Kavli Affiliate: Lars Bildsten
| First 5 Authors: Curtis McCully, Saurabh W. Jha, Richard A. Scalzo, D. Andrew Howell, Ryan J. Foley
| Summary:
Type Iax supernovae represent the largest class of peculiar white-dwarf
supernovae. The type Iax SN~2012Z in NGC 1309 is the only white dwarf supernova
with a detected progenitor system in pre-explosion observations. Deep
textit{Hubble Space Telescope} images taken before SN~2012Z show a luminous,
blue source that we have interpreted as a helium-star companion (donor) to the
exploding white dwarf. We present here late-time textit{HST} observations
taken $sim$1400 days after the explosion to test this model. We find the SN
light curve can empirically be fit by an exponential decay model in magnitude
units. The fitted asymptotic brightness is within $10%$ of our latest
measurements and approximately twice the brightness of the pre-explosion
source. The decline of the light curve is too slow to be powered by $^{56}$Co
or $^{57}$Co decay: if radioactive decay is the dominate power source, it must
be from longer half-life species like $^{55}$Fe. Interaction with circumstellar
material may contribute to the light curve, as may shock heating of the
companion star. Companion-star models underpredict the observed flux in the
optical, producing most of their flux in the UV at these epochs. A
radioactively-heated bound remnant, left after only a partial disruption of the
white dwarf, is also capable of producing the observed excess late-time flux.
Our analysis suggests that the total ejecta + remnant mass is consistent with
the Chandrasekhar mass for a range of type Iax supernovae.
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