NICER spectral-timing studies of a low-frequency QPO in the soft-intermediate state of MAXI J1535-571

We present the discovery and preliminary spectral-timing analysis of a low-frequency QPO feature in observations of MAXI J1535-571 in the soft-intermediate state, obtained by the Neutron Star Interior Composition Explorer (NICER). The feature is relatively broad and weak, with a weak harmonic and additional low-amplitude broadband noise. These characteristics likely make it a weak Type B QPO, like one previously identified in the soft-intermediate state of the transient black hole X-ray binary XTE J1550-564. The lag-energy spectrum of the QPO shows soft lags of about 50 ms with an inflected spectral shape, flattening above ∼4 keV. Preliminary QPO-phase-resolved spectral analysis suggests that the inflected lag-energy spectrum is due to the disk blackbody component lagging the Comptonized power-law emission by ∼27% of a QPO cycle. This large phase shift has similar amplitude but opposite sign to that seen in Rossi X-ray Timing Explorer data for the Type B QPO from the transient black hole X-ray binary GX 339-4. We suggest that the Type B QPOs from these systems may have the same origin, in the form of a precessing jet-like corona illuminating the disk, with the different lag signs depending on the observer inclination angle.