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GROND further followup observations of ATLAS18qqn/AT2018cow

ATel #11734; T.-W. Chen and P. Schady (both MPE Garching)
on 19 Jun 2018; 17:03 UT
Credential Certification: Ting-Wan Chen (jchen@mpe.mpg.de)

Subjects: Infra-Red, Optical, Transient

Referred to by ATel #: 11737, 11738, 11742, 11743, 11749, 11752, 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 04:50 UT on 2018-06-19 (MJD = 58288.202), 2.8 days after the ATLAS discovery. We derive the following magnitudes (all in the AB system) using aperture photometry with a aperture of the mean FWHM of the field stars:

g' = 13.65 +/- 0.01 mag,
r' = 14.14 +/- 0.01 mag,
i' = 14.48 +/- 0.03 mag,
z' = 14.67 +/- 0.03 mag,
J = 15.05 +/- 0.01 mag,
H = 15.40 +/- 0.02 mag, and
K = 15.68 +/- 0.02 mag.
Our photometry has not been corrected for host galaxy background contamination.

The above magnitudes are consistent with the blue, power-law spectral shape reported by Perley (ATel #11732), and imply a hotter temperature than implied by our first epoch of GROND observations (Chen & Rabus; ATel #11729), which was based on PSF photometry. Using aperture photometry, we now derive the following AB magnitudes for the observations taken on 2018-06-18:

g' = 13.42 +/- 0.01 mag,
r' = 13.89 +/- 0.01 mag,
i' = 14.14 +/- 0.03 mag,
z' = 14.33 +/- 0.03 mag,
J = 14.72 +/- 0.01 mag,
H = 15.14 +/- 0.02 mag, and
K = 15.29 +/- 0.02 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) = 0.08 mag in the direction of the transient (Schlafly & Finkbeiner 2011).

This object is now fading in all bands, e.g. 0.25 mag/day in GROND r band and 0.34 mag/day in GROND J band from 2018-06-18 to 2018-06-19.

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