TITLE: GCN CIRCULAR NUMBER: 21553 SUBJECT: LIGO/Virgo G298048: Pan-STARRS detections and izy photometry of the possible optical/NIR counterpart in NGC4993 DATE: 17/08/18 08:37:20 GMT FROM: S. J. Smartt at Queens U Belfast K. C. Chambers (IfA), M. E. Huber (IfA), S. J. Smartt, (QUB), K. W. Smith (QUB), D. R. Young, M. Coughlin (Harvard), T.-W. Chen (MPE), L. Denneau, H. Flewelling, A. Heinze, E. Kankare (QUB), T. Lowe, E. A. Magnier (IfA), A. Rest (STScI), B. Stalder (IfA), A. S. B. Schultz, C. W. Stubbs (Harvard) J. Tonry, C. Waters, R. J. Wainscoat, H. Weiland, M. Willman (IfA), D. E. Wright (QUB) We report Pan-STARRS imaging observations of the the skymap of G298048. A set of six images were taken, in twilight and at low airmass. We had planned to tile the northern part of the skymap, but after the discovery of the transient SSS17a/DLT17ck in GCN 21529 (Coulter et al) and 21531 (Valenti et al.) we immediately switched to cover the NGC4993 in 2x30s in each of Pan-STARRS i, z, y, ensuring the postion of the transient object was on a good, clean CCD cell. Difference images were produced by subtracting the Pan-STARRS1 3Pi reference image from these separate 30s exposures (Chambers et al. arXiv:1612.05560, and available at http://panstarrs.stsci.edu). We recover the transient as Pan-STARRS object PS17gl at coordinates RA = 13:09:48.08 DEC = -23:22:53.2 RA = 197.45033 DEC = -23.38144 Image log and photometry in the Pan-STARRS system (Tonry et al. 2012 Magnier et al. arXiv:1612.05242) : Date (UT) RA DEC Exp Filt Airmass 2017-08-18T05:33:01 198.347081 -23.075627 2000 30.0 y 2.83 17.28 0.13 2017-08-18T05:33:46 198.347076 -23.075658 2000 30.0 z 2.84 17.31 0.09 2017-08-18T05:34:31 198.347062 -23.075632 2000 30.0 i 2.86 17.23 0.10 2017-08-18T05:35:45 198.347073 -23.075633 2000 30.0 y 2.89 17.47 0.16 2017-08-18T05:36:30 198.347070 -23.075638 2000 30.0 z 2.91 17.21 0.08 2017-08-18T05:37:14 198.347060 -23.075617 2000 30.0 i 2.93 17.26 0.08 There is no evidence, within the uncertainties for fading between the exposures. If the Swope 1m photometry point of i=16 (Coulter et al.) is correct then this would imply substantial fading. But this needs checked, since DLT17ck was reported at R=17.3, is more consistent with the colours we measure : i - z = -0.02 +/- 0.02 z - y = -0.26 +/- 0.18 This is consistent with the DECam photometry by Nicholl et al. (GCN21541) and that within the uncertainties, the colours are consistent with 0, and being flat in Fnu as reported by the J-band detection of Tanvir & Levan (GCN21544). Further observations are planned with the same filter setup as long as the target is observable, or firmly ruled out as a counterpart to G298048.