The isolated neutron star X-ray pulsars RX J0420.0–5022 and RX J0806.4–4123 : 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 J0806.4−4123 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 × 10 19 cm −2 without significant changes between the two observations. Four XMM-Newton observations of RX J0420.0−5022 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 × 10 20 cm −2. 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 J0720.4−3125. For both targets we derive accurate X-ray positions from the Chandra data and present an optical counterpart candidate for RX J0420.0−5022 with B = 26.6 ± 0.3 mag from VLT imaging

Phase transformations induced by spherical indentation in ion-implanted amorphous silicon

The deformation behavior of ion-implanted (unrelaxed) and annealed ion-implanted (relaxed) amorphous silicon(a-Si) under spherical indentation at room temperature has been investigated. It has been found that the mode of deformation depends critically on both the preparation of the amorphous film and the scale of the mechanical deformation.Ex situmeasurements, such as Raman microspectroscopy and cross-sectional transmission electron microscopy, as well as in situ electrical measurements reveal the occurrence of phase transformations in all relaxed a-Si films. The preferred deformation mode of unrelaxed a-Si is plastic flow, only under certain high load conditions can this state of a-Si be forced to transform. In situ electrical measurements have revealed more detail of the transformation process during both loading and unloading. We have used ELASTICA simulations to obtain estimates of the depth of the metallic phase as a function of load, and good agreement is found with the experiment. On unloading, a clear change in electrical conductivity is observed to correlate with a “pop-out” event on load versus penetration curves

Effect of oxygen concentration on nanoindentation-induced phase transformations in ion-implanted amorphous silicon

The effect of the local oxygen concentration in ion-implanted amorphous Si (a-Si) on nanoindentation-inducedphase transformations has been investigated. Implantation of oxygen into the a-Sifilms has been used to controllably introduce an approximately constant concentration of oxygen, ranging from ∼10¹⁸ to ∼10²¹ cm⁻³, over the depth range of the phase transformed zones. Nanoindentation was performed under conditions that ensure a phase transformed zone composed completely of Si-III/XII in the nominally oxygen-free a-Si. The effect of the local oxygen concentration has been investigated by analysis of the unloading curves, Raman microspectroscopy, and cross-sectional transmission electron microscopy (XTEM). The formation of Si-III/XII is suppressed with increasing oxygen concentration, favoring a greater volume of a-Si within the zones. The Raman microspectroscopy and XTEM verify that the volume of Si-III/XII decreases with increasing O concentration. With the smaller volumes of Si-III/XII, the pop-out normally observed on load versus penetration depth curves during unloading decreases in magnitude, becoming more kinklike and is barely discernable at high concentrations of oxygen. The probability of forming any high pressure phases is reduced from 1 to ∼0.1 for a concentration of 10²¹ cm⁻³. We suggest that the bonding of O with Si reduces the formation of Si-III/XII during unloading through a similar mechanism to that of oxygen-retarded solid phase crystallization of a-Si.This project is funded by the Australian Research Council and WRiota Pty Ltd

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 $m_{50CCD}=28.56\pm0.13$ mag, which translates approximately to an unabsorbed flux of $F_{\lambda}=(1.7 \pm 0.3)e-20$ ergs/s/cm^2/A at 5150 A or an X-ray-to-optical flux ratio of $log(f_X/f_opt)=4.9$. This flux is a factor of $\approx 5$ 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 $m_{50CCD} > 29.6$ 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

Timing and spectral studies of the transient X-ray pulsar EXO 053109-6609.2 with ASCA and Beppo-SAX

We report timing and spectral properties of the transient Be X-ray pulsar EXO 053109--6609.2 studied using observations made with the ASCA and BeppoSAX observatories. Though there must have been at least one spin-down episode of the pulsar since its discovery, the new pulse period measurements show a monotonic spin-up trend since 1996. The pulse profile is found to have marginal energy dependence. There is also evidence for strong luminosity dependence of the pulse profile, a single peaked profile at low luminosity that changes to a double peaked profile at high luminosity. This suggests a change in the accretion pattern at certain luminosity level. The X-ray spectrum is found to consist of a simple power-law with photon index in the range of 0.4--0.8. At high intensity level the spectrum also shows presence of weak iron emission line.Comment: 12 pages, 8 figures, Accepted for publication in Ap

The continued spectral and temporal evolution of RX J0720.4-3125

RX J0720.4-3125 is the most peculiar object among a group of seven isolated X-ray pulsars (the so-called "Magnificent Seven"), since it shows long-term variations of its spectral and temporal properties on time scales of years. This behaviour was explained by different authors either by free precession (with a seven or fourteen years period) or possibly a glitch that occurred around $\mathrm{MJD=52866\pm73 days}$. We analysed our most recent XMM-Newton and Chandra observations in order to further monitor the behaviour of this neutron star. With the new data sets, the timing behaviour of RX J0720.4-3125 suggests a single (sudden) event (e.g. a glitch) rather than a cyclic pattern as expected by free precession. The spectral parameters changed significantly around the proposed glitch time, but more gradual variations occurred already before the (putative) event. Since $\mathrm{MJD\approx53000 days}$ the spectra indicate a very slow cooling by $\sim$2 eV over 7 years.Comment: seven pages, three figures, three tables; accepted by MNRA

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

How Common Are Magnetars? The Consequences of Magnetic-Field Decay

Ultramagnetized neutron stars or magnetars have been invoked to explain several astrophysical phenomena. We examine how the magnetic field of a magnetar will decay over time and how this decay affects the cooling of the object. We find that for sufficiently strong nascent fields, field decay alters the cooling evolution significantly relative to similarly magnetized neutron stars with constant fields. As a result, old magnetars can be expected to be bright in the soft X-ray band. The soft X-ray source RXJ~0720.4$-$3125 may well be the nearest such old magnetar.Comment: 7 pages, 1 figure, accepted for publication in Ap. J. Letter

M 33 X-7: ChASeM33 reveals the first eclipsing black hole X-ray binary

The first observations conducted as part of the Chandra ACIS survey of M 33 (ChASeM33) sampled the eclipsing X-ray binary M 33 X-7 over a large part of the 3.45 d orbital period and have resolved eclipse ingress and egress for the first time. The occurrence of the X-ray eclipse allows us to determine an improved ephemeris of mid-eclipse and binary period as HJD (2453639.119+-0.005) +- N x (3.453014+-0.000020) and constrain the eclipse half angle to (26.5+-1.1) degree. There are indications for a shortening of the orbital period. The X-ray spectrum is best described by a disk blackbody spectrum typical for black hole X-ray binaries in the Galaxy. We find a flat power density spectrum and no significant regular pulsations were found in the frequency range of 10^{-4} to 0.15 Hz. HST WFPC2 images resolve the optical counterpart, which can be identified as an O6III star with the help of extinction and colour corrections derived from the X-ray absorption. Based on the optical light curve, the mass of the compact object in the system most likely exceeds 9 M_sun. This mass, the shape of the X-ray spectrum and the short term X-ray time variability identify M 33 X-7 as the first eclipsing black hole high mass X-ray binary.Comment: 14 pages, 5 figures, ApJ accepte

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 $V=(660-1440) km s ^{-1}$. 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