2015A&A...578A...9H

We present near-infrared (NIR) time-series spectroscopy, as well as complementary ultraviolet (UV), optical, and NIR data, of the Type Ia supernova (SN Ia) iPTF13ebh, which was discovered within two days from the estimated time of explosion. The first NIR spectrum was taken merely 2.3 days after explosion and may be the earliest NIR spectrum yet obtained of a SN Ia. The most striking features in the spectrum are several NIR CI lines, and the CI λ1.0693µm line is the strongest ever observed in a SN Ia. Interestingly, no strong optical CII counterparts were found, even though the optical spectroscopic time series began early and is densely cadenced. Except at the very early epochs, within a few days from the time of explosion, we show that the strong NIR CI compared to the weaker optical CII appears to be general in SNe Ia. iPTF13ebh is a fast decliner with Δm₁₅(B)=1.79±0.01, and its absolute magnitude obeys the linear part of the width-luminosity relation. It is therefore categorized as a ``transitional'' event, on the fast-declining end of normal SNe Ia as opposed to subluminous/91bg-like objects. iPTF13ebh shows NIR spectroscopic properties that are distinct from both the normal and subluminous/91bg-like classes, bridging the observed characteristics of the two classes. These NIR observations suggest that composition and density of the inner core are similar to that of 91bg-like events, and that it has a deep-reaching carbon burning layer that is not observed in more slowly declining SNe Ia. There is also a substantial difference between the explosion times inferred from the early-time light curve and the velocity evolution of the SiII λ0.6355µm line, implying a long dark phase of ∼4 days.