Apsidal motion in the α CrB binary system revealed through X-ray and optical eclipse timing

Abstract

The α CrB system is among the brightest known eclipsing binaries in an eccentric orbit. In such systems, the periods between successive primary (Pprim) and secondary minima (Psec) typically differ, and the difference between the two periods provides information on apsidal motion in the system. However, with a binary period of 17.36 days and eclipse durations of more than 8 h, these eclipses are difficult to observe from the ground. We present new photometric observations of primary and secondary eclipses of α CrB obtained with the TESS satellite and we re-examine the available X-ray observations of α CrB at the time of secondary optical minimum. We also combine the optical and X-ray data to determine precise values of Pprim and Psec to explore the consequences for apsidal motion in the α CrB system. We have determined an internal structure constant that is in good agreement with theoretical expectations and we show that there is no need to invoke any substantial misalignment between the rotation and orbital axes of the primary.