Strong near-infrared carbon in the Type Ia supernova iPTF13ebh
  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 C I lines, and the C I {\lambda}1.0693
{\mu}m line is the strongest ever observed in a SN Ia. Interestingly, no strong
optical C II 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 C I compared to the weaker optical C II appears to be general in SNe Ia.
iPTF13ebh is a fast decliner with {\Delta}m15(B) = 1.79 $\pm$ 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 composition and density of the inner
core similar to that of 91bg-like events, and a deep reaching carbon burning
layer not observed in slower 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 Si II {\lambda}0.6355 {\mu}m line, implying a
long dark phase of ~ 4 days.
