The optical companion to the binary millisecond pulsar J1824-2452H in the globular cluster M28

TL;DR

This paper reports the optical identification of a companion star to a millisecond pulsar in globular cluster M28, revealing its variability and tidal distortion, and discusses implications for pulsar recycling in dense environments.

Contribution

It presents the first optical identification of the companion to PSR J1824-2452H in M28, showing variability and tidal distortion, and discusses the prevalence of such systems in globular clusters.

Findings

Companion star shows ~0.25 mag variability correlated with pulsar orbit.

The star is on the blue side of the main sequence, ~1.5 mag below the turn-off.

Optical identification increases known non-degenerate MSP companions in globular clusters.

Abstract

We report on the optical identification of the companion star to the eclipsing millisecond pulsar PSR J1824-2452H in the galactic globular cluster M28 (NGC 6626). This star is at only 0.2" from the nominal position of the pulsar and it shows optical variability (~ 0.25 mag) that nicely correlates with the pulsar orbital period. It is located on the blue side of the cluster main sequence, ~1.5 mag fainter than the turn-off point. The observed light curve shows two distinct and asymmetric minima, suggesting that the companion star is suffering tidal distortion from the pulsar. This discovery increases the number of non-degenerate MSP companions optically identified so far in globular clusters (4 out of 7), suggesting that these systems could be a common outcome of the pulsar recycling process, at least in dense environments where they can be originated by exchange interactions.