Looking for millisecond X-ray pulsations with NuSTAR

We report on NuSTAR observations of the recently active millisecond X-ray pulsar, J1739-285. The source undergoes period of X-ray activity every few years, during which type-I X-ray bursts can be observed. Oscillations in these bursts provided the first evidence for a spin period of 1122 Hz in 2007, making the source one of the fastest-spinning X-ray pulsars yet known. In September, 2019, the source was detected in outburst by the JEM-X X-ray monitor aboard INTEGRAL, and NuSTAR observations soon followed. We present the results of a thorough timing analysis of these follow-up observations, including searches for type-I X-ray bursts, pulsations, and quasi-periodic oscillations. In addition, we perform spectral modeling of the source in order to study the physical parameters of the system. We comment on the implications of our results on the spin history of the accreting neutron star and its accretion geometry.