A 2 day orbital period for a redback millisecond pulsar candidate in the globular cluster NGC 6397

Abstract

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.

Accepted for publication as a letter in MNRAS

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Cited by in corpus (4)

• A Census of X-ray Millisecond Pulsars in Globular Clusters
• Radio detection of an elusive millisecond pulsar in the Globular Cluster NGC 6397
• A time-series VI study of the variable stars of the globular cluster NGC 6397
• A VLITE Search for Millisecond Pulsars in Globular Clusters: Discovery of a Pulsar in GLIMPSE-C01