Pc1-2 Discrete regular daytime pulsation bursts at high latitudes

A family of related Pc1-2 (0.2–10 s) discrete daytime geomagnetic pulsations is presented using pulsation data obtained at Davis, Antarctica, a typical polar-cap station. The morphological properties of IPRP and Pclb pulsation regimes, which maximize in amplitude and frequency of occurrence under the projection of the polar cusp, are examined. Furthermore, two other variations of discrete pulsation bursts yet to be named are also presented, viz IPFP (Intervals of Pulsations with Falling Period) and IPAP (Intervals of Pulsations with Alternating Period) which are observed on rare occasions. It is also suggested that the Pc1b (0.2–5 s) should be extended to incorporate Pc2b (5–10 s) which from the results in this paper are physically the same phenomenon and could be collectively classified as IPCP (Intervals of Pulsations with Constant Period).     

The NAIAD experiment for WIMP searches at Boulby mine and recent results

The NAIAD experiment (NaI Advanced Detector) for weakly interacting massive particle (WIMP) dark matter searches at Boulby mine (UK) is described. The detector consists of an array of encapsulated and unencapsulated NaI(Tl) crystals with high light yield. Six crystals are collecting data at present. Data accumulated by four of them (10.6 kg × year exposure) have been used to set upper limits on the WIMP–nucleon spin-independent and WIMP–proton spin-dependent cross-sections. Pulse shape analysis has been applied to discriminate between nuclear recoils, as may be caused by WIMP interactions, and electron recoils due to gamma background. Various calibrations of crystals are presented.     

Theoretical and experimental lifetime and oscillator strength determination in singly ionized neodymium (Nd ii)

Radiative lifetime measurements were performed with time-resolved laser-induced fluorescence techniques for 24 levels of Nd  ii in the energy range 20 500–32 500 cm⁻¹. For 17 levels, no previous experimental data exist. These results have allowed the testing of new theoretical calculations with the relativistic Hartree–Fock method taking configuration interactions and core-polarization effects into account, and a satisfying agreement has been found for this complex ion. A new set of calculated oscillator strengths, accurate within a few per cent for the strongest transitions, is presented for 107 lines of astrophysical interest appearing in the wavelength range 358.0–1100.0 nm. These results will be useful to evaluate abundance values of neodymium in chemically peculiar stars in relation with cosmochronology.     

Circular Polarisation Observations of SS433

We present four epochs of observations of the Galactic X-ray binarySS433 using the Australian Telescope Compact Array (ATCA). These observations, taken on 4 February, 29 February, 23 March and 27 June 2000, at four frequencies ranging from 1–9 Ghz, are used to monitor the circular polarised emission from the source. We obtain circular polarisation spectra for the first three epochs with spectralindices α, (m _c ∝ν ^α), equal to 0.07±0.10, –1.07±0.09 and –0.18±0.05 and also observe a sign change in Stokes V. This change of handedness may indicate along-term restructuring of the magnetic field.     

X-ray and ultraviolet observations of the dwarf nova VW Hyi in quiescence

We present an analysis of X-ray and ultraviolet (UV) data of the dwarf nova VW Hyi that were obtained with XMM–Newton during the quiescent state. The X-ray spectrum indicates the presence of an optically thin plasma in the boundary layer that cools as it settles on to the white dwarf. The plasma has a continuous temperature distribution that is well described by a power law or a cooling flow model with a maximum temperature of 6–8 keV. We estimate from the X-ray spectrum a boundary layer luminosity of  8 × 10³⁰ erg s^-1  , which is only 20 per cent of the disc luminosity. The rate of accretion on to the white dwarf is  5 × 10⁻¹² M_⊙ yr⁻¹  , about half of the rate in the disc. From the high-resolution X-ray spectra, we estimate that the X-ray emitting part of the boundary layer is rotating with a velocity of 540 km s⁻¹, which is close to the rotation velocity of the white dwarf but is significantly smaller than the Keplerian velocity. We detect a 60-s quasi-periodic oscillation of the X-ray flux, which is likely to be due to the rotation of the boundary layer. The X-ray and the UV flux show strong variability on a time-scale of ∼1500 s. We find that the variability in the two bands is correlated and that the X-ray fluctuations are delayed by ∼100 s. The correlation indicates that the variable UV flux is emitted near the transition region between the disc and the boundary layer and that accretion rate fluctuations in this region are propagated to the X-ray emitting part of the boundary layer within ∼100 s. An orbital modulation of the X-ray flux suggests that the inner accretion disc is tilted with respect to the orbital plane. The elemental abundances in the boundary layer are close to their solar values.     

Tidal dwarfs in the M81 group: the second generation?

We present a quantitative star formation history derivation of the four suspected tidal dwarf galaxies in the M 81 group: Holmberg IX, BK3N,Arp-loop (A0952+69) and Garland using HST/WFPC2 images of these galaxies. We construct a library of synthetic Colour-Magnitude Diagrams(CMDs) based on theoretical isochrones and data-derived determinations of photometric errors. These synthetic CMDs were combined linearly andχ²-compared to observed photometry. All the galaxies show continuous star formation between about 20 and 200 Myr ago with star formation rates between 7.5⋅10^-3 M_⊙/yr and 7.67⋅10^-4 M_⊙/yr. The metallicity of the detected stars is spanning rather a wide range, being lower than solar abundance. We suppose, that all the galaxies were formed out of material from metal-poor outer part of the giant spiral galaxy M81after tidal interaction about 200 Myr ago. However, this suggestion requires significantly more deep color-magnitude diagrams to be sure with the scenario of the galaxy evolution. This revised version was published online in July 2006 with corrections to the Cover Date.