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Special Notice #421: SN 2016gkg - Unusual Type-II Supernova in NGC 613 (Scl)

September 22, 2016: SN 2016gkg (announced as AT2016gkg) was discovered by Victor Buso (Rosario, Argentina) on 2016 September 20.2484 UT at CCD magnitude (clear filter) 17.6 +/-0.5, rising to 14.5 +/-0.2 on Sep. 21.1398. His discovery was reported to the Transient Name Server by Sebastian Otero (Buenos Aires, Argentina). It was confirmed by ASAS-SN on Sep. 21.29 UT (B. Nicholls (Mt. Vernon Obs., New Zealand) et al., ATel #9521) and by ATLAS on Sep. 20.54 UT (J. Tonry (IfA, University of Hawaii) et al., ATel #9526).

SN 2016gkg was spectroscopically confirmed as a Type-II supernova by S. W. Jha (Rutgers), V. van Wyk, and P. Vaisanen (SALT/SAAO) from observations made on 2016 September 21.9 UT (ATel #9528).

Its optical behavior has been very unusual, with more than a magnitude decline in 24-48 hours, which is highly atypical for a Type-II supernova. It is for this reason that observations are requested as the supernova evolves. The most recent magnitude available is 15.73 V +/- 0.05 on 2016 Sep. 22.3 UT, using ASAS-SN data (Ping Che (KIAA-PKU) et al., ATel #9529).

V observations are requested; other bands may be added as time and inclination permit. As this supernova appears to be changing so quickly, multiple observations per night would be valuable.

Coordinates for SN 2016gkg:  R.A. 01 34 14.40   Dec. -29 26 24.2     (2000.0)
This position is 78.4" South and 49.1" West of the center of NGC 613 (ATel #9521).

Charts with comparison stars for SN 2016gkg may be created using the AAVSO Variable Star Plotter (VSP). Observers may find choosing the 'display DSS chart' when creating their charts very helpful, as the DSS image of the galaxy will be supraimposed on the star positions and magnitude labels. Also, be sure to tick the boxes requesting Rc and Ic comparison stars in addition to the V and B-V which are supplied by default.

Please report observations to the AAVSO International Database using the name SN 2016gkg.

Congratulations to Victor on his discovery!


This AAVSO Special Notice was compiled by Elizabeth O. Waagen.


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