SPIRITS 15c and SPIRITS 14buu: Two Obscured Supernovae in the Nearby Star-forming Galaxy IC 2163

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 m, 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 AV = 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 (M4.5 = −16.1 0.4 mag) event, SPIRITS 14buu, was serendipitously discovered in the same galaxy. The source displays an optical plateau lasting 80 days, and we suggest a scenario similar to the low-luminosity Type IIP SN 2005cs obscured by AV 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 18% of nearby core-collapse SNe are missed by currently operating optical surveys.