TITLE: GCN CIRCULAR NUMBER: 39272 SUBJECT: LIGO/Virgo/KAGRA S250206dm: Gemini-North spectroscopic observation of AT2025bbo and its host galaxy DATE: 25/02/11 00:33:06 GMT FROM: James Gillanders at University of Oxford jhgillanders.astro@gmail.com

M. E. Huber (IfA, University of Hawaii), J. H. Gillanders (Oxford), K. Chambers (IfA, University of Hawaii), S. J. Smartt (Oxford), M. Fulton (QUB), T.-W. Chen (NCU) and D. Magill (QUB) report on behalf of a larger collaboration:

We observed the transient AT2025bbo and its host galaxy with the GMOS-N spectrograph at the Gemini-North observatory under observing program ID GN-2025A-Q-133 (PI: M. E. Huber). AT2025bbo is a transient discovered during the Pan-STARRS coverage (Young et al., GCN 39210; Smartt et al., GCN 39244) of the skymap of S250206dm, the NSBH merger event detected by the LIGO-Virgo-KAGRA (LVK) Collaboration (GCN 39175).

Our epoch of observation commenced at 2025-02-10 05:44 UTC, corresponding to approximately 3.35 days after the recorded merger signal of S250206dm (LVK Collaboration, GCN 39175). We obtained 4x1200s exposures with the R400 grating, sampling a wavelength range of ~4500-9200 angstroms.

The source is coincident with the nucleus of the host galaxy (Smartt et al., GCN 39244), and we extracted a spectrum which would contain both the host light and any transient flux within the slit aperture. The spectrum is that of an early-type, elliptical galaxy, consistent with the Pan-STARRS and SDSS colours. The redshift from the Mg I and Na I lines is z = 0.070 +/- 0.006 (D_L = 316 Mpc for H0 = 70 km/s/Mpc). This confirms that the galaxy lies within the estimated luminosity distance range for S250206dm from the Bilby 3D skymap (373 +/- 104 Mpc; LVK Collaboration, GCN 39231).

The continuum of the spectrum does not show any obvious broad features from transient flux, nor any emission lines (e.g., H-alpha, [O III]) from AGN activity. It appears consistent with galaxy light, but further analysis is required to determine if any continuum from a transient source is present.

In Smartt et al. (GCN 39244), we advised caution on the reality of this event. Detections of AT2025bbo were reported by three other teams: Liu et al. (GCN 39429), Lee et al. (GCN 39252), and Ducoin et al. (GCN 39258). Upon further investigation, we find that the publicly available Pan-STARRS 3Pi reference frame (which all teams used to subtract the host) consists of only 2x40s frames at this location. While the header of the reference stack implies 24x40s, there is significant masking in the input images around the transient/galaxy position. This can be seen by selecting the warp option in the MAST user interface to access the image stamps, to display all 24 warp images. Liu et al. (GCN 39429) also reported an i-band detection, which has a good Pan-STARRS reference (630s total). Further work is required to determine if the flux excess, which all four teams detect, is due to transient flux on the galaxy core, or an artefact of the shallow reference r-band image.

We thank the staff of the Gemini Observatory for rapidly approving and executing these observations.

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