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

TL;DR

This paper reports the discovery of an 8.56 keV absorption line in a hyperluminous X-ray source in NGC 4045, suggesting either an ultrafast outflow or a cyclotron line, with implications for understanding extreme accretion or magnetic fields.

Contribution

The study presents the first detection of an absorption line at this energy in a hyperluminous X-ray source, exploring its possible origins as an ultrafast outflow or cyclotron feature.

Findings

Detection of an 8.56 keV absorption line with >3.3σ significance.

Line likely caused by highly ionized iron with a blue shift of 0.19c.

Line characteristics are difficult to reconcile with simple models.

Abstract

We report on the discovery of an absorption line at $E=8.56^{+0.05}_{-0.11}$ keV detected with a significance of $>3.3\sigma$ in the NuSTAR and XMM-Newton spectra of a newly discovered hyperluminous X-ray source (HLX, $L_{\rm X}>10^{41}$ ergs$^{-1}$) 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 0.19$c$, 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.