SPIRITS 15c and SPIRITS 14buu: Two Obscured Supernovae in the Nearby Star Forming Galaxy IC 2163
SPIRITS SPitzer InfraRed Intensive Transients Survey 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 $M_{[4.5] 17.1\pm0.4$ mag, Vega) and red [3.6] [4.5] 3.0 \pm 0.2$ mag) transients, SPIRITS 15c. The transient was detected in a dusty spiral arm of IC 2163 $D\approx35.5$ Mpc) Pre discovery ground based imaging revealed an associated, shorter duration transient in the optical and near IR (NIR) NIR spectroscopy showed a broad \approx 8400$ km s$ 1} double peaked emission line of He I at $1.083 \mu$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 \approx 200$ days. Assuming $A_V 2.2$ mag of extinction in SPIRITS 15c provides a good match between their optical light curves. The IR light curves and the extreme [3.6] [4.5] color cannot be explained using only a standard extinction law. Another luminous $M_{4.5} 16.1\pm0.4$ mag) event, SPIRITS 14buu, was serendipitously discovered in the same galaxy. The source displays an optical plateau lasting \gtrsim 80$ days, and we suggest a scenario similar to the low luminosity Type IIP SN 2005cs obscured by $A_V \approx 1.5$ mag. Other classes of IR luminous transients can likely be ruled out in both cases. If both events are indeed SNe, this may suggest \gtrsim 18\ of nearby core collapse SNe are missed by currently operating optical surveys.