Journal
ASTROPHYSICAL JOURNAL
Volume 802, Issue 1, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.1088/0004-637X/802/1/15
Keywords
acceleration of particles; ISM: individual objects (Cassiopeia A); ISM: supernova remnants; radiation mechanisms: non-thermal; X-rays: ISM
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Funding
- NASA [NNG08FD60C]
- NASA
- Division Of Physics
- Direct For Mathematical & Physical Scien [1352567] Funding Source: National Science Foundation
- Grants-in-Aid for Scientific Research [24740185] Funding Source: KAKEN
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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 similar to-3.35) spectrum than the outer filaments (Gamma similar to-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 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.
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