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An Evolving Broad Iron Line from the First Galactic Ultraluminous X-Ray Pulsar Swift J0243.6+6124

Jaisawal, Gaurava K. and Wilson-Hodge, Colleen A. and Fabian, Andrew C. and Naik, Sachindra and Chakrabarty, Deepto and Kretschmar, Peter and Ballantyne, David R. and Ludlam, Renee M. and Chenevez, Jérôme and Altamirano, Diego and Arzoumanian, Zaven and Fürst, Felix and Gendreau, Keith C. and Guillot, Sebastien and Malacaria, Christian and Miller, Jon M. and Stevens, Abigail L. and Wolff, Michael T. (2019) An Evolving Broad Iron Line from the First Galactic Ultraluminous X-Ray Pulsar Swift J0243.6+6124. Astrophysical Journal, 885 (1). Art. No. 18. ISSN 1538-4357.

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We present a spectral study of the ultraluminous Be/X-ray transient pulsar Swift J0243.6+6124 using Neutron Star Interior Composition Explorer (NICER) observations during the system's 2017–2018 giant outburst. The 1.2–10 keV energy spectrum of the source can be approximated with an absorbed cutoff power-law model. We detect strong, luminosity-dependent emission lines in the 6–7 keV energy range. A narrow 6.42 keV line, observed in the sub-Eddington regime, is seen to evolve into a broad Fe-line profile in the super-Eddington regime. Other features are found at 6.67 and 6.97 keV in the Fe-line complex. An asymmetric broad-line profile, peaking at 6.67 keV, is possibly due to Doppler effects and gravitational redshift. The 1.2–79 keV broadband spectrum from Nuclear Spectroscopic Telescope Array (NuSTAR) and NICER observations at the outburst peak is well described by an absorbed cutoff power law plus multiple Gaussian lines and a blackbody component. Physical reflection models are also tested to probe the broad iron-line feature. Depending on the mass accretion rate, we found emission sites that are evolving from ~5000 km to a range closer to the surface of the neutron star. Our findings are discussed in the framework of the accretion disk and its implication on the magnetic field, the presence of optically thick accretion curtain in the magnetosphere, jet emission, and the massive, ultrafast outflow expected at super-Eddington accretion rates. We do not detect any signatures of a cyclotron absorption line in the NICER or NuSTAR data.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Jaisawal, Gaurava K.0000-0002-6789-2723
Wilson-Hodge, Colleen A.0000-0002-8585-0084
Fabian, Andrew C.0000-0002-9378-4072
Naik, Sachindra0000-0003-2865-4666
Chakrabarty, Deepto0000-0001-8804-8946
Kretschmar, Peter0000-0001-9840-2048
Ballantyne, David R.0000-0001-8128-6976
Ludlam, Renee M.0000-0002-8961-939X
Chenevez, Jérôme0000-0002-4397-8370
Altamirano, Diego0000-0002-3422-0074
Fürst, Felix0000-0003-0388-0560
Guillot, Sebastien0000-0002-6449-106X
Malacaria, Christian0000-0002-0380-0041
Stevens, Abigail L.0000-0002-5041-3079
Wolff, Michael T.0000-0002-4013-5650
Additional Information:© 2019. The American Astronomical Society. Received 2019 June 3; revised 2019 September 15; accepted 2019 September 16; published 2019 October 25. We sincerely thank the referee for useful suggestions on the paper. This work was supported by NASA through the NICER mission and the Astrophysics Explorers Program, and made use of data and software provided by the High Energy Astrophysics Science Archive Research Center (HEASARC). This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 713683. A.L.S. is supported by an NSF Astronomy and Astrophysics Postdoctoral Fellowship under award AST-1801792. D.A. acknowledges support from the Royal Society. R.M.L. acknowledges the support of NASA through Hubble Fellowship Program grant HST-HF2-51440.001. Facilities: ADS - , HEASARC - , NICER - , NuSTAR - The NuSTAR (Nuclear Spectroscopic Telescope Array) mission, Swift - Swift Gamma-Ray Burst Mission. Software: HEAsoft (v6.24), XSPEC (v12.10.0; Arnaud 1996), Coplrefl (Ballantyne et al. 2012), xillver (García et al. 2013), relxill (García et al. 2014), Veusz.
Funding AgencyGrant Number
Marie Curie Fellowship713683
NSF Astronomy and Astrophysics FellowshipAST-1801792
NASA Hubble FellowshipHST-HF2-51440.001
NASA Einstein FellowshipUNSPECIFIED
NASA Postdoctoral ProgramUNSPECIFIED
Subject Keywords:Accretion ; High mass X-ray binary stars ; Neutron stars ; Pulsars
Issue or Number:1
Classification Code:Unified Astronomy Thesaurus concepts: Accretion (14); High mass X-ray binary stars (733); Neutron stars (1108); Pulsars (1306)
Record Number:CaltechAUTHORS:20191025-133026345
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Official Citation:Gaurava K. Jaisawal et al 2019 ApJ 885 18
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:99459
Deposited By: George Porter
Deposited On:25 Oct 2019 21:57
Last Modified:25 Oct 2019 21:57

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