"Resurrection" of the 1991T-like SN Ia ASASSN-18ro
ATel #12325; Subo Dong, Subhash Bose (KIAA-PKU), M. Stritzinger (Aarhus), K. Z. Stanek, C. S. Kochanek, T. A. Thompson (OSU), J. Prieto (UDP), B. Shappee (Hawaii), T. Holoien (Carnegie)
on 24 Dec 2018; 05:13 UT
Distributed as an Instant Email Notice Supernovae
Credential Certification: Subo Dong (dongsubo@pku.edu.cn)
Subjects: Optical, Supernovae
Referred to by ATel #: 12355
ASASSN-18ro (a.k.a., SN 2018evt, RA: 13:46:39.21 Dec: -09:38:35.75 ) was discovered by All-Sky Automated Survey for Supernovae (ASAS-SN; Shappee et al. 2014; Atel #11963) on UT 2018-08-11.00 at V ~ 16.5 mag (absolute magnitude M_V ~ -18.8 mag) in the host galaxy MCG -01-35-011 (z = 0.024971; A_V = 0.15 mag for Galactic extinction via NED). It was classified by ePESSTO as a SN Ia (best match SN Ia 1991T-like; Atel #11947).
Recent ASAS-SN observations of ASASSN-18ro show that it is at g ~ 16.4 mag on UT 2018-12-16.36 and g ~ 16.6 mag on 2018-12-22.09. Therefore, after 4 months of its discovery, the luminosity of ASASSN-18ro is still at about -19 mag, and such a high late-time luminosity is not compatible with the light curve of a typical (1991T-like) SN Ia. We note that ZTF and Gaia also recently detect ASASSN-18ro at a similar brightness, i.e. r = 16.24 +/- 0.02 mag on UT 2018-12-16.54 from ZTF (available via Lasair: https://lasair.roe.ac.uk/object/ZTF18actuhrs/) and G = 16.32 mag on UT 2018-12-17.18 from Gaia Transient Alerts (http://gsaweb.ast.cam.ac.uk/alerts/alert/Gaia18dwd/).
Re-inspection of the ePESSTO spectrum (taken with the ESO-NTT / EFOSC2-NTT at UT 2018-08-12.9995; available via TNS: https://wis-tns.weizmann.ac.il/object/2018evt) shows a prominent H-alpha emission line with FWHM ~ 1000 km/s (EW ~ 22 Angstrom). The archival 6dF spectrum (available via NED) at the nucleus of the host galaxy does not show any H-alpha emission. The H-alpha emission line likely origins from CSM interactions, which may also explain the high late-time luminosity of ASASSN-18ro. We note that some 1991T-like SNe Ia have been shown to have evidence of CSM interactions (e.g., Harris et al., 2018, ApJ, 868, 21; Graham et al., 2018, arXiv:1812.02757 and references therein).
Further follow-up observations are strongly encouraged.
