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Classification of AT2017ens/ATLAS17gqa as a Peculiar Superluminous Supernova

ATel #10587; D. A. Perley (LJMU), T.-W. Chen (MPE), A. V. Filippenko, T. G. Brink, W. Zheng (UC Berkeley)
on 21 Jul 2017; 01:45 UT
Credential Certification: Daniel Perley (dperley@dark-cosmology.dk)

Subjects: Optical, Supernovae, Transient

We report spectroscopic observations of the optical transient AT2017ens/ATLAS17gqa (Tonry et al. 2017, TNSTR, 636, 1), previously suggested (ATel #10583) from photometric observations to be a superluminous supernova candidate. Observations were taken with the Keck-I 10 m telescope on Mauna Kea, using the Low Resolution Imaging Spectrometer (LRIS) at 2017-07-20.27 UT. Our spectrum covers the range 310-1020 nm. The spectrum shows a blue continuum with a few broad features. Narrow emission lines at the rest wavelengths of H-alpha, H-beta, and H-gamma redshifted to z = 0.1086 are also present. No detections of [O II], [O III], or [N II] nebular emission are evident, suggesting that the Balmer lines originate from the transient and not an underlying host galaxy. We compared our spectrum to standard templates using several common classification programs (SNID, Superfit, GELATO). All of these favored a match with Type Ic-BL supernovae after peak brightness. However, at z = 0.1086, the photometry indicates a peak luminosity (M_g = -21 mag) substantially in excess of an ordinary SN Ic-BL, and Balmer emission would indicate the presence of significant hydrogen in the system. Further observations are encouraged, although the object is rapidly approaching solar conjunction. We thank M. Siebert, R. J. Foley, C. Kilpatrick, and Y.-C. Pan (UC Santa Cruz) for obtaining the Keck target-of-opportunity observations for us.