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.
@article{arxiv.1505.02096,
title = {Broadband X-ray Properties of the Gamma-ray Binary 1FGL J1018.6-5856},
author = {Hongjun An and Eric Bellm and Varun Bhalerao and Steven E. Boggs and Finn E. Christensen and William W. Craig and Felix Fuerst and Charles J. Hailey and Fiona A. Harrison and Victoria M. Kaspi and Lorenzo Natalucci and Daniel Stern and John A. Tomsick and William W. Zhang},
journal= {arXiv preprint arXiv:1505.02096},
year = {2015}
}
Comments
8 pages, 4 figures. Accepted for publication in ApJ