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ATLAS17gqa/AT2017ens GREAT followup: a spectroscopic classification of superluminous supernova candidate encouraged

ATel #10583; T.-W. Chen, T. Schweyer and P. Schady (MPE Garching)
on 19 Jul 2017; 14:21 UT
Distributed as an Instant Email Notice Supernovae
Credential Certification: Ting-Wan Chen (jchen@mpe.mpg.de)

Subjects: Infra-Red, Optical, Supernovae, Transient, Tidal Disruption Event

Referred to by ATel #: 10587

We observed the field of ATLAS17gqa/AT2017ens (Tonry et al. 2017, TNSTR, 636, 1) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2m MPG telescope at the ESO La Silla Observatory (Chile). The target was supplied by the ATLAS survey (see Tonry et al. 2011, PASP, 123, 58 and Tonry et al. ATel #8680) as part of the GREAT survey (GRond-Epessto-ATlas; see Chen et al. ATel #10510), and observed as part of the GREAT survey (Chen et al. ATel #10478).

Observations started at 23:54 UT on 2017-07-16 (MJD = 57950.996), 41.7 days after the ATLAS discovery. We derive the following magnitudes and 3-sigma limits (all in the AB system):

g' = 18.20 +/- 0.01 mag,
i' = 17.97 +/- 0.03 mag,
z' = 18.00 +/- 0.03 mag,
J = 18.48 +/- 0.10 mag,
H = 18.83 +/- 0.24 mag, and
K > 18.61 mag.

The source is unresolved in the r' band, and thus no GROND r'-band magnitude is given above. 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) = 0.02 mag in the direction of the transient (Schlafly & Finkbeiner 2011).

After correcting for Milky Way extinction, we fitted our 6-colour SED assuming a black body (no K-correction applied), which indicates a temperature of 7400 +/- 150 K. Based on our early GROND SED, the source had an incredibly high temperature of up to 21000 K at the start of 2017-06-08 (ATel #10478). Our temperature estimate of 7400 K on 2017-07-16 indicates that the source is still very hot after 42 days from discovery. The high temperature makes it very unlikely that this source is a type Ia supernova. Instead, it's very long rise time (3 weeks), luminosity and host galaxy properties are all suggestive of this being a new superluminous supernova. Further spectroscopic classification is required for this superluminous supernova candidate. Please be aware of the coordinates of this object of RA=12:04:09.36 Dec=-01:55:52.2, and so the source will be soon Sun constrained.

We acknowledge excellent help in obtaining these data from Sam Kim and Markus Rabus on La Silla.