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 early flux excess. This SN reaches a B -band absolute magnitude at M B = −19.68 ± 0.41 mag and a peak bolometric luminosity at L = (1.75 ± 0.37) × 10 43 erg s −1 , with a relatively small post-maximum decline rate [Δ m 15 ( B ) = 0.82 ± 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 element 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 Ni = 0.97 ± 0.20 M ☉ ) 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.