Phase-resolved spectrum of the Crab pulsar from NICER

Abstract

Context. Studies of the high-energy emission regions of rotation-powered pulsars are typically based on folded light curves (FLCs) and phase-resolved spectra (PRS). Aims. This work uses the NICER observatory to obtain the highest resolution FLC and PRS of the Crab pulsar at soft X-ray energies. Methods. NICER has accumulated about 347 ksec of data on the Crab pulsar. The data were processed using the standard analysis pipeline. Stringent filtering was done for spectral analysis. The individual detectors are calibrated in terms of the long-time light curve (LTLC), raw spectrum, and deadtime. The arrival times of the photons are established in reference to the Solar System barycenter, while the rotation frequency ν and its time derivative ν˙ are used to derive the rotation phase of each photon. Results. The LTLCs, raw spectra, and deadtimes of the individual detectors are statistically similar; the latter two show no evolution with epoch and the detector deadtime is independent of photon energy. The deadtime for the Crab pulsar, taking into account the two types of deadtime, is only ≈7–8% larger than that obtained using the cleaned events. Detector 00 behaves slightly differently from the rest, but can be used for spectral work. The PRS of the two peaks of the Crab pulsar are obtained at a resolution that is better than 1∕512 in rotation phase. The FLC very close to the first peak rises slowly and falls faster. The spectral index of the PRS is almost constant very close to the first peak. Conclusions. The high-resolution FLC and PRS of the peaks of the Crab pulsar provide important constraints for the formation of caustics in the emission zone.