The Extreme Stellar-Signals Project III. Combining Solar Data from HARPS, HARPS-N, EXPRES, and NEID

TL;DR

This study compares high-precision solar radial velocity measurements from four stabilized spectrographs, demonstrating their remarkable agreement and the potential for improved stellar variability analysis.

Contribution

It provides a comprehensive comparison of multiple high-resolution spectrographs observing the Sun simultaneously, highlighting their precision and instrumental systematics.

Findings

Residual intra-day scatter of 15-30 cm/s among instruments

Current instruments achieve unprecedented measurement precision

Solar surface flows can be isolated from orbital signals

Abstract

We present an analysis of Sun-as-a-star observations from four different high-resolution, stabilized spectrographs -- HARPS, HARPS-N, EXPRES, and NEID. With simultaneous observations of the Sun from four different instruments, we are able to gain insight into the radial velocity precision and accuracy delivered by each of these instruments and isolate instrumental systematics that differ from true astrophysical signals. With solar observations, we can completely characterize the expected Doppler shift contributed by orbiting Solar System bodies and remove them. This results in a data set with measured velocity variations that purely trace flows on the solar surface. Direct comparisons of the radial velocities measured by each instrument show remarkable agreement with residual intra-day scatter of only 15-30 cm/s. This shows that current ultra-stabilized instruments have broken through to a new level of measurement precision that reveals stellar variability with high fidelity and detail. We end by discussing how radial velocities from different instruments can be combined to provide powerful leverage for testing techniques to mitigate stellar signals.