Monitoring the Evolution of the X-ray Remnant of SN 1987A

Abstract

We report on the results of our monitoring program of the remnant of SN 1987A with the Advanced CCD Imaging Spectrometer (ACIS) on board the {\it Chandra X-ray Observatory}. Two new observations have been performed in AO2, bringing the total to four monitoring observations over the past two years. Over this time period, new techniques for correction of ``Charge Transfer Inefficiency (CTI)'' and for use of charge spreading to provide angular resolution somewhat better than the pixel size of the CCD detector have become available at Penn State. We have processed all four observations using sub-pixel resolution to obtain the highest possible angular resolution, and using our CTI correction software to provide more reliable spectral analysis and flux estimations. The high angular resolution images indicate that the X-ray bright knots are convincingly correlated with the optical spots, primarily at $\la$1 keV, while higher energy photons are very well correlated with radio images. Our data also provide marginal evidence for radial expansion of the X-ray remnant at a rate of 5200 $\pm$ 2100 km s$^{-1}$. The X-ray flux appears to linearly increase by $\sim$60% over the 18 month period of these observations. The spectrum is dominated by broad complexes of atomic emission lines and can be fit with a simple model of a plane-parallel shock with electron temperatures of $kT$ $\sim$ 2 $-$ 4 keV and a postshock electron density of $n_{e}$ $\sim$ 210 $-$ 420 cm$^{-3}$. The implied 0.5 $-$ 10 keV band luminosity in 2001 April is $\sim$1.3 $\times$ 10$^{35}$ ergs s$^{-1}$; as of that date, we still observe no direct evidence for the central point source, with an upper limit on the {\it observed} luminosity of $L_{X}$ $\sim$ 5.5 $\times$ 10$^{33}$ ergs s$^{-1}$ in the 2 $-$ 10 keV band.