Strong near-infrared carbon in the Type Ia supernova iPTF13ebh

TL;DR

This study presents early and comprehensive near-infrared, ultraviolet, and optical spectroscopy of the Type Ia supernova iPTF13ebh, revealing unique carbon features and bridging properties between normal and subluminous supernovae.

Contribution

It provides the earliest NIR spectrum of a SN Ia, highlighting strong NIR carbon lines and characterizing a transitional supernova with unique spectroscopic features.

Findings

Earliest NIR spectrum of a SN Ia obtained at 2.3 days post-explosion.

Strong NIR C I lines observed, especially at 1.0693 μm, with no optical C II counterparts.

iPTF13ebh exhibits properties bridging normal and subluminous SNe Ia, with a long dark phase of about 4 days.

Abstract

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.