297 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
Pulsar timing in extreme mass ratio binaries: a general relativistic approach
The detection of a pulsar (PSR) in a tight, relativistic orbit around a
supermassive or intermediate mass black hole - such as those in the Galactic
centre or in the centre of Globular clusters - would allow for precision tests
of general relativity (GR) in the strong-field, non-linear regime. We present a
framework for calculating the theoretical time-frequency signal from a PSR in
such an Extreme Mass Ratio Binary (EMRB). This framework is entirely
relativistic with no weak-field approximations and so able to account for all
higher-order strong-field gravitational effects, relativistic spin dynamics,
the convolution with astrophysical effects and the combined impact on the PSR
timing signal. Specifically we calculate both the spacetime path of the pulsar
radio signal and the complex orbital and spin dynamics of a spinning pulsar
around a Kerr black hole, accounting for spacetime curvature and frame
dragging, relativistic and gravitational time delay, gravitational light
bending, temporal and spatial dispersion induced by the presence of plasma
along the line of sight and relativistic aberration. This then allows for a
consistent time-frequency solution to be generated. Such a framework is key for
assessing the use of PSR as probes of strong field GR, helping to inform the
detection of an EMRB system hosting a PSR and, most essentially, for providing
an accurate theoretical basis to then compare with observations to test
fundamental physics.Comment: 19 pages, 15 Figures. Accepted for publication in MNRA
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
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
Unveiling the thermal and magnetic map of neutron star surfaces though their X-ray emission: method and light-curve analysis
Comment: 17 pages, 9 figures. Figures 3 and 5 (top panel) are provided separately as .png files. 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
Gravitational Burst Radiation from Pulsars in the Galactic centre and stellar clusters
Pulsars (PSRs) orbiting intermediate or supermassive black holes at the
centre of galaxies and globular clusters are known as Extreme Mass Ratio
Binaries (EMRBs) and have been identified as precision probes of strong-field
GR. For appropriate orbital parameters, some of these systems may also emit
gravitational radiation in a `burst-like' pattern. The observation of this
burst radiation in conjunction with the electromagnetic radio timing signal
would allow for multimessenger astronomy in strong-field gravitational regimes.
In this work we investigate gravitational radiation from these PSR-EMRBs,
calculating the waveforms and SNRs and explore the influence of this GW on the
pulsar radio signal. We find that for typical PSR-EMRBs, gravitational burst
radiation should be detectable from both the Galactic centre and the centre of
stellar clusters, and that this radiation will not meaningfully affect the
pulsar timing signal, allowing PSR-EMRB to remain `clean' test-beds of
strong-field GR.Comment: 15 pages, 9 figures, accepted for publication in MNRA
Orbital spin dynamics of a millisecond pulsar around a massive black hole with an general mass quadrupole
We investigate the spin dynamics of a millisecond pulsar (MSP) in compact
orbit around a Kerr-like massive black hole with an general mass quadrupole. We
use the Mathisson-Papetrou-Dixon formulation to compute the orbital and spin
evolution of the MSP, accounting for the non-linear interaction of the pulsar's
energy-momentum tensor on the background spacetime metric. We investigate how
the MSP spin and BH quadrupole moment manifest in the pulsar spin-orbital
dynamics. We discuss the astrophysical observational implications of these spin
and orbital dynamics on the timing of a radio pulsar in an Extreme Mass Ratio
Binary, e.g. a Galactic Centre pulsar. In particular, notable timing variations
in the Einstein delay and Roemer delay are observed, along with modifications
to the pulsar pulse profile.Comment: 11 pages, 11 figures, accepted for publication in MNRA
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