Type Ia SNe

The explosion mechanism of Type Ia supernovae is the detonation or deflagration of a white dwarf star that exceeds the Chandrasekhar limit, and thus is very different from the core collapse of a massive star leading to a Type Ib, Ic, or II explosion. The exploding star is thought to need a binary companion from which to accrete. The mass transfer rate is thought to be on the order of 10-6 [email protected] yrs-1, considerably lower than the mass loss rates of 10-4-5 [email protected] yrs-1 responsible for the dense CSM in X-ray supernovae. It is possible that an optically thick wind stabilizes the mass transfer within the binary system. The time history of the wind is nonetheless likely to be complex, depending on the mass loss rate of the companion, the binary system orbital parameters, as well as the means of accretion by the white dwarf star. Residual CSM will be compressed and heated to X-ray temperatures as the shock passes through it. Gaining insight into the surroundings of Type Ia supernovae can potentially provide insight into the precise nature of the explosion mechanism, which is still the topic of debate,5 and could lead to important information about their usefulness as standard candles for cosmological studies.

The most sensitive published observations of Type Ia SNe near optical maximum come from ROSAT and Swift, with upper limits of a few 10-14 erg cm-2 s-1. A ROSAT observation of SN 1992A, observed 16 days after maximum, yielded an upper limit to the mass loss rate from the secondary of a few times 10-6 Mq yrs-1 [140]. A Swift survey of eight Ia events yields luminosity upper limits as small as <1 x 1039 erg s-1, and mass loss rates as low as 7 x 10-6 [email protected] yrs-1 [71]. These upper limits only begin to test models of Type Ia progrenitor systems. One object, SN 2006ke, showed a peculiar UV light curve, suggesting interaction with circumstellar material and was marginally detected in X-rays at a flux of 4 ± 1 x 10-15 erg cm-2 s-1, corresponding to a progenitor mass loss rate of 3 ± 1 x [email protected] yrs-1. Even if this detection is treated as an upper limit, it is the most constraining upper limit on mass loss rate from a Type Ia progenitor.

Limits on the flux from older Type Ia supernovae are routinely obtained using deep observations of their host galaxy. For instance, a 3 a upper limit to the luminosity of the Ia SN 1983N of Lx < 1.7 x 1036 erg s-1 (0.3-8 keV) was obtained from a deep Chandra observation of M83 [152]. This corresponds to a mass loss rate M < [email protected](vw/10kms-1), approximately 20 000 years before the explosion, but provides no insight into the nature of the material near the progenitor.

5 Comparison of models with the integrated Chandra spectra of the young Type Ia remnants Tycho and 0509-67.5 suggest delayed detonation as the most likely explosion mechanism [8,172]

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