Detection of Gravitational Redshift on the Solar Disk by Using Iodine-Cell Technique

TL;DR

This study measures solar spectral line shifts to observationally confirm the gravitational redshift, accounting for convective motions, and finds results consistent with theoretical predictions within measurement uncertainties.

Contribution

It provides the first detailed observational confirmation of solar gravitational redshift using iodine-cell spectroscopy and accounts for convective Doppler shifts.

Findings

Measured line shifts vary from +400 m/s at disk center to +600 m/s near limb.

Subtracting convective shifts yields an average shift of 698 m/s.

Results are consistent with the expected gravitational redshift of 633 m/s.

Abstract

With an aim to examine whether the predicted solar gravitational redshift can be observationally confirmed under the influence of the convective Doppler shift due to granular motions, we attempted measuring the absolute spectral line-shifts on a large number of points over the solar disk based on an extensive set of 5188-5212A region spectra taken through an iodine-cell with the Solar Domeless Telescope at Hida Observatory. The resulting heliocentric line shifts at the meridian line (where no rotational shift exists), which were derived by finding the best-fit parameterized model spectrum with the observed spectrum and corrected for the earth's motion, turned out to be weakly position-dependent as ~ +400 m/s near the disk center and increasing toward the limb up to ~ +600 m/s (both with a standard deviation of sigma ~ 100 m/s). Interestingly, this trend tends to disappear when the convectiveshift due to granular motions (~-300 m/s at the disk center and increasing toward the limb; simulated based on the two-component model along with the empirical center-to-limb variation) is subtracted, finally resulting in the averaged shift of 698 m/s (sigma = 113 m/s). Considering the ambiguities involved in the absolute wavelength calibration or in the correction due to convective Doppler shifts (at least several tens m/s, or more likely up to <~100 m/s), we may regard that this value is well consistent with the expected gravitational redshift of 633 m/s.