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Classification of ATLAS17gxp/AT2017err with SALT and GREAT : a superluminous type IIn supernova at z = 0.107

ATel #10535; T.-W. Chen (MPE Garching), L. Wyrzykowski, M. Gromadzki, A. Hamanowicz (Warsaw University Astronomical Observatory, Poland), D. Buckley (South African Astronomical Observatory), S. J. Smartt, K. W. Smith, D. Young (QUB), C. Inserra (Southampton), P. Schady, T. Kruehler (MPE Garching), M. Fraser (UCD), J. Tonry, B. Stalder, L. Denneau., A. Heinze, H. Weiland (IfA, Univ. of Hawaii), A. Rest (STScI)
on 27 Jun 2017; 11:37 UT
Credential Certification: Ting-Wan Chen (jchen@mpe.mpg.de)

Subjects: Infra-Red, Optical, Supernovae, Transient

We report optical spectroscopy of ATLAS17gxp/AT2017err (Tonry et al. 2017, TNSTR, 649, 1). Target was supplied by the ATLAS survey (see Tonry et al. 2011, PASP, 123, 58 and Tonry et al. ATel #8680) and selected by the GREAT (GRond-Epessto-ATlas) survey (see Chen et al. ATel #10510). The spectra (380-830nm) were obtained using the Robert Stobie Spectrograph (RSS) on the South African Large Telescope (SALT) on 2017-06-23. Classifications were done with GELATO (Harutyunyan et al., 2008, A&A, 488, 383).

Survey Name | IAU Name  | RA (J2000)  | Dec (J2000) | Disc. Date | Source | Disc Mag |  z    | Type | Phase | Notes 
ATLAS17gxp  | SN2017err | 11:11:25.10 | +00:06:58.5 |  20170612  |  ATLAS  |  18.3   | 0.107 | SLSN-IIn | -1d  | (1) 

(1) The Balmer lines have a broad component (the broadest component has ~ 2400 km/s), strong H-alpha and H-beta, and relatively weak H-gamma are detected. The redshift determined from the narrow host emission [O II] 3727 line. According to the ATLAS lightcurve, it is still rising or around peak. At this redshift, the SN has g-band absolute magnitude < -20.4 mag and the underlying host galaxy (SDSS J111125.11+000658.9) is faint with M_g = -17.4 mag. The transient is close to the host centre but there are no strong [O III] lines detected.

We also report our observations with GROND (Greiner et al. 2008, PASP 120, 405), starting on 23:50 UT on 2017-06-17 (MJD = 57921.994), 5.7 days after the ATLAS discovery. We derive the following magnitudes (all in the AB system):

g' = 17.92 +/- 0.07 mag,
r' = 18.01 +/- 0.10 mag,
i' = 18.07 +/- 0.07 mag,
z' = 18.24 +/- 0.07 mag,
J = 18.96 +/- 0.13 mag, and
H = 19.14 +/- 0.19 mag

Given magnitudes are calibrated against SDSS field stars as well as 2MASS field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V) = 0.03 mag in the direction of the transients (Schlafly & Finkbeiner 2011).

After the Milky Way extinction correction, we fitted our colour SED assuming a black body (no K-correction applied), which indicates a temperature of 10800 +/- 400 K.

We acknowledge the excellent help in obtaining these data from Angela Hempel, Christina Gilligan and Vittorio Francesco Braga on La Silla.