Radio detection of an elusive millisecond pulsar in the Globular Cluster NGC 6397

TL;DR

The paper reports the discovery of a new millisecond pulsar in the globular cluster NGC 6397, with unique orbital characteristics, shedding light on faint binary pulsars and globular cluster evolution.

Contribution

It presents the first detection of a long-period eclipsing binary millisecond pulsar in NGC 6397, linking radio and X-ray sources and suggesting a new subgroup of faint, obscured binary pulsars.

Findings

Longest orbital period among eclipsing binaries in GCs

No X-ray flares detected during radio quiescence

Potential identification of a new subgroup of faint binary pulsars

Abstract

We report the discovery of a new 5.78 ms-period millisecond pulsar (MSP), PSR J1740-5340B (NGC 6397B), in an eclipsing binary system discovered with the Parkes radio telescope (now also known as Murriyang), Australia, and confirmed with the MeerKAT radio telescope in South Africa. The measured orbital period, 1.97 days, is the longest among all eclipsing binaries in globular clusters (GCs) and consistent with that of the coincident X-ray source U18, previously suggested to be a 'hidden MSP'. Our XMM-Newton observations during NGC 6397B's radio quiescent epochs detected no X-ray flares. NGC 6397B is either a transitional MSP or an eclipsing binary in its initial stage of mass transfer after the companion star left the main sequence. The discovery of NGC 6397B potentially reveals a subgroup of extremely faint and heavily obscured binary pulsars, thus providing a plausible explanation to the apparent dearth of binary neutron stars in core-collapsed GCs as well as a critical constraint on the evolution of GCs.