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GROND followup observations of ATLAS18qqn/AT2018cow

ATel #11729; T.-W. Chen (MPE Garching) and M. Rabus (PUC, Chile)
on 18 Jun 2018; 11:13 UT
Credential Certification: Ting-Wan Chen (jchen@mpe.mpg.de)

Subjects: Infra-Red, Optical, Transient

Referred to by ATel #: 11734, 11737, 11738, 11749, 11753, 11757, 11758, 11760, 11796

We observed the field of ATLAS18qqn/AT2018cow (Smartt et al. ATel #11727) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the MPG 2.2m telescope at the ESO La Silla Observatory (Chile).

Observations started at 23:54 UT on 2017-07-16 (MJD = 58287.150), 1.7 days after the ATLAS discovery. We derive the following magnitudes (all in the AB system):

g' = 14.11 +/- 0.01 mag,
r' = 13.89 +/- 0.01 mag,
i' = 14.14 +/- 0.03 mag,
z' = 14.34 +/- 0.03 mag,
J = 14.72 +/- 0.02 mag,
H = 15.09 +/- 0.02 mag, and
K = 15.32 +/- 0.02 mag.
Our photometry has not been corrected for host galaxy background contamination.

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) = 0.08 mag in the direction of the transient (Schlafly & Finkbeiner 2011).

After correcting for Milky Way extinction, we fitted SED assuming a black body, which indicates a temperature of 9200 +/- 600 K. This object may have risen 0.5 mag/day roughly compared between GROND-r and ATLAS-o bands.