Broadband X-ray Properties of the Gamma-ray Binary 1FGL J1018.6-5856

TL;DR

This study presents detailed X-ray observations of the gamma-ray binary 1FGL J1018.6-5856, revealing its orbital period, flux modulation, spectral properties, and suggesting it may not be accretion-powered.

Contribution

First precise measurement of the orbital period and spectral characteristics of 1FGL J1018.6-5856 using multiple X-ray observatories, updating previous gamma-ray based estimates.

Findings

Orbital period measured at 16.544 days, differing from earlier gamma-ray estimates.

X-ray flux correlates with spectral hardness across all phases.

Broadband spectra fit with an unbroken power-law, indicating a possible non-accretion system.

Abstract

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.