SN 2015bq: A Luminous Type Ia Supernova with Early Flux Excess

Abstract

We present optical and ultraviolet (UV) observations of a luminous type Ia supernova (SN Ia) SN 2015bq characterized by the early flux excess. This SN reaches a B-band absolute magnitude at $M_B=-19.68\pm 0.41$ mag and a peak bolometric luminosity at $L=(1.75\pm 0.37)\times 10^{43}$ erg s${}^{-1}$, with a relatively small post-maximum decline rate [$\Delta m_{15}(B)=0.82\pm 0.05$ mag]. The flux excess observed in the light curves of SN 2015bq a few days after the explosion, especially seen in the UV bands, might be due to the radioactive decay of ${}^{56}$Ni mixed into the surface. The radiation from the decay of the surface ${}^{56}$Ni heats the outer layer of this SN. It produces blue $U-B$ color followed by monotonically reddening in the early phase, dominated iron-group lines, and weak intermediate-mass elements absorption features in the early spectra. The scenario of enhanced ${}^{56}$Ni in the surface is consistent with a large amount of ${}^{56}$Ni ($M_{{}^{56}\mathrm{Ni}}$ = 0.97 $\pm 0.20$ $M_{\odot }$) synthesized during the explosion. The properties of SN 2015bq are found to locate between SN 1991T and SN 1999aa, suggesting the latter two subclasses of SNe Ia may have a common origin.

18 pages, 12 figures, accepted for publication in ApJ

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