289 research outputs found
Thermal emission from Isolated Neutron Stars and their surface magnetic field: going quadrupolar?
In the last few years considerable observational resources have been devoted
to study the thermal emission from isolated neutron stars. Detailed XMM and
Chandra observations revealed a number of features in the X-ray pulse profile,
like asymmetry, energy dependence, and possible evolution of the pulse profile
over a time scale of months or years. Here we show that these characteristics
may be explained by a patchy surface temperature distribution, which is
expected if the magnetic field has a complex structure in which higher order
multipoles contribute together with the dipole. We reconsider these effects
from a theoretical point of view, and discuss their implications to the
observational properties of thermally emitting neutron stars.Comment: 6 pages, 1 TeX file, 6 postscript figures; macro: elsart.cls.
Accepted for publication in Advances in Space Research. Manuscript Number:
JASR-D-04-00405R
Magnetars: the physics behind observations
Magnetars are the strongest magnets in the present universe and the
combination of extreme magnetic field, gravity and density makes them unique
laboratories to probe current physical theories (from quantum electrodynamics
to general relativity) in the strong field limit. Magnetars are observed as
peculiar, burst--active X-ray pulsars, the Anomalous X-ray Pulsars (AXPs) and
the Soft Gamma Repeaters (SGRs); the latter emitted also three "giant flares,"
extremely powerful events during which luminosities can reach up to 10^47 erg/s
for about one second. The last five years have witnessed an explosion in
magnetar research which has led, among other things, to the discovery of
transient, or "outbursting," and "low-field" magnetars. Substantial progress
has been made also on the theoretical side. Quite detailed models for
explaining the magnetars' persistent X-ray emission, the properties of the
bursts, the flux evolution in transient sources have been developed and
confronted with observations. New insight on neutron star asteroseismology has
been gained through improved models of magnetar oscillations. The long-debated
issue of magnetic field decay in neutron stars has been addressed, and its
importance recognized in relation to the evolution of magnetars and to the
links among magnetars and other families of isolated neutron stars. The aim of
this paper is to present a comprehensive overview in which the observational
results are discussed in the light of the most up-to-date theoretical models
and their implications. This addresses not only the particular case of magnetar
sources, but the more fundamental issue of how physics in strong magnetic
fields can be constrained by the observations of these unique sources.Comment: 81 pages, 24 figures, This is an author-created, un-copyedited
version of an article submitted to Reports on Progress in Physic
Accretion Rates in X--Ray Bursting Sources
We present estimates for the accretion rates in 13 X--ray bursting sources
which exhibit photospheric expansion, basing on theoretical models of
stationary, radiatively driven winds from neutron stars. The relatively high
values obtained, \Mdot_{acc}\magcir 10^{-9} \MS, are in accordance with
theoretical limits for unstable helium burning, and, at the same time, almost
never exceed the ``dynamical'' limit for stationary accretion, \sim 10
\Mdot_{Edd}. The only exceptions are 1820-30, already known to be a very
peculiar object, and 1608-522; there are indications, however, that in both
sources, accretion could be non--stationary.Comment: 21 pages, PlainTe
On the Nature of Photospheric Oscillations in Strong X--Ray Bursts
A possible sound origin for the photospheric oscillations in the X--ray
bursting sources 1608-522 and 2127+119 is suggested. It is shown that standing
sound waves in an expanding spherical envelope can have periods very close to
the observed ones. The quite large ratio, 10, of the periods in the two
sources is explained in terms of different wave regimes. The relevance of sound
oscillations to the observed QPO in type II bursts of the Rapid Burster is also
discussed.Comment: 14 pages, PlainTe
Winds from Neutron Stars and Strong Type I X--Ray Bursts
A model for stationary, radiatively driven winds from X--ray bursting neutron
stars is presented. General relativistic hydrodynamical and radiative transfer
equations are integrated from the neutron star surface outwards, taking into
account for helium nuclear burning in the inner, dense, nearly hydrostatic
shells. Radiative processes include both bremsstrahlung emission--absorption
and Compton scattering; only the frequency--integrated transport is considered
here. It is shown that each solution is characterized by just one parameter:
the mass loss rate \Mdot, or, equivalently, the envelope mass \Menv. We
found that, owing to the effects of Comptonization, steady, supersonic winds
can exist only for \Mdot larger than a limiting value \Mdot_{min}
\approx\Mdot_{E}. Several models, covering about two decades in mass loss
rate, have been computed for given neutron star parameters. We discuss how the
sequence of our solutions with decreasing \Menv can be used to follow the
time evolution of a strong X--ray burst during the expansion/contraction phase
near to the luminosity maximum. The comparison between our numerical results
and the observational data of Haberl {\it et al.\/} (1987) for the bursts in
4U/MXB 1820-30 gives an estimate for both the spectral hardening factor and the
accretion rate in this source.Comment: 32 pages (10 postsript figures available on request), PlainTe
The variable X-ray emission of PSR B0943+10
The old pulsar PSR B0943+10 (P=1.1 s, characteristic age tau=5 Myr) is the
best example of mode-switching radio pulsar. Its radio emission alternates
between a highly organized state with regular drifting subpulses (B mode) and a
chaotic emission pattern (Q mode). We present the results of XMM-Newton
observations showing that the X-ray properties of PSR B0943+10 depend on its
radio state (Hermsen et al. 2013). During the radio fainter state (Q mode) the
X-ray flux is more than a factor two larger than during the B-mode and X-ray
pulsations with about 50% pulsed fraction are detected. The X-ray emission of
PSR B0943+10 in the B-mode is well described by thermal emission with blackbody
temperature kT=0.26 keV coming from a small hot spot with luminosity of 7x10^28
erg/s, in good agreement with the prediction of the partially screened gap
model, which also explains the properties of the radio emission in this mode.
We derived an upper limit of 46% on the X-ray pulsed fraction in the B-mode,
consistent with the geometry and viewing angle of PSR B0943+10 inferred from
the radio data. The higher flux observed during the Q-mode is consistent with
the appearance of an additional component with a power-law spectrum with photon
index 2.2. We interpret it as pulsed non-thermal X-rays produced in the star
magnetosphere. A small change in the beaming pattern or in the efficiency of
acceleration of the particles responsible for the non-thermal emission can
explain the reduced flux of this component during the radio B-mode.Comment: Accepted for publication in MNRA
Polarized thermal emission from X-ray Dim Isolated Neutron Stars: the case of RX J1856.5-3754
The observed polarization properties of thermal radiation from isolated,
cooling neutron stars depend on both the emission processes at the surface and
the effects of the magnetized vacuum which surrounds the star. Here we
investigate the polarized thermal emission from X-ray Dim Isolated Neutron
Stars, taking RX J1856.5-3754 as a representative case. The physical conditions
of the star outermost layers in these sources is still debated, and so we
consider emission from a magnetized atmosphere and a condensed surface,
accounting for the effects of vacuum polarization as the radiation propagates
in the star magnetosphere. We have found that, for a significant range of
viewing geometries, measurement of the phase-averaged polarization fraction and
phase-averaged polarization angle at both optical and X-ray wavelengths allow
us to determine whether this neutron star has an atmosphere or a condensed
surface. Our results may therefore be relevant in view of future developments
of soft X-ray polarimeters.Comment: 12 pages, 12 figures, accepted for publication in MNRA
Modeling the broadband persistent emission of magnetars
In this paper, we discuss our first attempts to model the broadband
persistent emission of magnetars within a self consistent, physical scenario.
We present the predictions of a synthetic model that we calculated with a new
Monte Carlo 3-D radiative code. The basic idea is that soft thermal photons
(e.g. emitted by the star surface) can experience resonant cyclotron
upscattering by a population of relativistic electrons threated in the twisted
magnetosphere. Our code is specifically tailored to work in the
ultra-magnetized regime; polarization and QED effects are consistently
accounted for, as well different configurations for the magnetosphere. We
discuss the predicted spectral properties in the 0.1-1000 keV range, the
polarization properties, and we present the model application to a sample of
magnetars soft X-ray spectra.Comment: 14 pages, 7 figures, to be published in Advances in Space Research.
Proceedings of the conference "Frontieres of Space Astrophysics, Neutron
Stars & Gamma Ray Bursts", Cairo/Alexandria, 30 March- 4 April 200
X--Ray Spectra from Neutron Stars Accreting at Low Rates
The spectral properties of X--ray radiation produced in a static atmosphere
around a neutron star accreting at very low rates are investigated. Previous
results by Alme \& Wilson (1973) are extended to the range to include the typical luminosities, , expected from isolated neutron stars
accreting the interstellar medium. The emergent spectra show an overall
hardening with respect to the blackbody at the neutron star effective
temperature in addition to a significant excess over the Wien tail. The
relevance of present results in connection with the observability of
low--luminosity X--ray sources is briefly discussed.Comment: 14 pages (3 postscript figures available on request), PlainTex,
submitted to Ap
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