{
  "eventId": "LIGO/Virgo S230518h",
  "createdOn": 1684429050370,
  "submitter": "Volodymyr Savchenko at ISDC, University  of Geneva; LASTRO, EPFL <volodymyr.savchenko@unige.ch>",
  "circularId": 33815,
  "subject": "LIGO/Virgo S230518h: No counterpart candidates in INTEGRAL SPI-ACS prompt observation",
  "body": "V. Savchenko, C. Ferrigno (ISDC/UniGE, Switzerland)\nJ. Rodi (IAPS-Roma, Italy)\nA. Coleiro (APC, France)\nS. Mereghetti (INAF IASF-Milano, Italy)\n\non behalf of the INTEGRAL multi-messenger collaboration:\nhttps://www.astro.unige.ch/cdci/integral-multimessenger-collaboration\n\nUsing INTEGRAL/SPI-ACS realtime data (following [1]) we have performed\na search for a prompt gamma-ray counterpart of S230518h (GCN 33813).\n\nAt the time of the event (2023-05-18 12:59:08 UTC, hereafter T0),\nINTEGRAL was operating in nominal mode. The peak of the event\nlocalization probability was at an angle of 136 deg with respect to\nthe spacecraft pointing axis. This orientation implies strongly\nsuppressed (3.7% of optimal) response of ISGRI, somewhat suppressed\n(57% of optimal) response of IBIS/Veto, and strongly suppressed (35%\nof optimal) response of SPI-ACS.\n\nThe background within +/-300 seconds around the event was rather\nstable (excess variance 1.3).\n\nWe have performed a search for any impulsive events in INTEGRAL SPI-\nACS (as described in [2]) data.\n\nWe do not detect any significant counterparts and estimate a 3-sigma\nupper limit on the 75-2000 keV fluence of 3.8e-07 erg/cm^2 (within the\n50% probability containment region of the source localization) for a\nburst lasting less than 1 s with a characteristic short GRB spectrum\n(an exponentially cut off power law with alpha=-0.5 and Ep=600 keV)\noccurring at any time in the interval within 300 s around T0. For a\ntypical long GRB spectrum (Band function with alpha=-1, beta=-2.5, and\nEp=300 keV), the derived peak flux upper limit is ~3.7e-07 (5.8e-08)\nerg/cm^2/s at 1 s (8 s) time scale in 75-2000 keV energy range.\n\nWe report for completeness and in order of FAP, all excesses\nidentified in the search region. We find: 6 likely background\nexcesses:\n\nT-T0     | scale   | S/N | flux ( x 1e-06 erg/cm2/s)     | FAP    \n7.98     | 1.15    | 3.1 |    0.494 +/- 0.211  +/- 0.477  | 0.123\n-27.1    | 0.2     | 4.3 |      1.6 +/- 0.512  +/- 1.55   | 0.331\n213      | 0.8     | 4.3 |    0.815 +/- 0.254  +/- 0.787  | 0.618\n10.1     | 0.1     | 3.4 |     1.76 +/- 0.726  +/- 1.7    | 0.923\n128      | 2.1     | 3.2 |    0.386 +/- 0.156  +/- 0.373  | 0.945\n-28.9    | 0.3     | 3.3 |        1 +/- 0.415  +/- 0.965  | 0.991\n\n\nNote that FAP estimates (especially at timescales above 2s) may be\npossibly further affected by enhanced non-stationary local background\nnoise. This list excludes any excesses for which FAP is close to\nunity.\n\n\nAll results quoted are preliminary.\n\nThis circular is an official product of the INTEGRAL Multi-Messenger\nteam.\n\nOpen SPI-ACS data can be retrieved in here\n\nhttps://www.astro.unige.ch/mmoda/?T1=2023-05-18T12%3A59%3A00.000&T2=2023-05-18T12%3A59%3A18.000&T_format=isot&data_level=ordinary&instrument=spi_acs&product_type=spi_acs_lc&query_status=new&query_type=Real&time_bin=0.05&time_bin_format=sec\n\nNote that we send GCNs Circulars only when one merger contains at least one neutron star or a\nsignificance counterpart is found by any telescope. This policy may be revised, in which case we'll announce it in a GCN.\n\n[1] Savchenko et al. 2017, A&A 603, A46 \n[2] Savchenko et al. 2012, A&A 541A, 122S",
  "bibcode": "2023GCN.33815....1S"
}