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NICER Discovers mHz Oscillations in the "Clocked" Burster GS 1826-238We report the discovery with the Neutron Star Interior Composition Explorer (NICER) of mHz X-ray brightness oscillations from the "clocked burster" GS 1826−238. NICER observed the source in the periods 2017 June 20–29, July 11–13, and September 9–15, for a total useful exposure of 34 ks. Two consecutive dwells obtained on 2017 September 9 revealed highly significant oscillations at a frequency of 8 mHz. The fractional, sinusoidal modulation amplitude increases from 0.7% at 1 keV to ≈2% at 6 keV. Similar oscillations were also detected at lower significance in three additional dwells. The oscillation frequency and amplitude are consistent with those of mHz QPOs reported in other accreting neutron star systems. A thermonuclear X-ray burst was also observed on 2017 June 22. The burst properties and X-ray colors are both consistent with GS 1826 being in a soft spectral state during these observations, findings that are confirmed by ongoing monitoring with MAXI and SWIFT-BAT. Assuming that the mHz oscillations are associated with blackbody emission from the neutron star surface, modeling of the phase-resolved spectra shows that the oscillation is consistent with being produced by modulation of the temperature component of this emission. In this interpretation, the blackbody normalization, proportional to the emitting surface area, is consistent with being constant through the oscillation cycle. We place the observations in the context of the current theory of marginally stable burning and briefly discuss the potential for constraining neutron star properties using mHz oscillations.
Document ID
Document Type
Reprint (Version printed in journal)
Strohmayer, T. E. (NASA Goddard Space Flight Center Greenbelt, MD, United States)
Gendreau, K. C. (NASA Goddard Space Flight Center Greenbelt, MD, United States)
Altamirano, D. (University of Southampton Southampton, United Kingdom)
Arzoumanian, Z. (NASA Goddard Space Flight Center Greenbelt, MD, United States)
Bult, P. M. (NASA Goddard Space Flight Center Greenbelt, MD, United States)
Chakrabarty, D. (Massachusetts Institute of Technology (MIT) Cambridge, MA, United States)
Chenevez, J. (Technical University of Denmark Kongens Lyngby, Denmark)
Guillot, S. (Service de la Carte de la Vegetation CNRS Toulouse, France)
Guver, T. (İstanbul Üniversitesi İstanbul, Turkey)
Homan, J. (Eureka Scientific, Inc. Oakland, CA, United States)
Jaisawal, G. K. (Technical University of Denmark Kongens Lyngby, Denmark)
Keek, L. (Maryland Univ. College Park, MD, United States)
Mahmoodifar, S. (NASA Goddard Space Flight Center Greenbelt, MD, United States)
Miller, J. M. (Michigan Univ. Ann Arbor, MI, United States)
Ozel, F. (Arizona Univ. Tucson, AZ, United States)
Date Acquired
April 11, 2019
Publication Date
September 20, 2018
Publication Information
Publication: The Astrophysical Journal
Volume: 865
Issue: 1
ISSN: 0004-637X
Subject Category
Report/Patent Number
Funding Number(s)
Distribution Limits
Use by or on behalf of the US Gov. Permitted.

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