Broadband X-ray Imaging and Spectroscopy of the Crab Nebula and Pulsar with NuSTAR

TL;DR

This study uses NuSTAR to perform broadband X-ray imaging and spectroscopy of the Crab Nebula and pulsar, revealing spectral breaks, size-energy relationships, and confirming spectral steepening with no flux increase during a gamma-ray flare.

Contribution

First broadband NuSTAR X-ray imaging and spectroscopy of the Crab Nebula and pulsar, including spatially resolved spectral analysis and size-energy relationship measurements.

Findings

Spectral break at ~9 keV in the nebula's torus with steepening

Pulsed spectrum shows a break at ~12 keV with steepening

Nebula size decreases with energy following a power-law, consistent with theoretical models

Abstract

We present broadband (3 -- 78 keV) NuSTAR X-ray imaging and spectroscopy of the Crab nebula and pulsar. We show that while the phase-averaged and spatially integrated nebula + pulsar spectrum is a power-law in this energy band, spatially resolved spectroscopy of the nebula finds a break at $\sim$9 keV in the spectral photon index of the torus structure with a steepening characterized by $\Delta\Gamma\sim0.25$. We also confirm a previously reported steepening in the pulsed spectrum, and quantify it with a broken power-law with break energy at $\sim$12 keV and $\Delta\Gamma\sim0.27$. We present spectral maps of the inner 100\as\ of the remnant and measure the size of the nebula as a function of energy in seven bands. These results find that the rate of shrinkage with energy of the torus size can be fitted by a power-law with an index of $\gamma = 0.094\pm 0.018$, consistent with the predictions of Kennel and Coroniti (1984). The change in size is more rapid in the NW direction, coinciding with the counter-jet where we find the index to be a factor of two larger. NuSTAR observed the Crab during the latter part of a $\gamma$-ray flare, but found no increase in flux in the 3 - 78 keV energy band.