Effect of tides on the orbital evolution of the redback system PSR 1723-2837

Abstract

The standard model of stellar evolution in close binary systems assumes that during mass transfer episodes, the system is in a synchronized and circularized state. Remarkably, the redback system PSR J1723–2837 has an orbital period derivative P˙orb too large to be explained by this model. Motivated by this fact, we investigate the action of tidal forces in between two consecutive mass transfer episodes for a system under irradiation feedback, which is a plausible progenitor for PSR J1723–2837. We base our analysis on Hut’s treatment of equilibrium tidal evolution, generalized by considering the donor as a two layers object that may not rotate as a rigid body. We also analyse three different relations for the viscosity with the tidal forcing frequency. We found that the large value measured for P˙orb can be reached by systems where the donor star rotates slower (by few per cent) than the orbit just after mass transfer episodes. Van Staden & Antoniadis have observed this object and reported a lack of synchronism, opposite to that required by the Hut’s theory to account for the observed P˙orb⁠. Motivated by this discrepancy, we analyse photometric data obtained by the spacecraft Kepler second mission K2, with the purpose of identifying the periods present in PSR J1723–2837. We notice several periods close to those of the orbit and the rotation. The obtained periods pattern reveals the presence of a more complex phenomenology, which would not be well described in the frame of the weak friction model of equilibrium tides.