No abstract

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.

We present multiwavelength observations of the newly discovered X-ray transient XTE J1118+480 obtained in the rising phase of the 2000 April outburst. This source is located at unusually high Galactic latitude and in a very low absorption line of sight. This made the first EUVE spectroscopy of an X-ray transient outburst possible. Together with our HST, RXTE, and UKIRT data this gives unprecedented spectral coverage. We find the source in the low hard state. The flat IR-UV continuum appears to be a combination of optically thick disk emission and possibly synchrotron, while at higher energies, including EUV, a typical low hard state power-law is seen. EUVE observations reveal no periodic modulation, suggesting an inclination low enough that no obscuration by the disk rim occurs. We discuss the nature of the source and this outburst and conclude that it may be more akin to mini-outbursts seen in GRO J0422+32 than to a normal X-ray transient outburst.

No abstract

Using data obtained in 1994 June/July with the Extreme Ultraviolet Explorer deep survey photometer and in 2001 January with the Chandra X-ray Observatory Low Energy Transmission Grating Spectrograph, we investigate the extreme-ultraviolet (EUV) and soft X-ray oscillations of the dwarf nova SS Cyg in outburst. We find quasi-periodic oscillations (QPOs) at ν 0 ≈ 0.012 Hz and ν 1 ≈ 0.13 Hz in the EUV flux and at ν 0 ≈ 0.0090 Hz, ν 1 ≈ 0.11 Hz, and possibly ν 2 ≈ ν 0 +ν 1 ≈ 0.12 Hz in the soft X-ray flux. These data, combined with the optical data of Woudt & Warner for VW Hyi, extend the Psaltis, Belloni, & van der Klis ν high -ν low correlation for neutron star and black hole low-mass X-ray binaries (LMXBs) nearly two orders of magnitude in frequency, with ν low ≈ 0.08 ν high . This correlation identifies the high-frequency quasi-coherent oscillations (so-called "dwarf nova oscillations") of cataclysmic variables (CVs) with the kilohertz QPOs of LMXBs, and the low-frequency QPOs of CVs with the horizontal branch oscillations (or the broad noise component identified as such) of LMXBs. Assuming that the same mechanisms produce the QPOs in white dwarf, neutron star, and black hole binaries, we find that the data exclude the relativistic precession model and the magnetospheric and sonic-point beat-frequency models (as well as any model requiring the presence or absence of a stellar surface or magnetic field); more promising are models that interpret QPOs as manifestations of disk accretion onto any low-magnetic field compact object.