[vsnet-grb-info 23335] Fermi GBM-190816: Upper Limits from Swift/BAT Observations

[GCN Circulars]

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  <auc444 at psu.edu>

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.