720 research outputs found
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
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
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
Unifying neutron stars: getting to GUNS
The variety of the observational appearance of young isolated neutron stars
must find an explanation in the framework of some unifying approach. Nowadays
it is believed that such scenario must include magnetic field decay, the
possibility of magnetic field emergence on a time scale --
yrs, significant contribution of non-dipolar fields, and appropriate initial
parameter distributions. We present our results on the initial spin period
distribution, and suggest that inconsistences between distributions derived by
different methods for samples with different average ages can uncover field
decay or/and emerging field. We describe a new method to probe the magnetic
field decay in normal pulsars. The method is a modified pulsar current
approach, where we study pulsar flow along the line of increasing
characteristic age for constant field. Our calculations, performed with this
method, can be fitted with an exponential decay for ages in the range -- yrs with a time scale yrs. We
discuss several issues related to the unifying scenario. At first, we note that
the dichotomy, among local thermally emitting neutron stars, between normal
pulsars and the Magnificent Seven remains unexplained. Then we discuss the role
of high-mass X-ray binaries in the unification of neutron star evolution. We
note, that such systems allow to check evolutionary effects on a time scale
longer than what can be probed with normal pulsars alone. We conclude with a
brief discussion of importance of discovering old neutron stars accreting from
the interstellar medium.Comment: 6 pages, submitted to AN, proceedings of the workshop "The Fast and
the Furious: Energetic Phenomena in Isolated Neutron Stars, Pulsar Wind
Nebulae and Supernova Remnants" ESAC, Madrid, Spain 22nd - 24th May 201
Dynamical Comptonization in spherical flows: black hole accretion and stellar winds
The transport of photons in steady, spherical, scattering flows is
investigated. The moment equations are solved analytically for accretion onto a
Schwarzschild black hole, taking into full account relativistic effects. We
show that the emergent radiation spectrum is a power law at high frequencies
with a spectral index smaller (harder spectrum) than in the non--relativistic
case. Radiative transfer in an expanding envelope is also analyzed. We find
that adiabatic expansion produces a drift of injected monochromatic photons
towards lower frequencies and the formation of a power--law, low--energy tail
with spectral index .Comment: 11 pages with 3 ps figures, MNRAS to appea
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