William F. Welsh

The non‐radially pulsating primary of the cataclysmic variable GW Librae

ABSTRACT The dwarf nova GW Librae is the first cataclysmic variable dis covered to have a primary in awhite dwarf instability strip, making it the first multi-mode, nonradially-pulsatingstar knownto be accreting. The primaries of CVs, embedded in hot, bright accretion discs, are difficultto study directly. Applying the techniques of asteroseismology to GW Librae could thereforegive us an unprecedented look at a white dwarf that has undergone ∼ 10 9 years of accretion.However, an accreting white dwarf may have characteristics sufficiently different from thoseof single pulsating white dwarfs to render the standard models of white dwarf pulsations in-valid for its study.This paper presents amplitudespectra of GW Lib from a series of observingcampaignsconductedduring1997,1998and 2001.We find that t he dominantpulsationmodescluster at periods near 650, 370 and 230 s, which also appear in linear combinations with eachother. GW Lib’s pulsation spectrum is highly unstable on time-scales of months, and exhibitsclusters of signals very closely spaced in frequency, with separations on the order of 1 µHz.Key words: Stars: cataclysmic variables, white dwarfs, oscillations – Individual: GW Librae

High-Dispersion Spectroscopy of the X-Ray Transient RXTE J0421+560 (=CI Camelopardalis) during Outburst*

We obtained high-dispersion spectroscopy of CI Cam, the optical counterpart of XTE J0421+560, 2 weeks after the peak of its short outburst in 1998 April. The optical counterpart is a supergiant B[e] star that is emitting a two-component wind, a cool, low-velocity wind and a hot, high-velocity wind. The cool wind, which is the source of narrow emission lines of neutral and ionized metals, has a velocity of 32 km s-1 and a temperature near 8000 K. It is dense, roughly spherical, fills the space around the sgB[e] star, and, based on the size of an infrared-emitting dust shell around the system, extends to a radius between 13 and 50 AU. It carries away mass at a high rate, > 10-6 M☉ yr-1. The hot wind has a velocity in excess of 2500 km s-1 and a temperature of 1.7 ± 0.3 × 104 K. From an ultraviolet spectrogram of CI Cam obtained in 2000 March with Hubble Space Telescope, we derive a differential extinction E(B-V) = 0.85 ± 0.05. We show that the distance to CI Cam is greater than 5 kpc. Based on this revised distance, the X-ray luminosity at the peak of the outburst was L(2-25 keV) > 3.0 × 1038 ergs s-1, making CI Cam one of the most luminous X-ray transients. The ratio of quiescent luminosity to peak luminosity in the 2-25 keV band is Lq/Lp < 1.7 × 10-6. The compact star in CI Cam is immersed in the dense circumstellar wind from the sgB[e] star and burrows through the wind, producing little X-ray emission except for rare transient outbursts. This picture, a compact star traveling in a wide orbit through the dense circumstellar envelope of a sgB[e] star, occasionally producing transient X-ray outbursts, makes CI Cam unique among the known X-ray binaries. There is strong circumstantial evidence that the compact object is a black hole, not a neutron star. We speculate that the X-ray outburst was short because the accretion disk around the compact star is fed from a stellar wind and is smaller than disks fed by Roche lobe overflow.

Short-term emission line and continuum variations in mrk110

We present results of a variability campaign of Mrk 110 performed with the 9.2-m Hobby-Eberly Telescope (HET) at McDonald Observatory. The high

The Orbital Light Curve of Aquila X-1

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Cellular-Automaton Model for Flickering of Cataclysmic Variables

spectra cover most of the optical range. They were taken from 1999 November through 2000 May. The average interval between the observations was 7.3 days and the median interval was only 3.0 days. Mrk 110 is a narrow-line Seyfert 1 galaxy. During our campaign the continuum flux was in a historically low stage. Considering the delays of the emission lines with respect to the continuum variations we could verify an ionization stratification of the BLR. We derived virial masses of the central black hole from the radial distances of the different emission lines and from their widths. The calculated central masses agree within 20% . Furthermore, we identified optical Hei singlet emission lines emitted in the broad-line region. The observed line fluxes agree with theoretical predictions. We show that a broad wing on the red side of the [Oiii] λ 5007 line is caused by the Hei singlet line at 5016 A.

The ‘outside-in’ outburst of HT Cassiopeiae

We obtained R- and I-band CCD photometry of the soft X-ray transient/neutron-star binary Aql X-1 in 1998 June while it was at quiescence. We find that its light curve is dominated by ellipsoidal variations, although the ellipsoidal variations are severely distorted and have unequal maxima. After we correct for the contaminating flux from a field star located only 046 away, the peak-to-peak amplitude of the modulation is ≈0.25 mag in the R band, which requires the orbital inclination to be greater than 36°. The orbital period we measure is consistent with the 18.95 hr period measured by Chevalier & Ilovaisky. During its outbursts the light curve of Aql X-1 becomes single-humped. The outburst light curve observed by Garcia et al. agrees in phase with our quiescent light curve. We show that the single-humped variation is caused by a reflection effect, that is, by heating of the side of the secondary star facing toward the neutron star.

A Search for Ultrarapid Microvariability in the Seyfert Galaxy NGC 7469 with the Hubble Space Telescope

A new model is presented to elucidate the optical-ultraviolet flickering variability in cataclysmic variables. In this model, light fluctuations are produced by occasional flarelike events and subsequent avalanche flow in the accretion disk atmospheres. Flares are assumed to be ignited when the mass density exceeds a critical density. The disk then evolves to and stays at a self-organized critical state, in which seemingly chaotic fluctuations can be produced. We calculated fluctuating light curves by cellular-automaton simulations. The resultant power spectral density (PSD) consists of a flat part [S(f) ∝ f0, where f is frequency] at lower frequencies and a steep-decline part [S(f) ∝ f-β, with β 1-2] at higher frequencies. It is important that the PSD is rather insensitive to wavelength.

The Quiescent Accretion Disk in IP Pegasi at Near-Infrared Wavelengths

We present results from photometric observations of the dwarf nova system HT Cas during the eruption of 1995 November. The data include the first two-colour observations of an eclipse on the rise to outburst. They show that during the rise to outburst the disc deviates significantly from steady-state models, but the inclusion of an inner-disc truncation radius of about 4Rwd and a ‘flared’ disc of semi-opening angle of 10° produces acceptable fits. The disc is found to have expanded at the start of the outburst to about 0.41RL1, as compared with quiescent measurements. The accretion disc then gradually decreases in radius reaching < 0.32RL1 during the last stages of the eruption. Quiescent eclipses were also observed prior to and after the eruption and a revised ephemeris is calculated.

The orbital light curve of aquila X-1

The Faint Object Spectrograph on the Hubble Space Telescope (HST ) was used to obtain spectro- photometric observations of the Seyfert 1 galaxy NGC 7469. The spectra were obtained with a 90 s time resolution in order to resolve the fastest expected variability and to search for rapid characteristic time- scales (e.g., quasi-periodic oscillations or breaks in the power spectrum). The sensitivity of the obser- vations is such that a D1% spectral variation, or a D0.1% photometric variation, are detectable. The observations were taken through a square aperture and do not su†er contamination from the host 0A.86 galaxy and nearby bright starburst ring. During the 11.5 hr spanned by the observations, a 4% rise in the 1315 continuum was seen, with a corresponding smaller rise at longer wavelengths. Variations on Ae timescales of hours or less are of very low amplitude, at or below the Poisson noise levelEthe rms amplitude of the Nuctuations is D1% in the UV and 0.2% in the G160L zero-order photometric bandpass. No periodicities or characteristic timescales are present, nor do we see any evidence for delays between variations at di†erent wavelengths. The HST observations were made during an intense moni- toring campaign using IUE, RXT E, and ground-based observatories, and a comparison with the results of those observations is presented. The 4% rise seen in the HST data agrees well with the IUE light curve. The very small amplitude, rapid Nuctuations are roughly consistent with a power-law extrapo- lation of the power spectrum of the much longer timescale IUE observations. We were unable to deter- mine the amount of correlated variability between the HST and RXT E light curves because the amplitude of the Nuctuations are extremely small at these very rapid timescales. Subject headings: accretion, accretion disks E galaxies: individual (NGC 7469) E galaxies: photometry E galaxies: Seyfert E galaxies: starburst E ultraviolet: galaxies

On the Optical Pulsations in DQ Her Stars

We present near-infrared, H-band (1.45-1.85 μm) observations of an eclipsing dwarf nova, IP Peg, in quiescence. The light curves are composed of ellipsoidal variations from the late-type secondary star and emission from the accretion disk and the bright spot. The light curves have two eclipses: a primary eclipse of the accretion disk and the bright spot by the companion star, and a secondary eclipse of the companion star by the disk. The ellipsoidal variations of the secondary star were modeled and subtracted from the data. The resulting light curve shows a pronounced double-hump variation. The double-hump profile resembles those seen in the light curves of WZ Sge and AL Com and likely originates in the accretion disk. The primary eclipse was modeled using maximum entropy disk mapping techniques. The accretion disk has a flat intensity distribution and a cool brightness temperature (Tbr 3000 K) in the near-infrared. Superimposed on the face of the disk is the bright spot (Tbr 10,000 K); the position of the bright spot is different from the observed range of visible bright spot positions. The near-infrared accretion disk flux is dominated by optically thin emission. The secondary eclipse indicates the presence of some occulting medium in the disk, but the eclipse depth is too shallow to be caused by a fully opaque accretion disk.