monitoring variable stars. Most white dwarfs are thought to be stable photo- metrically and are often used as calibration standards for optical and ultra-violet.
15th European Workshop on White Dwarfs ASP Conference Series, Vol. 372, 2007 R. Napiwotzki and M. R. Burleigh
Monitoring Variable White Dwarfs with WASP F. Faedi,1 R. West,1 M. R. Burleigh,1 M. R. Goad,1 D. J. Christian,2 W. I. Clarkson,3,4 A. Collier Cameron,5 A. Evans,7 C. A. Haswell,3 C. Hellier,7 K. Horne,5 J. Irwin,8 S. R. Kane,5,6 T. A. Lister,5 A. J. Norton,3 D. Pollacco,2 I. Skillen,9 R. A. Street,2 and P. J. Wheatley10 1 Department
of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK 2 Department
of Physics and Astronomy, Queen’s University of Belfast, University Road, Belfast BT7 1NN, UK 3 Department
of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK 4 STScI,
3700 San Martin Drive, Baltimore, MD 21218, USA
5 School
of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, KY16 9SS, UK 6 Department
of Astronomy, University of Florida, 211 Bryant Space Science Center, Gainesville, FL 32611-2055, USA 7 Astrophysics
Group, Keele University, Keele, Staffordshire, ST5 5BG, UK 8 Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA, UK 9 ING,
Apartado de Correos 321, E-38700 Santa Cruz de La Palma, Tenerife, Spain 10 Department
of Physics, University of Warwick, Coventry CV4 7AL,
UK Abstract. The SuperWASP project is an ultra wide angle search for extra solar planetary transits. However SuperWASP can also be used extensively for monitoring variable stars. Most white dwarfs are thought to be stable photometrically and are often used as calibration standards for optical and ultra-violet observations. However, a few white dwarfs are intrinsically variable. For example, the ZZ Ceti stars are non-radial pulsators with variability time scales of a few hundred seconds. White dwarfs in close, detached binaries, often display optical variability such as eclipses, from which radii can be determined, or the effects of reflection and irradiation on a low mass companion. Rare magnetic white dwarfs often display star-spots which rotate in and out of view, revealing the spin period of the star. SuperWASP will monitor several hundred white dwarfs brighter than ∼14th magnitude on time scales from 10 minutes to months. The majority of these stars have never previously been tested for photometric variability. These observations will reveal new close binary systems, particularly those with optically undetectable very low mass (brown dwarf) companions, and enable us
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1.
Introduction
The SuperWASP-North telescope, was commissioned in 2004 with a complement of five cameras, and performed a pilot survey through May–October of that year acquiring 323,325 images covering 10,800 squares degrees of the Northern sky. Our observing strategy was to cyclically raster the sky in a series of fields centred on the current Local Sidereal Time (LST) and spaced by ∼ 1 hour in Right Ascension. Each observation lasted ∼ 1 minute (30 sec exposure, plus slew and telescope settling time). This strategy yielded well sampled light-curves with a typical cadence of 8 minutes. We obtained good quality photometry for stars in the range V ∼ 8 − 15, with a photometric precision of < 1% down to V ∼ 12. 1.1.
WD sample in the WASP 2004 Pilot Survey
Our white dwarf sample is the result of a cross-correlation between the McCook & Sion (1999) catalogue and WASP data taken from the 2004 observing season, and comprises 109 unique matches. In the full sky survey (starting 2006), we expect to observe between 400–500 white dwarfs. To demonstrate the potential of the WASP survey telescopes we present an analysis of a known variable white dwarf system MS Peg (with a magnitude V of ∼ 13.7, for which we can achieve a photometric precision ≃ 1.5% in a single exposure). The pre-cataclysmic variable MS Peg (WD 2256+249 = EG 232 = GD 245), designated 1SWASP J225848.13+251544.0, was observed routinely throughout the 2004 pilot season. MS Peg falls in an overlap region between two WASP cameras (cameras 1 and 4), hence we obtain two independent flux measurements at each epoch. Here we present data from camera 1 only, a total of 3486 epochs spanning 98 nights in the interval 25th May to 29th September 2004. 2.
Data Analysis
WASP data suffer from a number of systematic effects which need to be removed. This we achieve using the Tamuz algorithm (Tamuz et al. 2005), which searches for linear combinations of systematic effects in the data, for example, those associated with atmospheric extinction, detector efficiency, point spread function, etc. Using a large sample of light-curves, the algorithm searches for the best set of coefficients, here representing stellar extinction and air-mass, that minimise the global expression: S2 =
X (Fij − ci · aj )2 ij
2 σij
(1)
where ci is the extinction coefficient of the i-th star, aj is the airmass of the j-th image, Fij is the WASP flux of the star i in the image j, and σij is the uncertainty in the flux of star i in image j. It first derives the best-fitting extinction
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coefficients of each star of the sample assuming a given set of air-masses, and calculates the best-fitting air-masses taking into account the extinction coefficients. This algorithm deals with linear systematics effects in cases where the measurement uncertainties are unequal. We then proceed and search for the (1) (1) next linear effect hidden in the data: we call the first set of parameters ci , aj , (1)
(1)
(1)
we then remove this effect as follows: Fij = Fij − ci · aj . Our experience with the Tamuz algorithm suggests that four iterations are sufficient to remove the most significant linear effects present in the data. 3.
Preliminary Results
To demonstrate the power of this technique we have analysed five months of observations of the pre-cataclysmic variable white dwarf system MS Peg as observed by WASP in 2004. We searched the light-curve for periodic signals using the Lomb–Scargle algorithm (Press et al. 1992). The most significant peak in the periodogram (Figure 1, top panel) falls at 15004.8 seconds (4.1680 hours), consistent with the published period of 4.1679±0.0003 hours, (see Schmidt et al. 1995). The raw data were then folded on this period and binned into 100 phase bins; the resulting phase-folded light-curve shows a clear sinusoidal modulation with a peak-to-peak amplitude of ∼ 3% (Figure 1, bottom panel ). This is somewhat smaller than the amplitude reported by Schmidt et al. (1995), (∼ 10% in V and ∼ 5% in B), and may be due to dilution of the variable signal from MS Peg by light from another star falling within our photometry aperture (37.5” radius). Nevertheless this analysis amply demonstrates that the SuperWASP telescopes are capable of detecting variability in bright white dwarfs at the level of a few percent over an interval of hours or days. References McCook, G. P., & Sion, E. M. 1999, ApJS, 121, 1 Press, W. H., et al. 1992, Numerical Recipes, (Cambridge: Cambridge University Press) Schmidt, G. D., Smith, P. S., Harvey, D. A., & Grauer, A. D. 1995, AJ, 110, 398 Tamuz, O., Mazeh, T., & Zucker, S. 2005, MNRAS, 356, 1466
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arbitrary phase
Figure 1. In the top panel (power vs. frequency) we present the periodogram of the source MS Peg, the most significant peak falls at 15004.8 sec (4.1680 hours). In the bottom panel (flux vs. phase) we present the phasefolded light-curve with a sinusoidal modulation, the peak-to-peak amplitude is ∼ 3%.
