Deep MMT upper limit on transient optical emission in the host galaxy of FRB 20250316A

ATel #17112; Yuxin (Vic) Dong (Northwestern University) on behalf of the CHIME/FRB Collaboration
on 26 Mar 2025; 23:00 UT
Credential Certification: Kaitlyn Shin (kshin@mit.edu)

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The CHIME/FRB Collaboration reports on optical imaging of the CHIME-KKO localization (ATel #17086) of the nearby, bright FRB 20250316A and its most probable host galaxy NGC 4141 (ATel #17081). We obtained deep r-band observations starting at 23 March 2025 05:39:03 UT (PI: J. Rastinejad; 35 x 90-sec exposures) with the Binospec optical spectrograph mounted on the 6.5-m MMT telescope atop Mt. Hopkins, Arizona. Observations were taken in clear conditions with an average airmass of 1.2 and seeing of 1.0''. We measure a 3-sigma point-source limiting magnitude on the image of r > 25.8 mag (AB).

We obtained a second set of r-band observations starting at 24 March 2025 13:58:07 UT (PI: Y. Dong; 39 x 90-sec exposures) in 1.1'' seeing at an airmass of 1.7 to assess variability between the two epochs. The observations were taken ~32 hr after the first epoch and at comparable depth. We performed image subtraction between the two epochs and do not detect any significant residuals within the reported CHIME-KKO localization (ATel #17086) nor the Einstein Probe candidate X-ray source position (ATel #17100). The non-detection of any optical emission is consistent with previous reports by Troja et al, (GCN 39869), Jiang et al., (GCN 39864), Pereyra et al., (GCN 39858), Aryan et et al., (GCN 39839), Hashimoto et al., (Atel #17095), and Becerra et al., (ATel #17082), although our limits occur on a longer timescale and are significantly deeper.

Following image subtraction, we place a 3-sigma limit of r ~ 25 mag (AB) on the presence of transient optical emission ~8.23 days after the reported FRB. We emphasize that our observations are only sensitive to sources that vary between the two epochs, and are not sensitive to longer-timescale variability. However, visual inspection relative to optical imaging in DECaLS demonstrates no new sources in our data brighter than the nominal limit of the survey (r > 23.5 mag). At the distance of NGC 4141 (d~40 Mpc), the upper limit of r ~25 mag (AB) or M_r ~ -8 mag (accounting for Milky Way extinction) corresponds to an in-band luminosity of 3.3E38 erg/s. With this limit, we are sensitive to the presence of fast-evolving transients such as gamma-ray burst afterglows, fast blue optical transients, and even bright classical novae.

We thank MMT Observatory staff Skyler Self and Benjamin Weiner for scheduling and executing the observations, and Alexa C. Gordon and Jillian Rastinejad (Northwestern) for their assistance in rapid data reduction and analysis.