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Kepler's Supernova: An Overluminous Type Ia Event Interacting with a Massive Circumstellar Medium at a Very Late Phase

We have analyzed XMM-Newton, Chandra, and Suzaku observations of Kepler's supernova remnant (SNR) to investigate the properties of both the SN ejecta and the circumstellar medium (CSM). For comparison, we have also analyzed two similarly-aged, ejecta-dominated SNRs: Tycho's SNR, thought to be the remnant of a typical Type Ia SN, and SNR 0509-67.5 in the Large Magellanic Cloud, thought to be the remnant of an overluminous Type Ia SN. By simply comparing the X-ray spectra, we find that line intensity ratios of iron-group elements (IGE) to intermediate-mass elements (IME) for Kepler's SNR and SNR 0509-67.5 are much higher than those for Tycho's SNR. We therefore argue that Kepler is the product of an overluminous Type Ia SN. This inference is supported by our spectral modeling, which reveals the IGE and IME masses respectively to be ~0.95 M_sun and ~0.12 M_sun (Kepler's SNR), ~0.75 M_sun and ~0.34 M_sun (SNR 0509-67.5), and ~0.35 M_sun and ~0.70 M_sun (Tycho's SNR). We find that the CSM component in Kepler's SNR consists of tenuous diffuse gas (~0.3 M_sun) present throughout the entire remnant, plus dense knots (~0.035 M_sun). Both of these components have an elevated N abundance (N/H ~ 4 times the solar value), suggesting that they originate from CNO-processed material from the progenitor system. The mass of the diffuse CSM allows us to infer the pre-SN mass-loss rate to be ~1.5e-5 (V_w/10 km/s) M_sun/yr, in general agreement with results from recent hydrodynamical simulations. Since the dense knots have slow proper motions and relatively small ionization timescales, they were likely located a few pc away from the progenitor system. Therefore, we argue that Kepler's SN was an overluminous event that started to interact with massive CSM a few hundred years after the explosion. This supports the possible link between overluminous SNe and the so-called "Ia-CSM" SNe.