First Simultaneous Optical and EUV Observations of the Quasi-Coherent Oscillations of SS Cygni

Using EUV photometry obtained with the Extreme Ultraviolet Explorer (EUVE) satellite and UBVR optical photometry obtained with the 2.7-m telescope at McDonald Observatory, we have detected quasi-coherent oscillations (so-called ``dwarf nova oscillations'') in the EUV and optical flux of the dwarf nova SS Cygni during its 1996 October outburst. There are two new results from these observations. First, we have for the first time observed ``frequency doubling:'' during the rising branch of the outburst, the period of the EUV oscillation was observed to jump from 6.59 s to 2.91 s. Second, we have for the first time observed quasi-coherent oscillations simultaneously in the optical and EUV. We find that the period and phase of the oscillations are the same in the two wavebands, finally confirming the long-held assumption that the periods of the optical and EUV/soft X-ray oscillations of dwarf novae are equal. The UBV oscillations can be simply the Rayleigh-Jeans tail of the EUV oscillations if the boundary layer temperature kT_bb <~ 15 eV and hence the luminosity L_bb >~ 1.2e34 (d/75 pc)^2 erg/s (comparable to that of the accretion disk). Otherwise, the lack of a phase delay between the EUV and optical oscillations requires that the optical reprocessing site lies within the inner third of the accretion disk. This is strikingly different from other cataclysmic variables, where much or all of the disk contributes to the optical oscillations.Comment: 16 pages including 3 tables and 4 encapsulated postscript figures; LaTeX format, uses aastex.cls; accepted on 2001 August 2 for publication in The Astrophysical Journa

Correlation of the Quasi-Periodic Oscillation Frequencies of White Dwarf, Neutron Star, and Black Hole Binaries

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 nu_0 ~ 0.012 Hz and nu_1 ~ 0.13 Hz in the EUV flux and at nu_0 ~ 0.0090 Hz, nu_1 ~ 0.11 Hz, and possibly nu_2 ~ nu_0 + nu_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 nu_high-nu_low correlation for neutron star and black hole low-mass X-ray binaries (LMXBs) nearly two orders of magnitude in frequency, with nu_low ~ 0.08 nu_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.Comment: 15 pages including 4 encapsulated postscript figures; LaTeX format, uses aastex.cls; accepted on 2002 July 23 for publication in The Astrophysical Journa

ORFEUS II Far-UV Spectroscopy of AM Herculis

Six high-resolution (\lambda/\Delta\lambda ~ 3000) far-UV (\lambda\lambda = 910-1210 \AA) spectra of the magnetic cataclysmic variable AM Herculis were acquired in 1996 November during the flight of the ORFEUS-SPAS II mission. AM Her was in a high optical state at the time of the observations, and the spectra reveal emission lines of O VI \lambda\lambda 1032, 1038, C III \lambda 977, \lambda 1176, and He II \lambda 1085 superposed on a nearly flat continuum. Continuum flux variations can be described as per Gansicke et al. by a ~ 20 kK white dwarf with a ~ 37 kK hot spot covering a fraction f~0.15 of the surface of the white dwarf, but we caution that the expected Lyman absorption lines are not detected. The O VI emission lines have narrow and broad component structure similar to that of the optical emission lines, with radial velocities consistent with an origin in the irradiated face of the secondary and the accretion funnel, respectively. The density of the narrow- and broad-line regions is n_{nlr} ~ 3\times 10^{10} cm^{-3} and n_{blr} ~ 1\times 10^{12} cm^{-3}, respectively, yet the narrow-line region is optically thick in the O VI line and the broad-line region is optically thin; apparently, the velocity shear in the broad-line region allows the O VI photons to escape, rendering the gas effectively optically thin. Unexplained are the orbital phase variations of the emission-line fluxes.Comment: 15 pages, 6 Postscript figures; LaTeX format, uses aaspp4.sty; table2.tex included separately because it must be printed sideways - see instructions in the file; accepted on April 17, 1998 for publication in The Astrophysical Journa

Black Widow Pulsars: the Price of Promiscuity

The incidence of evaporating 'black widow' pulsars (BWPs) among all millisecond pulsars (MSPs) is far higher in globular clusters than in the field. This implies a special formation mechanism for them in clusters. Cluster MSPs in wide binaries with WD companions exchange them for turnoff-mass stars. These new companions eventually overflow their Roche lobes because of encounters and tides. The millisecond pulsars eject the overflowing gas from the binary, giving mass loss on the binary evolution timescale. The systems are only observable as BWPs at epochs where this evolution is slow, making the mass loss transparent and the lifetime long. This explains why observed BWPs have low-mass companions. We suggest that at least some field BWPs were ejected from globular clusters or entered the field population when the cluster itself was disrupted.Comment: 6 pages, 2 figures, MNRAS in pres

Implications of the HST/FGS parallax of SS Cygni on the disc instability model

We analyse the consequences of the recently measured parallax of SS Cygni (Harrison et al. 1999) on the accretion disc limit cycle model. Using the observed long term light curve of SS Cyg and d=166 pc, we obtain for the mean mass transfer rate 4.2*10^(17)g/s. In addition, we calculate the vertical structure of the accretion disc taking into account heating of the outer disc by the stream impact. Comparing the mean accretion rate derived from the observations with the calculated critical mass transfer rate, we find that the disc instability model disagrees with the observed long term light curve of SS Cyg as the mean mass transfer rate is greater or similar to the critical mass transfer rate. The failure of the model indicated by this result can be confirmed by considering that the accretion rate at the onset of the decline should be exactly equal to the value critical for stability. In contrast to this prediction of the model, we find that the accretion rate required to explain the observed visual magnitude at the onset of the decline must be significantly higher than the critical mass transfer rate. Our results strongly suggest that either the usually assumed temperature dependence of the viscosity parameter alpha is not a realistic description of the disc viscosity, that the mass transfer rate in SS Cyg noticeably increases during the outbursts or, finally, that the HST distance of 166 pc, is too high.Comment: 6 pages, 4 figures, accepted for publication in A&

Dynamics of Line-Driven Winds from Disks in Cataclysmic Variables. II. Mass Loss Rates and Velocity Laws

We analyze the dynamics of 2D stationary line-driven winds from accretion disks in cataclysmic variables (CVs), by generalizing the Castor, Abbott and Klein theory. In paper 1, we have solved the wind Euler equation, derived its two eigenvalues, and addressed the solution topology and wind geometry. Here, we focus on mass loss and velocity laws. We find that disk winds, even in luminous novalike variables, have low optical depth, even in the strongest driving lines. This suggests that thick-to-thin transitions in these lines occur. For disks with a realistic radial temperature, the mass loss is dominated by gas emanating from the inner decade in r. The total mass loss rate associated with a luminosity 10 Lsun is 10^{-12} Msun/yr, or 10^{-4} of the mass accretion rate. This is one order of magnitude below the lower limit obtained from P Cygni lines, when the ionizing flux shortwards of the Lyman edge is supressed. The difficulties with such small mass loss rates in CVs are principal, and confirm our previous work. We conjecture that this issue may be resolved by detailed nonLTE calculations of the line force within the context of CV disk winds, and/or better accounting for the disk energy distribution and wind ionization structure. We find that the wind velocity profile is well approximated by the empirical law used in kinematical modeling. The acceleration length scale is given by the footpoint radius of the wind streamline in the disk. This suggests an upper limit of 10 Rwd to the acceleration scale, which is smaller by factors of a few as compared to values derived from line fitting.Comment: 14 pages, 3 Postscript figures, also from http://www.pa.uky.edu/~shlosman/publ.html. Astrophysical Journal, submitte

A Luminous Recurrent Supersoft X-ray Source in NGC 300

We report the results of XMM-Newton observations for an especially luminous supersoft X-ray source (SSS) with bolometric luminosity of 10^39 erg/s in the spiral galaxy NGC 300. The source was detected as a SSS in 1992 and disappeared in subsequent X-ray observations. The source was active again during recent XMM-Newton observations. It appeared to be very soft (kT~60 eV) and very luminous (~10^38 - 10^39 erg/s). The two XMM-Newton observations also reveal that the source went from a ``high'' state to a ``low'' state in 6 days. We also found a 5.4-hr periodicity during the ``low'' state. We consider white dwarf, black hole, and neutron star models to explain the nature of the source.Comment: 5 pages, 3 figures, ApJL accepte

Discovery of a new cataclysmic variable through optical variability and X-ray emission

Aims: We present discovery observations of the new cataclysmic variable star (CV) 1RXS J092737.4-191529, as well as spectra and photometry of SY Vol. The selection technique that turned up these two CVs is described; it should be efficient for finding dwarf novae with high outburst duty cycles. Methods: Two very common observational features of CVs, namely optical variability and X-ray emission, are combined to select targets for follow-up observations. Long-slit spectra were taken to identify CVs in the sample. Results: Two out of three objects selected in this way are CVs. One of these is the known dwarf nova SY Vol, while the second system, 1RXS J092737.4-191529, is a new discovery. We present medium resolution spectra, $UBVRI$ magnitudes, and high-speed photometry for both these CVs. Rapid flickering in the light curve of 1RXS J092737.4-191529 confirms the mass transferring binary nature of this object; it is probably a dwarf nova that was in quiescence during our observations.Comment: 4 pages, 5 figures, accepted to A&

Chandra and FUSE spectroscopy of the hot bare stellar core H1504+65

H1504+65 is an extremely hot hydrogen-deficient white dwarf with an effective temperature close to 200,000 K. We present new FUV and soft X-ray spectra obtained with FUSE and Chandra, which confirm that H1504+65 has an atmosphere primarily composed of carbon and oxygen. The Chandra LETG spectrum (60-160 Angstroem) shows a wealth of photospheric absorption lines from highly ionized oxygen, neon, and - for the first time identified in this star - magnesium and suggests relatively high Ne and Mg abundances. This corroborates an earlier suggestion that H1504+65 represents a naked C/O stellar core or even the C/O envelope of an O-Ne-Mg white dwarf.Comment: 15 pages, 10 figures, accepted for publication in A&

The X-ray eclipse of the dwarf nova HT CAS observed by the XMM-Newton satellite: spectral and timing analysis

A cataclysmic variable is a binary system consisting of a white dwarf that accretes material from a secondary object via the Roche-lobe mechanism. In the case of long enough observation, a detailed temporal analysis can be performed, allowing the physical properties of the binary system to be determined. We present an XMM-Newton observation of the dwarf nova HT Cas acquired to resolve the binary system eclipses and constrain the origin of the X-rays observed. We also compare our results with previous ROSAT and ASCA data. After the spectral analysis of the three EPIC camera signals, the observed X-ray light curve was studied with well known techniques and the eclipse contact points obtained. The X-ray spectrum can be described by thermal bremsstrahlung of temperature $kT_1=6.89 \pm 0.23$ keV plus a black-body component (upper limit) with temperature $kT_2=30_{-6}^{+8}$ eV. Neglecting the black-body, the bolometric absorption corrected flux is $F^{\rm{Bol}}=(6.5\pm 0.1)\times10^{-12}$ erg s$^{-1}$ cm$^{-2}$, which, for a distance of HT Cas of 131 pc, corresponds to a bolometric luminosity of $(1.33\pm 0.02)\times10^{31}$ erg s$^{-1}$. The study of the eclipse in the EPIC light curve permits us to constrain the size and location of the X-ray emitting region, which turns out to be close to the white dwarf radius. We measure an X-ray eclipse somewhat smaller (but only at a level of $\simeq 1.5 \sigma$) than the corresponding optical one. If this is the case, we have possibly identified the signature of either high latitude emission or a layer of X-ray emitting material partially obscured by an accretion disk.Comment: Accepted for publication on Astronomy and Astrophysics, 200