Strong near-infrared carbon in the Type Ia supernova iPTF13ebh^⋆,⋆⋆

¹ Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
e-mail: hsiao@phys.au.dk
² Carnegie Observatories, Las Campanas Observatory, Colina El Pino, Casilla 601, Chile
³ Carnegie Observatories, 813 Santa Barbara St, Pasadena, CA 91101, USA
⁴ Florida State University, Tallahassee, FL 32306, USA
⁵ Physics Department, Texas Tech University, Lubbock, TX 79409, USA
⁶ University of Texas at Austin, 1 University Station C1400, Austin, TX 78712-0259, USA
⁷ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁸ Millennium Institute of Astrophysics, Casilla 36-D, Santiago, Chile
⁹ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
¹⁰ Gemini Observatory, Northern Operations Center, Hilo, HI 96720, USA
¹¹ Instituto de Astrofísica de La Plata (IALP), CCT-CONICET-UNLP, Paseo del Bosque S/N, B1900 FWA La Plata, Argentina
¹² Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo, Kashiwa, Chiba 277-8583, Japan
¹³ Department of Physics, Bishop’s University, Sherbrooke, Quebec, J1M 1Z7, Canada
¹⁴ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries vej 30, 2100 Copenhagen, Denmark
¹⁵ The Oskar Klein Centre, Physics Department, Stockholm University, Albanova University Center, 106 91 Stockholm, Sweden
¹⁶ Indian Institute of Astrophysics, Koramangala, Bangalore 560034, India
¹⁷ Las Cumbres Observatory Global Telescope Network, Goleta, CA 93117, USA
¹⁸ Physics Department, University of California, Santa Barbara, CA 93106, USA
¹⁹ Astronomy and Astrophysics Division, Physical Research Laboratory, Navrangapura, Ahmedabad − 380009 Gujarat, India
²⁰ Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA
²¹ Department of Physics and Astronomy, TexasA&M University, College Station, TX 77843, USA
²² Cahill Center for Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
²³ Computational Cosmology Center, Computational Research Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road MS 50B-4206, Berkeley, CA 94611, USA
²⁴ European Southern Observatory for Astronomical Research in the Southern Hemisphere (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching b. München, Germany
²⁵ University of Arizona, Steward Observatory, 933 North Cherry Avenue, Tucson, AZ 85719, USA
²⁶ Department of Particle Physics and Astrophysics, Weizmann Institute of Science, 76100 Rehovot, Israel
²⁷ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Chile
²⁸ School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK
²⁹ Spitzer Science Center, MS 314-6, California Institute of Technology, Pasadena, CA 91125, USA
³⁰ Space & Remote Sensing, MS B244, Los Alamos National Laboratory, Los Alamos, NM 87545, USA

Received: 7 November 2014
Accepted: 7 March 2015

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 iλ1.0693 μm 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 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 Si iiλ0.6355 μm line, implying a long dark phase of ~4 days.