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

TL;DR

This study provides a comprehensive analysis of type-C QPOs in MAXI J1348-630 using Insight-HXMT data, revealing new evolutionary features, spectral hardening, and hysteresis effects linked to outburst behaviors.

Contribution

It uncovers stable QPO frequencies, spectral hardening during state transitions, and hysteresis in frequency-flux relations, advancing understanding of QPO evolution in black hole binaries.

Findings

Type-C QPOs show a stable ~7 Hz frequency despite intensity variations.

Harder fractional rms spectrum with high-energy shift during state transition.

Hysteresis observed in QPO frequency-flux relation between main and mini-outbursts.

Abstract

Based on abundant data from Insight-HXMT, we conducted a detailed analysis of type-C quasi-periodic oscillations (QPOs) in the black hole X-ray binary MAXI J1348-630. Type-C QPOs were intensively detected over a broad energy band, with frequencies ranging from 0.24 to 10.3 Hz, and several new evolutionary features were identified. First, although type-C QPOs reappear intermittently, they show a stable characteristic frequency around 7 Hz. This implies a characteristic spatial scale for the QPO emission region, despite large variations in outburst intensity. Second, from the hard state to the hard-intermediate state, type-C QPOs display a harder fractional rms spectrum, with the rms peak shifting toward high energies (>20 keV) and an amplitude exceeding 10 %. This hard rms spectrum favors a high-energy origin for type-C QPOs. The spectral hardening occurs simultaneously with the weakening of the compact jet, suggesting a physical connection between these two processes. Finally, we observed hysteresis in the QPO frequency-flux relation, with the hysteresis loop evolving in opposite directions between the main and mini-outbursts. This offers a new perspective on the physical differences between the two outburst types, which may arise from variations in initial magnetic field conditions.