2025ApJ...990...87S

We report on a quasiperiodic variation at ∼1 Hz during a fast X-ray outburst of a high-mass X-ray binary MAXI J0709-159/LY CMa observed by the Neutron star Interior Composition ExploreR (NICER). The new X-ray transient MAXI J0709-159 was discovered on 2022 January 25. Due to the transient X-ray behavior characterized by the short (a few hours) outburst duration, rapid (≲1 s) variability with spectral change, and large luminosity swing from 10³² erg s^–1 to 10³⁷ erg s^–1, the object was considered to likely be a supergiant X-ray binary with a neutron star (NS) categorized as a supergiant fast X-ray transient. Follow-up NICER and NuSTAR observations confirmed that the position of the new X-ray object is consistent with a Be star, LY CMa, which has also been identified as a B supergiant. We analyzed the NICER data obtained from 3 hr to 6 days after the discovery. The light curve reveals that the X-ray activity continued for ∼7 hr in sparse short flares, each lasting ≲100 s, and the luminosity instantaneously reached up to ∼1 × 10³⁸ erg s^–1. The light-curve and spectral features reasonably agree with those expected from accretion of a clumpy stellar wind onto a magnetized NS. The variability power spectrum during the brightest flare shows a broad peak at 1.1 Hz resembling a quasiperiodic oscillation (QPO). If the QPO is attributed to the Keplerian orbital frequency at the inner edge of a transient accretion disk truncated by the NS magnetosphere, the NS surface magnetic field is estimated to be ∼10¹² G.