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

TL;DR

This study systematically investigates millihertz quasiperiodic oscillations in the low-mass X-ray binary GS 1826-238, revealing their occurrence at specific accretion rates and linking flux modulations to seed photon temperature variations.

Contribution

It provides the first comprehensive analysis of mHz QPOs in GS 1826-238, demonstrating their association with marginally stable nuclear burning at a specific accretion rate.

Findings

37 intervals with mHz QPOs identified

QPOs occur at ~0.1 Eddington accretion rate

Flux modulations linked to seed photon temperature variations

Abstract

We performed a systematic investigation of millihertz quasiperiodic oscillations (mHz QPOs) in the low-mass X-ray binary GS 1826$-$238 observed with {\it NICER} and {\it Insight}-HXMT. We discovered 37 time intervals exhibiting mHz QPOs out of 106 Good Time Interval (GTI) samples in the frequency range of 3$-$17 mHz at a significance level of $>99.99\%$. 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 $\sim 0.1 \dot{M}_{\rm Edd}$, 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.