NuSTAR Analysis of a Low-Frequency QPO in the Black Hole MAXI J1535-571

MAXI J1535-571 is a recently discovered transient X-ray black hole candidate that exhibited strong low-frequency quasi-periodic oscillations in its bright 2017-2018 outburst, as indicated by observations from the Neutron Star Interior Composition Explorer (NICER) (e.g., Stevens et al. 2018). To follow up on the behavior of these QPOs and their energy dependence, we present X-ray timing results from 12 observations obtained by the Nuclear Spectroscopic Telescope Array (NuSTAR). Due to NuSTAR's deadtime properties, we analyze the cospectra of MAXI J1535-571, rather than the power spectra, using tools developed by Bachetti and Huppenkothen (2018). The feature varies in frequency, with the peak occurring at an average frequency across observations of approximately 2.5 Hz; the QPO appears to be relatively broad and weak. We explore the energy dependence of the QPO and its lag behavior in the NuSTAR data, for comparison to NICER and the Type B QPO discovered by Stevens et al. (2018).