An active, asynchronous companion to a redback millisecond pulsar

TL;DR

This study presents long-term photometry of a redback millisecond pulsar's companion, revealing starspot activity, non-tidal locking, and constraints on system parameters, enhancing understanding of redback MSPs' optical variability.

Contribution

It provides detailed observational analysis of the companion star, highlighting starspots and non-tidal locking, which are novel insights into redback MSP systems.

Findings

Companion exhibits sporadic starspot activity.

The star is not tidally locked to the pulsar.

Constraints on irradiation efficiency and pulsar mass.

Abstract

PSR\,J1723$-$2837 is a "redback" millisecond pulsar (MSP) with a low-mass companion in a 14.8\,h orbit. The system's properties closely resemble those of "transitional" MSPs that alternate between spin-down and accretion-powered states. In this paper we report on long-term photometry of the 15.5\,mag companion to the pulsar. We use our data to illustrate that the star experiences sporadic activity, which we attribute to starspots. We also find that the companion is not tidally locked and infer $P_{\rm s}/P_{\rm b}= 0.9974(7)$ for the ratio between the rotational and orbital periods. Finally, we place constraints on various parameters, including the irradiation efficiency and pulsar mass. We discuss similarities with other redback MSPs and conclude that starspots may provide the most likely explanation for the often seen irregular and asymmetric optical lightcurves.