An 8.56 keV Absorption Line in the Hyperluminous X-Ray Source in NGC 4045: Ultrafast Outflow or Cyclotron Line?

Murray Brightman, Peter Kosec, Felix Fürst, Hannah Earnshaw, Marianne Heida, Matthew J Middleton, Daniel Stern, Dominic J Walton

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Abstract

We report on the discovery of an absorption line at E=8.56−0.11+0.05 keV detected with a significance of >3.3σ in the NuSTAR and XMM-Newton spectra of a newly discovered hyperluminous X-ray source (L X > 1041 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 from highly ionized iron with a blueshift of 0.19c, indicating an ultrafast outflow. However, the large equivalent width of the line ( EW=−0.22−0.09+0.08 keV) paired with the lack of other absorption lines detected is difficult to reconcile with models. An alternative explanation is that the line is due to a cyclotron resonance scattering feature produced by the interaction of X-ray photons with the powerful magnetic field of a neutron star.
Original languageEnglish
Article number138
Number of pages9
JournalThe Astrophysical Journal
Volume929
Issue number2
DOIs
Publication statusPublished - 21 Apr 2022

Keywords

  • 330
  • High-Energy Phenomena and Fundamental Physics

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