Pre-discovery detections and progenitor candidate for SPIRITS17pc in NGC 4388

ATel #11577; J. E. Jencson (Caltech), H. E. Bond (PSU), S. M. Adams (Caltech), M. M. Kasliwal (Caltech)
on 26 Apr 2018; 21:06 UT
Credential Certification: Jacob Jencson (jj@astro.caltech.edu)

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We report detections of pre-discovery outbursts of SPIRITS17pc, 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). SPIRITS17pc was discovered in Spitzer/IRAC images taken on UT 2017 Oct. 12.8 at [4.5] = 15.02 +- 0.03 mag (Vega). The [3.6] - [4.5] color is 0.52 +- 0.05 mag. Analysis of the pre-discovery light curve, based on image subtraction with archival reference image stacks from 2004-2008, indicates 2 additional, progressively brightening outbursts, peaking at [4.5] = 16.3 +- 0.1 mag and 15.8 +- 0.1 mag at 783 and 551 days before the discovery outburst, respectively.

At an RA and Dec of 12:25:44.43, +12:39:44.5, SPIRITS17pc is located 33.2 arcsec from the center of NGC 4388 (z=0.008419, d=18 Mpc, m-M=31.3 mag, from the NASA/IPAC Extragalactic Database; NED), a prominent member of the Virgo Cluster and known Seyfert 2 galaxy. At the distance to the putative host, the absolute magnitude at [4.5] is approximately -16.3 mag, in the luminosity range typically observed for core-collapse SNe at infrared wavelengths.

There is no obvious quiescent counterpart detected in the archival Spitzer/IRAC images used as references for subtraction, and due to bright background emission we are unable to place any meaningful limits on the mid-IR luminosity of the progenitor. We also examined the archival Hubble Space Telescope (HST) imaging of the site, which has optical to near-IR coverage with WFC3 in the F336W, F438W, F814W, F110W, and F160W filters taken on 2017 June 8 as part of the program part of program GO-12185 (PI J. Greene). By registering with a high resolution J-band image of the transient with Keck/NIRC2 taken on 2017 Dec. 8, we determined the precise position of SPIRITS17pc in the HST/WFC3 F110W frame to astrometric rms uncertainty of 0.15 WFC3 pixels (0.02 arcsec). We identify a point-like object consistent with the transient position. We measured photometry for the candidate from the WFC3 data using Dolphot (Dolphin 2000, PASP, 112, 1397) and found F336W = 25.52 +/- 0.22, F438W = 26.23 +/- 0.38, F814W = 24.00 +/- 0.05, F110W = 21.57 +/- 0.03, and F160W = 20.34 +/- 0.02 mag (Vega). Initial modelling of the broadband spectral energy distribution with DUSTY (Ivezic, Nenkova & Elitzur 1999) is consistent with a luminous, hot star with log(L/Lsun) ~ 5.3, T_eff ~ 20,000 K, active dust formation (dust temperature ~ 1800 K), and relatively low foreground host reddening at E(B-V) < 0.54 mag.

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