A NICER View of Spectral and Profile Evolution for Three X-ray Emitting Millisecond Pulsars*

We present two years of Neutron star Interior Composition Explorer (NICER) X-ray observations of three young and energetic rotation-powered millisecond pulsars: PSRs B1937+21, B1821−24, and J0218+4232. We fit Gaussians and Lorentzians to the pulse profiles for different energy sub-bands of the soft X-ray regime to measure the energy dependence of pulse separation and width. We find that the separation between pulse components of PSR J0218+4232 decreases with increasing energy at 3$\sigma$ confidence. Our phase-resolved spectral results provide updated constraints on the non-thermal X-ray emission of these three pulsars. The photon indices of the modeled X-ray emission spectra for each pulse component of PSR B1937+21 are inconsistent with each other at the 90\% confidence level.