A Systematic Study of Millihertz Quasiperiodic Oscillations in GS 1826−238

Abstract We performed a systematic investigation of millihertz quasiperiodic oscillations (mHz QPOs) in the low-mass X-ray binary GS 1826−238 observed with NICER and Insight-HXMT. We discovered 37 time intervals exhibiting mHz QPOs out of 106 Good Time Interval (GTI) samples in the frequency range of 3–17 mHz at a significance level of &gt;99.99%. The source remains in a soft state in our study. No significant differences are found between the samples with and without mHz QPOs according to positions in the color–color and hardness–intensity diagrams. These QPOs were discovered at an accretion rate of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>∼</mml:mo> <mml:mn>0.1</mml:mn> <mml:msub> <mml:mrow> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">Edd</mml:mi> </mml:mrow> </mml:msub> </mml:math> , similar to other sources. The broadband spectrum of GS 1826–238 can be modeled as a combination of a multicolor blackbody from the accretion disk and a Comptonization with seed photons emitted from the neutron star (NS) surface. The flux modulations of mHz QPOs are related to variations of the temperature of Comptonization seed photons, consistent with the marginally stable burning theory.