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.

NuSTAR（核光谱望远镜阵列，Nuclear Spectroscopic Telescope Array）于2012年7月至2014年8月期间对超新星1987A（SN1987A）开展多次观测，总曝光时长达2.7 Ms。本次观测得以测定该超新星遗迹在约25 keV以下的硬X射线辐射特性。3~25 keV区间的连续谱可通过多种双成分模型获得同等良好的拟合：双平面激波模型、带幂律谱的平面激波模型，或带单一下降型同步辐射成分的平面激波模型。我们认为最合理的模型为带幂律谱的平面激波模型，该幂律谱对应能量可达数十太电子伏特（TeV）的电子群体产生的同步辐射X射线。这一解释要求负责加速这些电子的激波受到宇宙线（cosmic-ray）压强的调制，从而产生从射电到X射线波段均符合的凹向上谱形，同时也能解释该模型所需的强磁场环境。观测到的铁发射线表明其铁丰度约为太阳丰度的0.9倍，约为大麦哲伦云（Large Magellanic Cloud，LMC）典型丰度的两倍，这意味着热辐射来自反向激波加热的抛射物质。纯热辐射模型则需要一个kT约为6 keV的高温成分。我们在中等X射线波段（3~8 keV）与硬X射线波段（8~20 keV）均探测到2012年至2014年间的辐射流量小幅增长。3~8 keV波段的流量增长速率略慢于此前的观测结果，其e折叠时间（e-folding time）约为5000天，与8~20 keV波段的e折叠时间相当。我们讨论了上述发现对极年轻超新星遗迹演化研究的重要意义。