The Structure of Stellar Coronae in Active Binary Systems

A survey of 28 stars using EUV spectra has been conducted to establish the structure of stellar coronae in active binary systems from the EMD, electron densities, and scale sizes. Observations obtained by the EUVE during 9 years of operation are included for the stars in the sample. EUVE data allow a continuous EMD to be constructed in the range log T~5.6-7.4, using iron emission lines. These data are complemented with IUE observations to model the lower temperature range. Inspection of the EMD shows an outstanding narrow enhancement, or ``bump'' peaking around log T~6.9 in 25 of the stars, defining a fundamental coronal structure. The emission measure per unit stellar area decreases with increasing orbital (or photometric) periods of the target stars; stars in binaries generally have more material at coronal temperatures than slowly rotating single stars. High electron densities (Ne>10^12 cm^-3) are derived at ~10 MK for some targets, implying small emitting volumes. The observations suggest the magnetic stellar coronae of these stars are consistent with two basic classes of magnetic loops: solar-like loops with maximum temperature around log T~6.3 and lower electron densities (Ne>10^9-10.5), and hotter loops peaking around log T~6.9 with higher electron densities (Ne>10^12). For the most active stars, material exists at much higher temperatures (log T>6.9) as well. However, current ab initio stellar loop models cannot reproduce such a configuration. Analysis of the light curves of these systems reveals signatures of rotation of coronal material, as well as apparent seasonal changes in the activity levels.Comment: 45 pages, 9 figures (with 20 eps files). Accepted for its publication in ApJ

On the binary nature of 1RXS J162848.1-415241

We present spectroscopy of the optical counterpart to 1RXS J162848.1-41524, also known as the microquasar candidate MCQC J162847-4152. All the data indicate that this X-ray source is not a microquasar, and that it is a single-lined chromospherically active binary system with a likely orbital period of 4.9 days. Our analysis supports a K3IV spectral classification for the star, which is dominant at optical wavelengths. The unseen binary component is most likely a late-type (K7-M) dwarf or a white dwarf. Using the high resolution spectra we have measured the K3 star's rotational broadening to be vsini = 43 +/- 3 km/s and determined a lower limit to the binary mass ratio of q(=M2/M1)>2.0. The high rotational broadening together with the strong CaII H & K / Halpha emission and high-amplitude photometric variations indicate that the evolved star is very chromospherically active and responsible for the X-ray/radio emission.Comment: 15 pages, 5 figures, accepted for publication in Ap

The U.K. Deep and Medium Surveys with Rosat: log N-Log S relation

We have carried out a soft X-ray survey of the sky using the ROSAT Position Sensitive Proportional Counter (PSPC) in a region of very low Galactic column density (NH = 6–9 × 1019 cm–2). The data consist of a deep > 70 ks pointing (the Deep Survey) and six pointings at lower sensitivity (13–20 ks; the Medium Survey). We detect a total of 141 sources over the 0.9 deg2 of sky area used. The faintest source detected has a flux of 3.2 × 10–15 erg cm–2 s–1 (0.5–2.0 keV). We present the source number-flux distribution, and we compare it with the distributions constructed from other ROSAT observations and from surveys carried out with X-ray instruments operating at higher energies. We estimate the contribution of the resolved sources to the soft X-ray background by direct comparison of their integrated spectrum with that of the diffuse background: 44 per cent of the extragalactic X-ray background between 0.5 and 2.0 keV is resolved directly into discrete sources whose average spectrum is steeper than that of the background. Integration of our number-flux relation to infinite flux produces a background resolved fraction of 44±565 per cent (0.5–2.0 keV). Limits on the slope of the log N–log S curve at very faint fluxes are set by the measured value of the X-ray background.KOM, ACF and RGM acknowledge the support of the Royal Society.Peer Reviewe

Limits on the spin-dependent WIMP-nucleon cross-sections from the first science run of the ZEPLIN-III experiment

We present new experimental constraints on the WIMP-nucleon spin-dependent elastic cross-sections using data from the first science run of ZEPLIN-III, a two-phase xenon experiment searching for galactic dark matter WIMPs based at the Boulby mine. Analysis of $\sim$450 kg$\cdot$days fiducial exposure revealed a most likely signal of zero events, leading to a 90%-confidence upper limit on the pure WIMP-neutron cross-section of $\sigma_n=1.8\times 10^{-2}$ pb at 55 GeV/$c^2$ WIMP mass. Recent calculations of the nuclear spin structure based on the Bonn CD nucleon-nucleon potential were used for the odd-neutron isotopes $^{129}$Xe and $^{131}$Xe. These indicate that the sensitivity of xenon targets to the spin-dependent WIMP-proton interaction is much lower than implied by previous calculations, whereas the WIMP-neutron sensitivity is impaired only by a factor of $\sim$2

Radio Astrometry of The Close Active Binary Hr 5110

The close active binary HR 5110 was observed at six epochs over 26 days using a global very long baseline interferometry array at 15.4 GHz. We used phase referencing to determine the position of the radio centroid at each epoch with an uncertainty significantly smaller than the component separation. After correcting for proper motion and parallax, we find that the centroid locations of all six epochs have barycenter separations consistent with an emission source located on the KIV secondary, and not in an interaction region between the stars or on the F primary. We used a homogeneous power-law gyrosynchrotron emission model to reproduce the observed flux densities and fractional circular polarization. The resulting ranges of mean magnetic field strength and relativistic electron densities are of the order of 10 G and 105cm-3, respectively, in the source region

A Multi-Wavelength Perspective of Flares on HR 1099: Four Years of Coordinated Campaigns

We report on four years of multiple wavelength observations of the RS CVn system V711 Tau (HR 1099) from 1993, 1994, 1996, and 1998. This combination of radio, ultraviolet, extreme ultraviolet, and X-ray observations allows us to view, in the most comprehensive manner currently possible, the coronal and upper atmospheric variability of this active binary system. We report on the changing activity state of the system as recorded in the EUV and radio across the four years of the observations, and study the high energy variability using an assemblage of X-ray telescopes. (Longer abstract in paper).Comment: manuscript is 110 pages in length; 36 figures tota