Strong near-infrared carbon in the Type Ia supernova iPTF13ebh

We present near-infrared (NIR) time-series spectroscopy, as well ascomplementary ultraviolet (UV), optical, and NIR data, of the Type Iasupernova (SN Ia) iPTF13ebh, which was discovered within two days fromthe estimated time of explosion. The first NIR spectrum was taken merely2.3 days after explosion and may be the earliest NIR spectrum yetobtained of a SN Ia. The most striking features in the spectrum areseveral NIR C i lines, and the C iλ1.0693 μm line is thestrongest ever observed in a SN Ia. Interestingly, no strong optical Cii counterparts were found, even though the optical spectroscopic timeseries began early and is densely cadenced. Except at the very earlyepochs, within a few days from the time of explosion, we show that thestrong NIR C i compared to the weaker optical C ii appears to be generalin SNe Ia. iPTF13ebh is a fast decliner with Δm15(B) =1.79 ± 0.01, and its absolute magnitude obeys the linear part ofthe width-luminosity relation. It is therefore categorized as a"transitional" event, on the fast-declining end of normal SNe Ia asopposed to subluminous/91bg-like objects. iPTF13ebh shows NIRspectroscopic properties that are distinct from both the normal andsubluminous/91bg-like classes, bridging the observed characteristics ofthe two classes. These NIR observations suggest that composition anddensity of the inner core are similar to that of 91bg-like events, andthat it has a deep-reaching carbon burning layer that is not observed inmore slowly declining SNe Ia. There is also a substantial differencebetween the explosion times inferred from the early-time light curve andthe velocity evolution of the Si iiλ0.6355 μm line, implying along dark phase of ~4 days.