A BeppoSAX observation of the supersoft source 1E 0035.4-7230

Results from a 37,000 s BeppoSAX Low-Energy Concentrator Spectrometer (LECS) observation of the supersoft source SMC 13 (=1E 0035.4-7230) in the Small Magellanic Cloud are reported. The BeppoSAX spectrum is fitted either with a blackbody spectrum with an effective temperature kT = 26-58 eV, an LTE white dwarf atmosphere spectrum with kT = 35-50 eV, or a non-LTE white dwarf atmosphere spectrum with kT = 25-32 eV. The bolometric luminosity is < 8 10^37 erg s-1 and < 3 10^37 erg s^-1 for the LTE and the non-LTE spectrum. We also applied a spectral fit to combined spectra obtained with BeppoSAX LECS and with ROSAT PSPC. The kT derived for the non-LTE spectrum is 27-29 eV, the bolometric luminosity is 1.1-1.2 10^37 erg s^-1. We can exclude any spectrally hard component with a luminosity > 2 10^35 erg s^-1 (for a bremmstrahlung with a temperature of 0.5 keV) at a distance of 60 kpc. The LTE temperature is therefore in the range 5.5+/-0.2 10^5 K and the non-LTE temperature in the range 3.25+/-0.16 10^5 K. Assuming the source is on the stability line for atmospheric nuclear burning, we constrain the white dwarf mass from the LTE and the non-LTE fit to ~1.1 M-solar and ~0.9 M-solar respectively. However, the temperature and luminosity derived with the non-LTE model for 1E 0035.4-7230 is consistent with a lower mass M~0.6-0.7 M-solar white dwarf as predicted by Sion and Starrfield (1994). At the moment, neither of these two alternatives for the white dwarf mass can be excluded.Comment: 6 pages, accepted by A&A March 30th 199

Luminous supersoft X-ray emission from the recurrent nova U Scorpii

BeppoSAX detected luminous 0.2-2.0 keV supersoft X-ray emission from the recurrent nova U Sco ~19-20 days after the peak of the optical outburst in February 1999. U Sco is the first recurrent nova to be observed during a luminous supersoft X-ray phase. Non-LTE white dwarf atmosphere spectral models (together with a ~0.5 keV optically thin thermal component) were fitted to the BeppoSAX spectrum. We find that the fit is acceptable assuming enriched He and an enhanced N/C ratio. This implies that the CNO cycle was active during the outburst, in agreement with a thermonuclear runaway scenario. The best-fit temperature is ~9 10^5 K and the bolometric luminosity those predicted for steady nuclear burning on a WD close to the Chandrasekhar mass. The fact that U~Sco was detected as a supersoft X-ray source is consistent with steady nuclear burning continuing for at least one month after the outburst. This means that only a fraction of the previously accreted H and He was ejected during the outburst and that the WD can grow in mass, ultimately reaching the Chandrasekhar limit. This makes U~Sco a candidate type Ia supernova progenitor.Comment: 4 pages, accepted by A&A Letters 15 June 199

A BeppoSAX observation of the super-soft source CAL87

We report on a BeppoSAX Concentrator Spectrometer observation of the super-soft source (SSS) CAL87. The X-ray emission in SSS is believed to arise from nuclear burning of accreted material on the surface of a white dwarf (WD). An absorbed blackbody spectral model gives a chi^2_v of 1.18 and a temperature of 42 +/- ^13 _11 eV. However, the derived luminosity and radius are greater than the Eddington limit and radius of a WD. Including an O viii edge at 0.871 keV gives a significantly better fit (at > 95% confidence) and results in more realistic values of the source luminosity and radius. We also fit WD atmosphere models to the CAL87 spectrum. These also give reasonable bolometric luminosities and radii in the ranges 2.7-4.8 10^{36} erg/s and 8-20 10^7 cm, respectively. These results support the view that the X-ray emission from CAL87 results from nuclear burning in the atmosphere of a WD.Comment: 4 pages. Accepted for publication in A&A (Letters

Discovery of complex narrow X-ray absorption features from the low-mass X-ray binary GX13+1 with XMM-Newton

We report the detection of a complex of narrow X-ray absorption features from the low-mass X-ray binary GX13+1 during 3 XMM-Newton observations in 2000 March and April. The features are consistent with being due to resonant scattering of the K_alpha and K_beta lines of He- and H-like iron (Fe xxv and Fe xxvi) and H-like calcium (Ca xx) K_alpha. Only the Fe xxvi K_alpha line has been previously observed from GX13+1. Due to the closeness in energy the Fe xxv and Fe xxvi K_beta features may also be ascribed to Nixxvii and Nixxviii K_alpha, respectively. We also find evidence for the presence of a deep (tau~0.2) Fe xxv absorption edge at 8.83 keV. The equivalent widths of the lines do not show any obvious variation on a timescale of a few days suggesting that the absorbing material is a stable feature of the system and present during a range of orbital phases

Discovery of a ~1 Hz quasi-periodic oscillation in the low-mass X-ray binary 4U 1746-37

We have discovered a ~1 Hz quasi-periodic oscillation (QPO) in the persistent X-ray emission and during type I X-ray bursts of the globular cluster source, dipper and low-mass X-ray binary (LMXB) 4U 1746-37. The QPO properties resemble those of QPOs found recently in the LMXB dippers 4U 1323-62, and EXO 0748-676, which makes 4U 1746-37 the third source known to exhibit this type of QPOs. We present evidence for X-ray spectral changes in this source similar to those observed in LMXBs referred to as atoll sources. We detect two states, a low intensity and spectrally hard state, and a higher intensity and spectrally soft state. This may explain the different spectral characteristics reported for 4U 1746-37 earlier. The high intensity state resembles the banana branch state of atoll sources. The QPOs are only seen in the low intensity state, and are absent when the source is in the banana branch. This strongly suggests that either the accretion disk or an extended central source change shape between the low intensity state and the banana branch. Twelve bursts were detected, of which 5 took place while the source was on the banana branch and 7 when the source was in the low intensity state. The bursts occurring on the banana branch had an e-folding time ~3 times longer than those which occurred in the low intensity state. Whereas previously detected dips showed only a decrease in count rate of ~15%, we found in one observation a dip in which the count rate dropped from ~200 counts per second to ~20 counts per second. This dip lasted only ~250 seconds, during which clear spectral hardening occured. This is the first time strong evidence for spectral changes during a dip are reported for this source.Comment: 17 pages, accepted for publication in Ap

Socioeconomic indicators of health inequalities and female mortality: a nested cohort study within the United Kingdom Collaborative Trial of Ovarian Cancer Screening (UKCTOCS)

Evidence is mounting that area-level socioeconomic indicators are important tools for predicting health outcomes. However, few studies have examined these alongside individual-level education. This nested cohort study within the control arm of the United Kingdom Collaborative Trial of Ovarian Cancer Screening (UKCTOCS) assesses the association of mutually adjusted individual (education) and area-level (Index of Multiple Deprivation-IMD 2007) socioeconomic status indicators and all-cause female mortality

An X-Ray Dip in the X-Ray Transient 4U 1630-47

An x-ray dip was observed during a 1996 Rossi X-Ray Timing Explorer observation of the recurrent x-ray transient 4U 1630-47. During the dip, the 2-60 keV x-ray flux drops by a factor of about three, and, at the lowest point of the dip, the x-ray spectrum is considerably softer than at non-dip times. We find that the 4U 1630-47 dip is best explained by absorption of the inner part of an accretion disk, while the outer part of the disk is unaffected. The spectral evolution during the dip is adequately described by the variation of a single parameter, the column density obscuring the inner disk.Comment: 13 pages, 4 figures, Accepted for publication in Ap

The Off State of GX 339-4

We report BeppoSAX and optical observations of the black hole candidate GX 339-4 during its X-ray `off' state in 1999. The broad-band (0.8-50 keV) X-ray emission can be fitted by a single power law with spectral index, \alpha ~1.6. The observed luminosity is 6.6e33 erg s^{-1} in the 0.5-10 keV band, which is at the higher end of the flux distribution of black hole soft X-ray transients in quiescence, comparable to that seen in GS 2023+338 and 4U 1630-47. An optical observation just before the BeppoSAX observation shows the source to be very faint at these wavelengths as well (B=20.1, V=19.2). By comparing with previously reported `off' and low states (LS), we conclude that the `off' state is actually an extension of the LS, i.e. a LS at lower intensities. We propose that accretion models such as the advection-dominated accretion flows are able to explain the observed properties in such a state.Comment: Accepted for publication in MNRA