The Optical Companion to the Binary Millisecond Pulsar J1824-2452H in the Globular Cluster M28

We report on the optical identification of the companion star to the eclipsing millisecond pulsar (MSP) PSR J1824-2452H in the galactic globular cluster M28 (NGC 6626). This star is at only 0farcs2 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 turnoff 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 (four out of seven), 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.