Sign of hard X-ray pulsation from the gamma-ray binary system LS 5039

TL;DR

This study presents tentative evidence of a 9-second X-ray pulsation in LS 5039, indicating the compact object is likely a highly magnetized neutron star, possibly a magnetar, which is novel for a binary system.

Contribution

The paper provides the first evidence suggesting the presence of a magnetar in a gamma-ray binary system, based on X-ray pulsation detection and magnetic field estimates.

Findings

Tentative 9 s X-ray pulsation detected.

Inferred magnetic field strength of ~10^{10} T.

Compact object likely a magnetar, not a black hole.

Abstract

To understand the nature of the brightest gamma-ray binary system LS 5039, hard X-ray data of the object, taken with the Suzaku and NuSTAR observatories in 2007 and 2016, respectively, were analyzed. The two data sets jointly gave tentative evidence for a hard X-ray periodicity, with a period of $\sim 9$ s and a period increase rate by $\sim 3 \times 10^{-10}$ s s$^{-1}$. Therefore, the compact object in LS 5039 is inferred to be a rotating neutron star, rather than a black hole. Furthermore, several lines of arguments suggest that this object has a magnetic field of several times $\sim 10^{10}$ T, two orders of magnitude higher than those of typical neutron stars. The object is hence suggested to be a magnetar, which would be the first to be found in a binary. The results also suggest that the highly efficient particle acceleration process, known to be operating in LS 5039, emerges through interactions between dense stellar winds from the massive primary star, and ultra-strong magnetic fields of the magnetar.