The Precursor of PSN J12355230+2755559 in NGC 4559

ATel #3865; Schuyler D. Van Dyk (Spitzer Science Center/Caltech), Mohan Ganeshalingam, Jeffrey M. Silverman, and Alexei V. Filippenko (UC Berkeley)
on 15 Jan 2012; 01:19 UT
Credential Certification: Schuyler D. Van Dyk (vandyk@ipac.caltech.edu)

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Schuyler D. Van Dyk (Spitzer Science Center/Caltech), Mohan Ganeshalingam, Jeffrey M. Silverman, and Alexei V. Filippenko (UC Berkeley) report identification of the precursor of the "SN impostor" PSN J12355230+2755559 in archival Hubble Space Telescope (HST) WFPC2 and ACS/HRC multi-band images, the former dating from 2001 May 25 and the latter from 2005 March 8 UT. Using the discovery images of the SN impostor made with the Katzman Automatic Imaging Telescope (KAIT) from 2012 January 11 UT, we have isolated a candidate star, using absolute astrometry with an uncertainty 0.7" (approximately an error circle of 15-pixel radius on both the WFPC2/PC or ACS/HRC images). Preliminary photometry using HSTphot with the F450W, F555W, and F814W WFPC2 images results in a brightness for the star of B=20.62, V=20.47, and I=20.03 mag (uncertainties are all 0.01 mag or smaller). Preliminary photometry using Dolphot with the F435W, F555W, and F814W HRC images results in B=20.39, V=20.27, and I=19.90 (again, uncertainties are all smaller than 0.01 mag). Although the astrometric uncertainty is larger than desirable, our confidence in the star's identification is strengthened by 1) the star being the brightest point source seen in the archival HRC F658N images (these contain the H-alpha emission line; the star is also the brightest such source in the F502N images, which contain the [O III] emission lines); and, 2) the possible indication of a brightening in all bands for the star between 2001 and 2005. The distance modulus to NGC 4559 is uncertain, but assuming the average value from the NASA/IPAC Extragalactic Database (NED), 29.57 mag, and assuming only the Galactic foreground extinction at V-band from Schlegel, Finkbeiner, & Davis (1998, ApJ, 500, 525), 0.06 mag, then the star had V absolute magnitude of -9.4, and absolute colors B-V=0.10 and V-I=0.36 (consistent with an early F spectral type) in 2005 prior to outburst. Alternatively, the object could be a reddened, compact star cluster. No obvious counterpart is seen in pre-outburst Spitzer Space Telescope IRAC images from 2004. Further analysis is ongoing.