Sometimes they come back: pulsations in A 0538-66 at 69 ms rediscovered with NICER

TL;DR

NICER observations of A 0538-66 revealed rare 69 ms pulsations, indicating a potentially very weak magnetic field in this high-mass X-ray binary, with implications for magnetic field evolution and accretion processes.

Contribution

This study reports the first detection of pulsations in A 0538-66 since 1980, providing new insights into its magnetic field strength and accretion behavior.

Findings

Detected 69 ms pulsations during a short NICER observation.

Estimated the neutron star magnetic field to be below ~2.7E10 G.

Discussed possible mechanisms for pulsation detection and magnetic field implications.

Abstract

We report on NICER X-ray observations of the Be X-ray binary A 0538-66 located in the Large Magellanic Cloud. Fast pulsations (69 ms) in this source were discovered in 1980 during a bright outburst in which it reached a luminosity of ~8E38 erg/s, but were never reobserved since then. We clearly detected the pulsations at P=69.3055 +/- 0.0005 ms with a pulsed fraction of ~20% during a short time interval (~11 minutes) on 2023 January 9, when A 0538-66 had a luminosity of ~8E36 erg/s (0.3-10 keV). The pulsations were not detected in other NICER observations (total exposure ~162.7 ks), during which A 0538-66 had a similar or lower luminosity. On 2023 February 8-9 the source exhibited a strong variability, with short flares reaching ~1E38 erg/s, but no periodic pulsations were detected. Assuming the magnetospheric radius lies within the corotation radius during pulsations, we estimate the neutron star magnetic field is below ~2.7E10 G. This would make A 0538-66 the high-mass X-ray binary with the weakest known magnetic field. We discuss implications for magnetic field evolution in accreting pulsars and propose that, alternatively, A 0538-66 has a stronger magnetic field and during the NICER detection, a centrifugal barrier may have been active while part of the plasma accumulated at the magnetosphere sporadically leaked through it via an instability mechanism, allowing accretion onto the polar caps.