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

Brightman, Murray and Kosec, Peter and Fürst, Felix and Earnshaw, Hannah and Heida, Marianne and Middleton, Matthew J. and Stern, Daniel and Walton, Dominic J. (2022) An 8.56 keV Absorption Line in the Hyperluminous X-Ray Source in NGC 4045: Ultrafast Outflow or Cyclotron Line? Astrophysical Journal, 929 (2). Art. No. 138. ISSN 0004-637X. doi:10.3847/1538-4357/ac5e37.

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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 (Lₓ > 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.

Item Type:Article
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URLURL TypeDescription Paper
Brightman, Murray0000-0002-8147-2602
Fürst, Felix0000-0003-0388-0560
Earnshaw, Hannah0000-0001-5857-5622
Heida, Marianne0000-0002-1082-7496
Middleton, Matthew J.0000-0002-8183-2970
Stern, Daniel0000-0003-2686-9241
Walton, Dominic J.0000-0001-5819-3552
Alternate Title:An 8.56 keV absorption line in the hyperluminous X-ray source in NGC 4045: ultra-fast outflow or cyclotron line?
Additional Information:© 2022. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2022 January 14; revised 2022 March 8; accepted 2022 March 14; published 2022 April 21. We thank the anonymous referee for their review of this manuscript and for their constructive input. We wish to thank the Swift PI, Brad Cenko, for approving the target of opportunity requests we made to observe NGC 4045 ULX, as well as the rest of the Swift team for carrying them out. We also acknowledge the use of public data from the Swift data archive. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. We also wish to thank the NuSTAR PI, Fiona Harrison, for approving the DDT request we made to observe NGC 4045 ULX, as well as the NuSTAR SOC for carrying out the observation. This work was also supported under NASA contract No. NNG08FD60C. NuSTAR is a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA). In addition, we wish to thank Belinda Wilkes, Director of the Chandra X-ray Center for approving the DDT request to observe NGC 4045 ULX and the Chandra team for carrying out the observation. Finally, we wish to thank Timothy Kallman for help in generating XSTAR models for XSPEC that were used in the analysis. This work was also based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. This research has made use of data and/or software provided by the High Energy Astrophysics Science Archive Research Center (HEASARC), which is a service of the Astrophysics Science Division at NASA/GSFC. The work of D.S. was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. Facilities: Swift - Swift Gamma-Ray Burst Mission, NuSTAR - , CXO - , XMM. - Software: NuSTARDAS, XMMSAS (Gabriel et al. 2004), XSPEC (Arnaud 1996), spex, (Kaastra et al. 1996), hendrics (Bachetti 2015).
Group:Space Radiation Laboratory, NuSTAR
Funding AgencyGrant Number
Subject Keywords:Ultraluminous x-ray sources; X-ray sources; X-ray transient sources; Neutron stars; Black holes
Issue or Number:2
Classification Code:Unified Astronomy Thesaurus concepts: Ultraluminous x-ray sources (2164); X-ray sources (1822); X-ray transient sources (1852); Neutron stars (1108); Black holes (162)
Record Number:CaltechAUTHORS:20220525-90131000
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Official Citation:Murray Brightman et al 2022 ApJ 929 138
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:114911
Deposited By: George Porter
Deposited On:31 May 2022 14:38
Last Modified:31 May 2022 14:38

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