Observations of ATLAS17gzd/AT2017esf with GREAT (GRond-Epessto-ATlas)

ATel #10510; T.-W. Chen, P. Schady, T. Kruehler, P. Wiseman, T. Schweyer, R. M. Yates (MPE Garching), J. Bolmer (MPE and ESO Vitacura), S. J. Smartt, K. W. Smith, D. Young (QUB), C. Inserra (Southampton), K. Maguire (QUB), E. Kankare (QUB), M. Sullivan (Southampton), S. Valenti (UC Davis), O. Yaron (Weizmann) J. Tonry, B. Stalder, L. Denneau., A. Heinze, H. Weiland (IfA, Univ. of Hawaii), A. Rest (STScI)
on 19 Jun 2017; 21:56 UT
Credential Certification: Ting-Wan Chen (jchen@mpe.mpg.de)

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The ''GREAT'' (GRond-Epessto-ATlas) survey of early superluminous supernova bumps project is a broad collaboration mainly coordinated by members of the GROND (Greiner et al. 2008, PASP 120, 405), ePESSTO (Smartt et al. 2015 A&A, 579, 40; www.pessto.org) and ATLAS team (Tonry et al. 2011, PASP, 123, 58; Tonry et al. ATel #8680). We aim to search for pre-peak light curve bumps of superluminous supernovae (e.g. Nicholl et al. 2015, ApJL, 807, L18; Nicholl & Smartt 2016, MNRAS, 457, L79; Smith et al. 2016, ApJL, 818, L8) in the low-z regime.

Targets are supplied by the ATLAS survey with a 2-4 day cadence, and we select our targets based on the presence of a faint underlying host galaxy in the PanSTARRS pre-explosion images. This is because the hosts of superluminous supernovae tend to be compact, star-forming dwarf galaxies with median of M_B ~ -17 mag (e.g. Chen et al. 2013ApJL, 763, L28; Lunnan et al. 2014, ApJ, 787, 138; Schulze et al. 2016, arXiv:161205978). We use simultaneous g'r'i'z'JHK GROND observations to estimate the photospheric temperature of the source from a fit of the spectral energy distribution assuming a black body. In those cases where we find a high temperature (> 20000 K) object, we then trigger ePESSTO for spectroscopic classification. This has the advantage that both the 2.2m MPG telescope and NTT are co-located at the ESO La Silla Observatory (Chile). We plan to report our GROND observations through ATels for those objects with an estimated black body temperature > 20000 K. For those objects that have a lower temperature, we will provide information through the ePESSTO NTT observation management tool.

Our most recent GREAT target is ATLAS17gzd/AT2017esf (Tonry et al. 2017, TNSTR, 653, 1), which we observed with GROND, starting on 00:07 UT on 2017-06-18 (MJD = 57922.005), 4.7 days after the ATLAS discovery on 2017-06-13 (pre-disc non-detection on 2017-06-10). We derive the following magnitudes and 3-sigma limit (all in the AB system):

g' = 18.01 +/- 0.01 mag,
r' = 18.33 +/- 0.01 mag,
i' = 18.59 +/- 0.02 mag,
z' = 18.76 +/- 0.02 mag,
J = 19.16 +/- 0.11 mag,
H = 19.70 +/- 0.29 mag, and
K > 19.37 mag.

Given magnitudes are calibrated against PS1 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) = 0.04 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 20200 +/- 1200 K.

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

Also as part of the GREAT survey, we have found several young supernovae occurring in faint host galaxies: ATLAS17avq/AT2017abw (type II?; ATel #10056), SN2017dgk (type Ic; ATel #10318), ATLAS17etw/SN2017dht (type II; ATel #10331, ATel #10334), PS17aio/AT2017vp (type II; ATel #10056), Gaia17bay/AT2017dfe (likely CV; ATel #10313, ATel #10321), ASASSN-17gu/ATLAS17ftg/AT2017eip (type Ia; ATel #10435).

We acknowledge the organisers and participants of MIAPP workshop ''Superluminous supernovae in the next decade'' for stimulating discussions of this project.