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

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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.