2013MNRAS.436.1225S

The Type Ia supernova (SN Ia) SN 2000cx was one of the most peculiar transients ever discovered, with a rise to maximum brightness typical of a SN Ia, but a slower decline and a higher photospheric temperature. 13yr later SN 2013bh (also known as iPTF13abc), a near identical twin, was discovered and we obtained optical and near-infrared photometry and low-resolution optical spectroscopy from discovery until about 1 month past r-band maximum brightness. The spectra of both objects show iron-group elements [CoII, NIII, FeII, FeIII and high-velocity features (HVFs) of TIII], intermediate-mass elements (SIII, SIIII and SII) and separate normal velocity features ( ∼ 12 000km/s) and HVFs ( ∼ 24 000kms-1) of CaII. Persistent absorption from FeIII and SIIII, along with the colour evolution, implies high blackbody temperatures for SNe 2013bh and 2000cx ( ∼ 12 000K). Both objects lack narrow Nai D absorption and exploded in the outskirts of their hosts, indicating that the SN environments were relatively free of interstellar or circumstellar material and may imply that the progenitors came from a relatively old and low-metallicity stellar population. Models of SN 2000cx, seemingly applicable to SN 2013bh, imply the production of up to ∼ 1 M_☉ of 56Ni and (4.3-5.5)x10^–3 M_☉ of fast-moving Ca ejecta.