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Description
The solar telescope connected to HARPS-N has observed the Sun since the summer of 2015. Such high-cadence, longbaseline data set is crucial for understanding spurious radial-velocity signals induced by our Sun and by the instrument. Aims. This manuscript describes the data reduction performed to obtain unprecedented radial-velocity precision for the three years of solar data released along with this paper. The nearly continuous observation of our Sun has allowed us to detect sub-m/s systematics in the HARPS-N solar data reduced by the current data reduction software. To improve the radial-velocity precision of the solar data, we reduced them using the new ESPRESSO data reduction software and developed new recipes to mitigate the detected systematics. The most significant improvement brought by the new data reduction is a strong decrease in the day-to-day radial-velocity scatter, from 1.28 to 1.09m/s; this is thanks to a more stable method to derive wavelength solutions, but also to the use of calibrations closer in time. We also demonstrate that the current HARPS-N data reduction software induces a long-term drift of 1.2m/s, due to the use of non-stable thorium lines. As a result, the old solar RVs are weakly correlated to the solar magnetic cycle, which is not expected. On the contrary, the newly derived radial velocities are much more correlated, with a Pearson correlation coefficient of 0.93. We also discuss a special correction for ghost contamination, to extract a calcium activity index free from instrumental systematics. Our work leads toward a better understanding of the instrumental and data reduction systematics affecting the HARPSN spectrograph. The new solar data released, representing an unprecedented time-series of 34550 high-resolution spectra and precise radial velocities will be crucial to understanding stellar activity signals of solar-type stars, with the goal of enabling the detection of other Earths.
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