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

TL;DR

This paper identifies a redback millisecond pulsar candidate in NGC 6397 with an unusually long orbital period of about 1.96 days, based on optical variability and multi-wavelength evidence.

Contribution

It reports the discovery of the longest orbital period redback candidate in a globular cluster, highlighting a new class of systems missed in previous pulsar searches.

Findings

Optical variability with a 1.96-day period suggests a redback system.

Candidate is the second redback in NGC 6397.

The orbital period is the longest among known redbacks in globular clusters.

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.