{
  "bibcode": "2019GCN.25434....1D",
  "body": "James DeLaunay (PSU) and Aaron Tohuvavohu (PSU) report:\n\nWe report the upper limits from additional deeper searches of the BAT\ndata for a counterpart to LVC-Fermi event GBM-190816 (GCN Circ.\n25406).\n\nAs reported in GCN Circ. 25415, Swift was in the middle of a slew at\nthe time of the event. The BAT FOV with >10% partial coding, contained\n~22.5% of the joint LVC-Fermi probability at LVC trigger time\n(2019-08-16T21:22:13.027 UTC). Event data was captured during the slew\nand is available from T0-65.5s to T0+150.36.\n\nUsing a likelihood based approach (DeLaunay et al., in prep), a search\nwas performed on the BAT event data for any short transient on the\nscale of 0.128s within +/-15s of the GBM-190816 event time. The search\nis performed by maximizing the likelihood of a power-law signal plus\nbackground model over the signal parameters (sky position, power-law\nindex, and spectral normalization) in each time bin being tested.\nLonger time scales would need to take into account the slewing effect\nand are not yet implemented in this search.\n\nNo significant counterparts were found using this method. Upper limits\ncan be set by finding what flux needs to be injected on top of the\nbackground to be detected above a certain threshold (signal-to-noise\n>~ 5 sigma) 90% of the time. For the upper limits we assume the same\nspectral shape as the hard template used by the GBM team for te\nclaimed detection (GCN Circ. 25406).\n\nThe sensitivity varies across the BAT field-of-view, so we report here\nthe average upper limit over the joint LVC-Fermi localization that\nlands within the >10% coded BAT field-of-view.\n\nAverage upper limit for Hard Spectral Template for 0.128s time scale:\n3.1e-7 erg/cm2/s (14-195 keV)\n\nWe also perform a search in events sensitive to all gamma-ray\ntransients, even those originating from outside of the coded\nfield-of-view. At T0, ~86% of the Fermi/LVC localization was above the\nEarth limb with respect to the spacecraft, and thus capable of\ndepositing flux onto the detector array. We find no evidence for a\ngamma-ray transient within T0 +/- 10. We set the following average\nsensitivity limit for a 1-s long sGRB occurring in the part of the\nFermi-LVC joint localization covered by the BAT field-of-view.\n\nAverage upper limit for Hard Spectral Template for 1.024s time scale:\n7.1 E-8 erg/cm2/s (14-195 keV)\n\nHowever, we cannot currently calculate an upper limit for the 63% of\nthe localization outside of the coded field-of-view due to the varying\nand unknown detector response function at wide off-boresight angles.\nUpper limits are typically an order of magnitude higher for events\noutside of the field-of-view. Work is ongoing to better characterize\nthis response.",
  "circularId": 25434,
  "createdOn": 1566415320000,
  "email": "auc444@psu.edu",
  "subject": "Fermi GBM-190816: Upper Limits from Swift/BAT Observations",
  "submitter": "Aaron Tohuvavohu at PSU/Swift  <auc444@psu.edu>"
}