NICER Discovers mHz Oscillations in the "Clocked" Burster GS 1826-238

Abstract

We report the discovery with the Neutron Star Interior Composition Explorer (NICER) of mHz X-ray brightness oscillations from the "clocked burster." 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 approximately 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 the source 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 black body 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 black body 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.

26 pages, 10 figures, accepted for publication in the Astrophysical Journal

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• Polarization properties of the weakly magnetized neutron star X-ray binary GS 1826-238 in the high soft state
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• Dense Matter in Neutron Stars with eXTP
• Burst-induced coronal cooling in GS 1826-24 The clock wagging its tail
• Discovery of millihertz quasi-periodic oscillations in the X-ray binary EXO 0748--676
• StrayCats: A catalog of NuSTAR Stray Light Observations
• Phase-resolved analyses of mHz quasi-periodic oscillations in 4U 1636-53 using Hilbert-Huang transform
• Detection of Millihertz Quasi-Periodic Oscillations in the X-Ray Binary 1RXS J180408.9$-$342058
• Constraining the geometry of the dipping atoll 4U 1624-49 with X-ray spectroscopy and polarimetry
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• Drifts of the marginally stable burning frequency in the X-ray binaries 4U 1608--52 and Aql X--1
• Discovery of an accretion-rate independent absolute RMS amplitude of millihertz quasi-periodic oscillations in 4U 1636-53
• AstroSat view of `Clocked` burster GS 1826-238:broad-band spectral nature of persistent and burst emission during soft state
• Revealing the spectral state transition of the Clocked Burster, GS 1826-238 with NuSTAR StrayCats
• The harmonic component of the millihertz quasi-periodic oscillations in 4U 1636-53
• Connecting the m-dots: accretion rates and thermonuclear burst recurrence times on neutron stars and white dwarfs
• A solution to the tension of burning on neutron stars and nuclear physics
• X-ray polarization of Z-type neutron star low-mass X-ray binaries -- II. Spectropolarimetric analysis