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NICER and Fermi GBM observations of the first galactic ultraluminous X-ray pulsar Swift J0243.6+6124

NICER and Fermi GBM observations of the first galactic ultraluminous X-ray pulsar Swift J0243.6+6124
NICER and Fermi GBM observations of the first galactic ultraluminous X-ray pulsar Swift J0243.6+6124
Swift J0243.6+6124 is a newly discovered Galactic Be/X-ray binary, revealed in late September 2017 in a giant outburst with a peak luminosity of 2x10^39(d/7 kpc)^2 erg s^-1 (0.1-10 keV), with no formerly reported activity. At this luminosity, Swift J0243.6+6124 is the first known galactic ultraluminous X-ray pulsar. We describe Neutron star Interior Composition Explorer (NICER) and Fermi Gamma-ray Burst Monitor (GBM) timing and spectral analyses for this source. A new orbital ephemeris is obtained for the binary system using spin-frequencies measured with GBM and 15-50 keV fluxes measured with the Neil Gehrels Swift Observatory Burst Alert Telescope to model the system's intrinsic spin-up. Power spectra measured with NICER show considerable evolution with luminosity, including a quasi-periodic oscillation (QPO) near 50 mHz that is omnipresent at low luminosity and has an evolving central frequency. Pulse profiles measured over the combined 0.2-100 keV range show complex evolution that is both luminosity and energy dependent. Near the critical luminosity of L~10^38 erg s^-1, the pulse profiles transition from single-peaked to double peaked, the pulsed fraction reaches a minimum in all energy bands, and the hardness ratios in both NICER and GBM show a turn-over to softening as the intensity increases. This behavior repeats as the outburst rises and fades, indicating two distinct accretion regimes. These two regimes are suggestive of the accretion structure on the neutron star surface transitioning from a Coulomb collisional stopping mechanism at lower luminosities to a radiation-dominated stopping mechanism at higher luminosities. This is the highest observed (to date) value of the critical luminosity, suggesting a magnetic fieeld of B~10^13 G.
0004-637X
1-20
Wilson-Hodge, Colleen A.
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Malacaria, Christian
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Jenke, Peter A.
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Jaisawal, Gaurava K.
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Kerr, Matthew
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Wolff, Michael T.
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Arzoumanian, Zaven
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Chakrabarty, Deepto
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Doty, John P.
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Gendreau, Keith C.
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Guillot, Sebastien
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Ho, Wynn
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LaMarr, Beverly
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Markwardt, Craig B.
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Ozel, Feryal
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Prigozhin, Gregory Y.
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Ray, Paul S.
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Ramos-Lerate, Mercedes
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Remillard, Ronald A.
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Strohmayer, Tod E.
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Vezie, Michael L.
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Wood, Kent S.
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Wilson-Hodge, Colleen A.
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Malacaria, Christian
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Jenke, Peter A.
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Jaisawal, Gaurava K.
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Kerr, Matthew
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Wolff, Michael T.
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Arzoumanian, Zaven
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Chakrabarty, Deepto
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Doty, John P.
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Gendreau, Keith C.
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Guillot, Sebastien
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Ho, Wynn
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LaMarr, Beverly
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Markwardt, Craig B.
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Ozel, Feryal
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Prigozhin, Gregory Y.
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Ray, Paul S.
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Ramos-Lerate, Mercedes
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Remillard, Ronald A.
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Strohmayer, Tod E.
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Vezie, Michael L.
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Wood, Kent S.
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Wilson-Hodge, Colleen A., Malacaria, Christian, Jenke, Peter A., Jaisawal, Gaurava K., Kerr, Matthew, Wolff, Michael T., Arzoumanian, Zaven, Chakrabarty, Deepto, Doty, John P., Gendreau, Keith C., Guillot, Sebastien, Ho, Wynn, LaMarr, Beverly, Markwardt, Craig B., Ozel, Feryal, Prigozhin, Gregory Y., Ray, Paul S., Ramos-Lerate, Mercedes, Remillard, Ronald A., Strohmayer, Tod E., Vezie, Michael L. and Wood, Kent S. (2018) NICER and Fermi GBM observations of the first galactic ultraluminous X-ray pulsar Swift J0243.6+6124. The Astrophysical Journal, 863 (1), 1-20. (doi:10.3847/1538-4357/aace60).

Record type: Article

Abstract

Swift J0243.6+6124 is a newly discovered Galactic Be/X-ray binary, revealed in late September 2017 in a giant outburst with a peak luminosity of 2x10^39(d/7 kpc)^2 erg s^-1 (0.1-10 keV), with no formerly reported activity. At this luminosity, Swift J0243.6+6124 is the first known galactic ultraluminous X-ray pulsar. We describe Neutron star Interior Composition Explorer (NICER) and Fermi Gamma-ray Burst Monitor (GBM) timing and spectral analyses for this source. A new orbital ephemeris is obtained for the binary system using spin-frequencies measured with GBM and 15-50 keV fluxes measured with the Neil Gehrels Swift Observatory Burst Alert Telescope to model the system's intrinsic spin-up. Power spectra measured with NICER show considerable evolution with luminosity, including a quasi-periodic oscillation (QPO) near 50 mHz that is omnipresent at low luminosity and has an evolving central frequency. Pulse profiles measured over the combined 0.2-100 keV range show complex evolution that is both luminosity and energy dependent. Near the critical luminosity of L~10^38 erg s^-1, the pulse profiles transition from single-peaked to double peaked, the pulsed fraction reaches a minimum in all energy bands, and the hardness ratios in both NICER and GBM show a turn-over to softening as the intensity increases. This behavior repeats as the outburst rises and fades, indicating two distinct accretion regimes. These two regimes are suggestive of the accretion structure on the neutron star surface transitioning from a Coulomb collisional stopping mechanism at lower luminosities to a radiation-dominated stopping mechanism at higher luminosities. This is the highest observed (to date) value of the critical luminosity, suggesting a magnetic fieeld of B~10^13 G.

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1806.10094 - Accepted Manuscript
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Accepted/In Press date: 19 June 2018
e-pub ahead of print date: 6 August 2018
Published date: 10 August 2018

Identifiers

Local EPrints ID: 421983
URI: http://eprints.soton.ac.uk/id/eprint/421983
ISSN: 0004-637X
PURE UUID: 71c416cf-a86d-4d83-8ab3-834650c2014d
ORCID for Wynn Ho: ORCID iD orcid.org/0000-0002-6089-6836

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Date deposited: 12 Jul 2018 16:30
Last modified: 17 Dec 2019 01:42

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Contributors

Author: Colleen A. Wilson-Hodge
Author: Christian Malacaria
Author: Peter A. Jenke
Author: Gaurava K. Jaisawal
Author: Matthew Kerr
Author: Michael T. Wolff
Author: Zaven Arzoumanian
Author: Deepto Chakrabarty
Author: John P. Doty
Author: Keith C. Gendreau
Author: Sebastien Guillot
Author: Wynn Ho ORCID iD
Author: Beverly LaMarr
Author: Craig B. Markwardt
Author: Feryal Ozel
Author: Gregory Y. Prigozhin
Author: Paul S. Ray
Author: Mercedes Ramos-Lerate
Author: Ronald A. Remillard
Author: Tod E. Strohmayer
Author: Michael L. Vezie
Author: Kent S. Wood

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