TITLE: GCN CIRCULAR NUMBER: 21886 SUBJECT: LIGO/Virgo G298048: ATLAS pre-discovery limits 601 to 16 days before first detection of SSS17a/DLT17ck DATE: 17/09/15 11:24:15 GMT FROM: S. J. Smartt at Queens U Belfast J. Tonry (IfA), K. W. Smith (QUB), L. Denneau, A. Heinze, B. Stalder, H. Weiland (IfA), C. W. Stubbs (Harvard), S. J. Smartt (QUB), A. Rest (STScI), K. C. Chambers (IfA), T.-W. Chen (MPE), M. Coughlin (Harvard), D. R. Young, (QUB), M. E. Huber (IfA), D. E. Wright (QUB), H. Flewelling, T. Lowe, E. A. Magnier, A. S. B. Schultz, C. Waters, R. J. Wainscoat, M. Willman (IfA) During its normal survey mode, the ATLAS survey project (Tonry et al. 2011, Stalder et al. 2017) frequently observed the field containing galaxy NGC4993 during the period between 57380.64463 and 57966.26370 (602 to 16 days before G298048; the LIGO Scientific Collaboration and the Virgo Collaboration GCN 21509) and the first discoveries and detections of the optical/NIR transient SSSa17a/DLT17ck (Coulter et al. GCN 21529, Allam et al. GCN 21530,Valenti et al. GCN21531, Melandri et al. 21532). We have completed a search for variability or eruptions at the position of the optical transient in the 414 images over this period. These images were taken typically with 4-5 images per night (30 second exposures). ATLAS observes in two wide-band filters, called "cyan" or "c:, which roughly covers the SDSS/Pan-STARRS g and r filters, and “orange” or "o", which roughly covers the SDSS/Pan-STARRS r and i. The position was observed 414 times in one or other filter and on each of these we forced flux measurements at the astrometric position of the transient on the difference image. We used the Pan-STARRS images for the astrometric postion of SSS17a/DLT17ck (Chambers et al. GCN 21553) We measured 5-sigma flux limits and any epochs with greater than 5-sigma detections. The 5-sigma flux limits were in the range 18.5 +/- 0.4 (AB mag, median and standard deviation) and (19.4 +/- 0.4). We found 44 images which formally had flux detections greater than 5-sigma, but on visual inspection we rule out these being real flux variability at the transient position. They all appear to be residuals from the host galaxy subtraction. With ATLAS, we rule out any variability down to 18.5 to 19.4 (filter dependent) during a period 601 to 16 days before discovery of the optical transient. The last image was on 57966.26370 (2017-08-01 06:19) with a 5-sigma limit of o < 18.2.