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We present the discovery of a low-frequency $\approx 5.7$ Hz quasi-periodicoscillation (QPO) feature in observations of the black hole X-ray binary MAXIJ1535-571 in its soft-intermediate state, obtained in September-October 2017 bythe Neutron Star Interior Composition Explorer (NICER). The feature isrelatively broad (compared to other low-frequency QPOs; quality factor$Q\approx 2$) and weak (1.9% rms in 3-10 keV), and is accompanied by a weakharmonic and low-amplitude broadband noise. These characteristics identify itas a weak Type A/B QPO, similar to ones previously identified in thesoft-intermediate state of the transient black hole X-ray binary XTE J1550-564.The lag-energy spectrum of the QPO shows increasing soft lags towards lowerenergies, approaching 50 ms at 1 keV (with respect to a 3-10 keV continuum).This large phase shift has similar amplitude but opposite sign to that seen inRossi X-ray Timing Explorer data for a Type B QPO from the transient black holeX-ray binary GX 339-4. Previous phase-resolved spectroscopy analysis of theType B QPO in GX 339-4 pointed towards a precessing jet-like coronailluminating the accretion disk as the origin of the QPO signal. We suggestthat this QPO in MAXI J1535-571 may have the same origin, with the differentlag sign depending on the scale height of the emitting region and the observerinclination angle.

A NICER Discovery of a Low-Frequency Quasi-Periodic Oscillation in the Soft-Intermediate State of MAXI J1535-571