GCN Circular 34097

V. Savchenko, C. Ferrigno (ISDC/UniGE, Switzerland)
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 S230527c (GCN 34086
Not found
 
 
).

At the time of the event (2023-06-27 01:53:37 UTC, hereafter T0), INTEGRAL was operating in nominal mode. The peak of the event
localization probability was at an angle of 94 deg with respect to the spacecraft pointing axis. This orientation implies strongly suppressed (8.9% of optimal) response of ISGRI, strongly suppressed (28% of optimal) response of IBIS/Veto, and near-optimal (81% of optimal) response of SPI-ACS.

The background within +/-300 seconds around the event was very stable (excess variance 1.2).

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

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 ~1.6e-07 (5.5e-08) erg/cm^2/s at 1 s (8 s) time scale in 75-2000 keV energy range.

We report for completeness and in order of FAP, all excesses identified in the search region. We find: 1 tentatively associated
excess:

T-T0     | scale   | S/N | flux ( x 1e-06 erg/cm2/s)     | FAP    
-0.221   | 2.4     | 3.4 |     1.38 +/- 0.347  +/- 0.328  | 0.00943 

3 likely background excesses:

T-T0     | scale   | S/N | flux ( x 1e-06 erg/cm2/s)     | FAP    
-24.6    | 0.4     | 3.6 |     3.49 +/- 0.855  +/- 0.829  | 0.349 
-56.4    | 1       | 3.3 |     2.04 +/- 0.538  +/- 0.485  | 0.643 
277      | 0.7     | 4.7 |     3.42 +/- 0.646  +/- 0.814  | 0.728 

Note that FAP estimates (especially at timescales above 2s) may be possibly further affected by enhanced non-stationary local background noise. This list excludes any excesses for which FAP is close to unity.

We note that no independent IBAS alerts happened in the vicinity.

SPI-ACS data can be retrieved in MMODA with this link:
https://www.astro.unige.ch/mmoda/?DEC=-29.74516667&RA=265.97845833&T1=2023-06-27T01%3A48%3A37.000&T2=2023-06-27T01%3A58%3A37.000&T_format=isot&data_level=ordinary&instrument=spi_acs&product_type=spi_acs_lc&query_status=new&query_type=Real&time_bin=2&time_bin_format=sec

INTEGRAL follow-up alert was distributed to SCIMMA through HERMES few minutes after the trigger GCN:
https://hermes.lco.global/message/cc05ca0b-1c31-444d-a809-900ec95c4690

All results quoted are preliminary.

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

Note that we send GCNs Circulars only when one of the following conditions is met: merger contains at least one neutron star, a
singificant counterpart is reported.

[1] Savchenko et al. 2017, A&A 603, A46
[2] Savchenko et al. 2012, A&A 541A, 122S

GCN pipeline version: local