THE X-RAY SPECTRUM OF SUPERNOVA REMNANT 1987A

摘要

We discuss the X-ray emission observed from supernova remnant 1987A with the Chandra X-Ray Observatory. We analyze a high-resolution spectrum obtained in 1999 October with the high-energy transmission grating (HETG). From this spectrum we measure the strengths and an average profile of the observed X-ray lines. We also analyze a high signal-to-noise ratio CCD spectrum obtained in 2000 December. The good statistics (≈9250 counts) of this spectrum and the high spatial resolution provided by the telescope allow us to perform spectroscopic analyses of different regions of the remnant. We discuss the relevant shock physics that can explain the observed X-ray emission. The X-ray spectra are well fitted by plane-parallel shock models with postshock electron temperatures of ≈2.6 keV and ionization ages of ≈6 x 10~(10) cm~(-3) s. The combined X-ray line profile has a FWHM of ?5≈5000 km s~(-1), indicating a blast-wave speed of ≈3500 km s~(-1). At this speed, plasma with a mean postshock temperature of ≈17 keV is produced. This is direct evidence for incomplete electron-ion temperature equilibration behind the shock. Assuming this shock temperature, we constrain the amount of collisionless electron heating at the shock front at T_(e0)/T_s = 0.11_(-0.01)~(+0.02). We find that the plasma has low metallicity (abundances are ≈0.1-0.4 solar) and is nitrogen enriched (N/O≈0.8 by number), similar to abundances found for the equatorial ring. Analysis of the spectra from different regions of the remnant reveals slight differences in the parameters of the emitting plasma. The plasma is cooler near the optical spot 1 (at position angle ≈30° and in the eastern half of the remnant, where the bright optical spots are found, than in the western half, consistent with the presence of slower (≈500 km s~(-1)) shocks entering denser ring material. There is an overall flux asymmetry between the two halves, with the eastern half being 15%-50% brighter (depending on how the center of the remnant is defined). However, our spectroscopic analysis shows that less than 5% of the overall X-ray emission could come from a slow shock component. Therefore the flux asymmetry cannot fully be due to X-rays produced by the blast wave entering the ring, but rather indicates an asymmetry in the global interaction with the circumstellar material interior to the ring.