TITLE: GCN CIRCULAR NUMBER: 39060 SUBJECT: EP250125a: Upper limits from Swift/BAT-GUANO DATE: 25/01/28 18:20:50 GMT FROM: Samuele Ronchini at PSU sjs8171@psu.edu

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

Swift/BAT did not detect EP250125a onboard (T0: 2025-01-25T02:37:31.749 UTC, EP GCN 39028).

The EP notice distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; [Tohuvavohu et al. 2020, ApJ, 900, 1](https://iopscience.iop.org/article/10.3847/1538-4357/aba94f)).

Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground.

Using the NITRATES analysis ([DeLaunay + Tohuvavohu 2022, ApJ, 941, 169](https://iopscience.iop.org/article/10.3847/1538-4357/ac9d38)), we searched for emission on 8 timescales from 0.128s to 16.384s in the interval [-20,+20] seconds around the trigger time. We find no evidence for a signal, and derive the following upper limits.

We quote the 5-sigma flux upper limits in the 15-350 keV band, computed at the EP250125a position RA = 175.36337 deg , Dec = -21.71347 deg provided in GCN 39029, for three spectral templates (soft, normal, and hard GRB-like templates described in [arXiv:1612.02395]) and for four time bins. In units of 10^-7 erg/s/cm^2:

|time_bin (s) |soft |normal|hard |-|-|-|- |0.256 |7.23 |5.47 | 5.08 |1.024 |3.68 |2.79 | 2.59 |4.096 |1.98 |1.50 | 1.39 |16.384 |1.21 |0.92 | 0.85

GUANO is a fully autonomous, extremely low latency, spacecraft commanding pipeline designed for targeted recovery of BAT event mode data around the times of compelling astrophysical events to enable more sensitive GRB searches.

A live reporting of Swift/BAT event data recovered by GUANO can be found at: [https://www.swift.psu.edu/guano/%5D()

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