The Unique Type Ia Supernova 2000cx in NGC 524

Abstract

We present extensive photometric and spectroscopic observations of the Type Ia supernova (SN Ia) 2000cx in the S0 galaxy NGC 524, which reveal it to be peculiar. Photometrically, SN 2000cx is different from all known SNe Ia, and its light curves cannot be fit well by the fitting techniques currently available. There is an apparent asymmetry in the $B$-band peak, in which the premaximum brightening is relatively fast (similar to that of the normal SN 1994D), but the postmaximum decline is relatively slow (similar to that of the overluminous SN 1991T). The color evolution of SN 2000cx is also peculiar: the $(B - V)_0$ color has a unique plateau phase and the $(V - R)_0$ and $(V - I)_0$ colors are very blue. Although the premaximum spectra of SN 2000cx are similar to those of SN 1991T-like objects (with weak Si II lines), its overall spectral evolution is quite different. The Si II lines that emerged near maximum $B$-band brightness stay strong in SN 2000cx until about three weeks past maximum. The change in the excitation stages of iron-peak elements is slow. Both the iron-peak and the intermediate-mass elements are found to be moving at very high expansion velocities in the ejecta of SN 2000cx. We discuss theoretical models for SN 2000cx. SN 2000cx may be an overluminous object like SN 1991T, but with a larger yield of $^{56}$Ni and a higher kinetic energy in the ejecta. We also briefly discuss the implications of our observations for the luminosity vs. light-curve width relation.Comment: Accepted to be published in the Oct issue of PAS

The Extremes of Thermonuclear Supernovae

“…2c), again not seen in other SNe Ia at similarly late epochs. The Si II λ 6355 velocity evolution shows the same flat trend as in SN 1991T, but in SN 2000cx the measured velocity is 2000-3000 km s −1 larger (Li et al, 2001b), hinting at an overall higher kinetic energy.…”

“…The photometry of SN 2000cx (Li et al, 2001b;Candia et al, 2003) provides no conclusive picture. The B-band decline is very slow [∆ m 15 (B) = 0.93], similar to that of luminous 91T-like objects.…”

Optical Light Curves of Supernovae

“…For most objects the rise and decline rates correlate fairly well [24,93], but SN2000cx violates this rule [66]. While it had a rapid rise, its light curve did decline more slowly than many other SNIa.…”

“…There have been several focused searches for SNIa, which have produced large sets of well-sampled light curves (see, e.g., [42,44,46,50,66,73,88,91,92,97,107,112]). These data samples have produced a detailed view of SNIa.…”

Late light curves of normally luminous Type Ia supernovae

“…The combined light curves can be seen in Figures 2 & 3, where they are normalized to be zero magnitude at 200days past explosion assuming an 18d rise time to peak light. The data set for SN 2000E includes photometry from Valentini et al (2003), and the data set for SN 2000cx includes data from Li et al (2001), Jha (2002), Candia et al (2003), and Sollerman et al (2004), where the data from our observations are plotted as the filled symbols. …”