SDWFS-MT-1: A self-obscured luminous supernova at z ≃ 0.2

We report the discovery of a 6 month long mid-infrared transient, SDWFS-MT-1 (aka SN 2007va), in the Spitzer Deep, Wide-Field Survey of the NOAO Deep Wide-Field Survey Boötes field. The transient, located in a z = 0.19 low-luminosity (M_[4.5] ≃-18.6 mag, L/L_* ≃ 0.01) metal-poor (12 + log(O/H) ≃ 7.8) irregular galaxy, peaked at a mid-infrared absolute magnitude of M_[4.5] ≃-24.2 in the 4.5 μm Spitzer/IRAC band and emitted a total energy of at least 10⁵¹ erg. The optical emission was likely fainter than the mid-infrared, although our constraints on the optical emission are poor because the transient peaked when the source was "behind" the Sun. The Spitzer data are consistent with emission by a modified blackbody with a temperature of ∼1350 K. We rule out a number of scenarios for the origin of the transient such as a Galactic star, active galactic nucleus activity, γ-ray burst, tidal disruption of a star by a black hole, and gravitational lensing. The most plausible scenario is a supernova (SN) exploding inside a massive, optically thick circumstellar medium, composed of multiple shells of previously ejected material. If the proposed scenario is correct, then a significant fraction (∼10%) of the most luminous SN may be self-enshrouded by dust not only before but also after the SN occurs. The spectral energy distribution of the progenitor of such an SN would be a slightly cooler version of η Carinae peaking at 20-30 μm.