Proper motion, spectra, and timing of PSR J1813-1749 using Chandra and NICER

PSR J1813-1749 is one of the most energetic rotation-powered pulsars known, producing a pulsar wind nebula (PWN) and gamma-ray and TeV emission, but whose spin period is only measurable in X-ray. We present analysis of two Chandra datasets that are separated by more than ten years and recent NICER data. The long baseline of the Chandra data allows us to derive a pulsar proper motion ${\mathrm{micro}}_{\text{R.A.}}=-(0_{{}^{.}}^{″}067\pm 0_{{}^{.}}^{″}010)$ yr^-1 and ${\mathrm{micro}}_{\text{decl.}}=-(0_{{}^{.}}^{″}014\pm 0_{{}^{.}}^{″}007)$ yr^-1 and velocity v _⊥ ≈ 900-1600 km s^-1 (assuming a distance d = 3 - 5 kpc), although we cannot exclude a contribution to the change in measured pulsar position due to a change in brightness structure of the PWN very near the pulsar. We model the PWN and pulsar spectra using an absorbed power law and obtain best-fit absorption N _H = (13.1 ± 0.9) × 10²² cm^-2, photon index Γ = 1.5 ± 0.1, and 0.3-10 keV luminosity L _X ≈ 5.4 × 10³⁴ erg s^-1(d/ 5 kpc)² for the PWN and Γ = 1.2 ± 0.1 and L _X « 9.3 × 10³³ erg s^-1(d/ 5 kpc)² for PSR J1813-1749. These values do not change between the 2006 and 2016 observations. We use NICER observations from 2019 to obtain a timing model of PSR J1813-1749, with spin frequency ν = 22.35 Hz and spin frequency time derivative $\stackrel{.}{v}=(-6.428\pm 0.003)\times {10}^{-11}$ Hz s^-1. We also fit ν measurements from 2009-2012 and our 2019 value and find a long-term spin-down rate $\stackrel{.}{v}=(-6.3445\pm 0.0004)\times {10}^{-11}$ Hz s^-1. We speculate that the difference in spin-down rates is due to glitch activity or emission mode switching.