We report on NuSTAR, XMM-Newton and Swift observations of the gamma-ray
binary 1FGL J1018.6-5856. We measure the orbital period to be 16.544+/-0.008
days using Swift data spanning 1900 days. The orbital period is different from
the 2011 gamma-ray measurement which was used in the previous X-ray study of An
et al. (2013) using ~400 days of Swift data, but is consistent with a new
gamma-ray solution reported in 2014. The light curve folded on the new period
is qualitatively similar to that reported previously, having a spike at phase 0
and broad sinusoidal modulation. The X-ray flux enhancement at phase 0 occurs
more regularly in time than was previously suggested. A spiky structure at this
phase seems to be a persistent feature, although there is some variability.
Furthermore, we find that the source flux clearly correlates with the spectral
hardness throughout all orbital phases, and that the broadband X-ray spectra
measured with NuSTAR, XMM-Newton, and Swift are well fit with an unbroken
power-law model. This spectrum suggests that the system may not be
accretion-powered.Comment: 8 pages, 4 figures. Accepted for publication in Ap
