Sign of Hard-X-Ray Pulsation from the $\gamma$-Ray Binary System LS 5039

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 $\sim 9\mathrm{s}$ and a period increase rate by $\sim 3\times {10}^{-10}\mathrm{s}{\mathrm{s}}^{-1}$. 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 $\sim {10}^{10}\mathrm{T}$, 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 ultrastrong magnetic fields of the magnetar.

Figure 1

Evidence of the pulsation from LS 5039. (a) Fourier analysis of the 10–30 keV Suzaku data, where red lines indicate signal strengths that arise with a post-trial probability of $1.0\times {10}^{-3}$ when considering the look-elsewhere effect. The data are divided into subsets with $\mathrm{\Delta }T=4096$, 8192, and 16 384 s, and the power spectra from individual subsets are averaged. (b) $Z^2$ periodograms in 10–30 keV, shown for $P_{\mathrm{NS}}=7–11\mathrm{s}$. Panel (b-1) shows the Suzaku result (though the search was conducted over 1–100 s), averaged over 86 subsets with $\mathrm{\Delta }T=5000\mathrm{s}$, and panel (b-2) shows that of NuSTAR from 30 subsets with $\mathrm{\Delta }T=10000\mathrm{s}$.

Figure 2

The 10–30 keV folded pulse profiles from Suzaku (a) and NuSTAR (b), obtained using the best-estimated orbital parameters and optimum $P_{\mathrm{NS}}$ of the respective observations (Table 1).

Figure 3

$Z^2$ periodograms calculated coherently using the entire data of Suzaku (a) and NuSTAR (b), after the orbital-motion corrections using the best-fit parameters in Table 1. Here up to the fourth harmonics are incorporated.