TITLE: GCN CIRCULAR NUMBER: 38333 SUBJECT: LIGO/Virgo/KAGRA S241125n: SAGUARO follow-up observations with the MMT DATE: 24/11/27 00:47:02 GMT FROM: Manisha Shrestha at University of Arizona mshrestha1@arizona.edu

Jillian C. Rastinejad (NU), Manisha Shrestha (UA), Griffin Hosseinzadeh (UCSD), David J. Sand (UA), Charles D. Kilpatrick (NU), Wen-fai Fong (NU), Bhagya Subrayan (UA), K. Azalee Bostroem (UA), Philip N. Daly (UA), Michael J. Lundquist (Keck), Kerry Paterson (MPIA) report on behalf of the SAGUARO collaboration:

We observed the field of the Swift/BAT trigger (DeLauney et al. GCN 38308) discovered within the localization region of the GW event S241125n (LIGO/Virgo/KAGRA GCN 38305) with the Binospec imager and spectrograph mounted on the MMT 6.5-meter telescope on Mount Hopkins, Arizona. We obtain 20x75 s imaging in the r-band at a mid-time of 2024-11-26 08:08:50 UT (1.30 days post-burst) and 30x60 s in the i-band at a mid-time of 2024-11-26 09:13:11 UT (1.34 days post-burst). Observations were taken at an average airmass of 1.3 and seeing of 1.2 - 1.8’’. Our observations covered 68.0 percent of the 5 arcmin BAT localization (DeLauney et al. GCN 38308) and the localization region of XRT source S241125n_X2 (Page et al. GCN 38324).

Calibrated to PS1 (Flewelling et al. 2020), our imaging reached limiting magnitudes of r>25.5 mag and i>25.5 mag (3 sigma; AB system). Within the error circle of S241125n_X2, we report magnitudes for 5 sources, as shown in the table below. The bright source (S241125n_X2_O5) is in the PS1 catalog, and the rest are below the PS1 detection limit. Therefore we cannot determine whether these are transient sources.

name RA (J2000) Dec (J2000) Comment rmag imag S241125n_X2_O1 3:53:20.4510 +69:33:06.688 Point Src 22.3 21.6 S241125n_X2_O2 3:53:19.8100 +69:33:07.471 Extended 24.1 22.6 S241125n_X2_O3 3:53:19.6728 +69:33:13.275 Faint 25.4 26.3 S241125n_X2_O4 3:53:19.0313 +69:33:10.008 Faint 25.4 24.6 S241125n_X2_O5 3:53:21.1088 +69:33:11.602 Point Src 18.9 18.4

The reported magnitudes are not corrected for the Milky Way galactic extinction value of E(B-V) = 0.4678 mag (Schlafly & Finkbeiner 2011). Further observations are planned. We thank Ryan Howie and Benjamin Weiner at the MMT for the rapid scheduling and execution of these observations.

*SAGUARO stands for Searches After Gravitational-waves Using ARizona's Observatories. It is a partnership between the University of Arizona and Northwestern University.

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