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SN2018cnf (ASASSN-18mr) is a type IIn supernova with an outburst in 2015 (PS15dkt)

ATel #11726; S. J. Prentice, K. Maguire (Queen's University Belfast), A. Pastorello, L. Tomasella (INAF-Padova Observatory), A. Reguitti (Padova University), A. Morales-Garoffolo (University of Cadiz), S. Geier (GTC), K. W. Smith, D. Wright, S. J. Smartt (Queen's University Belfast), M. Huber, K. C. Chambers, H. Flewelling, M. Willman, A. Schultz, T. Lowe, E. Magnier, C. Waters, R. J. Wainscoat (IfA, Hawaii)
on 17 Jun 2018; 14:30 UT
Distributed as an Instant Email Notice Supernovae
Credential Certification: Ken Smith (k.w.smith@qub.ac.uk)

Subjects: Optical, Supernovae, Transient

ASASSN-18mr (TNS #19408) is a g=17.7 mag transient in the host galaxy 2MASX J23393156-0308565, at a distance of 96 Mpc (z=0.023763) discovered by the All Sky Automated Survey for SuperNovae (ASAS-SN, Holoien, et al. 2017, MNRAS, 464, 2672) on 2018-06-14.

It is spatially coincident (within 0.4 arcsec) with a Pan-STARRS transient discovered in 2015. The Pan-STARRS Survey for Transients (PSST) reported PS15dkt at w = 20.72 +/- 0.05 on 2015-12-09 (MJD=57365.2). It is also detected in poor seeing images the night after (For PSST information, see http://star.pst.qub.ac.uk/ps1threepi/ Huber et al. ATel #7153 and Chambers et al. 2016, arXiv:1612.05560)

Summary details are as follows

 
Name        |  RA (J2000)  | Dec (J2000)  | Disc. Date |  Mag   | Abs Mag 
ASASSN-18mr | 23:39:31.21  | -03:08:55.18 |  20180614  |  17.7  | -17.4 
PS15dkt     | 23:39:31.23  | -03:08:54.80 |  20151209  | 20.72  | -14.3  

Photometry obtained on 2018-06-17.20 using LT+IO:O returns the following magnitudes: g 17.62 (0.02), g-r = 0.41 (0.03), suggesting that the object is still rising to the maximum light.

A spectrum taken on 2018-06-17.192 (UT) using SPRAT on the Liverpool Telescope (range 400-810 nm, resol. 1.8 nm) shows the characteristic interaction driven hydrogen emission lines of a type IIn SN.

The spectrum is dominated by relatively narrow H lines in emission, with a FWHM velocity of 2000 km/s. A weak Na ID feature (possibly blended with He I 5876) and a broad bump at 5200-5300 A, possibly due to a blend of Fe II lines, are also detected. The spectrum is somewhat similar to that of the type IIn SN 2009ip (e.g. Pastorello et al. 2013, ApJ, 767,1; Fraser et al. 2013, MNRAS, 433, 1312). Type IIn spectral features are in general interpreted as signature of interaction between the SN ejecta and circumstellar material. As a faint transient was observed 2.5 years before the explosion of ASASSN-18mr at the same location, this was likely a preSN major outburst, similar to those observed before the explosion of several Type IIn SNe.