625 research outputs found
A Coherent Timing Solution for the Nearby Isolated Neutron Star RX J0720.4-3125
We present the results of a dedicated effort to measure the spin-down rate of
the nearby isolated neutron star RX J0720.4-3125. Comparing arrival times of
the 8.39-sec pulsations for data from Chandra we derive an unambiguous timing
solution for RX J0720.4-3125 that is accurate to 5 years.
Adding data from XMM and ROSAT, the final solution yields
Pdot=(6.98+/-0.02)x10^(-14) s/s; for dipole spin-down, this implies a
characteristic age of 2 Myr and a magnetic field strength of 2.4e13 G. The
phase residuals are somewhat larger than those for purely regular spin-down,
but do not show conclusive evidence for higher-order terms or a glitch. From
our timing solution as well as recent X-ray spectroscopy, we concur with recent
suggestions that RX J0720.4-3125 is most likely an off-beam radio pulsar with a
moderately high magnetic field.Comment: 5 pages, 1 figure. Accepted for publication in ApJ
Antioxidant status in acute stroke patients and patients at stroke risk
Background and Purpose: Antioxidant enzymes like copper/ zinc superoxide dismutase (SOD), catalase and gluthatione peroxidase (GSHPx) are part of intracellular protection mechanisms to overcome oxidative stress and are known to be activated in vascular diseases and acute stroke. We investigated the differences of antioxidant capacity in acute stroke and stroke risk patients to elucidate whether the differences are a result of chronic low availability in arteriosclerosis and stroke risk or due to changes during acute infarction. Methods: Antioxidant enzymes were examined in 11 patients within the first hours and days after acute ischemic stroke and compared to risk- and age-matched patients with a history of stroke in the past 12 months ( n = 17). Antioxidant profile was determined by measurement of glutathione (GSH), malondialdehyde (MDA), SOD, GSHPx and minerals known to be involved in antioxidant enzyme activation like selenium, iron, copper and zinc. Results: In comparison to stroke risk patients, patients with acute ischemic stroke had significant changes of the GSH system during the first hours and days after the event: GSH was significantly elevated in the first hours (p < 0.01) and GSHPx was elevated 1 day after the acute stroke (p < 0.05). Selenium, a cofactor of GSHPx, was decreased (p < 0.01). GSHPx levels were negatively correlated with National Institutes of Health Stroke Scale (NIHSS) scores on admission (r = - 0.84, p < 0.001) and NIHSS scores after 7 days ( r = - 0.63, p < 0.05). MDA levels showed a trend for elevation in the first 6 h after the acute stroke ( p = 0.07). No significant differences of SOD, iron, copper nor zinc levels could be identified. Conclusions: Differences of antioxidant capacity were found for the GSH system with elevation of GSH and GSHPx after acute stroke, but not for other markers. The findings support the hypothesis that changes of antioxidant capacity are part of acute adaptive mechanisms during acute stroke. Copyright (C) 2004 S. Karger AG, Basel
A Probable Optical Counterpart for the Isolated Neutron Star RX J1308.6+2127
Using a very deep observation with HST/STIS, we have searched for an optical
counterpart to the nearby radio-quiet isolated neutron star RX J1308.6+2127
(RBS 1223). We have identified a single object in the 90% Chandra error circle
that we believe to be the optical counterpart. This object has
mag, which translates approximately to an unabsorbed
flux of ergs/s/cm^2/A at 5150 A or an
X-ray-to-optical flux ratio of . This flux is a factor of
above the extrapolation of the black-body fit to the X-ray
spectrum, consistent with the optical spectra of other isolated neutron stars.
Without color information we cannot conclude that this source is indeed the
counterpart of RX J1308.6+2127. If not, then the counterpart must have
mag, corresponding to a flux that is barely consistent with
the extrapolation of the black-body fit to the X-ray spectrum.Comment: 4 pages, 2 figures. Uses emulateapj5.sty, onecolfloat5.sty. Accepted
by ApJ Letter
VLT/FORS2 observations of the optical counterpart of the isolated neutron star RBS 1774
X-ray observations performed with ROSAT led to the discovery of a group
(seven to date) of X-ray dim and radio-silent middle-aged isolated neutron
stars (a.k.a. XDINSs), which are characterised by pure blackbody spectra
(kT~40-100 eV), long X-ray pulsations (P=3-12 s), and appear to be endowed with
relatively high magnetic fields, (B~10d13-14 G). RBS 1774 is one of the few
XDINSs with a candidate optical counterpart, which we discovered with the VLT.
We performed deep observations of RBS 1774 in the R band with the VLT to
disentangle a non-thermal power-law spectrum from a Rayleigh-Jeans, whose
contributions are expected to be very much different in the red part of the
spectrum. We did not detect the RBS 1774 candidate counterpart down to a 3
sigma limiting magnitude of R~27. The constraint on its colour, (B-R)<0.6,
rules out that it is a background object, positionally coincident with the
X-ray source. Our R-band upper limit is consistent with the extrapolation of
the B-band flux (assuming a 3 sigma uncertainty) for a set of power-laws F_nu
~nu^alpha with spectral indeces alpha<0.07. If the optical spectrum of RBS 1774
were non-thermal, its power-law slope would be very much unlike those of all
isolated neutron stars with non-thermal optical emission, suggesting that it is
most likely thermal. For instance, a Rayleigh-Jeans with temperature T_O = 11
eV, for an optically emitting radius r_O=15 km and a source distance d=150 pc,
would be consistent with the optical measurements. The implied low distance is
compatible with the 0.04 X-ray pulsed fraction if either the star spin axis is
nearly aligned with the magnetic axis or with the line of sight, or it is
slightly misaligned with respect to both the magnetic axis and the line of
sight by 5-10 degreesComment: 8 pages, 8 postscript figures, accepted for publication in Astronomy
& Astrophysic
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
HFPK 334: An unusual Supernova Remnant in the Small Magellanic Cloud
We present new Australia Telescope Compact Array (ATCA) radio-continuum and
XMM-Newton/Chandra X-ray Observatory (CXO) observations of the unusual
supernova remnant HFPK 334 in the Small Magellanic Cloud (SMC). The remnant
follows a shell type morphology in the radio-continuum and has a size of
20~pc at the SMC distance. The X-ray morphology is similar, however, we
detect a prominent point source close to the center of the SNR exhibiting a
spectrum with a best fit powerlaw with a photon index of . This central point source is most likely a background object and cannot
be directly associated with the remnant. The high temperature, nonequilibrium
conditions in the diffuse region suggest that this gas has been recently
shocked and point toward a younger SNR with an age of years.
With an average radio spectral index of we find that an
equipartition magnetic field for the remnant is 90~G, a value
typical of younger SNRs in low-density environments. Also, we report detection
of scattered radio polarisation across the remnant at 20~cm, with a peak
fractional polarisation level of 255\%.Comment: 19 pages, 6-figures, submitted to A
1RXS J214303.7+065419/RBS 1774: A New Isolated Neutron Star Candidate
We report on the identification of a new possible Isolated Neutron Star
candidate in archival ROSAT observations. The source 1RXS J214303.7+065419,
listed in the ROSAT Bright Survey as RBS 1774, is very soft, exhibits a thermal
spectrum well fitted by a blackbody at eV and has a low column
density, . Catalogue searches revealed
no known sources in other energy bands close to the X-ray position of RBS 1774.
Follow-up optical observations with NTT showed no peculiar object within the
X-ray error circle. The absence of any plausible optical counterpart down to
results in an X-ray to optical flux ratio in excess of 1000.Comment: LaTeX (A&A style files), 5 pages, 3 figures. Accepted for publication
in Astronomy and Astrophysics Letters. Minor correction
Discovery of periodic dips in the brightest hard X-ray source of M31 with EXTraS
We performed a search for eclipsing and dipping sources in the archive of the
EXTraS project - a systematic characterization of the temporal behaviour of
XMM-Newton point sources. We discovered dips in the X-ray light curve of 3XMM
J004232.1+411314, which has been recently associated with the hard X-ray source
dominating the emission of M31. A systematic analysis of XMM-Newton
observations revealed 13 dips in 40 observations (total exposure time 0.8
Ms). Among them, four observations show two dips, separated by 4.01 hr.
Dip depths and durations are variable. The dips occur only during
low-luminosity states (L erg s), while the
source reaches L erg s. We propose this
system to be a new dipping Low-Mass X-ray Binary in M31 seen at high
inclination (60-80), the observed dipping periodicity is
the orbital period of the system. A blue HST source within the Chandra error
circle is the most likely optical counterpart of the accretion disk. The high
luminosity of the system makes it the most luminous dipper known to date.Comment: 11 pages, 2 figures, 5 tables, accepted for publication in ApJ
INTEGRAL deep observations of the Small Magellanic Cloud
Deep observations of the Small Magellanic Cloud (SMC) and region were carried
out in the hard X-ray band by the INTEGRAL observatory in 2008-2009. The field
of view of the instrument permitted simultaneous coverage of the entire SMC and
the eastern end of the Magellanic Bridge. In total, INTEGRAL detected seven
sources in the SMC and five in the Magellanic Bridge; the majority of the
sources were previously unknown systems. Several of the new sources were
detected undergoing bright X- ray outbursts and all the sources exhibited
transient behaviour except the supergiant system SMC X-1. They are all thought
to be High Mass X-ray Binary (HMXB) systems in which the compact object is a
neutron star.Comment: 7 pages, 10 figures Accepted for publication in MNRA
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