We present deep ( > 2.4 Ms ) observations of the Cassiopeia A supernova remnant with 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 NuSTAR band while the TeV emission may be , suggesting that both hadronic and leptonic emission mechanisms may be at work .