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Proposed Progenitor of SN 2016adj (Cen A) is an Unrelated Star

ATel #8759; Ben Sugerman (Goucher College), Stephen Lawrence (Hofstra University)
on 1 Mar 2016; 12:47 UT
Credential Certification: Ben Sugerman (ben.sugerman@gmail.com)

Subjects: Optical, Supernovae

Referred to by ATel #: 8760

SN 2016adj in NGC 5128 (Centaurus A) was discovered on Feb 02.36 2016 UT by the BOSS survey (Marples et al., ATel #8651). The supernova field was imaged with the Hubble Space Telescope (HST) WFC3 UVIS camera on UT 2016 Feb 22.1 in the F438W and F814W filters as part of GO-14115 (P.I. Van Dyk). For SN 2016adj at this epoch we measure approximate B and I_c magnitudes of 18.1 and 12.5 respectively, with formal uncertainties of 0.05 mag (not including conversion from HST to Johnson/Cousins filters). The new F814W image was geometrically registered to a pre-explosion exposure taken on 2010 Jul 6 (GO-11360, P.I. O'Connell) using 122 common sources, with an RMS scatter of 2 milliarcsec. The SN centroid is 0.053 arcseconds (at PA 173) from the possible progenitor identified by Van Dyk et al. (ATel #8693) and Kelly et al. (ATel #8720), suggesting this source is not in fact the SN progenitor. When this star (I_c=23.1 mag, consistent with ATel #8693) is removed using point-spread function subtraction, a very faint source remains at the SN position, with an upper limit of I_c=26. No source is detected in F438W, yielding an upper limit of B=28. Assuming A_V of 2-4 mag as suggested by Stritzinger et al. (ATel #8657), the upper limits to the progenitor's absolute magnitude are M_B>-4.2 to -4.9 and M_I > -3.3 to -4.1, adopting a distance of 3.42 Mpc (Ferrarese et al. 2007, ApJ, 654, 186).