CHIME/FRB Discovery and Localization of the Swift-observed FRB 20241228A

On 2024 December 28, the Canadian Hydrogen Intensity Mapping Experiment (CHIME) FRB project detected the thus-far nonrepeating FRB 20241228A with a real-time signal-to-noise ratio of >50. Approximately 112 s later, the X-ray Telescope on board the Neil Gehrels Swift Observatory was on-source, the fastest follow-up to date of a nonrepeating fast radio burst (FRB). Using CHIME/FRB and two of the three CHIME/FRB Outriggers, we obtained a very-long-baseline interferometry (VLBI) localization for FRB 20241228A with a 1σ confidence ellipse of 11″ by 0″2. This represents the first published localization using both the CHIME k’niʔatn k’l⌣stk’masqt and CHIME Green Bank Observatory Outriggers. We associate FRB 20241228A with a star-forming galaxy at a redshift of z = 0.1614 ± 0.0002. The persistent X-ray luminosity limit at this source’s location and distance is <1.2 × 10⁴³ erg s⁻¹ in the 0.3−10 keV band, the most stringent limit of any nonrepeating FRB to date. The stellar mass (∼2.6 × 10¹⁰ M_⊙) and star formation rate (∼2.9 M_⊙ yr−1) of the host galaxy of FRB 20241228A are consistent with the broader FRB host-galaxy population. We measure significant scattering (∼1 ms at 600 MHz) and scintillation (∼15 kHz at 600 MHz) along the line of sight to this source, and suggest the scintillation screen is Galactic while the scattering screen is extragalactic. FRB 20241228A represents an exciting example of a new era in which we can harness VLBI localizations and rapid high-energy follow-up to probe FRB progenitors.