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NASA NTRS1981

BE Ursae Majoris /PG 1155 + 492/ - A unique cataclysmic-variable-like object

The variable spectrum of BE Ursae Majoris, which is similar to that of the cataclysmic variables (CVs), shows (1) an emission-line spectrum with the high-excitation CN 4650 A blend as the strongest optical feature, weaker H and He lines, and a strong Balmer jump, and (2) an absorption spectrum dominated by He. Although the object shows no evidence of photometric variability on either short or long time-scales, it has been shown by Kurochkin (1964, 1971) to have a 2.29-day, sinusoidal, 1.5-mag variation. The luminosity is dominated by a UV power-law component, though the 4000-10000 A continuum is flat. The hypothesis that this object is an active mass transfer binary is not supported by its photometric behavior, energy distribution and long period, despite the spectroscopic variability. Nevertheless, three components whose nature is not understood remain: (1) an ultraviolet source whose energy distribution is explainable neither by a uniform temperature primary star nor by an accretion disk; (2) a reprocessed component powered by the UV star; and (3) a component similar to a CV disk, producing the optically thick Balmer lines.

Ferguson, D. H.↗
NASA NTRS1981

The X-ray and optical characteristics of the cataclysmic variables V794 Aquilae

Observations of V794 Aql have revealed features that are consistent with its inclusion in the class of cataclysmic variables. The features include: (1) a 0.1-4.5 keV luminosity of 5 x 10 to the 30th ergs/sec for a d-value of 100 pc, with a hardness ratio of 11; (2) blue colors; (3) 0.5 mag variability on 40-min time scales, and 3.0 mag variability on the time scale of one day; and (4) a weak He II 4686 A, which argues against classification of V794 Aql as an AM Her-type variable. It is predicted that V794 Aql has a period shorter than six hours, with a strong possibility for eclipses, and is a strong UV emission source.

Szkody, P.↗
NASA NTRS2000

First XMM-Newton Observations of a Cataclysmic Variable II: Spectral Studies of OY Car

We present XMM-Newton X-ray spectra of the disc accreting cataclysmic variable OY Car, which were obtained during the performance verification phase of the mission. These data were taken 4 days after a short outburst. In the EPIC spectra we find strong Iron K(beta) emission with weaker Iron K(alpha) emission together with Silicon and Sulphur lines. The spectra are best fitted with a three temperature plasma model with a partial covering absorber. Multiple temperature emission is confirmed by the emission lines seen in the RGS spectrum and the H/He like intensity ratio for Iron and Sulphur which imply temperatures of approx. 7keV and approx. 3keV respectively.

Ramsay, Gavin↗
NASA NTRS2002

Two Types of Soft X-ray Spectra in Cataclysmic Variables

We present results of analyses of Chandra HETG soft X-ray spectra (Lambda = 1.5-25 A) of seven cataclysmic variables. We find that these spectra divide unambiguously into two distinct types. Spectra of the first type, consisting of EX Hya, V603 Aql, U Gem, and SS Cyg, are remarkably well fit by a simple cooling flow model, which assumes only steady-state isobaric radiative cooling. This model has only two free parameters, the maximum temperature, kT(sub max), which provides a rough measurement of the depth of the potential well, and the overall normalization, which provides a highly precise measurement of the total accretion rate. Spectra of the second type, consisting of V1223 Sgr, A Psc, and GK Per, are grossly inconsistent with a simple cooling flow model. They instead exhibit a hard continuum, and, in addition, show strong H-like and He-like ion emission but little Fe L-shell emission, which is consistent with expectations for line emission from a photoionized plasma. Using a simple photoionization model, we argue that the observed line emission for these sources can be driven entirely by the hard continuum. The physical significance of these two distinct types of X-ray spectra is also explored.

White, Nicholas E.↗
NASA NTRS1984

X-ray observations of a large sample of cataclysmic variable stars using the Einstein Observatory

This paper presents the results of an X-ray survey of 31 known or suspected cataclysmic variables. Eighteen of these close binary systems are detected with inferred luminosities in the 0.1-4.0 keV band of between 10 to the 30th and 10 to the 32nd erg/sec. The majority have relatively hard X-ray spectra (kT greater than 2 keV) irrespective of luminosity state. Of seven dwarf novae observed during optical outbursts only U Gem exhibited enhanced ultrasoft X-ray emission (kT of about 10 eV) in addition to weak, hard X-ray emission. Variability of the X-ray flux is observed in many of these stars, on time-scales ranging from tens of seconds to hours. The contribution to the flux from extended X-ray emission is investigated for SU UMa and GK Per. Several possibilities for the origin of the hard X-rays are considered.

Cordova, F. A.↗
NASA NTRS1987

Time-resolved IUE studies of cataclysmic variables. I - Eclipsing systems IP Peg, PG 1030+590, and V1315 Aql

IUE time-resolved spectra of the high-inclination cataclysmic variables IP Peg, PG 1030+590, and V1315 Aql are analyzed in order to determine the characteristics of the disk, hotspots, and white dwarfs. The UV continuum flux distributions are generally flatter than systems of low inclination and high mass-transfer rate, and the white dwarfs/inner disk appear to be relatively cool (15,000-19,000 K) for their orbital periods, possibly because the boundary layers are blocked from view. The continuum fluxes increase at spot phases, with the spot providing the dominant flux in IP Peg. The spot temperatures range from hot (20,000 K) in IP Peg, and perhaps in PG 1030+590, to cool (11,000 K) in V1315 Aql. The C IV emission lines show slightly larger decreases at spot phases than during eclipse, which implies an extended stream area.

Szkody, Paula↗
NASA NTRS1985

On the evolution of accretion disc flow in cataclysmic variables. III - Outburst properties of constant and uniform-alpha model discs

Attention is given to the stability and evolution of some simple accretion disk models in which the viscosity is prescribed by an ad hoc, uniform-alpha model. Emphasis is placed on systems in which the mass input rate from the secondary to the disk around the primary is assumed to be constant, although initial calculations with variable mass input rates are also performed. Time-dependent visual magnitude light curves constructed for cataclysmic binaries with a range of disk size, primary mass and mass input rate, and viscosity magnitude, are compared with the observed properties of various cataclysmic variable subclasses. The results obtained indicate that the observational differences between novae and dwarf novae may be due to mass input rate differences. The present models can reproduce the gross observational features of U-Gem-type dwarf nova outbursts.

Lin, D. N. C.↗
NASA NTRS1988

Quasiperiodicity in cataclysmic variable stars caused by solar-type magnetic cycles

Cyclical variations of orbital periods, quiescent magnitudes and outburst intervals in the activity of cataclysmic variable binary stars are inter-related and are ascribed to variations in radii of the secondaries, caused by solar-type (sunspot) magnetic cycles. In the nova remnant DQ Herculis the observed variations in orbital period and quiescent magnitude are consistent with this mechanism. But accretion onto the white dwarf, from an accretion disk acquired from its companion, cannot explain the observed variation of the 71-second oscillations.

Warner, Brian↗
NASA NTRS1990

LB 1800 - A bright eclipsing cataclysmic variable and a transient X-ray source

The star LB 1800 has been discovered to be a 13th mag high-excitation cataclysmic variable exhibiting novalike characteristics and partial eclipses in its light curve. It has subsequently been identified as the optical counterpart of the transient hard X-ray source 4U 0608-49 following analysis of HEAO 1 data. Both the radial velocity and photometric variations have a period of 5.56 hr, while the K-velocity of the relatively uncomplicated radial velocity curves is 134 + or - 9 km/s, leading to a mass function of 0.058 + or - 0.003 solar mass. The dynamical and eclipse solutions point to an orbital inclination of 87 deg + or - 3 deg and a mass ratio of 0.46 + or - 0.04 for the empirical secondary mass 0.55 solar mass. The implied white dwarf mass is therefore quite high at about 1.2 solar mass. The disk size, from timings of first and last contact, was estimated to be very close to that of the secondary star. Phase-dependent, double-peak emission lines are sometimes observed. It is argued that the orbital variations in the relative strengths of the red and blue peaks arise from changing hot-spot visibility.

Buckley, D. A. H.↗
NASA NTRS1982

IUE observations of cataclysmic variables

The four years of IUE operation have revealed that the ultraviolet region of the spectrum contributes a dominant share of the emerging energy from cataclysmic variables and provides important clues to the physical nature of these systems. The implications of the continuum flux distributions and line spectra for the determination of the accretion rates and mass loss rats are considered.

Szkody, P.↗
NASA NTRS1985

Soft X-rays, winds, and the cataclysmic variable boundary-layer problem

Models are presented for the effects of a stellar wind on the UV and soft X-ray spectra of cataclysmic variables (CVs). It is shown that P Cygni absorption lines observed from many CVs are consistent with the existence of a strong wind, and that such a wind can also account for the soft X-ray upper limits if the wind mass loss rate is about 10 to the -8th solar masses per yr. The implications of such a wind for the CV momentum and energy budget are discussed.

Kallman, T. R.↗
NASA NTRS1993

IUE observations of cataclysmic variable

Twenty two approved International Ultraviolet Explorer (IUE) programs were studied over a 14 year period. These programs are listed. The observations and subsequent analysis centered on cataclysmic variables (close binaries with a late main sequence star transferring material to a primary white dwarf via an accretion disk). The early studies highlighted the flux distribution of the accretion disk at outburst and quiescence, while later studies accomplished time-resolved observations throughout the orbital cycles, the study of the outflowing winds present at outburst, the study of the white dwarf in those systems with low accretion rate. There are 39 publications resulting from this work which are listed. These results include those for individual systems (Stepanian's star, Lanning 10, AM Her, MV Lyr, TV Col, VW Hyi, T Leo, IR Gem, TT Ari, Z Cam, BV Pup, IP Peg, PG1030+590, V1315 Aql, SW UMa, V426 Oph, WZ Sge, BY Cam, and U Gem) as well as review articles in journals and publications from reviews at meetings that summarize the impact of IUE on the study of accretion disks, white dwarfs, and hot spots resulting from stream impact as well as magnetic accretion columns.

Szkody, Paula↗
NASA NTRS1998

Recovery of 29 Second Oscillations in Hubble Space Telescope Eclipse Observations of the Cataclysmic Variable UX Ursae Majoris

Low-amplitude (approximately 0.5%) 29 s oscillations have been detected in Hubble Space Telescope Faint Object Spectrograph eclipse observations of the nova-like cataclysmic variable UX UMa. These are the same dwarf nova-type oscillations that were originally discovered in this system in 1972. The 29 s oscillations are seen in one pair of eclipse sequences obtained with the FOS/PRISM in 1994 November but not in a similar pair obtained with the FOS/GI60L grating in August of the same year. The oscillations in the PRISM data are sinusoidal to within the small observational errors and undergo an approximately - 360' phase shift during eclipses (i.e., one cycle is lost). The amplitudes are highest at pre-eclipse orbital phases and exhibit a rather gradual eclipse whose shape is roughly similar to, although perhaps slightly narrower than, LTX UMa's overall light curve in the PRISM bandpass (2000-8000 A). Spectra of the oscillations have been constructed from pre-, mid, and post-eclipse data segments of the November observations. The spectra obtained from the out-of-eclipse segments are extremely blue, and only lower limits can be placed on the temperature of the source that dominates the modulated flux at these orbital phases. Lower limits derived from blackbody (stellar atmosphere) model fits to these data are >or equal to 95,000 K (> or equal to 85,000 K); the corresponding upper limits on the projected area of this source are all less than 2% of the white dwarf (WD) surface area. By contrast, oscillation spectra derived from mid- eclipse data segments are much redder. Fits to these spectra yield temperature estimates in the range 20,000 K approximately greater T and T approximately less than 30,000 K for both blackbody and stellar atmosphere models and corresponding projected areas of a few percent of the WD surface area. These estimates are subject to revision if the modulated emission is optically thin. We suggest that the ultimate source of the oscillations is a hot, compact region near disk center, but that a significant fraction of the observed, modulated flux is due to reprocessing of the light emitted by this source in the accretion disk atmosphere. The compact source is occulted at orbital phases near mid-eclipse, leaving only part of the more extended reprocessing region(s) to produce the weak oscillations that persist even at conjunction. The highly sinusoidal oscillation pulse shape does not permit the identification of the compact comonent in this model with emission produced by a rotating disturbance in the inner disk or in a classical, equatorial boundary layer. Instead, this component could arise in a bright spot on the surface of the WD, possibly associated with a magnetic pole. However, a standard intermediate polar model can also be ruled out since UX UMa's oscillation period has been seen to change on timescales much shorter than the minimum timescale required to spin up the WD by accretion torques. A model invoking magnetically controlled accretion onto differentially rotating WD surface layers may be viable, but needs more theoretical work.

Knigge, Christian↗
NASA NTRS1987

Identification of US 943 as a halo cataclysmic variable

US 943 has been shown to exhibit large (greater than about 5 mag) amplitude variations on time scales of one to two years. New time-resolved CCD photometry presented herein shows it also to be variable on time scales of 20 to 40 minutes with amplitude variation of up to 1 mag. Its membership as a cataclysmic variable is suggested. This assignment would place US 943 at a galactic distance of about 700 pc.

Howell, Steve B.↗
NASA NTRS1985

X-ray emission from cataclysmic variables with accretion disks. I - Hard X-rays. II - EUV/soft X-ray radiation

Theoretical models explaining the hard-X-ray, soft-X-ray, and EUV emission of accretion-disk cataclysmic variables in terms of the disk boundary layer (DBL) are developed on the basis of a survey of the published observational data. The data are compared with model predictions in graphs for systems with high or low (greater than or less than 10-Pg/s) accretion rates. Good agreement is obtained both at low accretion rates, where an optically thin rarefied hot (Te = 10 to the 8th K) DBL radiates most of its energy as hard X-rays, and at high accretion rates, where an optically thick 100,000-K DBL radiates most of its energy in the EUV and as soft X-rays. Detailed analysis of the old nova V603 Aql suggests that previous models predicting more detections of soft-X-ray/EUV emissions from thick-DBL objects (Ferland et al., 1982) used inappropriate dwarf masses, interstellar column densities, or classical-nova space densities.

Patterson, J.↗