TITLE: GCN CIRCULAR NUMBER: 21262 SUBJECT: LIGO/Virgo G288732: Pan-STARRS coverage of the Fermi/LAT candidate position DATE: 17/06/22 16:11:05 GMT FROM: S. J. Smartt at Queens U Belfast K. W. Smith, K. C. Chambers (IfA), M. E. Huber (IfA), S. J. Smartt, T.-W. Chen (MPE), M. Coughlin (Harvard), 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),J. Tonry, L. Denneau, A. Heinze, B. Stalder, H. Weiland (IfA), C. W. Stubbs (Harvard), A. Rest (STScI), GCN 21227 (Omedi et al.) reported detection of a weak gamma-ray candidate with Fermi/lAT at position R.A.,Dec.=128.11, 43.39, (J2000) with a localization error of 0.24 degree, which is within the skymap of the LIGO GW Binary Merger Candidate G288732 (GCN 21221, Becsy et al.), discovered at 2017-06-08 02:01:16.492 UTC (57912.08421866) We report that we observed the Fermi/LAT postion with the Pan-STARRS1 telescope (Chambers et al. arXiv:1612.05560). We began taking data at 2017-06-09T 06:03:40 UT, centered on the Fermi position with a set of 8 dithered exposures in i and z band filters. The PS1 camera encloses a circle of 2.9 degree diamater, and therefore enclosed the whole of the Fermi/LAT error box. Difference images were produced by subtracting the Pan-STARRS1 3Pi reference image from these separate exposures and a nightly combined stack of the dithers (3Pi data described in Chambers et al. arXiv:1612.05560, and available at http://panstarrs.stsci.edu). Using techniques discussed in Smartt et al. (2016, MNRAS, 462, 4094), we located and vetted transients with quality filters and a machine learning algorithm on the difference images. No fast evolving transients (with variable lightcurves) were found during the first night of data (MJD ~ 57913.25 to 57913.30) in the individual 240s exposures, with limits of z > 18.5 +/- 0.5 (given the airmass range of 1.9 - 2.8 and poor image quality). Three transients were found in the stacked data covering several nights between 57913 and 57924. However all three are outside the error radius of 0.24 degrees from Fermi/LAT. In summary, no possible optical counterpart to the Fermi/LAT source is detected to i,z ~ 18.5 (within 24hrs) and i,z ~ 20.5 (daily stacked limits up to 5 days after). The detected sources are below (where AngSep = angular separation from the Fermi/LAT position in degrees) Name RA (J2000) Dec (J2000) Disc. MJD Disc Mag AngSep PS17diu 08 35 24.13 +44 00 29.5 57913.26 18.82 (z) 0.82 PS17dit 08 27 39.44 +42 36 36.2 57916.28 20.56 (i) 0.78 PS17djl 08 30 31.25 +43 55 04.2 57916.29 20.45 (i) 0.63 PS17diu is associated with SDSS J083524.25+440029.3, an r=19.36 mag galaxy with a host photoZ=0.083 (+/- 0.038) implying a transient M_i = -19.36. The lightcurve is relatively flat over 10 days. This is likely an old SN, and this is likely an old SN around peak. PS17dit is likely associated with 2MASX J08273960+4236311; a 18.50 mag galaxy with z=0.151, implying a transient at M = -18.8. The lightcurve is flat for 4 days, and this is likely an old SN around peak. PS17djl is likely associated with SDSS J083031.05+435504.3; an r=19.81 mag galaxy with A host photoZ=0.217 (+/- 0.051) implying a transient at M = -19.7. Again, the lightcurve is flat for 4 days, also suggesting this is likely an old SN around peak.