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Spectroscopic observation of ATLAS18sld/SN 2018ebt by NUTS (NOT Un-biased Transient Survey)

ATel #11891; S. Moran (Turku), Subo Dong, Subhash Bose (KIAA-PKU), S. Benetti (Padova), M. Fraser (UCD), M. Stritzinger (Aarhus U), S. Mattila, J. Harmanen, H. Kuncarayakti, T. Reynolds (Turku), A. Somero (MAGIC/Turku), F. Taddia (Stockholm), S. Dyrbye (NOT), S. J. Smartt, K. W. Smith, (Queen's University Belfast), J. Tonry, L. Denneau, B. Stalder, A. Heinze, H. Weiland, H. Flewelling (IfA, University of Hawaii), and A. Rest (STScI) on behalf of the NUTS and ATLAS collaboration
on 26 Jul 2018; 10:20 UT
Distributed as an Instant Email Notice Supernovae
Credential Certification: Subo Dong (dongsubo@pku.edu.cn)

Subjects: Optical, Supernovae

Referred to by ATel #: 11896, 11974, 12069

The Nordic Optical Telescope (NOT) Unbiased Transient Survey (NUTS; ATel #8992) reports the spectroscopic classification of ATLAS18sld/SN 2018ebt. The candidate was discovered by ATLAS (Atel #11727) on UT 2018-07-21 11:52:48 at 15.58 mag (orange-ATLAS).

The spectroscopic observation was performed with the 2.56 m Nordic Optical Telescope equipped with ALFOSC (range 350-950 nm; resolution 1.6 nm FWHM).

 
Survey Name   | IAU Name    | Discovery (UT)   | Discovery mag  | Observation (UT)         | Redshift          | Type           | Phase             
ATLAS18sld    | SN 2018ebt  | 2018-07-21.50    | 15.58          | 2018-07-24.06            | <~0.02-0.03       | likely Ic-BL.  | pre-/near peak 

Classification was performed using the SNID (Blondin and Tonry 2007, ApJ, 666, 1024) and GELATO (Harutyunyan et al. 2008, A&A, 488, 383) tools. The closest matches are SNe Ic-BL (e.g., 2012ap, 2006aj) with phases at a few days before the peak or around the peak, and the redshifts from these matches are uncertain, generally between 0.02 - 0.03 or less. There are also some matches with other SNe Ib/Ic. The most recent ATLAS observation (15.20 +/- 0.04 mag at MJD = 58324.50) shows that the SN is brighter than at discovery, and the light curve indicates that it might be approaching the peak, which is consistent with the estimated spectroscopic phase based on SNID/GELATO. At z ~ 0.02 - 0.03, the latest ATLAS measurement would indicate an absolute magnitude M_o ~ -20 - -21 (assuming that Galactic extinction A_o is similar to A_R = 0.6 from NED), while the redshift could be lower and then the object would be less luminous. Note that ATLAS18sld/SN 2018ebt was classified by Masayuki Yamanaka as a SN Ib/c at z=0.005 using the spectrum taken by 1.5-m Kanata telescope/HOWPol on UT 2018-07-23 13:47:51.

Spectral lines from the host galaxy will be needed to pin down the SN redshift/distance and its absolute magnitude. ATLAS18sld/SN 2018ebt is about 14'' away from the center of a faint galaxy GALEXASC J204152.80+641238.5 (unknown redshift). Given the apparent magnitude of the host (r=17.0 from PanSTARRS), and taking the largest plausible value for the distance (z=0.03), the host galaxy is likely fainter than r~-19.1. The relatively large separation from the center of its probable may indicate a low-metallicity environment.

Further follow-up observations are encouraged.