277 research outputs found
The Magnificent Seven: Magnetic fields and surface temperature distributions
Presently seven nearby radio-quiet isolated neutron stars discovered in ROSAT
data and characterized by thermal X-ray spectra are known. They exhibit very
similar properties and despite intensive searches their number remained
constant since 2001 which led to their name ``The Magnificent Seven''. Five of
the stars exhibit pulsations in their X-ray flux with periods in the range of
3.4 s to 11.4 s. XMM-Newton observations revealed broad absorption lines in the
X-ray spectra which are interpreted as cyclotron resonance absorption lines by
protons or heavy ions and / or atomic transitions shifted to X-ray energies by
strong magnetic fields of the order of 10^13 G. New XMM-Newton observations
indicate more complex X-ray spectra with multiple absorption lines. Pulse-phase
spectroscopy of the best studied pulsars RX J0720.4-3125 and RBS 1223 reveals
variations in derived emission temperature and absorption line depth with pulse
phase. Moreover, RX J0720.4-3125 shows long-term spectral changes which are
interpreted as due to free precession of the neutron star. Modeling of the
pulse profiles of RX J0720.4-3125 and RBS 1223 provides information about the
surface temperature distribution of the neutron stars indicating hot polar caps
which have different temperatures, different sizes and are probably not located
in antipodal positions.Comment: 10 pages, 8 figures, to appear in Astrophysics and Space Science, in
the proceedings of "Isolated Neutron Stars: from the Interior to the
Surface", edited by D. Page, R. Turolla and S. Zan
XMM-Newton Observations of Radio Pulsars B0834+06 and B0826-34 and Implications for Pulsar Inner Accelerator
We report the X-ray observations of two radio pulsars with drifting
subpulses: B0834 + 06 and B0826 - 34 using \xmm\. PSR B0834 + 06 was detected
with a total of 70 counts from the three EPIC instruments over 50 ks exposure
time. Its spectrum was best described as that of a blackbody (BB) with
temperature K and bolometric luminosity
of erg s. As it is typical in
pulsars with BB thermal components in their X-ray spectra, the hot spot surface
area is much smaller than that of the canonical polar cap, implying a
non-dipolar surface magnetic field much stronger than the dipolar component
derived from the pulsar spin-down (in this case about 50 times smaller and
stronger, respectively). The second pulsar PSR B0826 - 34 was not detected over
50 ks exposure time, giving an upper limit for the bolometric luminosity erg s. We use these data as well as the radio
emission data concerned with drifting subpulses to test the Partially Screened
Gap (PSG) model of the inner accelerator in pulsars.Comment: Accepted for publication by The Astrophysical Journa
The Turn-On of Mass Transfer in AM CVn Binaries: Implications for RX J0806+1527 and RX J1914+2456
We report on evolutionary calculations of the onset of mass transfer in AM
CVn binaries, treating the donor's evolution in detail. We show that during the
early contact phase, while the mass transfer rate, \Mdot, is increasing,
gravity wave (GW) emission continues to drive the binary to shorter orbital
period, \Porb. We argue that the phase where \Mdot > 0 and \nudot > 0
(\nu = 1/\Porb) can last between and yrs, significantly longer
than previously estimated. These results are applied to RX J0806+1527 (\Porb =
321 s) and RX J914+2456 (\Porb=569 s), both of which have measured \nudot >
0. \emph{Thus, a \nudot > 0 does not select between the unipolar inductor
and accretion driven models proposed as the source of X-rays in these systems}.
For the accretion model, we predict for RX J0806 that \ddot{\nu} \approx
\ee{1.0-1.5}{-28} Hz s and argue that timing observations can probe
at this level with a total yr baseline. We also place
constraints on each system's initial parameters given current observational
data.Comment: 5 pages, 3 figures, accepted to ApJ
Detection of pulsations and a spectral feature in the X-ray emission of the isolated neutron star 1RXS J214303.7+065419/RBS 1774
We report on the results of a deep XMM-Newton observation of RBS 1774, the
most recent dim isolated neutron star candidate found in the ROSAT archive
data. Spectral and timing analysis of the high-quality PN and MOS data confirm
the association of this source with an isolated neutron star. The spectrum is
thermal and blackbody-like, and there is evidence at a significance level >
4sigma that the source is an X-ray pulsar, with spin period of 9.437 s.
Spectral fitting reveils the presence of an absorption feature at ~0.7 keV, but
at this level data do not have enough resolution to allow us to discriminate
between an absorption line or an edge. We compare the newly measured properties
of RBS 1774 with those of other known dim isolated neutron stars, and discuss
possible interpretations for the absorption feature.Comment: 21 pages, 5 figures, ApJ accepte
Temperature distribution in magnetized neutron star crusts. II. The effect of a strong toroidal component
We continue the study of the effects of a strong magnetic field on the
temperature distribution in the crust of a magnetized neutron star (NS) and its
impact on the observable surface temperature. Extending the approach initiated
in Geppert et al.(2004), we consider more complex and, hence, more realistic,
magnetic field structures but still restrict ourselves to axisymmetric
configurations. We put special emphasis on the heat blanketing effect of a
toroidal field component. We show that asymmetric temperature distributions can
occur and a crustal field consisting of dipolar poloidal and toroidal
components will cause one polar spot to be larger than the opposing one. These
two warm regions can be separated by an extended cold equatorial belt. We
present an internal magnetic field structure which can explain both the X-ray
and optical spectra of the isolated NS RXJ 1856-3754. We investigate the
effects of the resulting surface temperature profiles on the observable
lightcurve which an isolated thermally emitting NS would produce for different
field geometries. The lightcurves will be both qualitatively (deviations from
sinusoidal shape) and quantitatively (larger pulsed fraction for the same
observational geometry) different from those of a NS with an isothermal crust.
This opens the possibility to determine the interna} magnetic field strengths
and structures in NSs by modeling their X-ray lightcurves and spectra. The
striking similarities of our model calculations with the observed spectra and
pulse profiles of isolated thermally emitting NSs is an indication for the
existence of strong magnetic field components maintained by crustal currents.Comment: 9 pages, 10 figures in jpg-forma
Spot-like Structures of Neutron Star Surface Magnetic Fields
There is growing evidence, based on both X-ray and radio observations of
isolated neutron stars, that besides the large--scale (dipolar) magnetic field,
which determines the pulsar spin--down behaviour, small--scale poloidal field
components are present, which have surface strengths one to two orders of
magnitude larger than the dipolar component. We argue in this paper that the
Hall--effect can be an efficient process in producing such small--scale field
structures just above the neutron star surface. It is shown that due to a
Hall--drift induced instability, poloidal magnetic field structures can be
generated from strong subsurface toroidal fields, which are the result of
either a dynamo or a thermoelectric instability acting at early times of a
neutron star's life. The geometrical structure of these small--scale surface
anomalies of the magnetic field resembles that of some types of
``star--spots''. The magnetic field strength and the length--scales are
comparable with values that can be derived from various observations.Comment: 4 pages, 2 figures, accepted by Astronomy & Astrophysics Letters;
language improved, 2nd para of Sect. 3 change
Imaging Performance of the XMM-Newton X-ray telescopes
The in-orbit imaging performance of the three X-ray telescopes on board of
the X-ray astronomy observatory XMM-Newton is presented and compared with the
performance measured on ground at the MPE PANTER test facility. The comparison
shows an excellent agreement between the on ground and in-orbit performance.Comment: 9 pages, 10 Postscript figures, for SPIE 4012, paper 8
Turning Points in the Evolution of Isolated Neutron Stars' Magnetic Fields
During the life of isolated neutron stars (NSs) their magnetic field passes
through a variety of evolutionary phases. Depending on its strength and
structure and on the physical state of the NS (e.g. cooling, rotation), the
field looks qualitatively and quantitatively different after each of these
phases. Three of them, the phase of MHD instabilities immediately after NS's
birth, the phase of fallback which may take place hours to months after NS's
birth, and the phase when strong temperature gradients may drive thermoelectric
instabilities, are concentrated in a period lasting from the end of the
proto--NS phase until 100, perhaps 1000 years, when the NS has become almost
isothermal. The further evolution of the magnetic field proceeds in general
inconspicuous since the star is in isolation. However, as soon as the product
of Larmor frequency and electron relaxation time, the so-called magnetization
parameter, locally and/or temporally considerably exceeds unity, phases, also
unstable ones, of dramatic changes of the field structure and magnitude can
appear. An overview is given about that field evolution phases, the outcome of
which makes a qualitative decision regarding the further evolution of the
magnetic field and its host NS.Comment: References updated, typos correcte
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