NuSTAR Observation of LS 5039

Abstract

Abstract LS 5039 is a high-mass γ-ray binary hosting a compact object of unknown type. NuSTAR observed LS 5039 during the entire 3.9-day binary period. We performed a periodic signal search up to 1000 Hz, which did not produce credible period candidates. We do see the 9.05 s period candidate, originally reported by Yoneda et al. using the same data, in the Fourier power spectrum, but we find that the statistical significance of this feature is too low to claim it as a real detection. We did not find significant bursts or quasiperiodic variability either. The modulation with orbital period is clearly seen and remains unchanged over a timescale of 10 years when compared to the earlier Suzaku light curve. The joint analysis of the NuSTAR and Suzaku XIS data shows that the 0.7–70 keV spectrum can be satisfactory described by a single absorbed power-law model with no evidence of a cutoff at higher energies. The slope of the spectrum is anticorrelated with the flux during the binary orbit. If LS 5039 hosts a young neutron star, its X-ray pulsations therefore appear to be outshone by the intrabinary shock emission. The lack of spectral lines and/or an exponential cutoff at higher energies suggests that the putative neutron star is not actively accreting. Although a black hole scenario still remains a possibility, the lack of variability or Fe Kα lines, which typically accompany accretion, makes it less likely.