Re-classification of Gaia17biu/SN 2017egm: the closest hydrogen-poor superluminous supernova yet found, located in a massive host galaxy

ATel #10498; Subo Dong, S. Bose, Ping Chen (KIAA-PKU), Stefano Benetti, A. Pastorello (INAF-Padova), Zheng Cai (UCSC), J. L. Prieto (UDP/MAS), Peter J. Brown (Mitchell Institute/Texas A&M), P. Ochner (Padova University), C. Ashall (LJMU), M. Stritzinger (Aarhus U), P. Lundqvist (Stockholm), S. Mattila (Turku), N. Elias-Rosa (INAF-Padova), R. S. Post (Post Astronomy), S. Villanueva Jr., K. Z. Stanek (OSU), R. A. Koff (Antelope Hills Observatory), on behalf of the NUTS collaboration and the ASAS-SN team.
on 16 Jun 2017; 15:22 UT
Distributed as an Instant Email Notice Supernovae
Credential Certification: Subo Dong (dongsubo@pku.edu.cn)

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Gaia17biu/SN 2017egm (RA: 10:19:05.62 Dec: +46:27:14.08) was discovered by Gaia on 2017-05-23 at 16.7 mag in a star-forming galaxy NGC 3191 (z = 0.030721; via NED). It was detected on images taken by the All-Sky Automated Survey for SuperNovae (ASAS-SN: Shappee et al. 2014) on 2017-05-20 and at later dates. It was previously classified as a Type II supernova by Xiang et al. (ATel #10442).

We took high-SNR spectra on 2017-05-30 with the 2.56 m Nordic Optical Telescope (NOT) equipped with ALFOSC as part of the Nordic Optical Telescope (NOT) Un-biased Transient Survey (NUTS; ATel #8992) and on 2017-06-03 with the 3m Shane telescope using the Kast spectrograph, respectively. Both spectra show the "W-shape" OII broad absorption features at rest-frame ~4100 and ~4400 A, which are characteristic for hydrogen-poor superluminous supernovae (SLSN-I; Quimby et al., 2011). There is also evidence of broad and weak spectral feature near ~6600 A, which shows resemblance with the spectral feature identified as CII 6580 in the pre-peak spectrum of SLSN-I Gaia16apd (e.g., Yan et al., 2017; Kangas et al., 2017). Spectrum taken on May 31 with the Liverpool Telescope (LT) is also consistent with SLSN-I. We triggered Swift/UVOT grism observation, and the UV spectrum taken on 2017-06-08 shows several broad absorption features similar to those in Gaia16apd (e.g., Yan et al., 2017).

We have taken Swift/UVOT imaging observations and conducted ground-based photometric monitoring using a number of telescopes affiliated with ASAS-SN and NUTS. Our light curves show rising fluxes for all optical and UV bands since its discovery. On June 15, it is at V = 15, suggesting an absolute magnitude of M_V ~ -20.6. Between June 2 and June 15, the supernova has been extremely blue with UVM2 - V between about -2.0 and -1.5 (Vega system), and preliminary fittings to the SEDs yield high blackbody temperatures (T_BB range from ~20,000 to ~15,000K) and blackbody radii R_BB ~ 10^15 cm. These are all consistent with a pre-peak SLSN-I.

The host galaxy NGC 3191 is at z = 0.030721 (~130 Mpc), placing SN 2017egm as the closest SLSN-I yet discovered, by more than factor of ~3. It has the highest apparent brightness among all known SLSNe-I. The vast majority of the SLSNe-I discovered to date have been in dwarf galaxies with stellar mass less than 2x10^9 M_Sun (e.g., Perley et al., 2016), in contrast, NGC 3191 is much more massive with a total stellar mass of about 5x10^10 M_Sun (Kelly & Kirshner 2012).

We are carrying out intensive multi-wavelength spectroscopic and photometric observations of this exceptional supernova.

We thank the Swift Acting PI Brad Cenko, the Observation Duty Scientists and the science planners.