The X-ray and extreme-ultraviolet flux evolution of SS Cygni throughout outburst

We present the most complete multiwavelength coverage of any dwarf nova outburst: simultaneous optical, Extreme Ultraviolet Explorer and Rossi X-ray Timing Explorer observations of SS Cygni throughout a narrow asymmetric outburst. Our data show that the high-energy outburst begins in the X-ray waveband 0.9–1.4 d after the beginning of the optical rise and 0.6 d before the extreme-ultraviolet rise. The X-ray flux drops suddenly, immediately before the extreme-ultraviolet flux rise, supporting the view that both components arise in the boundary layer between the accretion disc and white dwarf surface. The early rise of the X-ray flux shows that the propagation time of the outburst heating wave may have been previously overestimated.
The transitions between X-ray and extreme-ultraviolet dominated emission are accompanied by intense variability in the X-ray flux, with time-scales of minutes. As detailed by Mauche & Robinson, dwarf nova oscillations are detected throughout the extreme-ultraviolet outburst, but we find they are absent from the X-ray light curve.
X-ray and extreme-ultraviolet luminosities imply accretion rates of  3 × 10¹⁵ g s⁻¹  in quiescence,  1 × 10¹⁶ g s⁻¹  when the boundary layer becomes optically thick, and  ∼10¹⁸ g s⁻¹  at the peak of the outburst. The quiescent accretion rate is two and a half orders of magnitude higher than predicted by the standard disc instability model, and we suggest this may be because the inner accretion disc in SS Cyg is in a permanent outburst state.     

A Time Evolution Study of the Superhumps of the Dwarf Nova 1RXS J232953.9+062814 by Wavelet Transform

The time evolution behaviour of the superhumps of the dwarf nova1RXS J232953.9+ 062814 is investigated with the wavelet analysis method. On the basis of two nights CCD photometry performed during its first superoutburst as well as other published brightness data, we reveal the superhump's time-dependence as a function of periods and time. Our light curves, which phased in the rapid decay ending portion of the superoutburst and in the dawn of a following normal outburst, are important to help trace the superhump evolution for the star. Both the superhump period and the orbital period of the binary system are detected in the present data. We obtain P _sh=0.0458±0.0002d and P _orb=0.0450±0.0002d. They agree with those existing values. The two periods exchanged their roles during the superhump evolution. The general profile of brightness fading over the outbursts roughly followed an exponential decay law or a form of a five-order polynomial. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

TRIFFID observations of the cores of the three globular clusters M15, M92 and NGC 6712

We present the results of TRIFFID simultaneous V - and B -band observations of the cores of the globular clusters M15, M92 and NGC 6712. A variability search of their dense centres was made feasible through performing post-exposure image sharpening on the images, increasing the image resolution by a factor of ∼2. The isis implementation of the image subtraction technique developed by Alard & Lupton was then used to detect flux variations in our image sets. We have obtained periods for all observable variables (in our field of view) in NGC 6712 and we have found two new RR Lyrae variables (an RRab and an RRc). We have confirmed three variables in our field of view of the M92. For M15, we detect 48 variables in our field of view, 23 of which are new discoveries. We obtain periods and amplitudes for all variables and classify new ones based on the light-curve shape, the most significant period and the mean magnitude in the V band. Among the detected RR Lyrae we find 19 RRc, 12 RRab and two RRd types. In the subsequent analysis we find a marked increase in RRc over RRab variables in the core. In a refined procedure to search for fainter objects we find no dwarf novae in our field of view of M15. Simulations performed on the data set to quantify our sensitivity to such objects indicate that an upper limit of 10 dwarf novae (at 92 per cent probability) exist in our field of view. The implications this result has on globular clusters are discussed.     

The tidal stripping of satellites

We present an improved analytic calculation for the tidal radius of satellites and test our results against N -body simulations.
The tidal radius in general depends upon four factors: the potential of the host galaxy, the potential of the satellite, the orbit of the satellite and the orbit of the star within the satellite . We demonstrate that this last point is critical and suggest using three tidal radii to cover the range of orbits of stars within the satellite. In this way we show explicitly that prograde star orbits will be more easily stripped than radial orbits; while radial orbits are more easily stripped than retrograde ones. This result has previously been established by several authors numerically, but can now be understood analytically. For point mass, power-law (which includes the isothermal sphere), and a restricted class of split power-law potentials our solution is fully analytic. For more general potentials, we provide an equation which may be rapidly solved numerically.
Over short times (≲1–2 Gyr ∼1 satellite orbit), we find excellent agreement between our analytic and numerical models. Over longer times, star orbits within the satellite are transformed by the tidal field of the host galaxy. In a Hubble time, this causes a convergence of the three limiting tidal radii towards the prograde stripping radius. Beyond the prograde stripping radius, the velocity dispersion will be tangentially anisotropic.     

Dwarf galaxies in Clusters

The equilibrium of a self gravitating cylindrical polytrope with a general magnetic field and rotation has been discussed. This revised version was published online in July 2006 with corrections to the Cover Date.