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

With the 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 to measure the absolute spectral-line shifts on a large number of points over the solar disk based on an extensive set of 5188-5212 angstrom region spectra taken through an iodine cell with the Domeless Solar 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 approximate to+ 400 m s(-1) near the disk center and increasing toward the limb up to a approximate to+600 m s(-1) (both with a standard deviation of sigma approximate to 100 m s(-1)). Interestingly, this trend tends to disappear when the convective shift due to granular motions (approximate to -300 m s(-1) 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 an averaged shift of 698 m s(-1) (sigma=113 m s(-1)). Considering the ambiguities involved in the absolute wavelength calibration or in the correction due to convective Doppler shifts (at least several tens m s(-1), or more likely up to less than or similar to 100 m s(-1)), we may regard this value to be well consistent with the expected gravitational redshift of 633 m s(-1).