Locating the most energetic electrons in Cassiopeia A

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.

arXiv comment12 pages, 11 figures, accepted for publication in ApJ

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• The distribution of radioactive ${}^{44}$Ti in Cassiopeia A
• A cut-off in the TeV gamma-ray spectrum of the SNR Cassiopeia A
• A Systematic Survey for Broadened CO Emission Toward Galactic Supernova Remnants
• Revisiting INTEGRAL/SPI observations of 44Ti from Cassiopeia A
• X-ray Measurements of the Particle Acceleration Properties at Inward Shocks in Cassiopeia A
• Evidence for proton acceleration up to TeV energies based on VERITAS and Fermi-LAT observations of the Cas A SNR
• A Spatially Resolved Study of the Synchrotron Emission and Titanium in Tycho's Supernova Remnant with NuSTAR
• X-ray polarization detection of Cassiopeia A with IXPE
• Evidence for Rapid Adiabatic Cooling as an Origin of the Recombining Plasma in the Supernova Remnant W49B Revealed by NuSTAR Observations
• Non-thermal emission from the reverse shock of the youngest galactic Supernova remnant G1.9+0.3
• Dynamical Evolution and Radiative Processes of Supernova Remnants
• NuSTAR observations of the supernova remnant RX J1713.7$-$3946
• Richardson-Lucy deconvolution with a spatially Variant point-spread function of Chandra: Supernova Remnant Cassiopeia A as an Example
• Asymmetries of Heavy Elements in the Young Supernova Remnant Cassiopeia A
• Multi-Year X-ray Variations of Iron-K and Continuum Emissions in the Young Supernova Remnant Cassiopeia A
• Is Supernova Remnant Cassiopeia A a PeVatron?
• Vortical amplification of magnetic field at inward shock of supernova remnant Cassiopeia A
• Evidence for thermal X-ray emission from the synchrotron dominated shocks in Tycho's supernova remnant
• Kinematics of Supernova Remnants Using Multiepoch Maximum Likelihood Estimation: Chandra Observation of Cassiopeia A as an Example
• The auto and cross angular power spectrum of the Cas A supernova remnant in radio and X-ray
• NuSTAR observations of G11.2-0.3
• Chlorine and Potassium Enrichment in the Cassiopeia A Supernova Remnant
• Arcsecond-Scale X-ray Imaging and Spectroscopy of SS 433 with Chandra HETG
• A Study of Radio Knots within Supernova Remnant Cassiopeia A
• Spatio-temporal characterization of Cassiopeia A
• Nonthermal processes and particle acceleration in supernova remnants
• The forward and reverse shock dynamics of Cassiopeia A
• An Asymmetrical Model for High Energy Radiation of Cassiopeia A