NuSTAR observation of LS 5039

Abstract

LS 5039 is a high-mass γ-ray binary hosting a compact object of unknown type. NuSTAR observed LS 5039 during its 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. (2020) 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 also did not find significant bursts or quasi-periodic variability. The modulation with the orbital period is clearly seen and remains unchanged over a decade long timescale 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 cutoff at higher energies. The slope of the spectrum anti-correlates with the flux during the binary orbit. Therefore, if LS 5039 hosts a young neutron star, its X-ray pulsations appear to be outshined 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.

Galactic Observatory Science with the ASTRI Mini-Array at the Observatorio del Teide

“…The difference between the two periods would imply a fast spin-down of the pulsar, and suggests a magnetar nature for the compact object in LS 5039. However, a new analysis of the same data performed by Volkov et al (2021) resulted in a very low statistical significance of this periodic signal, which casts doubts on its firm detection. This means that, if the compact object in LS 5039 is indeed a young neutron star, the intrabinary shock emission dominates the X-ray pulsations.…”

INTEGRAL View of TeV Sources: A Legacy for the CTA Project

“…LS 5039 is a variable and periodic γ-ray source [46][47][48][49][50][51], having the shortest orbital period among the γ-ray binaries (3.9 days). At first, it was considered to be a microquasar [51], then the source showed evidence of X-ray pulsations with a period of 9s suggesting that LS 5039 hosts a pulsar, although no radio pulsations have been reported to date [52,53].…”

Phase-resolved hard X-ray emission of the high-mass binary LS 5039: a spectral hardening above 50 keV detected with INTEGRAL

“…In the 3-10 keV band, NuSTAR data have only allowed Yoneda et al (2020) to set a ∼ 0.03 upper limit on the pulsed fraction. Another combined analysis of NuSTAR and Suzaku data by Volkov et al (2021) has also revealed the 9 s pulsations, but the authors considered them not significant enough to conclude on the presence of a neutron star in the binary. Hoffmann et al (2009) analysed about 3×10 6 s (3 Ms) of INTEGRAL/ISGRI exposures of LS 5039 and were able to show that its 25-200 keV lightcurve follows the same pattern as the emission observed in the keV and TeV ranges.…”