General Relativistic Simulations of Accretion Induced Collapse of Neutron Stars to Black Holes. (arXiv:1209.0783v1 [astro-ph.HE])

General Relativistic Simulations of Accretion Induced Collapse of Neutron Stars to Black Holes. (arXiv:1209.0783v1 [astro-ph.HE]):
Neutron stars (NSs) in the astrophysical Universe are often surrounded by
accretion disks. Accretion of matter onto a NS may increase its mass above the
maximum value allowed by its equation of state, inducing its collapse to a
black hole (BH). Here we study this process for the first time, in 3D, and in
full general relativity. By considering three initial NS configurations, each
with and without a surrounding disk (of mass ~7% M_{NS}), we investigate the
effect of the accretion disk on the dynamics of the collapse and its imprint on
both the gravitational wave (GW) and electromagnetic (EM) signals that can be
emitted by these sources. We show in particular that, even if the GW signal is
similar for the accretion induced collapse (AIC) and the collapse of a NS in
vacuum (and detectable only for Galactic sources), the EM counterpart could
allow to discriminate between these two types of events. In fact, our
simulations show that, while the collapse of a NS in vacuum leaves no
appreciable baryonic matter outside the event horizon, an AIC is followed by a
phase of rapid accretion of the surviving disk onto the newly formed BH. The
post-collapse accretion rates, on the order of ~10^{-2} M_{sun} s^{-1}, make
these events tantalizing candidates as engines of short Gamma-Ray Bursts.

Radiative properties of magnetic neutron stars with metallic surfaces and thin atmospheres. (arXiv:1208.6582v1 [astro-ph.HE])

Radiative properties of magnetic neutron stars with metallic surfaces and thin atmospheres. (arXiv:1208.6582v1 [astro-ph.HE]):
The goal of this work is to develop a simple analytic description of the
emission properties (spectrum and polarization) of the condensed, strongly
magnetized surface of neutron stars. We have improved the method of van
Adelsberg et al. (2005) (arXiv:astro-ph/0406001) for calculating the spectral
properties of condensed magnetized surfaces. Using the improved method, we
calculate the reflectivity of an iron surface at magnetic field strengths B
\sim (10^{12} - 10^{14}) G, with various inclinations of the magnetic field
lines and radiation beam with respect to the surface and each other. We
construct analytic expressions for the emissivity of this surface as functions
of the photon energy, magnetic field strength, and the three angles that
determine the geometry of the local problem. Using these expressions, we
calculate X-ray spectra for neutron stars with condensed iron surfaces covered
by thin partially ionized hydrogen atmospheres. We develop simple analytic
descriptions of the intensity and polarization of radiation emitted or
reflected by condensed iron surfaces of neutron stars with strong magnetic
fields typical for isolated neutron stars. This description provides boundary
conditions at the bottom of a thin atmosphere, which are more accurate than
previously used approximations. The spectra calculated with this improvement
show absorption features different from those in simplified models. The
approach developed in this paper yields results that can facilitate modeling
and interpretation of the X-ray spectra of isolated, strongly magnetized,
thermally emitting neutron stars.

Constraining the Vela Pulsar's Radio Emission Region Using Nyquist-Limited Scintillation Statistics. (arXiv:1208.5485v1 [astro-ph.SR])

Constraining the Vela Pulsar's Radio Emission Region Using Nyquist-Limited Scintillation Statistics. (arXiv:1208.5485v1 [astro-ph.SR]):
Using a novel technique, we achieve ~100 picoarcsecond resolution and set an
upper bound of less than 4 km for the characteristic size of the Vela pulsar's
emission region. Specifically, we analyze flux-density statistics of the Vela
pulsar at 760 MHz. Because the pulsar exhibits strong diffractive
scintillation, these statistics convey information about the spatial extent of
the radio emission region. We measure both a characteristic size of the
emission region and the emission sizes for individual pulses. Our results imply
that the radio emission altitude for the Vela pulsar at this frequency is less
than 340 km.