TITLE: GCN CIRCULAR NUMBER: 25434 SUBJECT: Fermi GBM-190816: Upper Limits from Swift/BAT Observations DATE: 19/08/21 19:22:00 GMT FROM: Aaron Tohuvavohu at PSU/Swift James DeLaunay (PSU) and Aaron Tohuvavohu (PSU) report: We report the upper limits from additional deeper searches of the BAT data for a counterpart to LVC-Fermi event GBM-190816 (GCN Circ. 25406). As reported in GCN Circ. 25415, Swift was in the middle of a slew at the time of the event. The BAT FOV with >10% partial coding, contained ~22.5% of the joint LVC-Fermi probability at LVC trigger time (2019-08-16T21:22:13.027 UTC). Event data was captured during the slew and is available from T0-65.5s to T0+150.36. Using a likelihood based approach (DeLaunay et al., in prep), a search was performed on the BAT event data for any short transient on the scale of 0.128s within +/-15s of the GBM-190816 event time. The search is performed by maximizing the likelihood of a power-law signal plus background model over the signal parameters (sky position, power-law index, and spectral normalization) in each time bin being tested. Longer time scales would need to take into account the slewing effect and are not yet implemented in this search. No significant counterparts were found using this method. Upper limits can be set by finding what flux needs to be injected on top of the background to be detected above a certain threshold (signal-to-noise >~ 5 sigma) 90% of the time. For the upper limits we assume the same spectral shape as the hard template used by the GBM team for te claimed detection (GCN Circ. 25406). The sensitivity varies across the BAT field-of-view, so we report here the average upper limit over the joint LVC-Fermi localization that lands within the >10% coded BAT field-of-view. Average upper limit for Hard Spectral Template for 0.128s time scale: 3.1e-7 erg/cm2/s (14-195 keV) We also perform a search in events sensitive to all gamma-ray transients, even those originating from outside of the coded field-of-view. At T0, ~86% of the Fermi/LVC localization was above the Earth limb with respect to the spacecraft, and thus capable of depositing flux onto the detector array. We find no evidence for a gamma-ray transient within T0 +/- 10. We set the following average sensitivity limit for a 1-s long sGRB occurring in the part of the Fermi-LVC joint localization covered by the BAT field-of-view. Average upper limit for Hard Spectral Template for 1.024s time scale: 7.1 E-8 erg/cm2/s (14-195 keV) However, we cannot currently calculate an upper limit for the 63% of the localization outside of the coded field-of-view due to the varying and unknown detector response function at wide off-boresight angles. Upper limits are typically an order of magnitude higher for events outside of the field-of-view. Work is ongoing to better characterize this response.