TITLE: GCN CIRCULAR NUMBER: 39216 SUBJECT: CHIME/FRB source FRB 20250206A detected less than 1 minute after LIGO-Virgo-Kagra (LVK) S250206dm, however the probability of spatial coincidence is order 0.1% DATE: 25/02/07 21:26:31 GMT FROM: amanda.cook@mcgill.ca

The CHIME/FRB Collaboration reports the spatial and temporal proximity of the LIGO-Virgo-Kagra (LVK) compact-object merger candidate S250206dm (GCN 39175; likely either BH-NS or NS-NS) and the fast radio burst (FRB) 20250206A. FRB 20250206A was detected by CHIME/FRB at 2025 Feb 06, 21:26:27.956253 p/m 0.000008 UTC (topocentric at 400 MHz) about 52 seconds (having corrected for dispersive delay) after the LVK event's topocentric time. From recorded voltage data, we obtain a best-fit localization (Michilli et al. 2021) of RA, Decl. (J2000) 338.703, 12.1699 degrees, with 1-sigma uncertainties of 0.42, 0.28 arcminutes, respectively. Using LVK code for estimating the localization region of a given position(https://bit.ly/3CJsyu6) and the most updated Bilby skymap, the FRB position is at the 99.96% credible region of the Bilby localization, i.e., the FRB position is unlikely to be spatially coincident with the LVK localization.

Using our raw voltage data, the burst's structure-maximized dispersion measure (DM) is 207.117 +/- 0.003 pc cm^-3. With a Galactic disk DM estimate of 40 pc cm^-3 from NE2001 and assuming a Galactic halo DM of 38 pc cm^-3, (Yamasaki & Totani 2020), the residual extragalactic DM is ~130 pc cm^-3. Using a redshift-DM relationship for the intergalactic medium of z~ DM / 800 pc cm^-3 (Connor et al. 2024), this extragalactic DM corresponds to an upper limit on the luminosity distance (i.e., assuming no DM contribution from a host galaxy) of ~800 Mpc (assuming LambdaCDM parameters from Planck Collaboration 2018), consistent with the LVK-inferred luminosity distance of 348 +/- 114 Mpc (GCN 39184). The FRB was detected with a signal-to-noise ratio of 16.4 in one CHIME/FRB beam, and 15.7 in a second beam. A preliminary baseband fluence averaged over the entire 400-800 MHz CHIME observing band is 36 +/- 4 Jy ms (1-sigma uncertainty). The source does not appear to be a repeat burst from any previously detected CHIME FRB above a signal-to-noise ratio of 8.

The source has clear scattering, with a characteristic timescale of ~0.05-0.10 ms at 600 MHz (Fonseca et al. 2023). The Galactic scattering contribution in this direction using NE2001p at 600 MHz is expected to be ~1.5µs. The burst is consistent with being 100% linearly polarized and has a Faraday rotation measure (RM) of -62.07 +/- 0.05 rad m^-2, which was determined using RM-synthesis. The Galactic RM towards the FRB is estimated to be -37 +/- 11 rad m^-2(Hutschenreuter et al. 2020). The probability of chance coincidence (i.e., the likelihood that the LVK and CHIME/FRB events are temporally and spatially proximal by chance only) is difficult to estimate immediately given the large LVK uncertainty regions and CHIME/FRB's uneven exposure and sensitivity on the sky. With the strong caveat that we do not yet know the joint probability of coincidence (analysis ongoing), and that it is likely non-negligible, we are providing the above information because any potential electromagnetic counterpart will be transient. Due to on-site networking issues, this event was not sent out to the community as a VOEvent.

Below, we show the multi-beam event waterfall plot, where the panels represent adjacent beams in different rows and columns. We also show an overlay of the FRB position on top of the merger event (from GraceDB, theflat-resolution FITS file created from Bilby.fits.gz,1)

Multi-beam waterfall plot: https://storage.googleapis.com/chimefrb-dev.appspot.com/FRB20250206A/multibe...

Localization plot: https://storage.googleapis.com/chimefrb-dev.appspot.com/FRB20250206A/chime_p...

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