Sometimes They Come Back: Pulsations in A 0538–66 at 69 ms Rediscovered with NICER

We report on Neutron Star Interior Composition Explorer (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 ∼8 × 1038 erg s−1 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 ∼8 × 1036 erg s−1 (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 ∼1038 erg s−1, 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 ∼2.7 × 1010 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.