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 M_⊙, the SNR age is 14,000 yr, and the explosion energy is 8.1 × 10⁵⁰ 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 5 × 10¹⁰ cm^-3 s suggests that the observed emission is dominated by relatively dense (n_e ∼ 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 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.