SN 1987A: CONSTRAINING THE HARD X-RAY CONTINUUM AND ITS EVOLUTION WITH NUSTAR

NuSTAR observed SN1987A multiple times from July 2012 to August 2014 with a total exposure of 2.7 Ms. These observations allowed the determination of the hard X-ray emission from the remnant up to ∼ 25 keV. The continuum spectrum between 3 and 25 keV can be equally well described by several two-component models: two plane shocks, a plane shock with a power law, or a plane shock with a single rolling-off synchrotron component. We argue that the most plausible is a plane shock with a power-law, and that the power law represents synchrotron X-ray emission from an electron population with energies up to tens of TeV. This interpretation requires that the shock responsible for accelerating these electrons be modified by cosmic-ray pressure to produce the observed concave-up spectrum from radio to X-rays, and to explain the high magnetic field that this interpretation requires. The observed iron line implies an iron abundance of ∼ 0.9 solar, about twice the typical LMC abundances, suggesting that the thermal emission comes from reverse-shocked ejecta. A purely thermal description would require a hot component with kT ∼ 6 keV. We detect a slight increase of the flux from 2012 to 2014 in the medium X-ray range (3 – 8 keV) as well as in the hard X-range (8 – 20 keV). The rate of increase we find between 3 and 8 keV is somewhat slower than that found earlier, with an e-folding time of about 5000 days, comparable to that in the 8 – 20 keV band. We discuss the significance of these findings for the evolution of very young supernova remnants.