Janet H. Wood

THE ULTRAVIOLET PULSATIONS OF THE CATACLYSMIC VARIABLE AE AQUARII AS OBSERVED WITH THE HUBBLE SPACE TELESCOPE

We present the results of high time resolution UV spectroscopy and simultaneous high-speed UBVR photometry of AE Aqr. The UV spectra were obtained with the Faint Object Spectrograph aboard the Hubble Space Telescope (HST), and the photometry was carried out with the 82 sec telescope at McDonald Observatory. Our study focuses on the coherent 33 sceond oscillations, whose amplitude is found to be very large in the UV (40% of the mean quiescent level). The mean pulse profile has two broad unequal peaks spaced by half an oscillation cycle. The pulse profiles in the UV and optical bands appear quite similar in shape, with no discernible shifts. The orbital delay curve of the UV pulses establishes the white dwarf as their origin. The (UV+optical) spectrum of the pulsations is well described by a white dwarf atmosphere model with a temperature of about 26,000 K. We find no oscillations in the UV emission-line fluxes, nor in their velocities, down to a limit of 800 km/s. Based on the properties of the UV and optical pulsations we suggest that they originate in the X-ray heated magnetic polar caps of the white dwarf. Under this assumption we produce maximum entropy maps of the brightness distribution of the white dwarf surface. Using this model we are able to reproduce the observed mean pulse profile and interpret fluctuations in the oscillation amplitude as small fluctuations in the accretion rate. We find that the amplitudes and profiles of the pulses are not strongly affectd by the large aperiodic flares exhibited by the system. This suggests that the large flares are not related to the process of depositing material onto the white dwarf and argues against models that place their origin at the white dwarf magnetosphere.

Eclipse studies of the dwarf nova HT Cassiopeiae. I, Observations and system parameters

Optical eclipses are used to investigate the structure of the dwarf nova HT Cas. We present high-speed multicolor light curves covering four eclipses at UBR in 1982 September and 12 eclipses at UBVR in 1983 November-December. From the latter we derive high-quality mean light curves that cleanly separate the white dwarf and accretion disk eclipses. The bright spot is weak or absent during our observations, but a 0.3 mag orbital modulation, centered on mid-eclipse, is seen in the U-band. We define a purely photometric model of the system using contact timings of the white dwarf from the data presented here and measurements of the bright spot eclipses in other data (published by Patterson in 1981).

The X-Ray Eclipse of the Dwarf Nova HT Cassiopeiae: Results from ASCA and ROSAT HRI Observations

We report on a 1 day ASCA observation and ROSAT HRI observations of the eclipsing dwarf nova HT Cas. The presence of the X-ray eclipse, originally detected in a ROSAT PSPC observation, is confirmed at a much higher significance level. The quality of the ASCA light curve is high enough to allow detailed investigation of the eclipse depth, width, and shape. The eclipse is found to be deep, compatible with being total. The eclipse width is comparable to that of the white dwarf, as derived from optical light curves, and in fact may be narrower. The eclipse transition is also found to be short, which puts a limit of 1.15 times the white dwarf radius as the total size of the X-ray emission region. The out-of-eclipse spectrum of HT Cas is found to be consistent with a single temperature (kT ~ 10 keV), absorbed (NH ~ 3.3 × 1021 cm-2), thermal model.

Eclipse studies of the dwarf nova HT Cassiopeiae. II - White dwarf and accretion disk

The simultaneous UBVR light curves presented by Horne, Wood, & Stiening (1991, ApJ, 378) are used to investigate the white dwarf and accretion disk in the dwarf nova HT Cas. For the white dwarf, log g = 8 and pure hydrogen model atmospheres are used to fit the UBVR fluxes, subject to the constraint that the models are consistent with the observed UV flux. A temperature T W = 14000 ± 1000 K and a distance D = 125 ± 8 pc for E(B-V) = 0.0 are found

Hubble Space Telescope observations of the dwarf nova Z chamaeleontis through two eruption cycles

We have obtained the first high-speed photometry of the eclipsing dwarf nova Z Cha at ultraviolet wavelengths with the Hubble Space Telescope (HST). We observed the eclipse roughly every 4 days over two cycles of the normal eruptions of Z Cha, giving a uniquely complete coverage of its outburst cycle. The accretion disk dominated the ultraviolet light curve of Z Cha at the peak of an eruption; the white dwarf, the bright spot on the edge of the disk, and the boundary layer were all invisible. We were able to obtain an axisymmetric map of the accretion disk at this time only by adopting a flared disk with an opening angle of approximately 8 deg. The run of brightness temperature with radius in the disk at the peak of the eruption was too flat to be consistent with a steady state, optically thick accretion disk. The local rate of mass flow through the disk was approximately 5 x 10(exp -10) solar masses/yr near the center of the disk and approximately 5 x 10(exp -9) solar masses/yr near the outer edge. The white dwarf, the accretion disk, and the boundary layer were all significant contributors to the ultraviolet flux on the descending branches of the eruptions. The temperature of the white dwarf during decline was 18,300 K less than T(sub wd) less than 21,800 K, which is significantly greater than at minimum light. Six days after the maximum of an eruption Z Cha has faded to near minimum light at ultraviolet wavelenghts, but was still approximately 70% brighter at minimum light in the B band. About one-quarter of the excess flux in the B band came from the accretion disk. Thus, the accretion disk faded and became invisible at ultraviolet wavelengths before it faded at optical wavelenghts. The disk did, however, remain optically thick and obscured the lower half of the white dwarf at ultraviolet and possibly at optical wavelenghts for 2 weeks after the eruption ended. By the third week after eruptiuons the eclipse looked like a simple occultation of an unobscured, spherical white dwarf by a dark secondary star. The center of the accretion disk was, therfore, optically thin at ultraviolet wavelenghts and the boundary layer was too faint to be visible.

The stellar mass ratio of GK Persei

We study the absorption lines present in the spectra of the long-period cataclysmic variable GK Per during its quiescent state, which are associated with the secondary star. By comparing quiescent data with outburst spectra we infer that the donor star appears identical during the two states and the inner face of the secondary star is not noticeably irradiated by flux from the accreting regions. We obtain new values for the radial velocity semi-amplitude of the secondary star, KK ? 120:5 ^ 0:7 kms21, a projected rotational velocity, VK sin i ? 61:5 ^ 11:8 kms21; and consequently a measurement of the stellar mass ratio of GK Per, q ? MK/MWD ? 0:55 ^ 0:21. The inferred white dwarf radial velocities are greater than those measured traditionally using the wings of Doppler-broadened emission lines suspected to originate in an accretion disc, highlighting the unsuitability of emission lines for mass determinations in cataclysmic variables. We determine mass limits for both components in the binary, MK

An eclipse of the X-ray flux from the dwarf nova OY Carinae in quiescence

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Application of Realistic Model Atmospheres to Eclipse Maps of Accretion Disks: The Effective Temperature and Flare of the Disk in the Dwarf Nova Z Chamaeleontis

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Eclipse maps of the accretion disk in the classical nova V Persei

We present a phase-resolved ROSAT HRI X-ray light curve of the dwarf nova OY Car in quiescence. The X-ray flux is eclipsed at the same time as the optical eclipse of the primary, and the region of X-ray emission is comparable in size to the white dwarf. We use subsequent optical observations to update the orbital ephemeris of the system.

An XMM-Newton observation of the nova-like variable UX UMa : spatially and spectrally resolved two-component X-ray emission

In a previous paper we presented ultraviolet (λeff=1550 A) high-speed photometry of the eclipsing dwarf nova Z Cha obtained near the peak of a normal eruption, and we derived a maximum-entropy disk map from the eclipse light curve. In the present paper we report a new calculation of the disk map and temperature profile. The calculation differs from the previous one—and all other calculations of disk maps—in two ways: first, we have used realistic model atmospheres to convert the surface brightness of the disk to effective temperature, and second, we have included the effects of limb darkening, which is large at ultraviolet wavelengths and strongly affects the inferred temperature and geometry of the disk. Inclusion of limb darkening in the models made the distribution of surface brightness with radius much steeper than the distribution derived without limb darkening, but the use of realistic model spectra instead of blackbody spectra to convert the surface brightness to temperature flattened the distribution again so that the net effect was to leave the temperature distribution nearly unchanged. The distribution remains much flatter than the r−3/4 law predicted for optically thick, steady-state disks. In agreement with our earlier results, we find that the disk in Z Cha becomes flared during eruptions. The derived flare angle is large, 6°, but not as large as the flare needed for a disk with zero limb darkening. For comparison, model atmospheres for disks similar to the disk in Z Cha give photosphere heights corresponding to flare angles between 3° and 4°.