Physical origin of the QPO shoulder

Determine the physical origin of the QPO shoulder identified as a distinct variability component in the AstroSat/LAXPC observations of GRS 1915+105, and establish whether it represents a separate quasi-periodic oscillation coexisting with the QPO.

Background

The paper finds that the apparent energy-dependent shift of the QPO feature in GRS 1915+105 is better described by two Lorentzian components: a lower-frequency QPO and a higher-frequency QPO shoulder. The two components exhibit different fractional-rms amplitude and phase-lag spectra, suggesting that they may arise from separate variability processes. However, the shoulder cannot be definitively identified as a Type-B QPO because its quality factor is relatively low and additional characteristic signatures are absent. Its physical nature therefore remains unresolved, and the possibility that it represents a coexisting QPO is left for further investigation.

References

We therefore interpret the QPO shoulder as a distinct variability component whose physical origin remains uncertain, while noting that the possible coexistence of multiple QPOs is an intriguing possibility deserving further investigation.

— Complex Energy-Dependent Behaviour of Quasi-Periodic Oscillation Observed in GRS 1915+105  (2608.13068 - Sharma et al., 13 Aug 2026) in Section 5, Discussion, subsection “Alternative Explanation for Energy-Dependent QPO Frequency”