SuperWASP Variable Stars

Q1 Is there a unique classification for each lightcurve?

Not necessarily. Sometimes it can be difficult to distinguish between an EW type eclipsing binary whose lightcurve is folded at half the true period; a rotationally modulated star (whether due to star spots or ellipsoidal modulation) and a pulsating star. The folded lightcurves of all three will show a single maximum and a single minimum per cycle - but it is possible to distinguish most of them if you look closely, as described below.

The folded lightcurves of an EW type and a rotationally modulated star will be symmetrical in time either side of their peak, whilst the folded lightcurve of a pulsating star will generally have an asymmetric peak with a steeper rise in brightness and a shallower fall in brightness.

The folded lightcurve of an EW type generally has minima that are slightly narrower than the maxima whilst that of a rotationally modulated star is usually sinusoidal with equal width maxima and minima. Consequently, turning an EW type star's folded lightcurve upside down it will look different, whereas turning a rotationally modulated star's folded lightcurve upside down it will look the same!

Q2 What causes the apparently periodic structure in junk lightcurves?

This can be due to a number of factors - including systematic noise that occurs at the beginning and end of each night's observations, stray light from the Moon entering the telescopes, or other effects due to temperature changes in the detectors which change the focus of the telescopes slightly.

Q3 What are the numbers above each folded lightcurve?

These are the period (in seconds, in hours, and in days) at which the lightcurve has been folded and so corresponds to the period length of the cycle that is plotted. Periods detected will be between around an hour and a year or so.

Q4 What are the units of the flux on the vertical axis?

The flux is simply the SuperWASP count rate measured in white light. The faintest stars detected are of 15th magnitude and will give approximately 1 count per second. Brighter stars of 10th magnitude will give about 100 counts per second. A star that is just visible to the naked eye would be 5th magnitude and give about 10,000 counts per second.

Q5 Can we detect exoplanets in these lightcurves?

SuperWASP has detected lots (150+) of hot Jupiter exoplanets. These will typically have "dips" of <1% in brightness and repeat every few days. The period-searching which has produced the folded lightcurves in this project is not designed to find these types of variation though. Narrow eclipses of a few percent depth in brightness seen in some of these light curves will usually indicate grazing incidence eclipsing binaries (usually EA type).

Q6 Sometimes I see folded light curves with a short period modulation superimposed on the main one - what’s that?

Often light curves display systemmatic noise due to the way the data are obtained. This often varies with a period such as 1d, 1/2d , 1/3d, 1/4d, etc. So if a light curve is folded at a period close to (say) 11/3 days (=3.67d) then it may show 11 “clumps” of data per cycle, each with a short period modulation of 1/3d. Any such small number integer multiples will give rise to this effect.

Q7 Sometimes I see folded lightcurves with data points grouped into two traces - one above the other - what's that?

This happens because of the way the photometry is extracted from the images. An aperture is placed around the star position and the flux within the aperture is measured. But as slight focus shifts occur during a night or night-to-night, so sometimes part of another star's image may stray into the aperture (or out of the aperture) so increasing (or decreasing) the total flux within the aperture. The lightcurve therefore contains some data points with just flux from the target star but other data points with (or without) flux from other nearby stars.

Q8 Is there a duplication between this project and the other Zooniverse project called Variable Star Zoo?

No, not really. That other project uses the VVV survey (Vista Variables in the Via Lactea) to look for variable stars in the centre of the Milky Way. SuperWASP observes the whole sky except for the Galactic plane and Galactic centre, since the stars are too close together there to be resolved by SuperWASP's wide field cameras. Also - SuperWASP has a long base line (10 years) with high cadence observations (every few minutes) and focuses on bright stars (V ~ 8 - 15), whereas the VVV has a shorter baseline and worse cadence, but goes to much fainter magnitudes. So the two surveys (and hence the two Zooniverse projects) are very complementary.