TITLE: GCN CIRCULAR NUMBER: 20878 SUBJECT: LIGO/Virgo G277583: GROND follow up of ATLAS17cck DATE: 17/03/16 04:16:23 GMT FROM: Ting-Wan Chen at PESSTO T.-W. Chen and P. Wiseman (both MPE Garching) report: We observed the field of ATLAS17cck (Tonry et al., GCN #20877) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO La Silla Observatory (Chile). Observations started at 01:17 UT on 2017-03-16, 15 hours after the earliest detection reported in Tonry et al. (GCN #20877). They were performed at an average seeing of 1.4" and at an average airmass of 1.7. Based on the 24.5 minutes (griz) and 21.3 minutes (JHK) of exposures, we do not detect a source within the ATLAS17cck error circle of 2” radius down to the following 3 sigma limiting AB magnitudes: g' > 23.9 mag, r' > 23.9 mag, i' > 23.3 mag, z' > 23.1 mag, J > 21.1 mag, H > 20.6 mag, and K > 18.8 mag However, we note the detection of a bright, uncatalogued source located at RA, Dec = 07:44:50.33, +24:45:54.8, equivalent to 116.22165, +24.76602, with an r’-band magnitude of 16.2 mag. We believe this to be the same object as ATLAS17cck, and that it corresponds to the asteroid (1056) Azalea, which is expected at this location according to the IAU's Minor Planet Checker, 'http://www.minorplanetcenter.net/'. The given limits are derived based on images calibrated against Pan-STARRS/2MASS field stars, and are not corrected for the Galactic foreground extinction corresponding to a reddening of E_(B-V)= 0.04 in the direction of the reported object (Schlafly & Finkbeiner 2011). We acknowledge the excellent support provided by the observer, Paula Sarkis, at the telescope, and the support astronomer, Sam Kim, in obtaining these data. And Ying-Tung Chen for checking the asteroid position.