Chandra ACIS Survey of M33 (ChASeM33): X-Ray Imaging Spectroscopy of M33SNR 21, the Brightest X-Ray Supernova Remnant in M33

We present and interpret new X-ray data for M33SNR 21, the brightest X-ray supernova remnant (SNR) in M33. The SNR is in seen projection against (and appears to be interacting with) the bright H II region NGC 592. Data for this source were obtained as part of the Chandra ACIS Survey of M33 (ChASeM33) Very Large Project. The nearly on-axis Chandra data resolve the SNR into a ~5" diameter (20 pc at our assumed M33 distance of 817+/-58 kpc) slightly elliptical shell. The shell is brighter in the east, which suggests that it is encountering higher density material in that direction. The optical emission is coextensive with the X-ray shell in the north, but extends well beyond the X-ray rim in the southwest. Modeling the X-ray spectrum with an absorbed sedov model yields a shock temperature of 0.46^+0.01_-0.02 keV, with an ionization timescale of n_et=2.1^+0.2_-0.3×10¹² cm^-3 s and half-solar abundances (0.45^+0.12_-0.09). Assuming Sedov dynamics gives an average preshock H density of 1.7+/-0.3 cm^-3. The dynamical age estimate is 6500+/-600 yr, while the best-fit n_et value and derived n_e gives 8200+/-1700 yr; the weighted mean of the age estimates is 6700+/-600 yr. We estimate an X-ray luminosity (0.25-4.5 keV) of (1.2+/-0.2)×10³⁷ ergs s^-1 (absorbed), and (1.7+/-0.3)×10³⁷ ergs s^-1 (unabsorbed), in good agreement with the recent XMM-Newton determination. No significant excess hard emission was detected; the luminosity <~1.2×10³⁵ ergs s^-1 (2-8 keV) for any hard point source.