Pre-discovery detections and progenitor candidate for SPIRITS17qm in NGC 1365

ATel #11579; J. E. Jencson (Caltech), H. E. Bond (PSU), S. M. Adams (Caltech), M. M. Kasliwal (Caltech)
on 26 Apr 2018; 21:14 UT
Credential Certification: Jacob Jencson (jj@astro.caltech.edu)

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We report the detection of a pre-discovery outburst of SPIRITS17qm, discovered as part of the ongoing Spitzer InfraRed Intensive Transients Survey (SPIRITS) using the 3.6 and 4.5 micron imaging channels ([3.6] and [4.5]) of the Infrared Array Camera (IRAC) on the Spitzer Space Telescope (ATel #11575). SPIRITS17qm was discovered in Spitzer/IRAC images taken on UT 2017 Nov. 5 at [4.5] = 15.44 +\- 0.03 mag (Vega). The [3.6] - [4.5] color is 0.21 +\- 0.04 mag. Analysis of the pre-discovery light curve, based on image subtraction with archival reference images from 2004, indicates a previous infrared outburst peaking at [4.5] = 16.12 +\- 0.06 mag 1322 days before the discovery outburst.

At an RA and Dec of 03:33:38.85, -36:08:09.4, SPIRITS17qm is located 33.5 arcsec from the center of NGC 1365 (z=0.005457, d=18 Mpc, m-M=31.3 mag, from the NASA/IPAC Extragalactic Database; NED). At the distance to the putative host, the absolute magnitude at [4.5] is approximately -15.9 mag, in the luminosity range typically observed for core-collapse SNe at infrared wavelengths.

In the archival Spitzer/IRAC images used as references for subtraction, the transient is coincident with what appears to be a distinct source blended with a marginally resolved cluster. We also examined the archival Hubble Space Telescope (HST) imaging of the site, which includes two epochs with WFPC2/F606W taken on 2001 March 08 and 2001 June 09 as part of program SNAP-8597 (PI Regan). By registering with the Spitzer/IRAC [3.6] discovery image, we determined the position of SPIRITS17pc in the 2001 March 08 HST/WFPC2 frame to astrometric rms uncertainty of 3.0 WFPC2 pixels (0.3 arcsec). We identify a point-like object consistent with the transient position. The F606W magnitudes (Vega) for this object from the Hubble Legacy Archive source catalogs are 22.45 (2001-03-08) and 20.77 (2001-06-09). At the assumed distance to the host and correcting for Galactic extinction from NED with A_F606W = 0.49 mag, the absolute magnitudes are -9.3 and -11.0, indicating the source is luminous and highly variable, and thus, likely associated with SPIRITS17qm.

Continued monitoring in the optical, IR and radio, and analysis of the progenitor are ongoing.