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Neptunes in the Noise: Improved Precision in Exoplanet Transit Detection
AimИe Hall , Simon Hodgkin , Gabor Kovacs and Don Pollacco
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Institute of Astronomy, University of Cambridge 2 Department of Physics, University of Warwick
Figure 2 (right): Distribution of radii of all known exoplanets (grey), with those from Kepler (green) and SuperWASP (blue) also shown.

We present early results from a new analysis of raw data taken with the SuperWASP telescope[1]. This is shown to reduce the noise in the resulting lightcurves by up to a factor of 2 for the brightest stars compared to the existing pipeline. These new data are relatively free from systematics, and approach the millimagnitude level on transit duration timescales. Thus, we are able to detect smaller planets that pass in front of (transit) a star than those so far discovered by SuperWASP. Kepler analysis has suggested that small radius planets are more common than large ones, as seen in Figure 1 [2]. Figures 2 and 3 respectively show that current SuperWASP exoplanets are predominantly found to have large radii and are around stars dimmer than 11th magnitudes. By reducing the noise, our method can potentially find further, smaller planets around brighter stars which will be more valuable for follow-up.

Figure 3 (left): Distribution of magnitudes for stars found by SuperWASP to host transiting exoplanets.

Figure 1: Distribution of Kepler planet radii, showing that small radius planets are more common than larger ones.

Techniques used in reprocessing SuperWASP data Combining more flat field frames to reduce the photon noise contribution Ensuring chosen flat fields were not cloud-contaminated or saturated Co-located list driven photometry using soft-edged apertures Frame-to-frame photometric calibration using spatially dependent second order polynomial [3] Figure 4 shows our clear improvements made in lowering the rms noise when compared to the postsystematics correction results from the previous pipeline [4].

We used 7 SuperWASP fields that were observed on 30 nights between Nov 2011 and Feb 2012. Of these, only good quality frames were used in lightcurve generation. These have low average astrometric errors, source ellipticity and sky brightness. We also use the zero-point magnitudes to filter out frames suffering high extinction.
Figure 5: SuperWASP field distribution (logarithmic) Nov 2011 - Feb 2012.

On average, 70% of frames (4000-8000) are kept per field to create stellar lightcurves for stars with V magnitude > 12.

Figure 4: RMS-magnitude diagram for our SuperWASP analysis (black), the original pipeline (blue) and their pipeline's results after removing systematics (red).

With pure white noise statistics, the rms error in flux measurements will decrease with binning, and any deviation potentially implies residual correlated noise. Figure 6 show a field's flux rms error decreasing with binning for bright stars (8 < V < 11 , black) and the predicted binned rms for pure white noise (red). Deviations from white noise statistics here are 1mmag or less for bins of up to 2 hours.

Figure 6: RMS improvements with binning.

Candidate selection After reprocessing, the Box-Least-Squares algorithm (BLS) [5] fits trapezium transit models to the data to determine the best-fit period and depth of any variation. Where the BLS detection has a signal to pink noise SNR > 8, the results are analysed to remove candidates with periods close to 0.5,1 and 2 days, any with more than 80% of the in-transit data from a single night, or any with less than 3 transits. For the remaining candidates, we determined the stellar radius of those likely to be dwarfs via their reduced proper motion [6] and inspected the lightcurves of these visually to select the best candidates. These are represented in Figure 7, and show how, even with a limited dataset comprising of 30 nights, we have 53 candidates consistent with a transiting companion with twice Jupiter's radius or less.
Figure 7: Detected transit depths as a function of stellar radius (circles), coloured by stellar magnitude. The squares are the candidates shown below in Figure 8. Lines denote the predicted transit depth for planet with radius 2RJ (solid), RJ (dashed) and 0.5RJ (dotted).

Period: 2.54d

Period: 1.68d

Example Candidates Figure 8 shows phase-folded lightcurves for four candidates (seen as squares in Figure 7) with 3-10mmag depths around bright stars (V <10.5). The upper two plots show planet candidates compared to the two below which are likely to be binaries. The quality of these new lightcurves will enable us to easily distinguish between binary and planet candidates, and significantly reduce resource requirement for follow-up. XO-5b Single Transit Detection Figure 9 shows a phase curve of a single XO-5b transit detected in this analysis (SNR = 8.8), though it does not pass our selection tests as there Figure 9: XO-5b phase-folded lightcurve (black) and were not 3 or more transits. The BLS model has a mid-transit JD that differs from the literature by less than 6 minutes: just 3% of the duration. BLS model (red) for a single transit
Acknowledgements The author also thanks the WASP consortium*, Jonathan Irwin and Tom Louden. This research made use of the cross-match service provided by CDS and the VizieR catalogue access tool, Strasbourg.This work presented here is supported by a PhD studentship from Science and Technology Facilities Council.
*The WASP Consortium consists of astronomers primarily from the Queen's University Belfast, Keele, Leicester, The Open University, St Andrews, the Isaac Newton Group (La Palma), the Instituto de Astrofisica de Canarias (Tenerife) and the South African Astronomical Observatory. The WASP Cameras were constructed and are operated with funds made available from Consortium Universities and the UK's Science and Technology Facilities Council.

Period: 0.33d

Period: 0.31d

Figure 8: Phase-folded candidate lightcurves (black) and BLS model (red).
References

[1] D. L. Pollacco et al., Publ. Astron. Soc. Pac. 118 1407 (2006) [2] A. W. Howard et al., Ap. J. S. 201 15 (2012) [3] J. Irwin et al., MNRAS, 375 1449 (2007) [4] O. Tamuz, T. Mazeh and S. Zucker, MNRAS 356 1466 (2005) [5] G. KovАcs, S. Zucker and T. Mazeh, A&A, 391, 369 (2002) [6] Collier Cameron et al, MNRAS, 380, 3 (2007)