Spirits 16tn in ngc 3556: A heavily obscured and low-luminosity supernova at 8.8 mpc

We present the discovery by the SPitzer InfraRed Intensive Transients Survey (SPIRITS) of a likely supernova (SN) in NGC 3556 (M108) at only 8.8 Mpc, which, despite its proximity, was not detected by optical searches. A luminous infrared (IR) transient at M[4.5] = −16.7 mag (Vega), SPIRITS 16tn is coincident with a dust lane in the inclined, star-forming disk of the host. Using observations in the IR, optical, and radio, we attempt to determine the nature of this event. We estimate AV ≈ 8–9 mag of extinction, placing it among the three most highly obscured IR-discovered SNe to date. The [4.5] light curve declined at a rate of 0.013 mag day−1, and the [3.6] − [4.5] color grew redder from 0.7 to & 1.0 mag by 184.7 days post discovery. Optical/IR spectroscopy shows a red continuum, but no clearly discernible features are evident, preventing a definitive spectroscopic classification. Deep radio observations constrain the radio luminosity of SPIRITS 16tn to Lν . 1024 erg s−1 Hz−1 between 3–15 GHz, excluding many varieties of radio core-collapse SNe. A type Ia SN is ruled out by the observed red IR color, and lack of features normally attributed to Fe-peak elements in the optical and IR spectra. SPIRITS 16tn was fainter at [4.5] than typical stripped-envelope SNe by ≈ 1 mag. Comparison of the spectral energy distribution (SED) to SNe II suggests SPIRITS 16tn was both highly obscured, and intrinsically dim, possibly akin to the low-luminosity SN 2005cs. We infer the presence of an IR dust echo powered by an initial peak luminosity of the transient of 5 × 1040 erg s−1 . Lpeak . 4 × 1043 erg s−1, consistent with the observed range for SNe II. This discovery illustrates the power of IR surveys to overcome the compounding effects of visible extinction and optically sub-luminous events in completing the inventory of nearby SNe.