UC Berkeley

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. 13 yr 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 [Co II, Ni II, FeII, Fe III and high-velocity features (HVFs) of Ti II], intermediate-mass elements (Si II, Si III and S II) and separate normal velocity features (~12 000 km s-1) and HVFs (~24 000 km s-1) of Ca II. Persistent absorption from Fe III and Si III, along with the colour evolution, implies high blackbody temperatures for SNe 2013bh and 2000cx (~12 000 K). Both objects lack narrow Na I 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) × 10-3 M⊙ of fast-moving Ca ejecta. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.