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An unusually fast brightness decline in optical of young type II supernova SN 2016gkg from ASAS-SN follow-up observations

ATel #9529; Ping Chen, Subo Dong, S. Bose (KIAA-PKU), J. L. Prieto (Diego Portales; MAS), K. Z. Stanek, C. S. Kochanek, J. S. Brown, T. W.-S. Holoien, J. Shields (Ohio State), B. J. Shappee (Hubble Fellow, Carnegie Observatories), D. Bersier (LJMU), J. Brimacombe (Coral Towers Observatory), B. Nicholls (Mt. Vernon Obs., New Zealand)
on 22 Sep 2016; 14:24 UT
Credential Certification: Subo Dong (dongsubo@pku.edu.cn)

Subjects: Optical, Ultra-Violet, Supernovae

Referred to by ATel #: 9536, 9544, 9561, 9562, 9573, 9576

The All Sky Automated Survey for SuperNovae (ASAS-SN or "Assassin") collaboration reports an unusually fast brightness decline in optical for SN 2016gkg at an averaged rate of ~1 mag/d in V-band between UT 2016-09-21.7 (JD 2457653.2) and UT 2016-09-22.4 (JD 2457653.9). The light curve is shown at this URL.

SN 2016gkg was first reported by Victor Buso and Sebastian Otero, confirmed by ASAS-SN (Atel #9521) and ATLAS (Atel #9526), and spectroscopically classified as a young type II supernova (Atel #9528). Optical photometry by Buso & Otero, ASAS-SN and ATLAS suggest that SN 2016gkg rises rapidly by ~2+ mag between UT 2016-09-20 and UT 2016-09-21.

Using ASAS-SN data obtained from the quadruple 14-cm "Cassius" telescope in Cerro Tololo, Chile taken on UT 2016-09-22.3, we measure the supernova at V = 15.73 +/- 0.05, which is about 0.7 magnitude dimmer than the previous ASAS-SN epoch at about a day ago on 2016-09-21.3 (V~15.0). Such a fast brightness decline in optical is corroborated by observations from LCOGT 1m telescope in Cerro Tololo taken on UT 2016-09-22.1 (V = 15.54 +/- 0.03) and UT 2016-09-22.4 (V = 15.84 +/- 0.06). We also obtain Swift UVOT data on UT 2016-09-21.7 with V = 15.09 +/- 0.05. The ASAS-SN, LCOGT and Swift observations taken between UT 2016-09-21.7 and UT 2016-09-22.4 are all consistent with a brightness decline at a high rate of about 1 mag/d in V-band, and such a fast brightness decline in optical is very unusual for a type II supernova.

The LCOGT observations are performed with BVgri filters, and the Swift UVOT data are taken with all its optical and UV filters. Swift data show that the supernova is UV bright (UVW1-V ~ -1.3), consistent with a young type II supernova. All derived photometry are given in the table below. We also include the photometry previously reported by Buso & Otero, ASAS-SN (Atel #9521) and ATLAS (Atel #9526) in the table.

 
Obs. JD        Mag            Site         Filter 
2457651.7484   17.6  +/- 0.5  Buso & Otero Clear 
2457652.6398   14.5  +/-0.2  Buso & Otero Clear 
2457651.6653   >17.36         ASAS-SN      V 
2457652.7987   15.01 +/- 0.04 ASAS-SN      V 
2457653.7884   15.73 +/- 0.05 ASAS-SN      V 
2457652.04     15.94 +/- 0.13 ATLAS        o 
2457652.05     15.78 +/- 0.08 ATLAS        o 
2457653.6276   15.70 +/- 0.04 LCOGT        B 
2457653.8946   16.01 +/- 0.08 LCOGT        B 
2457653.6289   15.54 +/- 0.03 LCOGT        V 
2457653.8959   15.84 +/- 0.06 LCOGT        V 
2457653.6302   15.61 +/- 0.03 LCOGT        g 
2457653.8972   15.91 +/- 0.07 LCOGT        g 
2457653.6328   15.59 +/- 0.04 LCOGT        r 
2457653.8998   15.85 +/- 0.08 LCOGT        r 
2457653.6315   15.65 +/- 0.04 LCOGT        i 
2457653.1608   13.92 +/- 0.04 Swift        UVW2 
2457653.2318   15.28 +/- 0.25 Swift        UVM2 
2457653.1569   13.75 +/- 0.04 Swift        UVW1 
2457653.1588   13.97 +/- 0.04 Swift        U 
2457653.1598   15.21 +/- 0.04 Swift        B 
2457653.1645   15.09 +/- 0.05 Swift        V 

Follow-up observations are encouraged.

We thank the Swift Team for rapid approval and scheduling of ToO observations. We thank LCOGT and its staff for their continued support of ASAS-SN. ASAS-SN is supported by NSF grant AST-1515927, the Center for Cosmology and AstroParticle Physics (CCAPP) at OSU, and the Mt. Cuba Astronomical Foundation. For more information about the ASAS-SN project, see the ASAS-SN Homepage and the list of all ASAS-SN transients.

SN 2016gkg light curve