2022ApJ...924...35L

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) x 10⁴³ erg s^–1, with a relatively small post-maximum decline rate [Δm₁₅(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 ⁵⁶Ni mixed into the surface. The radiation from the decay of the surface ⁵⁶Ni 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 ⁵⁶Ni in the surface is consistent with a large amount of ⁵⁶Ni (M_^56Ni=0.97±0.20M_☉) 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.