Reflection Symmetry in the Folded Light Curve of the Crab Pulsar from NICER

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 analyzing a high resolution FLC of the Crab pulsar obtained at $1 - 10$ keV using the {\it{NICER}} 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 $\approx 0.0244$ just below their peaks; their mean difference is $-0.78 \pm 1.8$ photons/sec with a reduced $\chi^2$ of $0.93$. If the first curve is convolved by a Laplace function, the corresponding numbers are phase range of $\approx 0.0274$, mean difference of $-1.23 \pm 1.30$ and $\chi^2$ 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 $\approx 100$ photons/sec and width $\approx 0.1$ in phase, centered at phase $\approx 0.1$ beyond the second peak.