Investigation of systematic effects in Kepler data: Seasonal variations in the light curve of HAT-P-7b

TL;DR

This study analyzes Kepler data of HAT-P-7b to identify seasonal systematic effects in transit depth measurements, revealing about 1% variations that impact the accuracy of planetary radius estimates.

Contribution

It demonstrates the presence of seasonal systematic variations in Kepler transit data and highlights the importance of accounting for these effects in precise planetary measurements.

Findings

Seasonal variations in transit depth are about 1%.

No trends within individual seasons.

Systematic effects can lead to underestimated errors in radius measurements.

Abstract

With years of Kepler data currently available, it can now be attempted to measure variations in planetary transit depths over time. To do so, it is of primary importance to understand which systematic effects may affect the measurement of transits. We aim to measure the stability of Kepler measurements over years of observations. We present a study of the depth of about 500 transit events of the Hot Jupiter HAT-P-7b, using 14 quarters (Q0-Q13) of data from the Kepler Satellite. We find a systematic variation in the depth of the primary transit, related to quarters of data and recurring yearly. These seasonal variations are about 1%. Within seasons, we find no evidence for trends. We speculate that the cause of the seasonal variations could be unknown field crowding or instrumental artifacts. Our results show that care must be taken when combining transits throughout different quarters of Kepler data. Measuring the relative planetary radius of HAT-P-7b without taking these systematic effects into account leads to unrealistically low error estimates. This effect could be present in all Kepler targets. If so, relative radius measurements of all Hot Jupiters to a precision much better than 1% are unrealistic.