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We have performed a comprehensive spectral and timing analysis of the firstNuSTAR observation of the high-mass X-ray binary 4U 1538-522. The observationcovers the X-ray eclipse of the source, plus the eclipse ingress and egress. Weuse the new measurement of the mid-eclipse time to update the orbitalparameters of the system and find marginally-significant evolution in theorbital period, with $\dot{P}_{\rm orb}/P_{\rm orb} = \left(-0.95 \pm0.37\right) \times 10^{-6}$ yr$^{-1}$. The cyclotron line energy is foundapproximately 1.2 keV higher than RXTE measurements from 1997--2003, in linewith the increased energy observed by Suzaku in 2012 and strengthening the casefor secular evolution of 4U 1538-522's CRSF. We additionally characterize thebehavior of the iron fluorescence and emission lines and line-of-sightabsorption as the source moves into and out of eclipse.

The First NuSTAR Observation of 4U 1538-522: Updated Orbital Ephemeris and A Strengthened Case for an Evolving Cyclotron Line Energy