circumstellar media. The formal integration of the transfer
equation is performed by a generalization of the short characteristics (SC)
method to spherical coordinates. Accelerated Lambda Iteration (ALI) and Ng's
algorithm are used to converge towards a solution. By taking a logarithmically
spaced radial coordinate grid, the method has the natural capability of
treating problems that span several decades in radius, in the most extreme case
from the stellar radius up to parsec scale. Flux conservation is guaranteed in
spherical coordinates by a particular choice of discrete photon directions and
a special treatment of nearly-radially outward propagating radiation. The
algorithm works well from zero up to very high optical depth, and can be used
for a wide variety of transfer problems, including non-LTE line formation, dust
continuum transfer and high temperature processes such as compton scattering.
In this paper we focus on multiple scattering off dust grains and on non-LTE
transfer in molecular and atomic lines. Line transfer is treated according to
an ALI scheme for multi-level atoms/molecules, and includes both random and
systematic velocity fields. The algorithms are implemented in a multi-purpose
user-friendly radiative transfer program named RADICAL. We present two example
computations: one of dust scattering in the Egg Nebula, and one of non-LTE line
formation in rotational transitions of HCO in a flattened protostellar
collapsing cloud.Comment: 18 pages, 32 figure
Probing the surface magnetic field structure in RX J1856.5-3754
The evolution of magnetic field in isolated neutron stars is one of the most
important ingredients in the attempt to build a unified description of these
objects. A prediction of field evolution models is the existence of an
equilibrium configuration, in which the Hall cascade vanishes. Recent
calculations have explored the field structure in this stage, called the Hall
attractor. We use X-ray data of near-by, cooling neutron stars to probe this
prediction, as these sources are surmised to be close to or at Hall attractor
phase. We show that the source RX J1856.5-3754 might be closer to the attractor
than other sources of its class. Our modelling indicates that the properties of
surface thermal emission, assuming that the star is in the Hall attractor, are
in contradiction with the spectral data of RX J1856.5-3754.Comment: 9 pages, accepted to MNRA
On the Mathematical Character of the Relativistic Transfer Moment Equations
General--relativistic, frequency--dependent radiative transfer in spherical,
differentially--moving media is considered. In particular we investigate the
character of the differential operator defined by the first two moment
equations in the stationary case. We prove that the moment equations form a
hyperbolic system when the logarithmic velocity gradient is positive, provided
that a reasonable condition on the Eddington factors is met. The operator,
however, may become elliptic in accretion flows and, in general, when gravity
is taken into account. Finally we show that, in an optically thick medium, one
of the characteristics becomes infinite when the flow velocity equals . Both high--speed, stationary inflows and outflows may therefore
contain regions which are ``causally'' disconnected.Comment: 16 pages, PlainTex, accepted for publication in MNRA
Bare quark stars or naked neutron stars? The case of RX J1856.5-3754
In a cool neutron star (T less than or similar to 10(6) K) endowed with a rather highmagnetic field (B greater than or similar to 10(13) G), a phase transition may occur in the outermost layers. As a consequence, the neutron star becomes "bare,'' i.e., no gaseous atmosphere sits on the top of the crust. The surface of a cooling, bare neutron star does not necessarily emit a blackbody spectrum because the emissivity is strongly suppressed at energies below the electron plasma frequency, omega(p). Since omega(p) approximate to 1 keV under the conditions typical of the dense electron gas in the condensate, the emission from a T similar to 100 eV bare neutron star will be substantially depressed with respect to that of a perfect Planckian radiator atmost energies. Here we present a detailed analysis of the emission properties of a bare neutron star. In particular, we derive the surface emissivity for an Fe composition in a range of magnetic fields and temperatures representative of cooling isolated neutron stars, like RX J1856.5 - 3754. We find that the emitted spectrum is strongly dependent on the electron conductivity in the solid surface layers. In the cold electron gas approximation ( no electron-lattice interactions), the spectrum turns out to be a featureless depressed blackbody in the 0.1 - 2 keV band with a steeper low-energy distribution. When damping effects due to collisions between electrons and the ion lattice ( mainly due to electron-phonon interactions) are accounted for, the spectrum is more depressed at low energies and spectral features may be present, depending on the magnetic field strength. Details of the emitted spectrum are found, however, to be strongly dependent on the assumed treatment of the transition from the external vacuum to the metallic surface. The implications of our results for RX J1856.5 - 3754 and other isolated neutron stars are discussed
A tale of two populations: Rotating Radio Transients and X-ray Dim Isolated Neutron Stars
We highlight similarities between recently discovered Rotating Radio
Transients and X-ray Dim Isolated Neutron Stars. In particular, it is shown
that X-ray Dim Isolated Neutron Stars have a birthrate comparable to that of
Rotating Radio Transients. On the contrary, magnetars have too low a formation
rate to account for the bulk of the radio transient population. The
consequences of the recent detection of a thermal X-ray source associated with
one of the Rotating Radio Transients on the proposed scenarios for these
sources are also discussed.Comment: 5 pages, accepted to MNRAS Letter
VLT Observations of the Ultraluminous X-ray Source NGC 1313 X-2
We present archive ESO VLT photometric and spectroscopic data of the
Ultraluminous X-ray source NGC 1313 X-2. The superb quality of the VLT images
reveals that two distinct objects, with R magnitudes 23.7 and 23.6, are visible
inside the Chandra error box. The two objects, separated by 0.75 arcsec, were
unresolved in our previous ESO 3.6 m+EFOSC image. We show that both are stars
in NGC 1313, the first a B0-O9 main sequence star of ~20 Msun, while the second
a G supergiant of ~10 Msun. Irrespectively of which of the two objects the
actual counterpart is, this implies that NGC 1313 X-2 is a high mass X-ray
binary with a very massive donor.Comment: 7 pages, 4 figures, accepted for publication in ApJ Letter
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