SPIRITS16tn: Spitzer Discovery of a Possible Supernova in Messier 108 at 8.8 Mpc

ATel #9434; J. E. Jencson, S. Adams, M. M. Kasliwal, S. Tinyanont, Y. Cao, T. Prince (Caltech), R. M. Lau (Caltech/JPL), D. Perley (DARK), F. Masci, G. Helou, L. Armus, J. Surace, S. D. Van Dyk (IPAC), A. Cody (NASA Ames), M. L. Boyer, R. Khan (NASA Goddard), H. E. Bond, A. Monson (PSU), J. Bally (UC Boulder), E. Levesque (U. Washington), R. Williams (STSCI), P. A. Whitelock (SAAO, UCT), S. Mohamed (SAAO), R. D. Gehrz, S. Amodeo, D. Shenoy, R. Carlon, A. Cass, D. Corgan, D. Dykhoff, J. Faella, T. Gburek (U. Minnesota), N. Smith (U. Arizona), M. Cantiello (UCSB), N. Langer (U. Bonn), E. Ofek (WIS), J. Johansson (OKC/WIS), M. Parthasarathy (IIA), O. Fox (UC Berkeley), E. Hsiao (FSU), M. Phillips, N. Morrell, C. Gonzalez, C. Contreras (LCO), on behalf of the SPIRITS collaboration
on 31 Aug 2016; 01:02 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Mansi Manoj Kasliwal (mansi@astro.caltech.edu)

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We report the discovery of a possible, nearby supernova in Messier 108 (NGC 3556) designated as SPIRITS16tn. This luminous infrared transient was discovered during ongoing monitoring of nearby galaxies with the Spitzer InfraRed Intensive Transients Survey (SPIRITS; ATEL#6644, Kasliwal et al. 2016, ApJ submitted), using the Infrared Array Camera on the Spitzer Space Telescope.

On UT 2016 August 15.1, SPIRITS16tn had a 4.5um brightness of 13.04 +- 0.01 mag (Vega) and the [3.6] - [4.5] color was 0.67 +- 0.01 mag. We do not detect the source in images taken on UT 2016 March 3.63 to [3.6] > 17.6 and [4.5] > 17.7 mag. There are no known objects at this epoch and location in the MPC database. There is no obvious progenitor detected in archival Spitzer/IRAC imaging from UT 2011 February 7.6 to limiting magnitudes of [3.6] > 16.2 and [4.5] > 15.9 mag. There is also no progenitor detected at this location in archival HST imaging from UT 1994 July 4.3 to F606W ~ 25.8 mag.

At an RA (J2000) and Dec (J2000) of 11:11:20.40, +55:40:17.3, SPIRITS16tn is located approximately 89.9" from the center of the galaxy M108 (z=0.002332, d=8.8 Mpc, m-M=29.7, Sorce et al. 2014). The source is coincident with a dust lane. At the distance to the putative host, the absolute magnitude at [4.5] is -16.7 mag.

The target is currently unavailable for optical observing from the ground due to its proximity to the sun, but we encourage follow-up observations once the target becomes observable in ~ 2 months. We caution that we do not have spectroscopic confirmation that this source belongs to the reported host galaxy, and there is some chance it is an unrelated foreground or background event.