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Closest and brightest fast radio burst ever detected by astronomers The dazzling “RBFLOAT” radio burst, originating nearby in the Ursa Major constellation, offers the clearest view yet of the environment around these mysterious flashes. Credit: Danielle Futselaar
a neighborhood flash that rewrites observational access to FRBs
A fast radio burst (FRB) is an intense, millisecond-scale flash of radio emission that can briefly outshine every other radio source in its host galaxy. Some FRBs are so luminous that their radio signals are detectable across billions of light-years. Yet despite more than a decade of detections, the physical origin of these fleeting flares remains uncertain.
On March 16, 2025, an extraordinarily bright FRB was captured by the Canadian Hydrogen Intensity Mapping Experiment (CHIME) and its new companion stations, the CHIME Outriggers. Located roughly 130 million light-years away in the spiral galaxy NGC 4141 in Ursa Major, the burst is among the nearest and most luminous FRBs recorded to date. Its unusual brightness and proximity have earned it the informal label “RBFLOAT” — radio brightest flash of all time — and, crucially, have given astronomers an unprecedented window into the environment surrounding a fast radio burst.
"Cosmically speaking, this fast radio burst is just in our neighborhood," said Kiyoshi Masui, associate professor of physics and affiliate of MIT's Kavli Institute for Astrophysics and Space Research. "This means we get this chance to study a pretty normal FRB in exquisite detail."
CHIME, CHIME Outriggers and the leap in localization
CHIME is a large array of cylindrical, halfpipe-shaped antennae in British Columbia originally built to map hydrogen across cosmic time. Since beginning operations in 2018, CHIME has become one of the world’s most prolific FRB discoverers, reporting thousands of bursts across the sky. However, the parent CHIME array alone lacked the angular precision to always place bursts accurately inside their host galaxies.
To meet that challenge, scientists deployed the CHIME Outriggers: three smaller CHIME-style stations distributed across North America. Working coherently with the main CHIME array, the Outriggers form a continent-scale interferometer that can localize very bright, millisecond-duration radio flashes to sub-arcsecond precision. This network upgrade transforms CHIME from a powerful all-sky finder into a high-precision localizer for the brightest FRBs.
"Imagine we are in New York and there's a firefly in Florida that is bright for a thousandth of a second," said Shion Andrew, a Kavli graduate student at MIT. "Localizing an FRB to a specific part of its host galaxy is analogous to figuring out not just what tree the firefly came from, but which branch it's sitting on." The CHIME + Outriggers system made exactly that leap for RBFLOAT: it identified NGC 4141 as the host and located the burst at the edge of an active star-forming region.
Host galaxy environment and implications for progenitors
Follow-up imaging and spectroscopy placed RBFLOAT at the periphery of a star-forming complex in NGC 4141. That location is an important clue. One of the leading hypotheses for at least some FRBs is magnetars — young, highly magnetized neutron stars known to emit energetic flares across the electromagnetic spectrum. Because magnetars are often associated with regions of recent star formation, a burst located deep inside such a region would point strongly to a very young magnetar. RBFLOAT’s position on the edge suggests a slightly older progenitor, possibly a magnetar that has drifted or aged beyond the densest clusters of young stars.
"These are mostly hints," Masui said. "But the precise localization of this burst is letting us dive into the details of how old an FRB source could be. If it were right in the middle, it would only be thousands of years old — very young for a star. This one, being on the edge, may have had a little more time to bake."
The environment around an FRB imprints observable effects on the radio signal: dispersion and scattering by intervening plasma, Faraday rotation from magnetic fields, and absorption by local gas. Because RBFLOAT is both bright and nearby, these propagation signatures can be measured with higher signal-to-noise than for distant bursts. That improved fidelity enables tighter constraints on the density, magnetization, and structure of the FRB’s immediate surroundings, advancing models for how flares are produced and how they evolve with time.
Single event or repeating source? What the archive reveals
Since the discovery of the first FRB in 2007, astronomers have cataloged more than 4,000 bursts. Most are one-off events; a minority repeat sporadically, and a small subset show periodic or quasi-periodic activity. Determining whether repeating and nonrepeating FRBs arise from the same physical mechanisms remains a central question. One path to an answer is to search archival data at the source location for prior activity.
CHIME’s continuous monitoring and the Outriggers’ triggered recording enabled a thorough retrospective search. The collaboration examined six years of CHIME observations at the coordinates of NGC 4141 but found no prior bursts from that region. At least for the past six years, RBFLOAT appears to be a single, nonrepeating event. Given its closeness and intensity, the nonrepeating classification gives researchers a rare opportunity to map the surrounding environment of an FRB suspected not to repeat and compare it to repeaters.
"Right now we're in the middle of this story of whether repeating and nonrepeating FRBs are different," Masui noted. "These observations are putting together bits and pieces of the puzzle." Adam Lanman, an MIT physics postdoc and coauthor on the discovery paper, emphasized the increasing diversity: "As we're getting these much more precise looks at FRBs, we're better able to see the diversity of environments they're coming from."
Expert Insight
Dr. Elena Ruiz, a fictional astrophysicist specializing in compact object magnetospheres and a senior researcher at a major observatory, offers context: "RBFLOAT is a rare gift. A nearby, ultrabright FRB lets us probe the magneto-ionic environment with detail normally reserved for much closer steady sources. If magnetars are responsible, the peripheral location suggests a population of FRB-producing neutron stars that are older than those embedded in dense stellar nurseries. Continued multiwavelength follow-up — radio polarization, X-ray monitoring, and deep optical spectroscopy — will help distinguish between competing progenitor models."
Her assessment highlights a practical roadmap: combine high-time-resolution radio data with X-ray and optical surveys to search for persistent counterparts, shock signatures, or remnants that can link an FRB to a specific evolutionary path.
Technology, funding and the path forward
RBFLOAT showcases how instrumental upgrades expand scientific reach. The CHIME Outriggers network was funded by the Gordon and Betty Moore Foundation and the U.S. National Science Foundation, while CHIME’s original construction received support from the Canada Foundation for Innovation and the provinces of Quebec, Ontario, and British Columbia. This mix of philanthropic and public investment enabled the continent-spanning interferometry necessary for high-precision FRB localization.
Looking ahead, the CHIME Collaboration expects to localize hundreds of FRBs per year as the Outriggers remain operational and the array continues to optimize real-time triggers. As localization rates grow, so will the statistical sample tying FRBs to host-galaxy properties: stellar age, metallicity, star-formation rate, and local magnetic environment. That demographic mapping is essential to determine whether repeating and nonrepeating FRBs share a single progenitor class or instead arise from multiple channels, such as young magnetars, interacting binaries, or exotic compact-object collisions.
Conclusion
RBFLOAT — the exceptionally bright, nearby fast radio burst detected in NGC 4141 — represents a milestone in FRB science. By combining CHIME’s wide-field sensitivity with the angular precision of the CHIME Outriggers, astronomers pinned the burst to the edge of a star-forming region and found no evidence of prior repeating activity in six years of archival data. These observations refine progenitor hypotheses, favoring models that can operate in less centrally concentrated star-forming environments and emphasizing magnetars as a plausible source class, potentially at a range of ages.
Beyond the specific case of RBFLOAT, the result demonstrates how technical upgrades and coordinated follow-up can convert fleeting millisecond events into rich astrophysical probes. As CHIME and other next-generation radio facilities expand their localization capabilities, the astrophysics community is poised to resolve whether FRBs are a single phenomenon or a family of distinct cosmic flashes — and what these enigmatic bursts reveal about extreme physics, neutron-star evolution, and the magnetized interstellar medium.
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