CHIME/FRB Outrigger Sample Overview

• CHIME/FRB Outrigger Sample is a collection of fast radio bursts precisely localized using CHIME and dedicated VLBI outriggers for arcsecond-to-milliarcsecond astrometry.
• It employs a dual-stage localization pipeline with FPGA-based channelization and GPU-driven correlation to achieve subarcsecond precision and robust host galaxy associations.
• The sample underpins cosmological studies by constraining baryon content and probing FRB progenitor diversity through innovative calibration and real-time data processing.

The CHIME/FRB Outrigger Sample refers to the collection of fast radio bursts (FRBs) detected and precisely localized using the Canadian Hydrogen Intensity Mapping Experiment (CHIME) in conjunction with its dedicated network of outrigger Very Long Baseline Interferometry (VLBI) stations, designed to obtain arcsecond-to-milliarcsecond FRB positions at the time of discovery. This sample combines CHIME’s high FRB detection rate with transformative, real-time positional accuracy, enabling unambiguous host galaxy associations and opening powerful new avenues for extragalactic astronomy and cosmological inference.

1. Outrigger Sample Definition and Instrumental Implementation

The CHIME/FRB Outrigger Sample is comprised of FRBs detected in real-time by the CHIME/FRB backend (operating over 400–800 MHz) and immediately localized using one or more dedicated outrigger VLBI stations. The initial implementation includes three outrigger sites: KKO (66 km from CHIME), Green Bank Observatory (3370 km baseline), and Hat Creek Observatory (956 km), each employing a cylindrical reflector with architecture closely matched to CHIME to ensure beam compatibility and phase stability (Lanman et al., 12 Feb 2024, Collaboration et al., 7 Apr 2025).

Localization is achieved in two stages: (1) CHIME’s baseband pipeline processes full-array voltage data to obtain arcminute-level positions; (2) The outrigger(s) record short (∼100 ms) triggered baseband captures upon FRB detection, enabling VLBI correlation and fringe-fitting to provide localization precision from 1–2 arcseconds (single outrigger) to ∼50 milliarcseconds (full array). Calibration utilizes in-beam calibrators, pulsars, and compact radio sources, with phase referencing and correction for both geometric and ionospheric delays (Lanman et al., 12 Feb 2024, Pearlman, 26 Nov 2024, Collaboration et al., 7 Apr 2025).

The digital backends at each site use FPGA-based channelization (F-engine) and GPU-based cross-correlation (X-engine), capturing the polyphase-filtered, channelized voltages needed for offline correlation and high-precision astrometry. Data rates can reach up to terabits per second, but ring buffers and targeted recording during triggers enable tractable data volumes (Leung et al., 2020, Leung et al., 8 Mar 2024).

2. Localization Pipeline, Calibration, and Astrometric Performance

VLBI localization in the Outrigger sample relies on robust delay modeling and calibration. After a trigger, baseband buffers from CHIME and outriggers are aligned and correlated off-line. The visibility phase is modeled as:

$$\phi(\nu) = 2\pi \nu\,\tau_{\text{geo}} + 2\pi K\, \Delta \mathrm{TEC}/\nu + \phi_{\text{inst}}(\nu)$$

where $\tau_{\text{geo}}$ encodes the positional offset, $\Delta\mathrm{TEC}$ is the differential ionospheric total electron content, and $\phi_{\text{inst}}$ are instrumental phase terms (Lanman et al., 12 Feb 2024, Collaboration et al., 7 Apr 2025).

Calibration proceeds via phase referencing: bright in-beam calibrators (often cataloged compact radio sources or pulsars) are identified in each snapshot; their known positions allow for correction of both clock errors and differential ionospheric delays. Pulsar gating is used to maximize SNR and exploit well-determined ephemerides, with gating windows defined by polynomial timing solutions:

$P(t) = \sum_{n=0}^{N} a_n (t - t_0)^n$

The filtered visibilities are used to extract delay solutions and baseline offsets via Markov Chain Monte Carlo (MCMC) or maximum likelihood fits. Analysis of commissioning data indicates subarcsecond localizations (rms ≈ 1.16″) with a single outrigger (KKO), and recent results demonstrate routine localizations down to ∼50 milliarcseconds as the full array comes online (Lanman et al., 12 Feb 2024, Collaboration et al., 7 Apr 2025).

A key systematic in astrometry is clock stability; rigorous calibration and interpolation procedures (e.g., using Allan variance models of clock noise, weighed smoothing and repeated calibrator crossings) ensure that residual delay errors remain well below ∼200 ps, necessary for milliarcsecond accuracy (Cary et al., 2021).

3. Sample Properties and Host Galaxy Identification

The first published Outrigger sample consists of 81 FRBs localized with the CHIME–KKO baseline, achieving positional accuracies of 2″ × ~60″ and enabling robust host associations via the PATH (Probabilistic Association of Transients to their Hosts) framework (Collaboration et al., 16 Feb 2025). This process cross-matches radio localization ellipses to optical counterparts from DECaLS and Pan-STARRS, assigning posterior host probabilities $P(O|x)$. For high-confidence ("gold") sample events ($P(O|x) > 0.9$), targeted spectroscopy (Lick, Keck, Gemini) yields host redshifts, with 19 spectroscopically confirmed hosts, mostly at $z < 0.2$. The most nearby event, FRB 20231229A, is at a distance of 90 Mpc.

Host identifications within galaxy clusters and groups were enabled, e.g., FRBs 20230203A, 20230703A, and 20231206A. For the Outrigger sample, this significantly increases the low-redshift anchor for FRB–host association statistics and Macquart relation studies ($\textrm{DM}$ vs. $z$) (Collaboration et al., 16 Feb 2025, Lanman et al., 8 Sep 2025).

A notable discovery is the association of FRB 20231128A with a luminous persistent radio source candidate, with the probability of chance coincidence $P_{cc} \sim 0.008$—if confirmed as compact, it would be the nearest such source known (Collaboration et al., 16 Feb 2025).

4. Cosmological and Astrophysical Applications

The Outrigger sample enables key advances in using FRBs as cosmological probes. The increase in FRBs localized to $z < 0.2$ anchors the Macquart relation linking extragalactic DM to redshift, through the formula:

$$\frac{\textrm{DM}_{\rm host}}{1+z} = \textrm{DM}_{\rm tot} - \left[ \langle \textrm{DM}_{\rm IGM}(z)\rangle + \textrm{DM}_{\rm MW} \right]$$

where $\textrm{DM}_{\rm IGM}(z)$ is the mean intergalactic medium (IGM) contribution and $\textrm{DM}_{\rm MW}$ is the Milky Way component (Collaboration et al., 16 Feb 2025, Lanman et al., 8 Sep 2025).

A subset of Outrigger-localized FRBs has been used to constrain baryon fractions $f_g$ within galaxy groups and clusters by modeling DM contributions of the intracluster medium (ICM) and intragroup medium (IGrM) along the sightline. Electron density models (e.g., mNFW or UPP profiles) are integrated to calculate predicted DMs, which are then compared to observations to set limits on $f_g$ as a function of radius. Empirically, the baryon content inside $R_{500}$ is generally consistent with eROSITA X-ray observations, while $R_{200}$-scale measurements show mild depletion relative to cosmic values (Lanman et al., 8 Sep 2025).

A case paper, FRB 20230703A—intersecting three galaxy groups—finds the observed extragalactic DM is lower than expected if all halos retain the cosmic baryon fraction, implying significant gas depletion in at least one group (Lanman et al., 8 Sep 2025).

5. Sample Diversity, Repetition, and Progenitor Constraints

Analysis of localized events within the Outrigger sample has begun to probe the diversity of FRB progenitors and environments. The high-precision localization of FRB 20250316A to 13 pc (at 40 Mpc distance) exemplifies the capability to place FRBs within specific ISM environments of the host, facilitating resolved studies of density, metallicity, and star formation (Collaboration et al., 23 Jun 2025). For FRB 20250316A, the absence of repeated bursts down to limits inconsistent with known repeaters, and the ∼190 pc offset from the center of the nearest star-forming region, argues for diversity in FRB channels (suggesting, e.g., older/kicked magnetar progenitors versus young star-forming environments).

Population studies based on the Outrigger and main CHIME samples suggest that the majority of one-off FRBs may eventually reveal repetition with sufficient observing time, but high-fluence, non-repeating events like FRB 20250316A remain outliers with significant implications for population synthesis models (McGregor et al., 2023, Collaboration et al., 23 Jun 2025).

6. Future Prospects, Scaling, and Impact

With the phased deployment and commissioning of all three Outrigger stations, the project is positioned to deliver hundreds of FRBs per year with VLBI-level astrometric precision of ∼50 mas, fully sufficient for unambiguous host identification and detailed environmental studies (Collaboration et al., 7 Apr 2025). The deployment of advanced VLBI calibration pipelines (including real-time pulsar gating and in-beam calibrators), robust software correlators, and multi-node data transport infrastructures is critical for sustained science operations and routine sub-arcsecond astrometry (Leung et al., 8 Mar 2024, Pearlman, 26 Nov 2024).

A large, growing Outrigger sample will enable statistical mapping of the baryon content in galaxy groups and clusters, systematic investigation of FRB host offsets and environment, and population-level studies of repetition rates and energetics. The sample's expansion will also solidify FRBs as probes of the low-density IGM, offering sharp constraints on the cosmic baryon budget and feedback processes in massive haloes (Lanman et al., 8 Sep 2025).

In summary, the CHIME/FRB Outrigger Sample constitutes the first statistically significant, arcsecond–to–milliarcsecond–localized FRB catalog in the era of wide-field VLBI array surveys. It underpins an emerging field in which FRB physics, host galaxy demographics, and cosmological structure are studied within a unified, precisely calibrated framework.