A lack of 9-s periodicity in the follow-up NuSTAR observation of LS 5039

TL;DR

The second NuSTAR observation of LS 5039 did not confirm the previously reported 9-second periodicity, suggesting it was noise, but confirmed phase-dependent spectral features.

Contribution

This study provides a follow-up analysis that refutes the earlier periodic signal claim and reinforces phase-dependent spectral characteristics of LS 5039.

Findings

No detection of the 9-s periodicity in the second observation.

Confirmation of phase-dependent spectral features.

The previous periodic signal was likely a noise fluctuation.

Abstract

The Nuclear Spectroscopic Array (NuSTAR) observed the gamma-ray binary LS 5039 for a second time in order to check for the presence of a periodic signal candidate found in the data from the previous NuSTAR observation. We do not detect the candidate signal in the vicinity of its previously reported frequency, assuming the same orbital ephemeris as in our previous paper. This implies that the previously reported periodic signal candidate was a noise fluctuation. We also perform a comparison of the lightcurves from the two NuSTAR observations and the joint spectral fitting. Our spectral analysis confirms the phase-dependence found from a single NuSTAR observation at a higher significance level.