1,721,062 research outputs found
Eclipse mapping of the accretion stream in UZ Fornacis
No full textWe present a new method to map the surface brightness of the accretion streams in AM Herculis systems from observed light curves. Extensive tests of the algorithm show that it reliably reproduces the intensity distribution of the stream for data with a signal-to-noise ratio greater than or similar to 5. As a first application, we map the accretion stream emission of C IV lambda 1550 in the polar UZ Fornacis using HST FOS high state spectra. We find three main emission regions along the accretion stream: (1) On the ballistic part of the accretion stream, (2) on the magnetically funneled stream near the primary accretion spot, and (3) on the magnetically funneled stream at a position above the stagnation region
Eclipse mapping of the accretion stream in UZ Fornacis
No full textWe present a new method to map the surface brightness of the accretion streams in AM Herculis systems from observed light curves. Extensive tests of the algorithm show that it reliably reproduces the intensity distribution of the stream for data with a signal-to-noise ratio greater than or similar to 5. As a first application, we map the accretion stream emission of C IV lambda 1550 in the polar UZ Fornacis using HST FOS high state spectra. We find three main emission regions along the accretion stream: (1) On the ballistic part of the accretion stream, (2) on the magnetically funneled stream near the primary accretion spot, and (3) on the magnetically funneled stream at a position above the stagnation region
A search for variable white dwarfs in large-area time-domain surveys: A pilot study in SDSS Stripe 82
No full textWe present a method to reliably select variable white dwarfs from large-area time-domain surveys and apply this method in a pilot study to search for pulsating white dwarfs in the Sloan Digital Sky Survey Stripe 82. From a sample 400 high-confidence white dwarf candidates, we identify 24 which show significant variability in their multi-epoch Stripe 82 data. Using colours, we further selected a sample of pulsating white dwarf (ZZ Ceti) candidates and obtained high-cadence follow-up for six targets. We confirm five of our candidates as cool ZZ Cetis, three of which are new discoveries. Among our 24 candidates we also identify: one eclipsing binary, two magnetic white dwarfs and one pulsating PG1159 star. Finally, we discuss the possible causes for the variability detected in the remaining targets. Even with sparse multi-epoch data over the limited area of Stripe 82, we demonstrate that our selection method can successfully identify various types of variable white dwarfs and efficiently select high-confidence ZZ Ceti candidates
The history and source of mass-transfer variations in AM Herculis
No full textThe optical brightness of magnetic cataclysmic variables without accretion discs is a direct measure of the near-instantaneous mass-transfer rates from the late-type secondary stars to the magnetic white dwarfs in these semi-detached binary systems. We derive the mass-transfer history of the magnetic cataclysmic variable AIM Herculis from its long-term visual light curve and from bolometric corrections obtained from a number of X-ray observations covering various accretion states. On average. less than 25% of the maximum observed mass-transfer rate occurs. Assuming that the mass-transfer rate is modulated by stellar starspots on the secondary stars, we convert the derived mass-transfer rates into spot filling factors at the L-1-point. A statistical model for the coverage fraction and size distribution of random spots near the L-1-point in AM Her suggests that the spot filling factor is roughly 0.5 for a fitted power-law law distribution of starspot radii, i.e. about half the surface of the star near the L-1-point is covered with spots. This density can only be explained if the spottedness of the L-1-point is unusual - for instance if spot groups are forced to wander towards the L-1-point - or if a large-scale magnetic spot group produced by an alpha (2)-dynamo slowly drifts in and out of the L-1-region. The former solution predicts that the occurrence of long-term high- and low-states is random and the latter that the long-term light curves of polars are quasi-periodic; the light curve of AM Her may suggest periods of order a decade. Finally, we discuss the relevance of this result to the mass-transfer variations of other cataclysmic variables
Supersoft X-ray binaries: an observational update
No full textRecent observational results on supersoft X-ray binaries are reviewed, with emphasis on the galactic source QR And and on the enigmatic LMC source RX J0439.8-6809. (C) 2000 Elsevier Science B.V. All rights reserved
The response of a dwarf nova disc to real mass transfer variations
No full textWe present simulations of dwarf nova outbursts taking into account realistic variations of the mass loss rate from the secondary. The mass transfer variation has been derived from 20 years of visual monitoring and from X-ray observations covering various accretion states of the discless cataclysmic variable AM Herculis. We find that the outburst behaviour of a fictitious dwarf nova with the same system parameters as AM Her is strongly influenced by these variations of the mass loss rate. Depending on the mass loss rate, the disc produces either long outbursts, a cycle of one long outburst followed by two short outbursts, or only short outbursts. The course of the transfer rate dominates the shape of the outbursts because the mass accreted during an outburst cycle roughly equals the mass transferred from the secondary over the outburst interval. Only for less than 10% of the simulated time, when the mass transfer rate is nearly constant, the disc is in a quasi-stationary state during which it periodically repeats the same cycle of outbursts. Consequently, assuming that the secondary stars in non-magnetic CV's do not differ from those in magnetic ones, our simulation indicates that probably all dwarf novae are rarely in a stationary state and are constantly adjusting to the prevailing value of the mass transfer rate from the secondary
On the occurrence of dwarf nova outbursts in post novae
No full textWe show that irradiation of the accretion disc by the white dwarf limits the occurrence of dwarf nova outbursts in post nova accretion discs. After the nova explosion, the white dwarf has to cool for up to similar to 100 yr - depending on the orbital period (i.e., disc size) and the temperature of the white dwarf after the nova-eruption - before the disc can begin producing dwarf nova outbursts. During this time the inner disc is maintained in a hot, ionised state. Using these calculations, we interpret the long-term photometric variability of the post nova V446 Her (Nova Her 1960) which shows today regular dwarf nova outbursts. As the white dwarf in V446 Her continues to cool over the next similar to 10-20 yr, we predict an increase in the amplitude of outbursts and a decrease of the outburst frequency, because the decreasing irradiation of the accretion disc should allow an increasing annular extent of the accretion disc to participate in limit cycle oscillations
Multi-wavelength spectrophotometry of EX Hydrae
No full textWe present phase-resolved infrared and optical spectrophotometry of the intermediate polar EXHya supplemented by archival ultraviolet data. The spin-modulated emission from the accretion funnel and the emission from the accretion disk or ring contain substantial optically thin components. The white dwarf dominates the unmodulated flux in the ultraviolet and is identified by numerous absorption lines. Metal absorption in the accretion curtain may add to the observed spectral features. The secondary star is of spectral type M4 +/- 1 and is detected by its ellipsoidal modulation. We derive a distance of 65 +/- 11 pc which makes EX Hydrae one of the closest cataclysmic variables with a known distance. The luminosity derived from the integrated overall spectral energy distribution is 3 x 10(32) erg s(-1). The accretion rate of 3 x 10(15) g s(-1) (for an 0.6 M-. white dwarf) is in reasonable agreement with the rates expected from angular momentum loss by gravitational radiation and from the observed spin-up of the white dwarf
CVcat: An interactive database on cataclysmic variables
No full textCVcat is a database that contains published data on cataclysmic variables and related objects. Unlike in the existing online sources, the users are allowed to add data to the catalogue. The concept of an "open catalogue" approach is reviewed together with the experience from one year of public usage of CVcat. New concepts to be included in the upcoming AstroCat framework and the next CVcat implementation are presented. CVcat can be found at http://www.cvcat.org
Magnetic white dwarfs in the Early Data Release of the Sloan Digital Sky Survey
No full textWe have identified 7 new magnetic DA white dwarfs in the
Early Data Release of the Sloan Digital Sky Survey. Our selection
strategy has also recovered all the previously known magnetic white
dwarfs contained in the SDSS EDR, KUV 03292+0035 and
HE 0330–0002. Analysing the SDSS fibre spectroscopy of the magnetic
DA white dwarfs with our state-of-the-art model spectra, we find
dipole field strengths 1.5\,\mbox{MG}\le B_\mathrm{d}\le 63 MG and
effective temperatures 8500\le\mbox{T_\mathrm{eff}}\le 39\,000 K. As a conservative
estimate, we expect that the complete SDSS will increase the number of
known magnetic white dwarfs by a factor 3
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