Evolution of the Reverse Shock Emission from SNR 1987A

Abstract

We present new ( 2004 July) G750L and G140L Space Telescope Imaging Spectrograph ( STIS) data of the H alpha and Ly alpha emission from supernova remnant ( SNR) 1987A. With the aid of earlier data, from 1997 October to 2002 October, we track the local evolution of Ly alpha emission and both the local and global evolution of H alpha emission. The most recent observations allow us to directly compare the H alpha and Ly alpha emission from the same slit position and at the same epoch. Consequently, we find clear evidence that, unlike H alpha, Ly alpha is reflected from the debris by resonant scattering. In addition to emission that we can clearly attribute to the surface of the reverse shock, we also measure comparable emission, in both H alpha and Ly alpha, that appears to emerge from supernova debris interior to the surface. New observations taken through slits positioned slightly eastward and westward of a central slit show a departure from cylindrical symmetry in the H alpha surface emission. Using a combination of old and new observations, we construct a light curve of the total H alpha flux, F, from the reverse shock, which has increased by a factor of similar to 4 over about 8 yr. However, due to large systematic uncertainties, we are unable to discern between the two limiting behaviors of the flux: F proportional to t ( self-similar expansion) and F proportional to t(5) ( halting of the reverse shock). Such a determination is important for constraining the rate of hydrogen atoms crossing the shock, which is relevant to the question of whether the reverse shock emission will vanish in less than or similar to 7 yr. Future deep, low- or moderate-resolution spectra are essential for accomplishing this task.