TITLE: GCN CIRCULAR NUMBER: 39887 SUBJECT: FRB 20250316A: Gemini Imaging and Spectroscopy of the Sub-arcsecond Localization Region of FRB 20250316A DATE: 25/03/27 21:54:43 GMT FROM: Wen-fai Fong at Northwestern University wfong@northwestern.edu

S. Simha (U. Chicago-Northwestern), T. Eftekhari (Northwestern) report on behalf of the CHIME/FRB Collaboration:

The CHIME/FRB Collaboration reports on optical imaging and spectroscopy of the sub-arcsecond CHIME Outrigger localization (ATel #17114, GCN #39886) of the nearby, bright FRB 20250316A (ATel #17081). We obtained deep g-band observations starting at 24 March 2025 12:39:44 UT (PI: T. Eftekhari; 15 x 120-sec exposures) with the Gemini-North Multi-Object Spectrograph (GMOS-N) mounted on the 8-m Gemini North telescope at Maunakea, Hawai'i. Observations were taken in clear conditions with an average airmass of 1.6 and seeing of 0.8". We do not detect any strong, possible transient (point source) optical emission at the FRB location (1-sigma; ATel #17114) and measure a 3-sigma point-source limiting magnitude at this location of g > 23.8 mag (AB). However, we detect a clump of extended optical emission in the host galaxy, offset from the FRB localization region by ~1.2" (measured from the brightest pixel in the g-band image). At a distance to NGC 4141 of ~40 Mpc, the corresponding projected physical distance between the FRB and this clump is ~250 pc.

We also obtained long-slit optical spectroscopy with GMOS-N starting at 25 March 2025 10:41:44 (PI: T. Eftekhari; 8 x 900-sec exposures) at an airmass of ~1.33. We used a 1" slit width, the B480 grating, and the GG455 blocking filter at central wavelengths of 640 and 650 nm. The slit was oriented at a position angle of 10.6 degrees East of North to cover both the FRB localization region and the nearby extended optical emission. While we do not detect any transient spectral features or features at the FRB position, we detect strong nebular emission from the nearby optical clump, including H-beta, H-alpha, and [OIII] at a common redshift of z~0.0065, consistent with emission from a star-forming region within the host.

We thank Gemini Observatory staff, including Jennifer Andrews, for assistance with planning and executing the observations.

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