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SN 2013ej is a Highly Polarized Type II-Plateau Supernova

ATel #5275; Posted by D. C. Leonard (San Diego State University), G. Pignata (Universidad Andres Bello), L. Dessart (Laboratoire d'Astrophysique de Marseille), D. Hillier (University of Pittsburgh), H. G. Khandrika, A. A. Rachubo, R. D. Fahad, S. J. Hadden (San Diego State University), L. Gonzalez (University of California, San Diego)
on 13 Aug 2013; 22:00 UT
Credential Certification: D. C. Leonard (leonard@astro.caltech.edu)

Subjects: Optical, Supernovae

We report photometric and spectropolarimetric observations of the young Type II supernova SN 2013ej (PSN J01364816+1545310; CBET #3606) that establish its membership in the (photometrically defined) Type-II Plateau subclass and reveal unusually strong polarization at a very early phase.

We present the following optical (B, V, R_C, I_C) photometry from observations taken at Mount Laguna Observatory's 1-m reflector:

UT            | Day*  | B     | V     | R_C   | I_C    
2013-07-31.37 |  5.99 | 12.64 | 12.65 | 12.58 | 12.47  
2013-08-03.37 |  8.99 | 12.59 | 12.52 | 12.44 | 12.30  
2013-08-06.41 | 12.03 | 12.69 | 12.53 | 12.41 | 12.28  
2013-08-10.44 | 16.06 | 12.83 | 12.50 | 12.34 | 12.19  

*Day since discovery, 2013-07-25.38 UT (CBET #3609), which is likely within 2 days of explosion (ATEL #5237).

The data were calibrated against local standard stars provided by Foley et al. (2003, PASP, 115, 1220); uncertainties are estimated to be ~0.03 mag. The data demonstrate that SN 2013ej has begun a photometric plateau in the V, R_C, and I_C bands, and so we propose a Type II-Plateau (II-P) classification for this core-collapse event. From comparison with light curve templates derived from observations of SN 1999gi (Leonard et al. 2002, AJ, 124, 2490), we conclude that B-band maximum was achieved within about one day of our 2013-08-03.37 (~day 9) observation.

We obtained spectropolarimetry of SN 2013ej on Aug. 1.35 UT (~day 7) with the ESO Very Large Telescope (+ FORS2 in polarimetric mode; range 430-920 nm, resolution 1.2 nm). The flux spectrum resembles those of SNe II-P at similarly early epochs, presenting a smooth, blue continuum with broad P-Cyg features corresponding to H_alpha, H_beta, He I (587.6 nm), and N II (P-Cyg absorptions near 450.0 nm and 550.0 nm; see Dessart & Hillier 2006, A&A 447, 691) all evident. In addition, we note a strong absorption feature centered at 615.0 nm, in the blue wing of the H_alpha P-Cyg trough. This feature may be attributed to Si II (635.5 nm), although it is unusually strong for this early epoch. This may point to peculiar ionization conditions at the photosphere, perhaps related to a non-standard red supergiant envelope at explosion. An alternative origin for such a blue notch in the H_alpha trough is circumstellar interaction (Chugai et al. 2007, ApJ, 662, 1136). The ejecta velocity, as deduced from the minimum of H_beta, is about 10,500 km/s.

The observed spectropolarimetry (uncorrected for interstellar polarization -- ISP) are characterized by significant polarization that decreases from ~1.3% at 430 nm to ~1.0% at 920 nm, with marked modulations of up to ~0.6% occurring across all strong P-Cyg line features. To our knowledge, these are the earliest spectropolarimetry data yet obtained for this class of SN. Such strong early-time polarization is unusual for an SN II-P (e.g. Leonard et al. 2001, ApJ, 553, 861) but not without precedent: SN 2012aw showed moderate polarization (~0.5%) at an early phase (~day 17) that exhibited a similar wavelength dependence (ATEL #4033). While some portion of the observed continuum polarization of SN 2013ej may be due to (an as-yet unknown) ISP, the strength of the line features suggests that much of it is intrinsic to the SN. This implies substantial asymmetries in the electron-scattering atmosphere of the outer ejecta at these early times (Dessart & Hillier 2011, MNRAS, 415, 3497).

Further spectropolarimetry observations from VLT are planned, and we gratefully acknowledge the effort of the entire observing staff at ESO-Paranal for the prompt execution of our spectropolarimetry ToO request. D.C.L. thanks the NSF for support through grants AST-1009571 and AST-1210311, and L.D. thanks the Agence Nationale de la Recherche for support through grant ANR-11-BS56-007.