SN 2000cx and SN 2013bh: Extremely Rare, Nearly Twin Type Ia Supernovae

TL;DR

SN 2000cx and SN 2013bh are extremely rare, nearly identical Type Ia supernovae with peculiar spectral and photometric features, providing insights into their high-temperature explosions and progenitor environments.

Contribution

This study presents detailed photometric and spectroscopic analysis of two rare, nearly twin Type Ia supernovae, revealing their peculiarities and environmental characteristics.

Findings

Both supernovae show high blackbody temperatures (~12000 K).

Spectra reveal iron-group and intermediate-mass elements with high-velocity features.

Models suggest they produce up to ~1 M_Sun of Ni-56 and have low-metallicity progenitors.

Abstract

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. Thirteen years later SN 2013bh (aka iPTF13abc), a near identical twin, was discovered and we obtained optical and near-IR 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, Fe II, 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 (~12000 km/s) and HVFs (~24000 km/s) of Ca II. Persistent absorption from Fe III and Si III, along with the colour evolution, imply high blackbody temperatures for SNe 2013bh and 2000cx (~12000 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_Sun of Ni-56 and (4.3-5.5)e-3 M_Sun of fast-moving Ca ejecta.