The full evolution of the type-C quasi-periodic oscillations in MAXI J1348-630 revealed by Insight-HXMT

Type-C quasi-periodic oscillations (QPOs) in black hole X-ray binaries serve as sensitive probes of accretion geometry and variability, although their full evolution throughout the entire outburst and the physics driving the evolution are not yet fully understood. Utilizing the intensive, broadband observations by Insight-HXMT of MAXI J1348-630, we aim to comprehensively investigate the properties and evolution of type-C QPOs during both its main 2019 outburst and subsequent mini-outbursts. We performed a comprehensive timing and spectral analysis of Insight-HXMT data. The evolution of QPO frequency, fractional rms, and energy dependence was tracked across spectral states, and correlations with X-ray flux and spectral shape were examined. (1) The type‑C QPO frequency rose from 0.24 Hz to 7.28 Hz during the main outburst rise and subsequently exhibited a nearly constant value of ∼7 Hz when the QPOs reappeared across different spectral states. (2) The fractional rms spectrum underwent a pronounced hardening following the transition from the hard to the hard-intermediate state. (3) The correlation between QPO frequency and X-ray flux exhibited strong hysteresis between the rise and decay phases; notably, the hysteresis loop reversed direction between the main and mini-outbursts. (4) In contrast, the frequency and hardness followed a tight, nearly single-track anticorrelation. Our results provide a complete picture of type-C QPO evolution. The stable reappearance frequency at ∼7 Hz indicates that the Compton region may reform at a consistent characteristic scale across different spectral states. Results (3) and (4) suggest that the type‑C QPO frequency evolution is governed more by spectral shape than by X-ray luminosity, while the reversed hysteresis provides a new perspective on the differences between the main and mini-outbursts, possibly originating from variations in the initial conditions.