TITLE: GCN CIRCULAR NUMBER: 20881 SUBJECT: SUBJECT: LIGO/Virgo G277583: ATLAS coverage and transients over the first 2 days DATE: 17/03/16 17:15:00 GMT FROM: S. J. Smartt at Queens U Belfast S. J. Smartt, K. W. Smith, J. Tonry, L. Denneau, A. Heinze, B. Stalder, H. Weiland (IfA), C. W. Stubbs (Harvard), A. Rest (STScI), K. C. Chambers (IfA), T.-W. Chen (MPE), M. Coughlin (Harvard), M. E. Huber (IfA), D. E. Wright, D. R. Young, E. Kankare (QUB), H. Flewelling, T. Lowe, E. A. Magnier, A. S. B. Schultz, C. Waters, R. J. Wainscoat, M. Willman (IfA), Further to GCN20877, we report ATLAS observations of the skymap (cWB_plus_LIB.fits) of G277583 (GCN 20860), with event time 2017-03-13 22:40:09.593 == 57825.94498372 We started observing at MJD=57826.54, 14hrs after the LIGO alert and covered approximately 3600 sq degrees during the first 2 days. This corresponds to a probability coverage of approximately 60% of the cWB_plus_LIB.fits skymap (final number currently being refined, map plot will be available on GraceDB). The limiting magnitude in the 30 sec exposures in the orange filters is o ~ 18.7 We find the following four transients which are within the 90% contour for which we have no previous detections. We also list, for reference old objects which are within the skymap but discovered before G277583 We apologise for the earlier release of ATLAS17cck in GCN 20877, which is indeed minor planet Azalea (GCN 20879, Littlefield & Garnavich). The object passed through our automated positional scatter and ephemeris filters for removing known and unknown moving objects and therefore did not trigger automatic rejection. This was due to an artificial magnitude split introduced into our local version (at QUB) of the minor planet database, with objects brighter than a threshold maximum magnitude not being checked. The magnitude calculation was an oversimplification, and has been removed. Additional checks have also been introduced to prevent leakage of minor planets into the stationary list. Name RA DEC MJD Discovery Mag f PC Host/Comment ATLAS17cdb 05:14:50.41 -32:09:46.9 57827.25431835 18.55 o 90 orphan ATLAS17byq 17:07:47.34 +09:49:20.8 57826.61285455 17.61 o 90 2MASX J17074707+0949172 ATLAS17byo 16:33:19.87 +23:43:52.4 57826.58056845 17.43 o 80 z=0.033, SDSS J163319.94+234356.4 ATLAS17cgg 08:03:55.23 +26:31:12.3 57826.42460955 18.02 o 50 z=0.0217, IC 0491 f = filter PC = probability contour within which the object lies Old objects from before G277583 Name RA DEC MJD Discovery Mag f PC Host/Comment ATLAS17bza 17:19:36.88 -25:01:04.2 57826.57606615 16.49 o 70 AT2017aby (Gaia17agz), b=7, likely Galactic ATLAS17bkc 15:27:56.20 +11:42:17.0 57816.61101455 18.40 c 80 ATLAS17bhd 06:26:20.16 -01:22:47.5 57808.37305585 16.55 c 90 b=-6.2, probable CV, faint source in PS1 ATLAS17bib 09:44:23.18 +34:16:03.1 57805.44415665 18.87 c 90 AT 2017blg ATLAS17bkt 15:19:18.86 +28:13:44.4 57817.57088835 16.54 c 90 AT2017cak, ATel 10152 ATLAS17cgp 16:32:23.53 -02:36:07.2 57816.65313705 18.30 c 40 AT2017ly ATLAS17bkb 15:20:07.83 +11:52:37.5 57816.61101455 18.25 c 90 AT2017byj, ATel 10148 ATLAS17btj 12:14:16.03 -38:05:48.1 57807.56503375 18.66 c 80 AT2017bzg ATel 10152 ATLAS17bhq 08:38:57.62 +28:24:40.0 57805.40020845 19.07 c 60 AT2017bhe