Pre-discovery limits on SPIRITS17lb, a probable supernova in IC 2163

ATel #10489; J. E. Jencson, S. Adams, M. M. Kasliwal (Caltech), and H. E. Bond (PSU)
on 14 Jun 2017; 00:38 UT
Credential Certification: Jacob Jencson (jj@astro.caltech.edu)

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We report pre-discovery limits on SPIRITS17lb, 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 #10488). SPIRITS17lb was discovered in Spitzer/IRAC images taken on UT 2017 May 28.69 at [4.5] = 14.53 +- 0.03 mag (Vega). The [3.6] - [4.5] color is 1.08 +- 0.05 mag. The source was detected in both Spitzer channels, but we do not have a second epoch to confirm. There is no evidence it is a moving object between the Spitzer detections, and there are no known objects at this epoch and location in the MPC database. At an RA and Dec of 06:16:27.78, -21:22:51.7, SPIRITS17lb is located approximately 19.0" from the center of IC 2163 (z=0.009223, d=35.4 Mpc, m-M=32.75, from the NASA/IPAC Extragalactic Database; NED), and is coincident with a spiral arm of the galaxy. At the distance to the putative host, the absolute magnitude at [4.5] is approximately -18.2 mag.

The source was not detected in reference subtracted images taken on UT 2016 December 29.94 to [3.6] > 18.3 and [4.5] > 17.7 mag, constraining the explosion date to 149.75 days before first detection. We do not detect the transient in optical images taken on UT 2017 March 4.19 with the GRBCam on the 60-inch telescope at Palomar Observatory (Cenko et al. 2005, PASP, 118, 1396) to g > 20.6 mag, r > 20.1 mag, and i > 19.8 mag (AB). The optical non-detections may further constrain the explosion date to 85.5 days before first detection, unless the event is highly dust obscured.

There is no obvious progenitor detected in archival Spitzer/IRAC imaging from UT 2005 February 22.71, but due to bright background emission from the coincident spiral arm we are only able to place weak limits on the luminosity of the progenitor to [3.6] > 14.3 mag (-18.5 mag absolute) and [4.5] > 14.2 mag (-18.6 mag absolute). We also examined the archival Hubble Space Telescope (HST) imaging of the site. The highest quality frames covering the position of SPIRITS17lb were taken on UT 2012 December 04 with the Wide Field Camera 3 (WFC3) in the F625W and F814W bands as part of program SNAP-13029 (PI A. Filippenko). Using 21 point-like objects in common with the Spitzer/IRAC [3.6] discovery image, we determined the position of SPIRITS17lb in the HST/WFC3 frames to formal astrometric root-mean-square uncertainty of 1.4 WFC3 pixels for the F625W image and 1.2 WFC3 pixels for the F814W image. There is no obvious progenitor candidate in either image within the 3-sigma error circle from the alignment. Aperture photometry at the position of the transient within an 0.2'' aperture gives 3-sigma upper limits on the progenitor luminosity of F625W > 25.5 mag (-7.4 mag absolute) and F814W > 25.0 mag (-7.9 mag absolute), assuming only Galactic foreground extinction toward the transient position from Schlafly & Finkbeiner (2011, ApJ, 737, 103; from NED). A full analysis of all available, archival HST imaging of the transient position is underway.

Additional follow-up observations, where possible, are highly encouraged. Please contact the authors if additional information (such as finding charts) is needed. 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.