{
  "bibcode": "2017GCN.20854....1M",
  "body": "Daniele Malesani (DARK/NBI), Elena Pian (SNS), L. Amati (INAF-IASF Bo), \nL. A. Antonelli, S. Ascenzi (INAF-OAR), M.T. Botticella (INAF-OAC), M. \nBranchesi (Urbino University/INFN Firenze), S. Campana (INAF-OABr), E. \nCappellaro (INAF OAPd), S. Covino, P. D'Avanzo (INAF-OABr), V. D'Elia \n(INAF-ASDC), F. Getman, A. Grado (INAF-OAC), G. Greco (Urbino \nUniversity, INFN Firenze), L. Limatola (INAF-OAC), M. Lisi (INAF-OAR), \nA. Melandri (INAF-OABr), L. Nicastro, E. Palazzi (INAF-IASF Bo), S. \nPiranomonte, L. Pulone (INAF-OAR), A. Rossi (INAF-IASF Bo), G. Stratta \n(Urbino University/INFN Firenze), G. Tagliaferri (INAF-OABr), V. Testa \n(INAF-OAR), L. Tomasella, S. Yang (INAF-OAPd), E. Brocato (INAF-OAR), on \nbehalf of the GRAvitational Wave Inaf TeAm (GRAWITA), report:\n\nWe observed the location of \"Source 23\" (Evans et al., LVC Circ. GCN \n20841) within the error uncertainty of the LIGO/Virgo event G275697 (LVC \nCirc. 20763), using the Nordic Optical Telescope (NOT) equipped with the \nALFOSC imaging camera. Observations were carried out at high air mass \n(average 2.1) around twilight using the SDSS i filter. The mean epoch of \nthe observation is March 10.26458 UT (10.47 days after the GW event). \nTwo images with 2 min exposure and and eight with 1 min were secured (12 \nmin in total) in the SDSS i filter.\n\nThe location of XRT Source 23 lies close to an unrelated bright star (R \n~ 11), which complicates the analysis inside the XRT error circle. Two \nobjects, both visible in the Pan-STARRS archival images, are detected \nconsistent with the XRT error circle. Their properties are summarized below:\n\nO1:\nRA(J2000) = 21:33:15.28\nDec(J2000) = +39:41:11.6\ni = 19.76 +- 0.08 AB (NOT data)\ni = 19.90 +- 0.15 AB (Pan-STARRS data)\n\nO2:\nRA(J2000) = 21:33:14.74\nDec(J2000) = +39:41:07.6\ni = 21.94 +- 0.16 AB (NOT data)\ni = 21.90 +- 0.15 AB (Pan-STARRS data)\n\nPhotometry was computed against nearby stars from the Pan-STARRS \ncatalog. For the Pan-STARRS images, we re-measured the photometry to \nproperly account for the glare of the bright star, so the quoted values \ndiffer from those reported in the Pan-STARRS catalog. These values are \nvery sensitive to the choice of the background area, hence the large \nuncertainties.\n\nNeither object shows significant variability.\n\nNo other objects are detected within the XRT error area, down to a \n3-sigma limiting magnitude i = 21.5 (AB).",
  "circularId": 20854,
  "createdOn": 1489332475000,
  "email": "malesani@dark-cosmology.dk",
  "subject": "LIGO/Virgo G275697: GRAWITA NOT optical observations of Swift/XRT Source 23",
  "submitter": "Daniele Malesani at DARK/NBI  <malesani@dark-cosmology.dk>",
  "eventId": "LIGO/Virgo G275697"
}