TITLE:   GCN CIRCULAR
NUMBER:  20434
SUBJECT: LIGO/Virgo G268556: Early SWASP limits of ATLAS17aeu
DATE:    17/01/11 22:56:15 GMT
FROM:    Danny Steeghs at U of Warwick/GOTO  <D.T.H.Steeghs@warwick.ac.uk>

D.Steeghs, D.Pollacco, K.Ulaczyk, R.Cutter, R.West, A.Levan (U. Warwick), D.K. Galloway, E.Rol, E.Thrane (Monash U.), V.Dhillon, M.Dyer, S.Littlefair, E.Daw, J.Maund, J.Mullaney (U. Sheffield), G.Ramsay (Armagh O.), P.O'Brien, R.Starling (U. Leicester)

On behalf of the GOTO collaboration:

We have analysed our SWASP imaging related to G268556 (GCN #20364, spanning MJD 57758.043-57758.210) for evidence of ATLAS17aeu. Our first imaging window following G268556 spans 9-5 hours before the reported detections in Tonry et al. (GCN #20382) and end 1 hour before GRB170105A (GCN #20387). We do not find any evidence for an optical source at the ATLAS17aeu position. We combined 3 sets of images not affected by clouds and achieve the following 5-sigma upper limits:

MJD (mid):              g limit:    r limit:
57758.05948013   >17.7       >17.0
57758.09198558   >17.6       >16.9
57758.20992235   >17.0       >16.3

These photometric limits were derived by crossmatching field stars with the APASS survey. Considering the reported optical flux evolution of ATLAS17aeu (GCN #20382, #20393, #20397, #20407, #20416) and extrapolating to earlier times, we would have expected to detect ATLAS17aeu if its peak optical flux preceded GRB170105A. Our non-detections thus further corroborate associating ATLAS17aeu with the optical afterglow of GRB170105A.