Revisiting Cassiopeia A after a decade: the first spatially resolved synchrotron X-ray variability above 15 keV by NuSTAR

As the youngest (~ 350 years old) nearby (~ 3.4 kpc) Galactic core-collapse supernova remnant (SNR), Cassiopeia A (Cas A) is a rare test bed for a plethora of astrophysical theories. Of particular interest is cosmic ray acceleration at Cas A. A general argument is that supernovae and their remnants may account for a large fraction of the Galactic cosmic ray protons up to a few PeV. Ultra-high-energy (above 100 TeV, UHE) gamma rays from pion decays are the smoking gun of proton acceleration; however, the UHE flux of Cas A and other Galactic SNRs falls below the sensitivity of the current UHE observatories. The limited angular resolution of gamma-ray telescopes (> 0.1°) poses another challenge in locating the proton acceleration sites in SNRs. An alternative yet unambiguous evidence of particle acceleration lies in sub-arcminute-scale hard X-ray observation of synchrotron emission from the ultrarelativistic primary and secondary leptons. The leptons are accelerated to such high energies at the SNR blast wave in a strongly amplified (> 100s uG) magnetic field. While the strong magnetic field is optimal for efficient particle acceleration, it also entails extremely fast synchrotron cooling for leptons. The interplay between these two competing phenomena can exhibit year-scale variability of hard X-ray emission in young SNRs, as observed with Chandra in RX J1713.7-3946, Tycho, and Cas A. An important caveat of those observations is that the X-ray continuum probed by Chandra (below 6 keV) must contain non-negligible thermal emission, limiting the attribution of the observed variability to particle acceleration and synchrotron cooling. In this work, we present, for the first time, the spatially resolved synchrotron X-ray variability of Cas A observed by NuSTAR in 15 - 50 keV, safely eliminating the possibility of thermal contribution. We compare our new NuSTAR observation of Cas A with the 2.4 Ms of archival NuSTAR data from 10 years ago to measure the changes in the synchrotron flux and spectral shape from different parts of the remnant. We discuss the implications of these results for the recent theory of modified non-linear diffusive shock acceleration.