2005ApJ...622L.117H

We present observations with theChandra X-Ray Observatory of the supernova remnant (SNR) B0049-73.6 in the Small Magellanic Cloud (SMC). The remnant shows outer emission from swept-up SMC gas and a bright ejecta-dominated ring in the interior. The X-ray spectrum of the outer shell shows normal SMC abundances and allows us to estimate the current blast wave speed at 570 km/s from Sedov models. The swept-up mass is equal to 170 M_☉, the SNR age is 14,000 yr, and the explosion energy is 8.1x10⁵⁰ ergs. The brightest parts of the inner ring are dominated by O- and Ne-rich heavy-element ejecta. B0049-73.6 is thus a remnant of a core-collapse explosion. More diffuse interior ejecta emission shows less prominent O and Ne lines. The total mass of O within the heavy-element ejecta is about 0.3 M_☉. The filling fraction of the O-rich gas is less than 1%, and its average ionization timescale of 5x10¹⁰/cm3 s suggests that the observed emission is dominated by relatively dense (n_e∼0.2/cm3) ejecta. The bright inner ring most likely is composed of the innermost ejecta swept up by a central ``nickel bubble'' resulting from radioactive energy input from freshly synthesized ⁵⁶Ni. This is in a qualitative agreement with theoretical predictions of such centrally located Fe-Ni bubbles, and it confirms the important role of the Ni bubble effect in determining the innermost ejecta structure in core-collapse supernovae.