GCN Circular 20393
M. M. Kasliwal (Caltech), S. M. Adams (Caltech), H. Vedantham (Caltech), V. Bhalerao (IUCAA), S. B. Cenko (NASA/GSFC), R. Quimby (SDSU) report on behalf of the GROWTH (Global Relay of Observatories Watching Transients Happen) and iPTF (intermediate Palomar Transient Factory) collaborations We observed the position of the fast-fading ATLAS17aeu (Tonry et al. LVC GCN#20382) with the Large Format Camera on the Palomar 200-inch Hale Telescope. At 2017-01-07 08:05:33 UTC, we found that ATLAS17aeu had faded further to i=21.9 +/- 0.3 mag (AB mags, calibration wrt SDSS). If we fit a power law to the decay, the best-fit slope is 1.1 +/- 0.2 and best-fit t_zero is 57758.322 +/- 0.043 (i.e. 21.5 hr after GW trigger, LVC GCN#20364). We caution that we have not corrected for the filter differences between ATLAS and LFC. Incidentally, this t_zero is consistent with GRB070105A reported by POLAR (GCN#20387) and detected by ASTROSAT (GCN#20389). The hypothesis that this is the afterglow of the GRB appears consistent with the X-ray detection reported by Swift (Evans et al. LVC GCN#20390).