Donald Wayne Hoard

AR URSAE MAJORIS: THE FIRST HIGH-FIELD MAGNETIC CATACLYSMIC VARIABLE

We identify the luminous soft X-ray source AR UMa as a magnetic cataclysmic variable containing a white dwarf with the highest field yet detected in an accreting binary. IUE and optical spectroscopy, optical photometry, and circular polarimetry and spectropolarimetry define remarkably distinct accretion states of this binary. Circular polarization is nearly absent in the high state, but the low state exhibits values which vary between 2% and 5% on the orbital period of 1.932 hr. The UV continuum contains a broad absorption feature near 1300 A, while optical spectropolarimetry during the low state reveals a number of strongly polarized dips. These are interpreted as Zeeman components of hydrogen Lyα and another atmospheric species, possibly He I, in a photospheric magnetic field of ~ 230 MG.The radial velocity curve of the low-state optical emission lines shares the period of the optical photometry and polarimetry and is phased appropriately for an origin on the irradiated secondary star. While the high state exhibits prominent UV line emission typical of the magnetic variables, the strength of the UV continuum does not vary appreciably with a change in accretion state. This, combined with the high soft X-ray luminosity and lack of circular polarization, indicates that accretion occurs largely in the form of dense filaments which avoid a standoff shock and thermalize their kinetic energy below the white dwarf photosphere. We suggest that these phenomena may play a role in the apparent lack of high-field systems with easily detectable circular polarization during high-accretion states.

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 in the high-field magnetic cataclysmic variable AR Ursae Majoris

AR UMa is the highest field example known of the magnetic cataclysmic variables, with a polar field Bp = 230 MG. We report here the results of a long program of photometry, spectroscopy, and spectropolarimetry of the object that was undertaken with the aim of gaining a better understanding of the role that the white dwarf field plays in shaping the properties of the magnetic systems. Only three accretion episodes, reaching mV ≤ 15 and covering a total of 9 months, were detected during the 4 yr of available photometry. Fortunately, the onset of one episode coincided with a week-long observing run, resulting in spectrophotometry and linear and circular spectropolarimetry that document the event. The long intervals of quiescence have enabled a determination of the orbital ephemeris that is now sufficiently precise to phase observations 20 yr old. The high-state observations lead to a number of conclusions regarding the system geometry. The white dwarf magnetic axis is inclined rather little to the spin axis, but the dipole is twisted in azimuth such that it lies nearly perpendicular to the stellar line of centers. Tomographic analysis of a wide variety of atomic species indicates that the white dwarf likely accretes in twin funnels that split off a ballistic gas stream reaching 30%-50% of the distance between the stars. A narrow high-velocity (KNHV ≈ 700 km s-1) emission-line component offset in phase from the main features suggests the existence of an additional gas stream, but the interpretation of this is not yet resolved. The system is viewed from a moderate inclination, which places the disfavored (retrograde) magnetic pole on the observed hemisphere at all times. We have investigated the possible effect that the white dwarf magnetosphere might have in restricting mass flow through L1, and therefore in giving rise to the unique, protracted low-accretion states of this object. However, the mechanisms considered are too weak and/or would give rise to consequences that are at odds with low-state observations. At the same time, the high magnetic field in AR UMa has yielded a new, powerful observational tool: phase-resolved Zeeman spectroscopy of the emission lines produced in the accretion stream(s). Future high-quality observations and sophisticated modeling of these features hold promise for three-dimensional reconstructions of the gas flow in high-field magnetic variables.

Chandra Spectra of the Prototype Dwarf Nova U Geminorum at Quiescence

We have observed the prototypical dwarf nova U Gem at quiescence with the Chandra medium- and high-resolution gratings to accomplish the first study of resolved X-ray emission lines in a disk-accreting cataclysmic variable. Doppler tomograms constructed from optical spectra obtained close in time to the X-ray observation show a typical quiescent disk structure with an irradiated secondary but no prominent disk hot spot. The unprecedented spectral resolution of Chandra over past X-ray telescopes reveals prominent narrow emission lines of O, Ne, Mg, Si, S, and Fe. The line fluxes, ratios, and widths indicate that X-ray emission arises from a range of temperatures in a high-density (greater than 1014 cm-3) gas, moving at low (less than 300 km s-1) velocity, with a small (less than 107 cm) scale height compared to the white dwarf radius. Simple models with cooling flows, cooling flows plus isothermal zones, and thermal conduction give reasonable agreement with the low-temperature emission lines, but a good fit to the entire range of lines will require a better understanding of all the parameters that affect the boundary layer.

Observations of the SW Sextantis star UU Aquarii

AbstractWe present 14 nights of medium resolution (1–2 A) spectroscopy of the eclipsing cataclysmic variable UU Aquarii obtained during a high accretion state in 1995 August–October. UU Aqr appears to be an SW Sextantis (SW Sex) star, as noted by Baptista, Steiner & Horne, and we discuss its spectroscopic behaviour in the context of the SW Sex phenomenon. Emission-line equivalent width curves, Doppler tomography, and line profile simulation provide evidence for the presence of a bright spot at the impact site of the accretion stream with the edge of the disc, and a non-axisymmetric, vertically and azimuthally extended absorbing structure in the disc. The absorption has maximum depth in the emission lines around orbital phase 0.8, but is present from φ≈0.4 to φ≈0.95. An origin is explored for this absorbing structure (as well as for the other spectroscopic behaviour of UU Aqr) in terms of the explosive impact of the accretion stream with the disc.

RXTE, ROSAT, EUVE, IUE, and Optical Observations through the 45 Day Supercycle of V1159 Orionis

A complete 45 day supercycle of the cataclysmic variable V1159 Ori comprising a superoutburst and eight normal outbursts was observed. Coverage included ground-based optical observations as well as observations with RXTE for 38 days, ROSAT for 34 days, IUE for 27 days, and Extreme Ultraviolet Explorer (EUVE) for 10 days. The resulting light curves reveal that the optical and UV light variations are inversely correlated with the RXTE and ROSAT fluxes, with the largest change in intensity occurring in the ROSAT bandpass. There is no evidence for a strong EUV/soft X-ray component during outburst. An outflowing wind is evident from the C IV line profile during each brief outburst as well as the superoutburst. The transitions from outburst states of the disk to quiescent states take place on timescales of hours. Accretion disk models can fit the UV line and continuum energy distributions near outburst only if the disk radial temperature profile is modified from the standard case to produce a hotter distribution in the outer annuli. The high mass transfer rate, the hot disk distribution, and the similarity of outbursts and superoutbursts argue for outside-in outbursts in this system.

Identification of the soft X-ray source wga j1802.1+1804 with a new magnetic cataclysmic variable

We have discovered a bright (V ~ 14.5) cataclysmic variable during observations of the soft X-ray sources in the list of Singh et al. The optical source, which is coincident with the X-ray position of WGA J1802.1+1804, shows all the characteristics of a magnetic AM Herculis-type system: circular polarization, He II strength greater than Hβ, multiple line components, and a consistent photometric, polarimetric, spectroscopic, and X-ray period of 113 minutes. The X-ray spectrum shows a dominant soft blackbody (kT = 20-45 eV) and a weaker bremsstrahlung component (kT > 1 keV), while the circular polarization is relatively low (4% in the red).

Observations of the Magnetic Cataclysmic Variable VV Puppis with the Far Ultraviolet Spectroscopic Explorer

We present the first far-ultraviolet (FUV) observations of the magnetic cataclysmic variable VV Puppis, obtained with the Far Ultraviolet Spectroscopic Explorer satellite. In addition, we have obtained simultaneous ground-based optical photometric observations of VV Pup during part of the FUV observation. The shapes of the FUV and optical light curves are consistent with each other and with those of past observations at optical, extreme-ultraviolet, and X-ray wavelengths. Time-resolved FUV spectra during the portion of VV Pups orbit when the accreting magnetic pole of the white dwarf can be seen show an increasing continuum level as the accretion spot becomes more directly visible. The most prominent features in the spectrum are the O VI λλ1031.9, 1037.6 emission lines. We interpret the shape and velocity shift of these lines in the context of an origin in the accretion funnel near the white dwarf surface. A blackbody function with Tbb 90,000 K provides an adequate fit to the FUV spectral energy distribution of VV Pup.

Time-resolved, multi-color photometry and spectroscopy of Virgo 4 (OU Vir): A high orbital inclination, short orbital period dwarf nova

We present multi-color photometry and time resolved spectroscopy of OU Vir. The analysis of the quiescent light curve shows that OU Vir is characterized by i) strong cycle-to-cycle brightness variations, and ii) hot spot modulated light curve with grazing eclipse of the impact region. Colors are derived both in- and out- of eclipse. The time-resolved spectroscopy allows us to produce the radial velocity curve from the H accretion disk emission line which possibly reveals only weak evidence for hot spot line emission. The hot spot is believed to be a turbulent optically thick region, producing mostly continuum emission.

Extreme Ultraviolet and Optical Observations of The AM Herculis Type Cataclysmic Variable V884 Herculis (RX J1802.1+1804)

We observed V884 Herculis (RX J1802.1+1804), during a high accretion state, from 1997 September 17-22 UT using the Extreme Ultraviolet Explorer (EUVE) satellite. Simultaneously, multiple ground-based optical telescopes obtained photometric observations. Although the EUV flux was too low for spectral study, the light curve from the Deep Survey instrument provides useful information on the system geometry. The phased, folded EUV light curve shows a brief total eclipse similar to previous ROSAT PSPC data followed by a broad dip. The optical light curve shows a lower amplitude of variability and a smoother structure. The similarity of the optical light curves on all six nights, along with previous magnetic measurements, and the coincidence of the periods determined from the optical and EUVE data, confirm the synchronism of this system and its classification as an AM Her type magnetic cataclysmic variable. Doppler tomography of 1995 high state spectra reveals a short, ballistic accretion stream with little emission from an irradiated secondary star. An optical light curve obtained in 1998 June shows the system during a lowered accretion state, with a mean magnitude that is ~1.5 mag fainter than the high state. The low state light curve displays a smaller amplitude of orbital variation but only slight changes in accretion geometry. A spectrum obtained 15 days later shows that the optical continuum had returned to a high state, although the line strengths appear different from past high states.