664 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
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
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
The impulsive phase of magnetar giant flares: assessing linear tearing as the trigger mechanism
Giant -ray flares comprise the most extreme radiation events observed
from magnetars. Developing on (sub)millisecond timescales and generating vast
amounts of energy within a fraction of a second, the initial phase of these
extraordinary bursts present a significant challenge for candidate trigger
mechanisms. Here we assess and critically analyse the linear growth of the
relativistic tearing instability in a globally twisted magnetosphere as the
trigger mechanism for giant -ray flares. Our main constraints are given
by the observed emission timescales, the energy output of the giant flare
spike, and inferred dipolar magnetic field strengths. We find that the minimum
growth time of the linear mode is comparable to the -folding rise time, i.e.
ms. With this result we constrain basic geometric parameters of
the current sheet. We also discuss the validity of the presumption that the
-folding emission timescale may be equated with the growth time of an MHD
instability.Comment: 15 pages, 4 figures, MNRAS in pres
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
An unified timing and spectral model for the Anomalous X-ray Pulsars XTE J1810-197 and CXOU J164710.2-455216
Anomalous X-ray pulsars (AXPs) and soft gamma repeaters (SGRs) are two small
classes of X-ray sources strongly suspected to host a magnetar, i.e. an
ultra-magnetized neutron star with $B\approx 10^14-10^15 G. Many SGRs/AXPs are
known to be variable, and recently the existence of genuinely "transient"
magnetars was discovered. Here we present a comprehensive study of the pulse
profile and spectral evolution of the two transient AXPs (TAXPs) XTE J1810-197
and CXOU J164710.2-455216. Our analysis was carried out in the framework of the
twisted magnetosphere model for magnetar emission. Starting from 3D Monte Carlo
simulations of the emerging spectrum, we produced a large database of synthetic
pulse profiles which was fitted to observed lightcurves in different spectral
bands and at different epochs. This allowed us to derive the physical
parameters of the model and their evolution with time, together with the
geometry of the two sources, i.e. the inclination of the line-of-sight and of
the magnetic axis with respect to the rotation axis. We then fitted the
(phase-averaged) spectra of the two TAXPs at different epochs using a model
similar to that used to calculate the pulse profiles ntzang in XSPEC) freezing
all parameters to the values obtained from the timing analysis, and leaving
only the normalization free to vary. This provided acceptable fits to
XMM-Newton data in all the observations we analyzed. Our results support a
picture in which a limited portion of the star surface close to one of the
magnetic poles is heated at the outburst onset. The subsequent evolution is
driven both by the cooling/varying size of the heated cap and by a progressive
untwisting of the magnetosphere.Comment: 15 pages, 12 figures, accepted for publication in Ap
Radiative acceleration and transient, radiation-induced electric fields
The radiative acceleration of particles and the electrostatic potential
fields that arise in low density plasmas hit by radiation produced by a
transient, compact source are investigated. We calculate the dynamical
evolution and asymptotic energy of the charged particles accelerated by the
photons and the radiation-induced electric double layer in the full
relativistic, Klein-Nishina regime. For fluxes in excess of , the radiative force on a diluted plasma
(n\la 10^{11} cm) is so strong that electrons are accelerated rapidly
to relativistic speeds while ions lag behind owing to their larger inertia. The
ions are later effectively accelerated by the strong radiation-induced double
layer electric field up to Lorentz factors , attainable in the
case of negligible Compton drag. The asymptotic energies achieved by both ions
and electrons are larger by a factor 2--4 with respect to what one could
naively expect assuming that the electron-ion assembly is a rigidly coupled
system. The regime we investigate may be relevant within the framework of giant
flares from soft gamma-repeaters.Comment: 14 pages, 7 figures, ApJ, in press (tentatively scheduled for the v.
592, 2003 issue
Spectral variability in transonic discs around black holes
Transonic discs with accretion rates relevant to intrinsically bright
Galactic X-ray sources (-) exhibit
a time dependent cyclic behaviour due to the onset of a thermal instability
driven by radiation pressure. In this paper we calculate radiation spectra
emitted from thermally-unstable discs to provide detailed theoretical
predictions for observationally relevant quantities. The emergent spectrum has
been obtained by solving self-consistently the vertical structure and radiative
transfer in the disc atmosphere. We focus on four particular stages of the disc
evolution, the maximal evacuation stage and three intermediate stages during
the replenishment phase. The disc is found to undergo rather dramatic spectral
changes during the evolution, emitting mainly in the 1-10 keV band during
outburst and in the 0.1-1 keV band off-outburst. Local spectra, although
different in shape from a blackbody at the disc effective temperature, may be
characterized in terms of a hardening factor . We have found that is
rather constant both in radius and in time, with a typical value .Comment: 10 pages Latex with 11 ps figures. Accepted for publication in MNRA
Patientâs assessment and prediction of recovery after stroke: a roadmap for clinicians
Background and purposeIn neurorehabilitation clinical practice, assessment is usually more oriented to evaluate patient's present status, than to plan interventions according to predicted outcomes. Therefore, we conducted an extensive review of current prognostic models available in the literature for recovery prediction of many functions and constructs, after stroke. We reported results in the form of a practical guide for clinicians, with the aim of promoting the culture of early clinical assessment for patient stratification, according to expected outcome.Recommendations for clinical practiceClinical assessment should be directed both towards the objective evaluation of the present health status, and to the prediction of expected recovery. The use of specific outcome measures with predictive value is recommended to help clinicians with the definition of sound therapeutic goals
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