Phase-resolved X-ray spectroscopy of the Vela pulsar with NICER observations

Abstract

PSR J0835-4510 (a.k.a. the Vela pulsar) is the best studied pulsar in the Southern hemisphere given its high radio brightness and large glitch activity. However, its X-ray brightness is comparatively lower, making the study of its X-ray profile difficult. We aim to investigate the X-ray emission of the Vela pulsar coming from the surface of the neutron star to infer the properties of its hotspots and assess whether they share a common spectral shape. We also aim to prove whether the 2021 glitch affected the hotspots. We used ephemeris obtained from high-cadence radio observations with the radio telescopes of the Argentine Institute of Radio Astronomy to combine all the NICER observations of the Vela pulsar after its 2019 glitch and before its 2024 glitch. We obtained a folded pulse profile for three different energy ranges and subsequently studied their phase-resolved energy spectra. We additionally analyzed a Chandra observation to quantify the pulsar's contribution to the total soft X-ray emission detected by NICER, which cannot spatially resolve the pulsar from the surrounding pulsar wind nebula (PWN). The pulsar contribution to the total observed emission represents less than 40-45% of the flux below 0.9 keV, dropping to 3-4% in the 0.9-2.0 keV range and further down to ~2% above 2 keV, where the PWN fully dominates. We find that the X-ray folded profile of the Vela pulsar was not measurably perturbed by the 2021 glitch. We studied separately seven regions of the pulse profile. We found that all of them are well modeled by one or two blackbodies. The temperatures of these hotspots are between $0.79(7) \times 10^{6}$ K $\leq T \leq 4.8^{+0.8}_{-0.6} \times 10^{6}$ K, while their sizes vary between $0.048^{+0.016}_{-0.009}$ km $\leq R \leq 3.9^{+0.9}_{-0.6}$ km, and their luminosities between $5\times10^{30}$ erg/s $\lesssim L < 1\times10^{32}$ erg/s.