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We report on the discovery of an absorption line at $E=8.56^{+0.05}_{-0.11}$keV detected with a significance of $>3.3\sigma$ in the NuSTAR and XMM-Newtonspectra of a newly discovered hyperluminous X-ray source (HLX, $L_{\rmX}>10^{41}$ ergs$^{-1}$) in the galaxy NGC 4045 at a distance of 32 Mpc. Thesource was first discovered serendipitously in a Swift/XRT observation of thegalaxy, and Swift monitoring reveals a highly variable source changing by overan order of magnitude from maximum to minimum. The origin of the absorptionline appears likely to be by highly ionized iron with a blue shift of 0.19$c$,indicating an ultrafast outflow (UFO). However, the large equivalent width ofthe line (EW$=-0.22^{+0.08}_{-0.09}$ keV) paired with the lack of otherabsorption lines detected are difficult to reconcile with models. Analternative explanation is that the line is due to a cyclotron resonancescattering feature (CRSF), produced by the interaction of X-ray photons withthe powerful magnetic field of a neutron star.

An 8.56 keV absorption line in the hyperluminous X-ray source in NGC 4045: ultra-fast outflow or cyclotron line?