TITLE: GCN CIRCULAR NUMBER: 20377 SUBJECT: LIGO/Virgo G268556: ATLAS imaging of the skymap DATE: 17/01/05 23:24:44 GMT FROM: S. J. Smartt at Queens U Belfast J. Tonry, L. Denneau, A. Heinze, B. Stalder, H. Weiland (IfA), C. W. Stubbs (Harvard), K. W. Smith, S. J. Smartt, (QUB), D. R. Young (QUB), A. Rest (STScI), K. C. Chambers (IfA), M. Coughlin (Harvard), M. E. Huber (IfA), D. E. Wright (QUB), H. Flewelling, E. A. Magnier, A. S. B. Schultz, C. Waters, R. J. Wainscoat (IfA) We report the following observations of the skymap for LIGO/Virgo G268556 with the ATLAS telescope system ATLAS is a twin 0.5m telescope system on Haleakala and Mauna Loa (see Tonry et al. ATel 8680 and Tonry 2011, PASP, 123, 58). The first unit is operational on Haleakala and is robotically surveying the sky in two filters cyan and orange (denoted c and o, all mags are in AB system). More information is on http://www.fallingstar.com The alert for G268556 was received 6.5 hours after the event, too close to sunrise for targeted observations on 57757 in Hawaii. The next night we started observing the LIGO/VIRGO localisation region at 57758.296 (2016-01-05.296 UT) starting at a pointing centre of RA=80, DEC=-30, moving north and east across the lobe to RA=170,80. A total of 1250 square degrees were covered. Sets of dithered 8x30sec exposures in the cyan filter were taken, at a 5-sigma sensitivity of c = 19.0 for each exposure. Difference images are automatically created with a reference image built during early 2016. These data are currently being processed and results will be reported soon. Maps will be uploaded to GraceDB. During the previous night (2016-01-04 UT) ATLAS did its routine survey of a 20 degree wide declination strip between RA = 334 to 213 deg (wrapped through 0), and DEC = +15 to +35 deg. This intersected a small part of the LIGO/VIRGO map (containing 9% of the probability) between 30mins to 3hrs after the detection time of G268556. ATLAS also surveyed this strip four days prior on 57753, but the sky was partly cloudy. No obvious transient is visible in this 9% region. Furthermore, ATLAS has been scanning the northern region containing the highest probability contours during the last three weeks. We report here bright transients found before the detection of G268556. They will mostly be still visible on the sky map region, but are mostly field supernovae (or CVs) and unrelated to the GW source. IAU Name | ATLAS Name | RA (J2000) | Dec (J2000) | Disc. MJD | Disc. Date | Disc Mag | Notes AT2016iyx | ATLAS16dzp | 08:43:24.74 | +37:32:45.8 | 57742.47 | 20161220.47 | 17.28 o | SN2016izg | ATLAS16eak | 07:53:12.77 | +12:53:56.4 | 57736.47 | 20161214.47 | 17.10 o | ASASSN-16pb (Ia) AT2016izk | ATLAS16eaw | 08:49:04.77 | +14:24:49.8 | 57744.57 | 20161222.57 | 16.62 c | ASASSN-16pc (CV) AT2016jae | ATLAS16eay | 09:42:34.50 | +10:59:35.3 | 57744.61 | 20161222.61 | 17.42 c | AT2016jax | ATLAS16ecb | 08:32:53.76 | +42:08:01.6 | 57746.49 | 20161224.49 | 17.60 c | AT2016jaw | ATLAS16ecc | 08:48:21.88 | +24:31:51.9 | 57745.52 | 20161223.52 | 17.98 c | | ATLAS16ecj | 08:50:02.45 | +46:39:49.0 | 57742.48 | 20161220.48 | 18.50 o | | | ATLAS17aay | 09:34:07.69 | +31:46:25.4 | 57745.55 | 20161223.55 | 18.44 c | |