{
  "bibcode": "2019GCN.26605....1S",
  "body": "G. P. Smith (Birmingham), M. Nicholl (Birmingham), K. Sharon (Michigan), \nM. Bianconi (Birmingham), W. P. Varricatt (UKIRT), S. Benigni (UKIRT), \nand E. J. Ridley (Birmingham) report on behalf of the Gravitationally \nLensed Gravitational Wave Hunters:\n\nAs part of our search for strongly-lensed gravitational waves and their \nelectromagnetic counterparts, we observed a contiguous area of 0.75 \nsquare degrees within the sky localization of the gravitational wave \ntrigger S191216ap (GCN #26454) with the WFCAM instrument on UKIRT \nthrough the z-band.  The observations were centered on the position of \nthe HAWC sub-threshold detection of gamma-ray flux reported by \nMartinez-Castellanos (GCN #26472), and encompassed the full extent of \ntheir 68% confidence region.  The observations comprise two epochs:\n\nEpoch  Date(UT)    Start(UT)  End(UT)   Airmass  FWHM(arcsec)\n1      2019-12-20  04:45:21   05:45:57  1.3-1.7  1.2-1.6\n2      2019-12-21  04:31:26   05:43:34  1.3-1.8  1.6-1.8\n\nWe estimate that Epoch 1 reaches a 5 sigma point source sensitivity of \nAB~21.5.  Epoch 2 is less sensitive than Epoch 1, with noticeable \nvariations in Epoch 2 between the four observations required to achieve \ncontiguous coverage with WFCAM.\n\nA preliminary comparison of Epoch 1 with templates derived from \nPan-STARRS1 data (nominal sensitivity of AB=22.3) identified two \ncandidate transient sources.  Neither source is known to the Transient \nName Server, and Minor Planet Center.  The celestial coordinates and \nestimated z-band apparent AB magnitudes of these two candidates are:\n\nName      RA(J2000)    Dec(J2000)   AB(Epoch1)  AB(Epoch2)\nGLGWc19a  21:32:45.50  +05:19:58.0  20.8+/-0.1  ~22\nGLGWc19b  21:35:20.11  +04:55:19.8  20.6+/-0.1  >20(tentative)\n\nNeither source is associated with an obvious candidate gravitational \nlens (i.e. massive galaxy, group or cluster) to the depth of the PS1 \ndata.  However, GLGWc19a is 7 arcsec West, 1 arcsec North of the center \nof an edge-on disk galaxy (possible host?) that is located at \n21:32:45.97 +5:19:57.0.\n\nThese sources are observable at Airmass<2 for ~30 minutes following the \nend of evening twilight in the Northern Hemisphere in the next few \nnights, before becoming unobservable through the winter months.  We \ntherefore encourage urgent follow-up observations of GLGWc19a, GLGWc19b \nand the candidate host galaxy adjacent to GLGWc19a.\n\nWe thank the Director of UKIRT, Klaus Hodapp, for quickly approving our \nurgent DDT proposal.  We are also grateful to Mike Irwin for kindly \nexpediting the processing of our data at Cambridge Astronomy Survey Unit \n(CASU).\n\nThis circular is citable.",
  "circularId": 26605,
  "createdOn": 1577551233000,
  "email": "gps@star.sr.bham.ac.uk",
  "subject": "LIGO/Virgo S191216ap: Two candidate counterparts from UKIRT/WFCAM z-band observations",
  "submitter": "Graham P Smith at U of Birmingham  <gps@star.sr.bham.ac.uk>",
  "eventId": "LIGO/Virgo S191216ap"
}