Noise properties of the CoRoT data

Abstract

In this short paper, we study the photometric precision of stellar light curves obtained by the CoRoT satellite in its planet-finding channel, with a particular emphasis on the time scales characteristic of planetary transits. Together with other articles in the same issue of this journal, it forms an attempt to provide the building blocks for a statistical interpretation of the CoRoT planet and eclipsing binary catch to date. After pre-processing the light curves so as to minimise long-term variations and outliers, we measure the scatter of the light curves in the first three CoRoT runs lasting more than 1 month, using an iterative non-linear filter to isolate signal on the time scales of interest. The behaviour of the noise on 2 h time scales is described well by a power-law with index 0.25 in R-magnitude, ranging from 0.1 mmag at R = 11.5 to 1 mmag at R = 16, which is close to the pre-launch specification, though still a factor 2−3 above the photon noise due to residual jitter noise and hot pixel events. There is evidence of slight degradation in the performance over time. We find clear evidence of enhanced variability on hour time scales (at the level of 0.5 mmag) in stars identified as likely giants from their R magnitude and B − V colour, which represent approximately 60 and 20% of the observed population in the directions of Aquila and Monoceros, respectively. On the other hand, median correlated noise levels over 2 h for dwarf stars are extremely low, reaching 0.05 mmag at the bright end.

Transiting exoplanets from the CoRoT space mission

“…With that model, we computed the probability that a background eclipsing object is located at a distance 0.4 arcsec from a given CoRoT target, with an amplitude that produces an apparent transit depth lower than 5 × 10 −4 and with an SNR above the CoRoT detection threshold (defined in Aigrain et al 2009). We obtain an average of 4 × 10 −4 object in the simulation of the LRa01 field (≈10 4 stars) that exhibits such a small and detectable transit, a probability compatible with the upper limit we find here.…”

“…Because the transit signal is proportional to the planet's projected surface, the first published CoRoT results (Barge et al 2008;Alonso et al 2008;Deleuil et al 2008;Aigrain et al 2008;Moutou et al 2008) were focused on the population of rather massive planets, one of which has even been quoted as "the first inhabitant of the brown-dwarf desert", with a well-defined mass (21.7 ± 1 M Jup ) and a well-defined radius (1.0 ± 0.1 R Jup ) ). However, CoRoT has the capability of detecting significantly smaller planets, and analysis of the noise on the light curves (LC hereafter) indeed shows that in many cases it is not far from the photon noise limit (see Aigrain et al 2009). In the same line, blind tests performed by different teams of the CoRoT consortium on actual LCs where transits were added did confirm that the performances of CoRoT are such that ≈2 R Earth hot SuperEarth planets 1 are within reach.…”

Transiting exoplanets from the CoRoT space mission

“…To circumvent the issue of varying data weights associated with different time sampling, the oversampled section of the light curve was rebinned to 512 s sampling. Outliers were then identified and clipped out using a moving median filter (see Aigrain et al 2009, for details). Finally, we also discarded two segments of the light curve, in the CoRoT date ranges before 3031 days, and from 3152 to 3153.54 days (see Fig.…”

Transiting exoplanets from theCoRoTspace mission

“…The light curve was first pre-processed to remove out-oftransit variability as follows. Outliers were identified using an iterative non-linear filter (see Aigrain et al 2009), and a straight line was fitted to the region around each transit. The effect of contamination reported in Sect.…”