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

Abstract

We report on the discovery of an absorption line at E = 8.56 +0.05 −0.11 keV detected with a significance of > 3.3σ in the NuSTAR and XMM-Newton spectra of a newly discovered hyperluminous X-ray source (HLX, L X > 10 41 erg s −1 ) in the galaxy NGC 4045 at a distance of 32 Mpc. The source was first discovered serendipitously in a Swift/XRT observation of the galaxy, and Swift monitoring reveals a highly variable source changing by over an order of magnitude from maximum to minimum. The origin of the absorption line appears likely to be by highly ionized iron with a blue shift of 0.19c, indicating an ultrafast outflow (UFO). However, the large equivalent width of the line (EW= −0.22 +0.08 −0.09 keV) paired with the lack of other absorption lines detected are difficult to reconcile with models. An alternative explanation is that the line is due to a cyclotron resonance scattering feature (CRSF), produced by the interaction of X-ray photons with the powerful magnetic field of a neutron star.