The gravitational redshift of Sirius B

TL;DR

This paper accurately measures the gravitational redshift of Sirius B using a novel spectroscopic method, confirming its mass aligns with theoretical predictions and previous dynamical measurements.

Contribution

A new observational strategy using HST to measure Sirius B's gravitational redshift relative to Sirius A, reducing systematic uncertainties.

Findings

Measured gravitational redshift of 80.65 km/s

Derived white dwarf mass of 1.017 solar masses

Results agree with dynamical mass and theoretical models

Abstract

Einstein's theory of General Relativity predicts that the light from stars will be gravitationally shifted to longer wavelengths. We previously used this effect to measure the mass of the white dwarf Sirius B from the wavelength shift observed in its H-alpha line based on spectroscopic data from the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST), but found that the results did not agree with the dynamical mass determined from the visual-binary orbit. We have devised a new observing strategy using STIS where the shift is measured relative to the H-alpha line of Sirius A rather than comparing it to a laboratory based rest wavelength. Sirius A was observed during the same orbit with HST. This strategy circumvents the systematic uncertainties which have affected previous attempts to measure Sirius B. We measure a gravitational redshift of 80.65 +/- 0.77 km/s. From the measured gravitational redshift and the known radius, we find a mass of 1.017 +/- 0.025 Msol which is in agreement with the dynamical mass and the predictions of a C/O white dwarf mass-radius relation with a precision of 2.5 per cent.