QPO time lags in MAXI J1348-630 and the origin of type-B QPOs in black-hole binaries

We present the results of the timing analysis of a set of 2019 NICER observations of the bright black-hole transient MAXI J1348-630. In these observations, a stable-frequency type-B QPO is observed. Adding all data yields a high-statistics power density spectrum that allows a detailed energy-dependent analysis. We extracted the QPO rms and phase lag spectra with high energy resolution from 0.8 to 12 keV, sampling the poorly explored energy region below 2 keV. The QPO fractional rms increases with energy from 0.5% to 10%. In addition to the known flattening at high energies, it also flattens at energies below 2 keV. Taking the 2-2.5 keV band as reference, the QPO phase lags increase towards higher energies up to 0.6 rad (corresponding to a time lag of 22 ms), as already known, but also surprisingly increase towards lower energies up to 0.8 rad (29 ms). Both increases appear monotonic, with a possible evidence of a feature between 6 and 7 keV. The energy spectrum can be fitted with a typical black-hole binary spectrum in the intermediate state. We discuss the physical origin of the time lags within Comptonization models.