An Fe-Ni bubble in the Small Magellanic Cloud supernova remnant B0049-73.6

We present observations with the Chandra 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(-1) from Sedov models. The swept-up mass is equal to 170, the SNR age is 14,000 yr, and the explosion energy is 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-circle dot. The filling fraction of the O- rich gas is less than 1%, and its average ionization timescale of 5 x 10(10) cm(-3) s suggests that the observed emission is dominated by relatively dense (n(e) similar to 0.2 cm(-3)) 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 Ni-56. 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.