Locating the most energetic electrons in Cassiopeia A

TL;DR

Deep NuSTAR observations of Cassiopeia A reveal that the highest energy X-ray emission originates from interior knots and outer filaments, challenging previous assumptions about shock-dominated emission and suggesting complex particle acceleration processes.

Contribution

This study provides the first spatially resolved imaging of Cassiopeia A above 15 keV, identifying distinct interior and outer regions contributing to high-energy emission.

Findings

High-energy emission is dominated by interior knots and outer filaments.

Spectral differences suggest interior knots are in the remnant's 3D interior.

The morphology of >15 keV emission differs from radio and low-energy X-rays.

Abstract

We present deep ($>$2.4 Ms) observations of the Cassiopeia A supernova remnant with {\it NuSTAR}, which operates in the 3--79 keV bandpass and is the first instrument capable of spatially resolving the remnant above 15 keV. We find that the emission is not entirely dominated by the forward shock nor by a smooth "bright ring" at the reverse shock. Instead we find that the $>$15 keV emission is dominated by knots near the center of the remnant and dimmer filaments near the remnant's outer rim. These regions are fit with unbroken power-laws in the 15--50 keV bandpass, though the central knots have a steeper ($\Gamma \sim -3.35$) spectrum than the outer filaments ($\Gamma \sim -3.06$). We argue this difference implies that the central knots are located in the 3-D interior of the remnant rather than at the outer rim of the remnant and seen in the center due to projection effects. The morphology of $>$15 keV emission does not follow that of the radio emission nor that of the low energy ($<$12 keV) X-rays, leaving the origin of the $>$15 keV emission as an open mystery. Even at the forward shock front we find less steepening of the spectrum than expected from an exponentially cut off electron distribution with a single cutoff energy. Finally, we find that the GeV emission is not associated with the bright features in the {\it NuSTAR} band while the TeV emission may be, suggesting that both hadronic and leptonic emission mechanisms may be at work.