Gaussian process analysis of type-B quasiperiodic oscillations in the black hole X-ray binary MAXI J1348-630

Abstract

We analyzed Insight-HXMT data of the black hole X-ray binary MAXI J1348-630 during the type-B QPO phase of its 2019 outburst. Using the Gaussian process method, we applied an additive composite kernel model consisting of an SHO, a DRW, and an additional white noise (AWN) to data from three energy bands: LE (1-10 keV), ME (10-30 keV), and HE (30-150 keV). We find that for the DRW component, correlations on the timescale of $\tau_{\rm DRW}\sim10$ s are absent in the LE band, while they persist in the ME and HE bands over the full duration of the light curves. This energy-dependent behavior may reflect thermal instabilities, with the shorter correlation timescale in the disk compared to the corona. Alternatively, it may reflect variable Comptonizations of seed photons from different disk regions. Inner-disk photons are scattered by a small inner corona, producing soft X-rays. Outer-disk photons interact with an extended, jet-like corona, resulting in harder emission. The QPO is captured by an SHO component with a stable period of $\sim 0.2$ s and a high quality factor of $\sim 10$. The absence of significant evolution with energy or time of the SHO component suggests a connection between the accretion disk and the corona, which may be built by coherent oscillations of disk-corona driven by magnetorotational instability. The AWN components are present in all the three-band data and dominate over the DRW and SHO components. We interpret the AWN as another fast DRW with its $\tau_{\rm DRW} < 0.01$ s. It may trace high-frequency fluctuations that occur in both the inner region of the accretion disk and the corona. Overall, our work reveals a timescale hierarchy in the coupled disk-corona scenario: fast DRW < SHO < disk DRW < corona DRW.