2007ApJ...670.1260H

We set sensitive upper limits to the X-ray emission of four Type Ia supernovae (SNe Ia) using the Chandra X-Ray Observatory. SN 2002bo, a normal, although reddened, nearby SN Ia, was observed 9.3 days after explosion. For an absorbed, high-temperature bremsstrahlung model the flux limits are 3.2x10^–16 ergs/cm2/s (0.5-2 keV band) and 4.1x10^–15 ergs/cm2/s (2-10 keV band). Using conservative model assumptions and a 10 km/s wind speed, we derive a mass-loss rate of M{dot}∼2x10^–5 M_☉/yr, which is comparable to limits set by the nondetection of Hα lines from other SNe Ia. Two other objects, SN 2002ic and SN 2005gj, observed 260 and 80 days after explosion, respectively, are the only SNe Ia showing evidence for circumstellar interaction. The SN 2002ic X-ray flux upper limits are ∼4 times below predictions of the interaction model currently favored to explain the bright optical emission. To resolve this discrepancy, we invoke the mixing of cool dense ejecta fragments into the forward shock region, which produces increased X-ray absorption. A modest amount of mixing allows us to accommodate the Chandra upper limit. SN 2005gj is less well studied at this time. Assuming the same circumstellar environment as for SN 2002ic, the X-ray flux upper limits for SN 2005gj are ∼4 times below the predictions, suggesting that mixing of cool ejecta into the forward shock has also occurred here. Our reanalysis of Swift and Chandra data on SN 2005ke does not confirm a previously reported X-ray detection. The host galaxies NGC 3190 (SN 2002bo) and NGC 1371 (SN 2005ke) each harbor a low-luminosity [L_X~(3-4)x10⁴⁰ ergs/s] active nucleus in addition to widespread diffuse soft X-ray emission.