Spectroscopic observations of the eclipsing Polar MN Hya (RX J0929--24)

We present low--medium resolution optical spectroscopy of the eclipsing AM Her system MN Hya (RX J0929--24). We determine the magnetic field strength at the primary accretion region of the white dwarf to be 42MG from the spacing of cyclotron features visible during ~0.4--0.7. From spectra taken during the eclipse we find that the secondary has a M3--4 spectral type. Combined with the eclipse photometry of Sekiguchi, Nakada & Bassett and an estimate of the interstellar extinction we find a distance of ~300--700pc. We find unusual line variations at phase ~0.9: Halpha is seen in absorption and emission. This is at the same point in the orbital phase that a prominent absorption dip is seen in soft X-rays.Comment: accepted by MNRAS, 7 pages, 5 fig

Economic geology of the Avoca mineralized belt S.E. Ireland and Parys Mountain, Anglesey

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The X-ray eclipse of OY Car resolved with XMM-Newton: X-ray emission from the polar regions of the white dwarf

We present the XMM-Newton X-ray eclipse lightcurve of the dwarf nova OY Car. The eclipse ingress and egress are well resolved for the first time in any dwarf nova placing strong constraints on the size and location of the X-ray emitting region. We find good fits to a simple linear eclipse model, giving ingress/egress durations of 30+/-3 sec. Remarkably this is shorter than the ingress/egress duration of the sharp eclipse in the optical as measured by Wood et al. (1989) and ascribed to the white dwarf (43+/-2 sec). We also find that the X-ray eclipse is narrower than the optical eclipse by 14+/-2 sec, which is precisely the difference required to align the second and third contact points of the X-ray and optical eclipses. We discuss these results and conclude that X-ray emission in OY Car most likely arises from the polar regions of the white dwarf. Our data were originally reported by Ramsay et al (2001), but they did not make a quantitative measurement of eclipse parameters. We have also corrected important timing anomalies present in the data available at that time.Comment: 6 pages, 5 figures; accepted for publication in MNRA

Disorder effect in low dimensional superconductors

The quasiparticle density of states (DOS), the energy gap, the superfluid density $\rho_s$, and the localization effect in the s- and d-wave superconductors with non-magnetic impurity in two dimensions (2D) are studied numerically. For strong (unitary) scatters, we find that it is the range of the scattering potential rather than the symmetry of the superconducting pairing which is more important in explaining the impurity dependences of the specific heat and the superconducting transition temperature in Zn doped YBCO. The localization length is longer in the d-wave superconducting state than in the normal state, even in the vicinity of the Fermi energy.Comment: 2 pages, uuencoded compressed postscript file, IRC-940610

ROSAT observations of V471 Tauri, showing that stellar activity is determined by rotation, not age

I present pointed ROSAT PSPC observations of the pre-cataclysmic binary V471 Tauri. The hard X-ray emission (>0.4keV) is not eclipsed by the K star, demonstrating conclusively that this component cannot be emitted by the white dwarf. Instead I show that its spectrum and luminosity are consistent with coronal emission from the tidally spun-up K star. The star is more active than other K stars in the Hyades, but equally active as K stars in the Pleiades with the same rotation periods, demonstrating that rotation--and not age--is the key parameter in determining the level of stellar activity. The soft X-ray emission (<0.4keV) is emitted predominately by the white dwarf and is modulated on its spin period. I find that the pulse-profile is stable on timescales of hours and years, supporting the idea that it is caused by opacity of accreted material. The profile itself shows that the magnetic field configuration of the white dwarf is dipolar and that the magnetic axis passes through the centre of the star. There is an absorption feature in the lightcurve of the white dwarf, which occurs at a time when our line-of-sight passes within a stellar radius of the K star. The column density and duration of this feature imply a volume and mass for the absorber which are similiar to those of coronal mass ejections of the Sun.Comment: 7 pages, 7 figures, accepted for publication in MNRAS. Also available at http://www.star.le.ac.uk/~pjw