# High-Precision X-ray Timing of Three Millisecond Pulsars with NICER: Stability Estimates and Comparison with Radio

## TL;DR

This study demonstrates high-precision X-ray timing of millisecond pulsars using NICER, achieving stability comparable to theoretical limits and providing insights into pulsar timing noise without the interstellar medium effects present in radio observations.

## Contribution

First high-precision X-ray timing analysis of millisecond pulsars with NICER, showing stability estimates and comparison with radio data to understand timing noise.

## Key findings

- NICER achieves timing uncertainties near theoretical limits.
- Timing stability of ^{-14} for PSR B1937+21.
- No observed break point in _z within NICER data span.

## Abstract

The Neutron Star Interior Composition Explorer (NICER) is an X-ray astrophysics payload on the International Space Station. It enables unprecedented high-precision timing of millisecond pulsars without the pulse broadening and delays due to dispersion and scattering within the interstellar medium that plague radio timing. We present initial timing results from a year of data on the millisecond pulsars PSR B1937+21 and PSR J0218+4232, and nine months of data on PSR B1821-24. NICER time-of-arrival uncertainties for the three pulsars are consistent with theoretical lower bounds and simulations based on their pulse shape templates and average source and background photon count rates. To estimate timing stability, we use the $\sigma_z$ measure, which is based on the average of the cubic coefficients of polynomial fits to subsets of timing residuals. So far we are achieving timing stabilities $\sigma_z \approx 3 \times 10^{-14}$ for PSR B1937+21 and on the order of $10^{-12}$ for PSRs B1821$-$24 and J0218+4232. Within the span of our \textit{NICER} data we do not yet see the characteristic break point in the slope of $\sigma_z$; detection of such a break would indicate that further improvement in the cumulative root-mean-square (RMS) timing residual is limited by timing noise. We see this break point in our comparison radio data sets for PSR B1821-24 and PSR B1937+21 on time scales of $> 2$ years.