A new evolutionary phase of supernova remnant 1987A

We have been monitoring the supernova remnant (SNR) 1987A with Chandra observations since 1999. Here we report on the latest change in the soft X-ray light curve of SNR 1987A. For the last 1.5yr (since day 8000), the soft X-ray flux has significantly flattened, staying (within uncertainties) at f _X 5.7 × 10^-12ergcm^-2s^-1 (corresponding to L _X 3.6 × 10³⁶ergs^-1) in the 0.5-2keV band. This remarkable change in the recent soft X-ray light curve suggests that the forward shock is now interacting with a decreasing density structure, after interacting with an increasing density gradient over 10yr prior to day 8000. Possibilities may include the case that the shock is now propagating beyond a density peak of the inner ring. We briefly discuss some possible implications on the nature of the progenitor and the future prospects of our Chandra monitoring observations.