University of Minnesota
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Robert Gehrz

SPIRITS 15c and SPIRITS 14buu: Two Obscured Supernovae in the Nearby Star-Forming Galaxy IC 2163
J. E. Jencson, M. M. Kasliwal, J. Johansson, C. Contreras, S. Castellón, H. E. Bond, A. J. Monson, F. J. Masci, A. M.; Cody, J. E. Andrews, J. Bally, Y. Cao, O. D. Fox, T. Gburek, R. D. Gehrz, W. Green, G. Helou, E. Hsiao, N. Morrell, M. Phillips, T. , 2016, ApJ, 837, 167.

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SPitzer InfraRed Intensive Transients Survey—SPIRITS—is an ongoing survey of nearby galaxies searching for infrared (IR) transients with Spitzer/IRAC. We present the discovery and follow-up observations of one of our most luminous (M [4.5] = −17.1 ± 0.4 mag, Vega) and reddest ([3.6] − [4.5] = 3.0 ± 0.2 mag) transients, SPIRITS 15c. The transient was detected in a dusty spiral arm of IC 2163 (D ≈ 35.5 Mpc). Pre-discovery ground-based imaging revealed an associated, shorter-duration transient in the optical and near-IR (NIR). NIR spectroscopy showed a broad (≈8400 km s−1), double-peaked emission line of He i at 1.083 μm, indicating an explosive origin. The NIR spectrum of SPIRITS 15c is similar to that of the Type IIb SN 2011dh at a phase of ≈200 days. Assuming an A V = 2.2 mag of extinction in SPIRITS 15c provides a good match between their optical light curves. The NIR light curves, however, show some minor discrepancies when compared with SN 2011dh, and the extreme [3.6]–[4.5] color has not been previously observed for any SN IIb. Another luminous (M 4.5 = −16.1 ± 0.4 mag) event, SPIRITS 14buu, was serendipitously discovered in the same galaxy. The source displays an optical plateau lasting gsim80 days, and we suggest a scenario similar to the low-luminosity Type IIP SN 2005cs obscured by A V ≈ 1.5 mag. Other classes of IR-luminous transients can likely be ruled out in both cases. If both events are indeed SNe, this may suggest that gsim18% of nearby core-collapse SNe are missed by currently operating optical surveys.