TITLE: GCN CIRCULAR NUMBER: 38311 SUBJECT: LIGO/Virgo/KAGRA S241125n: No counterpart candidates in INTEGRAL SPI-ACS prompt observation, comparisong with Swift/BAT counterpart DATE: 24/11/25 21:06:32 GMT FROM: Volodymyr Savchenko at UNIGE, EPFL <volodymyr.savchenko@epfl.ch> 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 S241125n (GCN 38305). At the time of the event (2024-11-25 01:01:16.78 UTC, hereafter T0), INTEGRAL was operating in nominal mode. The peak of the event localization probability was at an angle of 104 deg with respect to the spacecraft pointing axis. This orientation implies strongly suppressed (6.4% of optimal) response of ISGRI, strongly suppressed (29% of optimal) response of IBIS/Veto, and near-optimal (80% of optimal) response of SPI-ACS. *Position of counterpart reported by Swift/BAT (GCN 38308) at the time of the event was at 25 deg with respect to the spacecraft pointing axis. This orientation implies suppressed response of all INTEGRAL all-sky instruments.* The background within +/-300 seconds around the event was rather 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 1.6e-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 ~1.4e-07 (3.4e-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: 8 likely background excesses: |T-T0 | scale | S/N | flux ( x 1e-06 erg/cm2/s) | FAP |---|---|---|---|---| |7.16 | 0.35 | 3 | 0.475 +/- 0.188 +/- 0.377 | 0.44 |-48.7 | 1.85 | 3.1 | 2.03 +/- 0.814 +/- 1.61 | 0.453 |125 | 2 | 3.5 | 2.06 +/- 0.783 +/- 1.63 | 0.454 |12.1 | 0.1 | 3.8 | 1.1 +/- 0.358 +/- 0.871 | 0.539 |-185 | 2 | 3.5 | 2.05 +/- 0.783 +/- 1.62 | 0.634 |-32 | 0.7 | 3.2 | 0.347 +/- 0.133 +/- 0.275 | 0.712 |104 | 0.05 | 6.6 | 2.84 +/- 0.528 +/- 2.25 | 0.739 |38.3 | 0.05 | 4.4 | 1.85 +/- 0.516 +/- 1.46 | 0.868 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. *One of the listed excesses is 0.8 s from Swift/BAT counterpart of the LVK event. It also features comparable detection timescale (0.1s). Tentatively, FAP of associating this event to the Swift/BAT event is at moderately significantly low level of 0.036. However, 0.8s delay can not be explained by light travel time between INTEGRAL and Swift. If these events are indeed related, the difference in timing could potentially be explained by spectral evolution.* 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=2024-1...). INTEGRAL follow-up alert was NOT distributed to SCIMMA through HERMES few minutes after the trigger GCN since the event, not expected to be EM-bright, did not pass the cuts. 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 View this GCN Circular online at https://gcn.nasa.gov/circulars/38311. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...