Swift observations of 1FGL J1018.6-5856

TL;DR

This study analyzes Swift X-ray observations of the gamma-ray binary 1FGL J1018.6-5856, revealing its flux modulation period, phase shifts, and a correlation between flux and spectral hardness, contributing to understanding its emission mechanisms.

Contribution

It provides the first detailed X-ray timing and spectral analysis of 1FGL J1018.6-5856, highlighting phase-dependent flux variations and flux-hardness correlations.

Findings

X-ray flux modulated at 16.57-day period

Maximum flux phase shifted to 0.3--0.4, not aligned with gamma-ray peak

Flux correlates with spectral hardness in 0.5--10 keV band

Abstract

We report on X-ray properties of the gamma-ray binary 1FGL J1018.6-5856 using observations obtained with the Swift X-ray telescope. Using 54 observations made between MJD 55575 and 55984, we find that the X-ray flux is modulated at a period of 16.57 +/- 0.11 days, consistent with previous reports based on gamma-ray data. We find that the X-ray maximum at phase 0 previously reported may not be a persistent feature of the source: the dramatic increases at phase 0 were detected only for ~100 days and not thereafter. Rather, the persistent sinusoidal maximum seems to be at phase 0.3--0.4, and is misaligned with the gamma-ray (GeV) peak. We also find evidence that the source's X-ray flux is correlated with the spectral hardness in the 0.5--10 keV band. Such a correlation has also been reported in the gamma-ray binaries LS 5039 and LS I+61d303 and can help us to understand the X-ray emission mechanisms of the sources.