[vsnet-grb-info 23308] Fermi GBM-190816: Location-dependent upper limits from INTEGRAL/SPI-ACS prompt observation

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  25407
SUBJECT: Fermi GBM-190816: Location-dependent upper limits from INTEGRAL/SPI-ACS prompt observation
DATE:    19/08/20 09:08:59 GMT
FROM:    Volodymyr Savchenko at ISDC,U of Geneve  <savchenk at in2p3.fr>

V. Savchenko, C. Ferrigno (ISDC/UniGE, Switzerland)
S. Molkov (IKI, Russia),
J. Rodi (IAPS-Roma, Italy)
A. Coleiro (APC, France)
S. Mereghetti (INAF IASF-Milano, Italy)

on behalf of the INTEGRAL multi-messenger collaboration:
https://www.astro.unige.ch/cdci/integral-multimessenger-collaboration

Using INTEGRAL/SPI-ACS realtime data (following [1]) we have performed
a search for a prompt gamma-ray counterpart of GBM-190816 (GCN 25406).
At the time of the event (2019-08-16 21:22:14.6 UTC, hereafter T0),
INTEGRAL was operating in nominal mode. The peak of the event
localization probability was at an angle of 98 deg with respect to the
spacecraft pointing axis. This orientation implies strongly suppressed
(7.7% of optimal) response of ISGRI, strongly suppressed (22% of
optimal) response of IBIS/Veto, and near-optimal (87% of optimal)
response of SPI-ACS. The background within +/-300 seconds around the
event was very stable(excess variance 1.1).

We have performed a search for impulsive events in INTEGRAL SPI-
ACS data, as described in [2].

We do not detect any significant counterparts and estimate a 3-sigma
upper limit on the 75-2000 keV fluence of 1.8e-07 erg/cm^2 (within the
50% probability containment region of the source localization) for a
burst lasting less than 1 s with a characteristic short GRB spectrum
(an exponentially cut off power law with alpha=-0.5 and Ep=600 keV)
occurring at any time in the interval within 300 s around T0. For a
typical long GRB spectrum (Band function with alpha=-1, beta=-2.5, and
Ep=300 keV), the derived peak flux upper limit is ~1.5e-07 (5.5e-08)
erg/cm^2/s at 1 s (8 s) time scale in 75-2000 keV energy range.

For the mean reported distance 362 Mpc this corresponds to the limit
on the total isotropic equivalent energy in 1 s of 2.8e+48 erg for the
short GRB spectrum and for a long GRB spectrum isotropic equivalent
luminosity in 1 s (8 s) of 2.4e+48 erg/s (8.6e+47 erg/s)

Since the GW localization region includes regions with particularly large
range of SPI-ACS sensitivities, the upper limit from SPI-ACS is very
location-dependent.

We find 10 excesses in the search region, reported below in order
of FAP. The excesses are likely due to background noise.

scale   | T     | S/N | luminosity ( x 1e+48 erg/s)    | FAP
0.45    | 1.73  | 3   |     13.6 +/- 6.57   +/- 13.8   | 0.0917
0.1     | 0.901 | 3.1 |     29.1 +/- 14.1   +/- 29.5   | 0.171
0.35    | 12.9  | 3.7 |     18.6 +/- 7.47   +/- 18.9   | 0.191
0.45    | 15.9  | 3.5 |     15.5 +/- 6.58   +/- 15.7   | 0.25  
1.6     | -126  | 3.8 |     9.12 +/- 3.48   +/- 9.25   | 0.344
4.3     | 197   | 3.4 |     5.27 +/- 2.12   +/- 5.35   | 0.411
0.15    | -5.32 | 3.3 |     25.4 +/- 11.5   +/- 25.7   | 0.421
0.2     | -8.25 | 3.2 |     21.2 +/- 9.9    +/- 21.6   | 0.625
0.35    | 40.7  | 3.5 |     17.7 +/- 7.47   +/- 18     | 0.789
0.05    | 231   | 8.6 |     11.9 +/- 2.08   +/- 12.1   | 0.819

Where uncertainties are statistical and systematic. Systematic
uncertainty includesdependency on the unknown location and
assumed 20% systematics in the response.

For any given fluence measured by Fermi/GBM, INTEGRAL observation
can constrainthe event location. In particular, the GW localization lobe
near RA=12h corresponds to considerably less constraining upper limit,
corresponding to the directions through the bottomof the spacecraft.
Hence INTEGRAL non-detection suggests that a source in the GW localization
lobe near RA=12h of is more likely compatible with the possible GBM
detection:

https://doi.org/10.5281/zenodo.3372187

However we caution that this conclusion depends on unknown event spectrum
and fluence.

All results quoted are preliminary.

This circular is an official product of the INTEGRAL Multi-Messenger team.