A NICER View of Spectral and Profile Evolution for Three X-Ray-emitting Millisecond Pulsars
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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- April 2020
- DOI:
- 10.3847/1538-4357/ab718f
- arXiv:
- arXiv:2001.11513
- Bibcode:
- 2020ApJ...892..150R
- Keywords:
-
- Millisecond pulsars;
- Neutron stars;
- X-ray astronomy;
- 1062;
- 1108;
- 1810;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 16 pages, 13 figures, accepted for publication in ApJ