Study of millihertz quasiperiodic oscillations in the 2023 outburst of 4U 0115+63 using NuSTAR

The Be/X-ray binary 4U 0115+63 underwent a Type II giant outburst in 2023, reaching a peak luminosity of ∼ 10^ ̊m erg ̊m s^. We analyzed two NuSTAR observations, one during a high state and one during a low state. The high-state data reveal quasiperiodic oscillations (QPOs) at 1. 1 and 71. 9^, mHz, while the low-state data show peaks at 2. 4, mHz, and 67. 6^, mHz. Focusing on the ∼1, mHz and ∼2, mHz QPOs, we found no significant correlation between their frequencies and luminosity. Common models such as the beat frequency model, Keplerian frequency model, disk precession, and disk thermal instability failed to explain these QPOs. Additionally, the energy dependence of the two QPOs differed: the rms of ∼1, mHz QPO decreased with energy below 10, keV and then increased, and its phase-resolved spectra softened at the peak phase. In contrast, the rms of ∼2, mHz QPO remained stable, and its spectra exhibited no phase-dependent variations. This suggests that the two QPOs are driven by different physical (albeit unknown) mechanisms.