SW Sex Stars Research Articles

Evidence of Magnetic Accretion in an SW Sextantis Star: Discovery of Variable Circular Polarization in LS Pegasi

We report on the discovery of variable circular polarization in the SW Sextantis star LS Pegasi. The observed modulation has an amplitude of ≈0.3% and a period of 29.6 minutes, which we assume as the spin period of the magnetic white dwarf. We also detected periodic flaring in the blue wing of Hβ, with a period of 33.5 minutes. The difference between both frequencies is just the orbital frequency, so we relate the 33.5 minute modulation to the beat between the orbital and spin period. We propose a new accretion scenario in SW Sex stars, based on the shock of the disk-overflown gas stream against the white dwarf's magnetosphere, which extends to the corotation radius. From this geometry, we estimate a magnetic field strength of B1 ~ 5-15 MG. Our results indicate that magnetic accretion plays an important role in SW Sex stars, and we suggest that these systems are probably intermediate polars with the highest mass accretion rates.

A Self-occulting Accretion Disk in the SW Sextantis Star DW Ursae Majoris

We present the ultraviolet spectrum of the SW Sextantis star and nova-like variable DW Ursae Majoris in an optical low state, as observed with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope (HST). The data are well described by a synthetic white dwarf (WD) spectrum with Teff = 46,000 ± 1000 K, log g = 7.60 ± 0.15, v sin i = 370 ± 100 km s-1, and Z/Z☉ = 0.47 ± 0.15. For this combination of Teff and log g, WD models predict MWD = 0.48 ± 0.06 M☉ and RWD = (1.27 ± 0.18) × 109 cm. Combining the radius estimate with the normalization of the spectral fit, we obtain a distance estimate of d = 830 ± 150 pc. During our observations, DW UMa was approximately 3 mag fainter in V than in the high state. A comparison of our low-state HST spectrum with a high-state spectrum obtained with the International Ultraviolet Explorer shows that the former is much bluer and has a higher continuum level shortward of 1450 A. Since DW UMa is an eclipsing system, this suggests that an optically thick accretion disk rim blocks our view of the WD primary in the high state. If self-occulting accretion disks are common among the SW Sex stars, we can account for (1) the preference for high-inclination systems within the class and (2) their V-shaped continuum eclipses. Moreover, even though the emission lines produced by a self-obscured disk are generally still double-peaked, they are weaker and narrower than those produced by an unobscured disk. This may allow a secondary line emission mechanism to dominate and produce the single-peaked, optical lines that are a distinguishing characteristic of the SW Sex stars.

Orbital Period of the Low‐Inclination SW Sextantis Star V442 Ophiuchi

V442 Ophiuchi is a low-inclination (noneclipsing) nova-like cataclysmic variable. We have obtained medium resolution (1-3.5 A), time-resolved optical spectroscopy of this interacting binary star spanning an interval of 4 yr, from 1995 August to 1999 June. Using an Hα radial velocity curve constructed from our combined spectroscopic data sets, we have determined that Porb = 0.1243 days for V442 Oph, breaking the long-standing 1 day alias ambiguity in its orbital period. V442 Oph shares many of the characteristics of the group of cataclysmic variables known as the SW Sextantis stars. Its Doppler tomograms and orbital phase-dependent emission-line behavior, including the occurrence of a prominent transient absorption event, are consistent with those of other SW Sex stars. V442 Oph also displays single-peaked rather than double-peaked emission lines, strong He II λ4686 emission, and a phase offset of +0.08 between the Hα emission-line radial velocities and the phasing of He II. We discuss the role of V442 Oph as one of three known low-inclination examples of SW Sex stars. Like the other low-inclination SW Sex stars, LS Pegasi and V795 Herculis, the Balmer emission lines in V442 Oph have high velocity components that extend to ≈±1900 km s-1 and follow an apparent S-wave pattern in the trailed spectrum. Finally, we compare two models for the SW Sex stars involving accretion stream overflow, in the context of a low-inclination system in which our line of sight likely extends over the entire face of the disk.

Outbursts of EX Hydrae: mass-transfer events or disc instabilities?

We present the 45-yr record of the light curve of EX Hya, and discuss the characteristics of its 15 observed outbursts. We then concentrate on the 1998 outburst, reporting the first outburst X-ray observations. We discover an X-ray beat-cycle modulation, indicating that an enhanced accretion stream couples directly with the magnetosphere in outburst, confirming our previous prediction. Optical eclipse profiles late in outburst show that the visible light is dominated by an enhanced mass-transfer stream overflowing the accretion disc. We are uncertain whether the enhanced mass transfer is triggered by a disc instability, or by some other cause. While in outburst, EX Hya shows some of the characteristics of SW Sex stars.

S193: another non-eclipsing SW Sex star

ABSTRA C T We present time-resolved optical spectroscopy of the cataclysmic variable S193. The emission lines are remarkably similar to those of V795 Her and exhibit high-velocity S-waves and complex absorptions that are modulated with the orbital period. Evidence for transient anomalous spectral features is seen during the first two nights of our run. We propose that S193 and V795 Her are non-eclipsing SW Sex stars. Finally, we show that the ‘disc overflow’ model fails to explain the Balmer line orbital behaviour in these low-inclination systems.

LS Pegasi: A Low‐Inclination SW Sextantis–Type Cataclysmic Binary with High‐Velocity Balmer Emission‐Line Wings 11Based in part on observations obtained at MDM Observatory, operated by Dartmouth College, Columbia University, the Ohio State University, and the University of Michigan.

ABSTRACTWe present time‐resolved spectroscopy and photometry of the bright cataclysmic variable LS Peg (=S193; V ≈ 13.0—Szkody et al.). The Balmer lines exhibit broad, asymmetric wings Doppler‐shifted by about 2000 km s−1 at the edges, while the He i lines show phase‐dependent absorption features strikingly similar to SW Sextantis stars, as well as emission through most of the phase. The Ciii/N iii emission blend does not show any phase dependence. From velocities of Hα emission lines, we determine an orbital period of 0.174774 ± 0.000003 days (=4.1946 hr), which agrees with Szkody's value of approximately 4.2 hr. No stable photometric signal was found at the orbital period. A noncoherent quasi‐periodic photometric signal was seen at a period of 20.7 ± 0.3 minutes.The high‐velocity Balmer wings most probably arise from a stream reimpact point close to the white dwarf. We present simulated spectra based on a kinematic model similar to the modified disk‐overflow scenario of Hellier & Robinson. The models reproduce the broad line wings, though some other details are unexplained.Using an estimate of dynamical phase based on the model, we show that the phasing of the emission‐ and absorption‐line variations is consistent with that in (eclipsing) SW Sex stars. We therefore identify LS Peg as a low‐inclination SW Sex star.Our model suggests i = 30°, and the observed absence of any photometric signal at the orbital frequency establishes i<60°. This constraint puts a severe strain on interpretations of the SW Sex phenomenon which rely on disk structures lying slightly out of the orbital plane.

The mass of the white dwarf in the old nova BT Mon

We present spectrophotometry of the eclipsing old nova BT Mon (Nova Mon 1939). By detecting weak absorption features from the secondary star, we find its radial velocity semi-amplitude to be K_R = 205+/-5 km/s and its rotational velocity to be vsin i = 138+/-5 km/s. We also measure the radial velocity semi-amplitude of the primary star to be K_R = 170+/-10 km/s. From these parameters we obtain a mass of 1.04+/-0.06 M_sun for the white dwarf primary star and a mass of 0.87+/-0.06 M_sun for the G8V secondary star. The inclination of the system is found to be 82.2+/-3.2 deg and we estimate that the system lies at a distance of 1700+/-300pc. The high mass of the white dwarf and our finding that BT Mon was probably a fast nova together constitute a new piece of evidence in favour of the thermonuclear runaway model of classical nova outbursts. The emission lines are single peaked throughout the orbital cycle, showing absorption around phase 0.5, high velocity S-wave components and large phase offsets in their radial velocity curves. In each of these respects, BT Mon is similar to the SW Sex stars. We also find quasi-periodic flaring in the trailed spectra, which makes BT Mon a candidate intermediate polar.

Observations of the SW Sextantis Star BH Lyncis in a High State

We present time-resolved photometry and medium resolution (2 A) spectroscopy of the SW Sextantis star BH Lyncis obtained during a high state of the system. Previous detailed observations have been obtained only during low and intermediate states. In addition, we present the first International Ultraviolet Explorer low-resolution (6 A) spectrum of BH Lyn. A revised photometric ephemeris, HJD(min) = 2,447,180.3366(2) + 0.15587535(6)E, is derived based on new and previously published eclipse timings. The amplitude of the pre-eclipse hump in light curves of BH Lyn is shown to vary, suggesting that the degree to which the incoming accretion stream impacts the edge of the disk changes over time. We estimate system parameters using the V-band eclipse width and He II λ4686 emission-line radial velocity (i ≈ 79°, q ≈ 0.45, MWD ≈ 0.7 M☉, M2 ≈ 0.3 M☉, R2 ≈ 0.4 R☉, a ≈ 1.2 R☉). Doppler tomograms constructed for the prominent emission lines in BH Lyn show a lack of emission at the expected locations of the bright spot and accretion stream trajectories. The differences between the low and high states of BH Lyn are discussed, along with the possible origin of a significant amount of the emission-line flux in a wind from the disk and/or white dwarf primary star. Finally, a model involving the presence of vertically extended bulges in the accretion disk is introduced to explain the characteristic phase-dependent absorption feature found in emission lines of the SW Sex stars.

Accretion Disks in the SW Sex Stars

AbstractWe present recent results of time-resolved optical spectroscopy on two SW Sex stars, PG 0859+415 and BH Lyncis.

V1315 Aquilae and the Nature of SW Sextantis Stars

Optical spectroscopy of the nova-like variable V1315 Aql shows strong single-peaked emission lines, high-velocity s-waves in the wings of the lines, and two distinct absorption components. In order to explain all the features, I suggest that disk overflow accretion has to be combined with an accretion disk wind. The absorption is probably caused by the accretion stream as it flows over the disk and by P Cygni troughs produced by the wind. If correct, this model can account for the peculiarities of the emission lines in SW Sex stars. Both disk overflow and the strong wind might result from a high mass transfer rate.

The Nova-Like Variables

AbstractWe review optical observations and theoretical models of the non-magnetic nova-like variables (UX UMa, VY Scl and SW Sex stars). A brief discussion of the classification scheme is followed by a statistical overview of the observed properties. The most important features of each of the sub-classes are then reviewed, concluding with a summary of the theoretical models invoked to understand these systems.

V795 Her: an SW Sex star in the period gap?

We present optical photometry and spectroscopy of the nova-like variable V795 Her. The emission lines exhibit an extremely complex structure consisting of several time-variable emission and absorption components. We confirm the spectroscopic modulation of 0.1082 d which we attribute to the orbital period of the system. The equivalent widths (EWs) of all the emission lines are strongly modulated on this period, showing a pronounced minimum around phase 0.5. We associate this minimum with the transit of an absorption feature which drives dramatic line changes, seen as transient double peaks and P Cygni type profiles. The absorption depth grows with the excitation level in Balmer and Hei lines, but both the He ii λ4686 and the metallic lines (e.g. Cii λ4267 and the Bowen blend) are entirely in emission. The centroid of the Hα emission (which is contaminated very little by the absorption feature) is delayed by 83° with respect to the high-excitation lines. We assume that the latter trace the orbital motion of the primary, enabling us to define an absolute zero-phase. High-velocity S-waves are clearly observed in all Balmer lines, with amplitudes of about 1750 km s−l and maximum redshift at about phase 0. However, our high-resolution spectra suggest that these can be separated into two phased components, with γ-velocities of ∼ ±750 km s−1 and K ∼1000 km s−1, probably formed in accretion columns on the white dwarf. In addition, the H i cores exhibit the presence of low-velocity (K = 268 km s−1) S-waves, roughly phased with their high-amplitude partners. On the other hand, our R-band photometry is dominated by flickering with no evidence for modulation with the orbital period nor the previously reported 0.1165-d periodicity. Based on the spectral properties, we propose that V795 Her is a new SW Sex star, observed at intermediate inclination. The complex behaviour of V795 Her can be qualitatively explained invoking an intermediate polar scenario with a synchronously rotating white dwarf.

PX Andromedae and the SW Sextantis phenomenon

We show that the emission-line peculiarities of PX And and other SW Sex stars can be explained by an accretion stream which overflows the initial impact with the accretion disk and continues to a later reimpact. The overflowing stream is seen projected against a brighter disk and produces the 'phase 0.5 absorption' features. Emission from the reimpact site produces the high-velocity line wings which alternate from red to blue on the orbital cycle. We conclude that substantial disk overflow is the property distinguishing SW Sex stars from other cataclysmic variables.