617 research outputs found
The isolated neutron star X-ray pulsars RX J0420.0â5022 and RX J0806.4â4123 : new X-ray and optical observations
We report on the analysis of new X-ray data obtained with XMM-Newton and Chandra from two ROSAT-discovered X-ray dim isolated neutron stars (XDINs). RX J0806.4â4123 was observed with XMM-Newton in April 2003, 2.5 years after
the first observation. The EPIC-pn data confirm that this object is an X-ray pulsar with 11.371 s neutron star spin period. The X-ray spectrum is consistent with absorbed black-body emission with a temperature kT = 96 eV and N H = 4 Ă 10 19 cm â2 without significant changes between the two observations. Four XMM-Newton observations of RX
J0420.0â5022 between December 2002 and July 2003 did not confirm the 22.7 s pulsations originally indicated in ROSAT data, but clearly reveal a 3.453 s period. A fit to the X-ray spectrum using an absorbed black-body model yields kT = 45 eV, the lowest value found from the small group of XDINs and N H = 1.0 Ă 10 20 cm â2. Including a broad absorption line improves the quality of the spectral fits considerably for both objects and may indicate the presence of absorption features similar to those reported from RBS1223, RX J1605.3+3249 and RX J0720.4â3125. For both targets we derive accurate X-ray positions from the Chandra data and present an optical counterpart candidate for RX J0420.0â5022 with B = 26.6 ± 0.3 mag from VLT imaging
On the compactness of the isolated neutron star RX J0720.4-3125
The data from all observations of RX J0720.4-3125 conducted by XMM-Newton
EPIC-pn with the same instrumental setup in 2000-2012 were reprocessed to form
a homogenous data set of solar barycenter corrected photon arrival times
registered from RX J0720.4-3125. A Bayesian method for the search, detection,
and estimation of the parameters of an unknown-shaped periodic signal was
employed as developed by Gregory & Loredo (1992).
A number of complex models (single and double peaked) of light curves from
pulsating neutron stars were statistically analyzed. The distribution of phases
for the registered photons was calculated by folding the arrival times with the
derived spin-period and the resulting distribution of phases approximated with
a mixed von Mises distribution, and its parameters were estimated by using the
Expected Maximization method. Spin phase-resolved spectra were extracted, and a
number of highly magnetized atmosphere models of an INS were used to fit
simultaneously, the results were verified via an MCMC approach. The
phase-folded light curves in different energy bands with high S/N ratio show a
high complexity and variations depending on time and energy.
They can be parameterized with a mixed von Mises distribution, i.e. with
double-peaked light curve profile showing a dependence of the estimated
parameters (mean directions, concentrations, and proportion) upon the energy
band, indicating that radiation emerges from at least two emitting areas.
The genuine spin-period of the isolated neutron star RX J0720-3125 derived as
more likely is twice of that reported in the literature (16.78s instead of
8.39s).
The gravitational redshift of RX J0720.4-3125 was determined to
and the compactness was estimated to
.Comment: Comments: 19 pages, 15 figures and 5 tables, Astronomy and
Astrophysics accepted. arXiv admin note: text overlap with arXiv:1108.389
The Determination of the `Diffusion Coefficients' and the Stellar Wind Velocities for X-Ray Binaries
The distribution of neutron stars (NS's) is determined by stationary solution
of the Fokker-Planck equation. In this work using the observed period changes
for four systems: Vela X-1, GX 301-2, Her X-1 and Cen X-3 we determined D, the
'diffusion coefficient',-parameter from the Fokker-Planck equation. Using
strong dependence of D on the velocity for Vela X-1 and GX 301-2, systems
accreting from a stellar wind, we determined the stellar wind velocity. For
different assumptions for a turbulent velocity we obtained . It is in good agreement with the stellar wind velocity determined by
other methods. We also determined the specific characteristic time scales for
the 'diffusion processes' in X-ray pulsars. It is of order of 200 sec for
wind-fed pulsars and 1000-10000 sec for the disk accreting systems.Comment: 8 pages, Latex, no figures, accepted for publication to Astronomical
and Astrophysical Transactions (1995). Admin note 20Feb2000: original
(broken) version now paper.tex.orig in source; fixed version with two bad
equations set in verbatim used for PS, paper.tex in sourc
XMM-Newton observations of the isolated neutron star RX J0806.4-4123
The isolated neutron star RX J0806.4-4123 was observed with XMM-Newton in
November 2000. The data from the three EPIC instruments allowed us (i) to
derive an improved X-ray position to an accuracy of 2-3'', (ii) to accumulate
the first medium-resolution soft X-ray spectra of high statistical quality and
(iii) to find a candidate for the neutron star rotation period. Although this
period of 11.3714 s is formally detected at a 3.5-\sigma level in the EPIC-pn
data, the similar pulse profiles deduced from all three EPIC instruments
increase the confidence that the period is real. The pulsed fraction of ~6%
would then be the weakest X-ray flux modulation detected from dim isolated
neutron stars. We fitted the X-ray spectra with blackbody and neutron star
atmosphere models and discuss the results with respect to the brightness limit
placed by optical images. The reduced size of the error circle on the X-ray
position should allow deeper searches for an optical counterpart.Comment: 7 pages, 6 figures, accepted by A&
Multiwavelength appearance of Vela Jr.: Is it up to expectations?
Vela Jr. is one of the youngest and likely nearest among the known galactic
supernova remnants (SNRs). Discovered in 1997 it has been studied since then at
quite a few wavelengths, that spread over almost 20 decades in energy. Here we
present and discuss Vela Jr. properties revealed by these multiwavelength
observations, and confront them with the SNR model expectations. Questions that
remained unanswered at the time of publication of the paper of Iyudin et al.
(2005), e.g. what is the nature of the SNR's proposed central compact source
CXOU J085201.4-461753, and why is the ISM absorption column density apparently
associated with RX J0852.0-4622 much greater than the typical column of the
Vela SNR, can be addressed using the latest radio and X-ray observations of
Vela Jr.. These, and other related questions are addressed in the following.Comment: 8 pages, 7 figures. Accepted for publication in ESA SP-622,
Proceedings of the 6th INTEGRAL Workshop held in Moscow, Russia, July 02-08,
200
Constraints on the Equation-of-State of neutron stars from nearby neutron star observations
We try to constrain the Equation-of-State (EoS) of supra-nuclear-density
matter in neutron stars (NSs) by observations of nearby NSs. There are seven
thermally emitting NSs known from X-ray and optical observations, the so-called
Magnificent Seven (M7), which are young (up to few Myrs), nearby (within a few
hundred pc), and radio-quiet with blackbody-like X-ray spectra, so that we can
observe their surfaces. As bright X-ray sources, we can determine their
rotational (pulse) period and their period derivative from X-ray timing. From
XMM and/or Chandra X-ray spectra, we can determine their temperature. With
precise astrometric observations using the Hubble Space Telescope, we can
determine their parallax (i.e. distance) and optical flux. From flux, distance,
and temperature, one can derive the emitting area - with assumptions about the
atmosphere and/or temperature distribution on the surface. This was recently
done by us for the two brightest M7 NSs RXJ1856 and RXJ0720. Then, from
identifying absorption lines in X-ray spectra, one can also try to determine
gravitational redshift. Also, from rotational phase-resolved spectroscopy, we
have for the first time determined the compactness (mass/radius) of the M7 NS
RBS1223. If also applied to RXJ1856, radius (from luminosity and temperature)
and compactness (from X-ray data) will yield the mass and radius - for the
first time for an isolated single neutron star. We will present our
observations and recent results.Comment: refereed NPA5 conference proceedings, in pres
The Puzzles of RX J1856.5-3754: Neutron Star or Quark Star?
We discuss recent Chandra and XMM-Newton observations of the bright isolated
neutron star RX J1856.5-3754 and suggest that the absence of any line features
is due to effects of a high magnetic field strength (~10^13 G). Using different
models for the temperature distribution across the neutron star surface
assuming blackbody emission to fit the optical and X-ray spectrum and we derive
a conservative lower limit of the "apparent" neutron star radius of 16.5 km x
(d/117 pc). This corresponds to the radius for the "true" (de-redshifted)
radius of 14 km for a 1.4 Msun neutron star, indicating a stiff equation of
state at high densities. A comparison of the result with mass-radius diagrams
shows that quark stars and neutron stars with quark matter cores can be ruled
out with high confidence.Comment: 6 page, 2 figures, "The Restless High-Energy Universe" Proceedings of
the symposium dedicated to six years of successful BeppoSAX operations
Amsterdam, May 5-8, 200
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
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