Optical observations of the high-mass X-ray binary MAXI J0709−159/LY Canis Majoris

Abstract We report on the optical spectroscopic monitoring of the X-ray transient MAXI J0709−159 (identified as the Be star LY CMa) performed for about 1.5 months after the X-ray detection with MAXI. The observed spectrum shows a double-peaked H$\alpha$ line with a peak-to-peak separation of $\sim$230 km s$^{-1}$, suggestive of the Be disk origin. We also detected a broad wing of the H$\alpha$ line with a line-of-sight velocity of $\gtrsim$900 km s$^{-1}$, which could be explained by the accretion disk of the compact object or a stellar wind from the Be star. Initially the H$\alpha$ line showed an asymmetric profile with an enhanced blue peak; the blue peak then decreased over $\sim$3 weeks to a similar strength to the red peak. We suggest that the evolution of the blue peak is associated with the X-ray activity and generated by the turbulence of the Be disk due to the passage of the compact object. We also investigated flux variation using the archival TESS data and found quasi-periodic variations with frequencies of $\sim$1 and $\sim$2 d$^{-1}$, which were likely caused by the pulsation of the B star. The overall variability properties on timescales of $\sim$days were similar to those in Be X-ray binaries, rather than supergiant X-ray binaries.