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Spectroscopic classification of ATLAS17mwz (SN 2017hrq) by SALT as a probable Type IIn SN.

ATel #10953; Ping Chen (KIAA-PKU), D. A. H. Buckley (SAAO), M. Gromadzki (Warsaw), Andrea Pastorello (Padova), Subo Dong, S. Bose (KIAA-PKU), J.-L. Prieto (Diego Portales; MAS), M. Stritzinger (Aarhus U), K. Z. Stanek (OSU)
on 12 Nov 2017; 00:45 UT
Credential Certification: Subo Dong (dongsubo@pku.edu.cn)

Subjects: Optical, Supernovae, Transient

ATLAS17mwz ( SN 2017hrq ) was discovered by the ATLAS survey (see Tonry et al. 2011, PASP, 123, 58) on UT 2017-10-29.47 at 17.0 mag (ATLAS orange filter). Onori et al. (Atel #10944) reported an ePESSTO spectrum taken on Nov. 8, 2017, and based on narrow Balmer lines at z = 0.055, they classified it as a luminous Type IIn SN or a nuclear transient associated with the host's central black hole.

ATLAS17mwz (SN 2017hrq) was detected on images taken by the All-Sky Automated Survey for SuperNovae (ASAS-SN: Shappee et al. 2014, ApJ, 788, 48) on UT 2017-10-13.3, and at later dates. The ASAS-SN detections triggered our spectroscopic observations for classification with SALT (+RSS).

We obtained our first SALT/RSS spectrum of ATLAS17mwz (SN 2017hrq) on UT 2017-11-04.9 (spectral range 380.0-920.0 nm) with an (averaged) spectral resolution of 350 and exposure time of 1000s. The spectrum consists of a blue continuum superposed with narrow Balmer lines in emission, consistent with the findings of Onori et al. (Atel #10944), and several narrow He-I emission lines.

On UT 2017-11-10.9, we obtained additional SALT/RSS spectra (spectral range 463.0-766.0 nm with a resolution of ~1100 and an exposure time of 2x1200s). The spectrum exhibits narrow H-alpha and H-beta emission lines composed of both an unresolved component (<300 km/s) and a resolved broader component (FWHM ~= 1800 km/s). A He-I (rest 706.5 nm) emission feature also exhibits a two-component profile with similar velocities. Additionally, narrow He-I (rest 587.6 nm and 667.8 nm) features are also present.

The Balmer and He-I line profiles are indicative of possible SN+CSM interactions, consistent with an SN IIn.

Follow-up observations were also obtained with the Las Cumbres Observatory (LCO) 1-m telescope located at Cerro Tololo Inter-American Observatory (CTIO) on UT 2017-11-07.1 UT. These observations indicate the position of the SN is offset from the host nucleus by ~2", which is consistent with both ATLAS astrometry of the transient and the DSS astrometry of the host's center.