ASASSN-16jt and pre-outburst transient AT 2016cvk resemble the unusual transient SN 2009ip

ATel #9445; J. S. Brown (Ohio State), J. L. Prieto (Diego Portales; MAS), B. J. Shappee (Hubble Fellow, Carnegie Observatories), Subo Dong (KIAA-PKU), N. Morrell (Las Campanas Observatory), K. Z. Stanek, T. W.-S. Holoien, C. S. Kochanek, J. Shields (Ohio State), D. Bersier (LJMU), S. Bose, Ping Chen (KIAA-PKU), J. Brimacombe (Coral Towers Observatory), P. Marples (Leyburn Observatory, Australia)
on 1 Sep 2016; 16:29 UT
Distributed as an Instant Email Notice Supernovae
Credential Certification: Jonathan Brown (brown@astronomy.ohio-state.edu)

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In ATEL #9439 we reported the discovery of a new transient source, ASASSN-16jt, and note its proximity to a previously reported transient (AT 2016cvk). Based on the coordinates in the two reportings, the two sources appeared to be separated by ~1". However, based on archival LCOGT imaging of AT 2016cvk, we find that the position of the two sources may in fact be coincident, strongly indicating that AT 2016cvk was a prior outburst of ASASSN-16jt. This figure shows a comparison of the location transient in various images. The left panel shows an image from J. Brimacombe on 2016-08-31, the middle panel shows an image from P. Marples on 2016-08-30, and the right panel shows an LCOGT image from 2016-06-29. The red circle has a radius of 5" and is centered on the position of the transient in the J. Brimacombe image. The ASAS-SN non-detection limits from mid-June and selected deep limits from July and August are shown in the table below:

 
Obs. UT Date         V mag 
2016-05-28.39        >17.0 
2016-06-06.27        >17.5 
2016-06-08.28        >17.5 
2016-06-16.23        >17.0 
2016-06-19.23        >16.6 
2016-06-21.33        >16.0 
-------------------------- 
2016-07-07.31        >17.2 
2016-07-26.34        >17.4 
2016-08-07.32        >17.5 
2016-08-26.24        >17.8 

At the distance of the host galaxy (z=0.010783, d=41.3 Mpc, via NED), the ASAS-SN V-band limit on UT 2016-06-16.23 translates to an limit on the absolute magnitude M_V > -16.1. At the epoch of discovery, AT 2016cvk had an approximate absolute magnitude M_V ~ -15, while presently ASASSN-16jt has M_V ~ -17. We note that SN 2009ip showed similar behavior in 2012 (Mauerhan et al. 2013, MNRAS, 430, 1801).

We obtained optical spectra of ASASSN-16jt/AT2016cvk on June 18 (first outburst) and on September 1 (current outburst) with WFCCD mounted on the du Pont 2.5-m telescope at Las Campanas Observatory. A comparison of the spectra can be found here. The spectrum obtained on Jun. 18 shows striking resemblance to the spectra of the Type IIn SN 2009ip obtained in 2012 before its luminous peak in late September, with relatively broad Balmer lines in emission and P-Cygni absorption troughs at 6600 km/s, indicating fast-moving ejected material. The spectrum obtained on Sep. 1 (ongoing outburst) shows a bluer continuum, broad (~6000 km/s) Balmer lines (with narrow components on top) in emission only (no P-Cygni profiles), and also He I lines and a broad He II 4686 emission component (FWHM ~ 6000 km/s). Although the luminosities of both outbursts are in the range of normal Type II SN and we see ejecta moving at fast velocities in the spectra (few 1000 km/s), detailed follow-up is needed to determine the actual physical nature of these outbursts. Given the resemblance to SN 2009ip, these outbursts/explosions are most likely associated to the final death throes of a very massive star with a dense CSM.

Follow-up observations are encouraged.

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.