Broadband Properties of the Harmonic of Type-C Quasiperiodic Oscillation in MAXI J1348-630

Abstract Harmonics are common features of quasiperiodic oscillations (QPOs) in black hole X-ray binaries; however, their physical origins remain poorly understood. Using broadband Insight-HXMT data, we investigated the Type-C QPO harmonic in MAXI J1348−630. The harmonic is significantly detected exclusively during the hard intermediate state and displays prominent energy-dependent properties: while it is strong in the soft X-ray band (<10 keV) with its fractional rms amplitude even exceeding that of the fundamental QPO, it is much weaker in the hard X-ray band (>10 keV), where its rms amplitude is generally several times lower than that of the fundamental. Furthermore, the harmonic shows no significant phase coupling with the fundamental in the soft X-ray band, whereas strong coupling is present in the hard X-ray band. These features point to a complex, energy-dependent origin for the harmonic. We propose that the hard X-ray harmonic may arise from nonlinear distortion of the fundamental waveform within the corona, while the soft X-ray harmonic is likely produced via a distinct physical process, such as the reflection emission from the inner disk.