Pei Jin, Mariano Méndez, Federico García, Diego Altamirano, Ruican Ma
We present a detailed timing analysis of the transition from the hard-intermediate state (HIMS) to the soft-intermediate state (SIMS) in MAXI J1820+070 using NICER observations. This transition is marked by a sharp drop of the broadband noise across both the soft and hard X-ray bands, the disappearance of the type-C quasi-periodic oscillation (QPO), the quenching of the steady, optically thick, compact jet, the appearance of a type-B QPO, and the detection of discrete, optically thin, radio ejections. For the first time, we detected a QPO at ∼3.5–5.9 Hz in the 2–12 keV power density spectrum of MAXI J1820+070 roughly half a day before the transition. It then appeared to evolve smoothly into the type-B QPO observed immediately after the transition. The location of this additional QPO component in the broadband root mean square (rms), as opposed to the QPO frequency plot, is consistent with that of the type-B QPOs in GX 339-4 and GRO J1655-40. This suggests a possible connection between this additional QPO in the HIMS and the type-B QPO in the SIMS. This result, together with recent findings in Swift J1727.8-1613, suggests that QPOs with these characteristics can emerge prior to the HIMS-to-SIMS transition and are not confined exclusively to the SIMS. If this additional QPO feature is the precursor of the type-B QPO in the SIMS, its presence prior to the transition (in contrast to the bright, optically thin, radio ejections appear at the transition itself) would imply that there might not be any direct physical connection between the type-B QPO and the discrete radio ejections. Our results further suggest a link between the disappearance of the type-C QPO, the drop in the broadband noise, and the emergence of discrete radio ejections at the HIMS-to-SIMS transition. We speculate that the simultaneous presence of such a QPO, non-harmonically related to the type-C QPO in the HIMS could be compatible with a spine–sheath outflow structure.