Two NuSTAR observations of gamma-ray binary LS 5039: pulsation search and spectral variations with the orbital phase.

The Nuclear Spectroscopic Array (NuSTAR) observed the gamma-ray binary LS 5039 twice to look for a periodic signal. We do not detect the any periodic signal in the first observation alone and conclude that a 9-s candidate reported by Yoneda et al. (2021) is likely to be a noise fluctuation. With our second NuSTAR observation, we also fail to detect the proposed candidate signal, which was interpreted as an evidence of a magnetar. We present this result as well as the outcome of a more relaxed period search, which takes both observations into consideration. We also perform a comparison of the binary lightcurves from the two NuSTAR observations and the joint spectral fitting, establishing the phase dependence at a higher significance level and probing the existence of a spectral cutoff at higher energies.