A NICER View of Spectral and Profile Evolution for Three X-Ray-emitting Millisecond Pulsars
Enoto, Teruaki; Bogdanov, Slavko; Ray, Paul S.; Arzoumanian, Zaven; Guillot, Sebastien; Kerr, Matthew; Markwardt, Craig B.; Gendreau, Keith C.; Ransom, Scott; Lommen, Andrea; Wood, Kent S.; Harding, Alice; Deneva, Julia; Ludlam, Renee; Rowan, Dominick M.; Ghazi, Zaynab; Lugo, Lauren; Spano, Elizabeth; Venter, Christo; Lewandowska, Natalia
United States, South Africa, France, Japan
Abstract
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 >3σ 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.