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

Dominick M. Rowan, Zaynab Ghazi, Lauren Lugo, Elizabeth Spano, Andrea Lommen, Alice Harding, Christo Venter, Renee Ludlam, Paul S. Ray, Matthew Kerr, Zaven Arzoumanian, Slavko Bogdanov, Julia Deneva, Sebastien Guillot, Natalia Lewandowska, Craig B. Markwardt, Scott Ransom, Teruaki Enoto, Kent S. Wood, Keith Gendreau

Published 2020-01-30Version 1

We present two years of Neutron star Interior Composition Explorer (NICER) X-ray observations of three energetic rotation-powered millisecond pulsars (MSPs): 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 $\gt 3\sigma$ confidence. The 95% upper limit on pulse separation evolution for PSRs B1937+21 and B1821-24 is less than 2 milliperiods per keV. 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, suggesting different emission origins for each pulse. We find that the PSR B1821-24 and PSR J0218+4232 emission spectra are invariant with phase at the 90% confidence level. We describe the implications of our profile and spectral results in the context of equatorial current sheet emission models for these three MSPs with non-thermal, magnetospheric X-ray emission.