Reflection symmetry in the folded light curve of the Crab pulsar from NICER

Abstract

ABSTRACT The rotation powered pulsars Crab, Vela, and Geminga have double peaked folded light curves (FLC) at gamma-ray energies that have an approximate reflection symmetry. Here, this aspect is studied at soft X-ray energy by analysing a high-resolution FLC of the Crab pulsar obtained at 1–10 keV, using the Neutron star Interior Composition Explorer observatory. The rising edge of the first peak of the FLC and the reflected version of the falling edge of the second peak are compared in several ways, and phase ranges are identified where the two curves are statistically similar. The best matching occurs when the two peaks are aligned, but only in a small phase range of ≈0.0244 just below their peaks; their mean difference is −0.78 ± 1.8 photons s−1 with a reduced χ2 of 0.93. If the first curve is convolved by a Laplace function, the corresponding numbers are phase range of ≈0.0274, mean difference of −1.23 ± 1.30 and χ2 of 0.76. These phase ranges are much smaller than those over which the reflection symmetry has been perceived. Therefore, the only way the two edges can have a mirror relation over a substantial phase range is if one invokes a broad and faint emission component of amplitude ≈100 photons s−1 and width ≈0.1 in phase, centred at phase ≈0.1 beyond the second peak.