TITLE: GCN CIRCULAR NUMBER: 38351 SUBJECT: LIGO/Virgo/KAGRA S241125n: Preliminary flux estimate and spectral analysis of the Swift/BAT-GUANO candidate counterpart DATE: 24/11/27 19:23:19 GMT FROM: Jimmy DeLaunay at Penn State delauj2@gmail.com

James DeLaunay (PSU), Aaron Tohuvavohu (Caltech), Samuele Ronchini (PSU), Gayathri Raman (PSU), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC) report:

We have performed a preliminary spectral study on the sub-threshold GRB candidate that was found to be spatially and temporally coincident with the GW event S241125n (GCN 38308).

This candidate was identified in BAT-GUANO data by the NITRATES pipeline in an analysis time bin that starts at T_GW + 11.264 s and has a duration of 0.512 s. The NITRATES analysis uses Comptonized spectral templates that range from soft to hard GRB spectra. The maximum likelihood template found in the search had Epeak = 49 keV and spectral index = -1.2 (gamma on results webpage is -1 x spectral index). The best fit 15-350 keV flux for this spectral template at this position and analysis time bin is 1.1(-0.3, +0.2)E-7 erg/cm2/s with a 90% error bar. Note that the parameters of the template that maximized the likelihood in the low-latency search are not necessarily the true parameters of the signal.

Due to the low S/N of this event, error bars on Epeak and the spectral index are large and correlated. The spectral index is unconstrained in the range of -0.4 to -2.0, but the allowed values of Epeak vary with it. With a spectral index fixed at -0.6, the 90% confidence limits of Epeak are approximately 25 to 80 keV. With a spectral index fixed at -1.6, the 90% confidence limits of Epeak are approximately 20 to 300 keV. The flux estimate varies with spectral index and Epeak.

In the most recent joint localization map of the candidate GRB and S241125n, ~84% of the localization probability is within 5 arcminutes of the most likely position. The other 16% is spread out across the sky and is multimodal. Some of this remaining probability is also within BAT’s coded field of view and some is outside of it. The response changes greatly whether in or outside the coded field of view, which drives substantial range in the flux estimate depending on sky position. The Southern lobes of the joint localization are outside of the BAT’s coded field of view and thus have higher estimated flux values. This region is most likely excluded by external upper limits from Fermi-GBM (GCN 38316) and Konus-Wind (GCN 38321). The most likely position cannot be confidently excluded by these upper limits.

All values in this circular are preliminary.

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