A 2 day orbital period for a redback millisecond pulsar candidate in the globular cluster NGC 6397
Manuel Pichardo Marcano, L.E. Rivera Sandoval, Thomas J. Maccarone, Yue Zhao, Craig O. Heinke
We report optical modulation of the companion to the X-ray source U18 in the
globular cluster NGC 6397. U18, with combined evidence from radio and X-ray
measurements, is a strong candidate as the second redback in this cluster,
initially missed in pulsar searches. This object is a bright variable star with
an anomalous red color and optical variability (\sim 0.2 mag in amplitude) with
a periodicity \sim 1.96 days that can be interpreted as the orbital period.
This value corresponds to the longest orbital period for known redback
candidates and confirmed systems in Galactic globular clusters and one of the
few with a period longer than 1 day.