Abstract
Fast radio bursts (FRBs) are millisecond-duration radio transients whose origins remain unknown. As the vast majority of bursts are one-off events, it is necessary to pinpoint FRBs precisely within their host galaxies at the time of detection. Here we use two purpose-built outrigger telescopes to localize FRB 20210603A at the time of its detection by the Canadian Hydrogen Intensity Mapping Experiment (CHIME). Our very-long-baseline interferometry stations localized the burst to a 0.2" × 2" final ellipse in the disk of its host galaxy SDSS J004105.82+211331.9. A spatially resolved spectroscopic follow-up revealed recent star formation (Hα emission) on kiloparsec scales near the burst position. The excess dispersion measure is consistent with expectations from the nearly edge-on disk of the host galaxy, demonstrating the utility of FRBs as probes of the interstellar medium in distant galaxies. The excess dispersion measure, rotation measure and scattering are consistent with expectations for a pulse travelling from deep within its host galactic plane, strengthening the link between the local environment of FRB 20210603A and the disk of its host galaxy. Finally, this technique demonstrates a way to overcome the trade-off between angular resolution and field of view in FRB instrumentation, paving the way towards plentiful and precise FRB localizations.
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Data availability
Calibrated visibilities, dynamic spectra for producing the figures and Markov-chain Monte Carlo chains for the localization analysis are available upon request and will be hosted by the time of publication as downloadable HDF5 files from the repository of the Canadian Advanced Network for Astronomical Research for CHIME/FRB at https://www.canfar.net/storage/list/AstroDataCitationDOI/CISTI.CANFAR/24.0086/data. Optical images, spectra and photometric data are immediately available as fits files at https://github.com/tcassanelli/frb-vlbi-loc.
Code availability
The code used for beamforming, VLBI localization and polarization analysis are available on Github: https://github.com/CHIMEFRB/baseband-analysis. The scattering timescale was measured using fitburst54, which is available at https://github.com/CHIMEFRB/fitburst. Code for interpreting burst properties and for producing the figures and tables in this paper from the results of our analyses is available at https://github.com/tcassanelli/frb-vlbi-loc. In our analyses, we also made use of open-source software including Astropy101, baseband102, difxcalc1143, Matplotlib103, NumPy104, SciPy105, h5py106, emcee17, corner107, cartopy108, IRAF86,87 and Prospector16.
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Acknowledgements
We would like to dedicate this work to our colleague Jing Luo, who passed away on 15 February 2022. Jing worked for several years on the commissioning and maintenance of the 10 m telescope at Algonquin Radio Observatory. His expertise in radio pulsars, radio observations and hardware were indispensable in accomplishing this scientific milestone. We acknowledge that CHIME is located on the traditional, ancestral and unceded territory of the Syilx/Okanagan people. CHIME is funded by a grant from the 2012 Leading Edge Fund of the Canada Foundation for Innovation (Project No. 31170) and by contributions from the provinces of British Columbia, Québec and Ontario. The CHIME/FRB Project is funded by a grant from the 2015 Innovation Fund of the Canada Foundation for Innovation (Project No. 33213) and by contributions from the provinces of British Columbia and Québec, and by the Dunlap Institute for Astronomy and Astrophysics at the University of Toronto. Further support was provided by the Canadian Institute for Advanced Research (CIFAR), McGill University and the McGill Space Institute thanks to the Trottier Family Foundation and the University of British Columbia. The Dunlap Institute is funded through an endowment established by the David Dunlap family and the University of Toronto. We thank the staff of DRAO, which is operated by the National Research Council of Canada, for their gracious hospitality, support and expertise. ARO10 is operated by the University of Toronto. TONE is at Green Bank Observatory, which is supported by the National Science Foundation (NSF), and is operated by Associated Universities, Inc. under a cooperative agreement. We would like to thank the staff at Green Bank Observatory for logistical support during the construction and operations of TONE. This work is based on observations obtained at the international Gemini Observatory, a programme of NSF’s NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the NSF. PyRAF is a product of the Space Telescope Science Institute, which is operated by AURA for NASA on behalf of the Gemini Observatory partnership: the NSF (United States), National Research Council (Canada), Agencia Nacional de Investigación y Desarrollo (Chile), Ministerio de Ciencia, Tecnología e Innovación (Argentina), Ministério da Ciência, Tecnologia, Inovações e Comunicações (Brazil) and the Korea Astronomy and Space Science Institute (Republic of Korea). This work is also based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and the French Alternative Energies and Atomic Energy Commission/Dapnia, at the CFHT, which is operated by the National Research Council of Canada, the Institut National des Science de l’Univers of the French National Centre for Scientific Research and the University of Hawaii. The observations at the CFHT were performed with care and respect from the summit of Maunakea which is a significant cultural and historic site. A.B.P. is a Banting Fellow, a McGill Space Institute Fellow and a Fonds de Recherche du Quebec – Nature et Technologies (FRQNT) postdoctoral fellow. A.P.C. is a Vanier Canada Graduate Scholar. B.M.G. acknowledges the support of the Natural Sciences and Engineering Research Council of Canada (NSERC; Grant No. RGPIN-2015-05948) and of the Canada Research Chairs programme. C.L. was supported by the US Department of Defense through the National Defense Science & Engineering graduate fellowship programme and is an NHFP Einstein Fellow. E.P. acknowledges funding from a Veni Fellowship from the Netherlands Organisation for Scientific Research. F.A.D is funded by a four-year doctoral fellowship at the University of British Columbia. FRB research at the University of British Columbia is funded by an NSERC discovery grant and by the Canadian Institute for Advance Research. The CHIME baseband system was funded in part by an award granted by the John R. Evans Leaders Fund from the Canada Foundation for Innovation to I.S. FRB research at West Virginia University is supported by the NSF (Grant Nos. 2006548 and 2018490). J.B.P. is supported by the Major Research Instrumentation Program of the NSF (Grant No. 2018490). J.M.P. is a Kavli Fellow. K. Shin is supported by the NSF Graduate Research Fellowship Program. K.W.M. is supported by the NSF (Grant No. 2008031). M.B. is supported by an FRQNT doctoral research award. M.D. is supported by a Canada Research Chair, Killam Fellowship and NSERC Discovery Grant, by CIFAR and by the FRQNT Centre de Recherche en Astrophysique du Québec. P. Scholz is a Dunlap Fellow. S. Cary would like to thank K. McLeod from Wellesley College for her supervision and feedback, which was essential for the host galaxy analysis. S. Chatterjee is a member of the NANOGrav Physics Frontiers Center, which is supported by the NSF (Award No. PHY-1430284). U.P. is supported by NSERC (Funding References RGPIN-2019-067, CRD 523638-201 and 555585-20), the Research Excellence Program of the Ontario Research Fund, CIFAR, the Simons Foundation, Thoth Technology Inc. and the Alexander von Humboldt Foundation. This research is supported by the Ministry of Science and Technology of Taiwan (Grant No. 110-2112-M-001-071-MY3). V.M.K. holds the Lorne Trottier Chair in Astrophysics & Cosmology and a Distinguished James McGill Professorship and receives support from an NSERC Discovery grant (RGPIN 228738-13), an R. Howard Webster Foundation Fellowship from CIFAR and the FRQNT Centre de Recherche en Astrophysique du Québec. Z.P. is a Dunlap Fellow.
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Authors and Affiliations
Contributions
T.C. led the full instrument adaptation, triggering system and data management of the ARO10 telescope, wrote the main text and methods sections, and prepared several figures and tables. C.L. wrote the VLBI software correlator and analysis pipeline used to localize the FRB, designed and built the digital back end of the TONE array, and led the data analysis, scientific interpretation and writing of the paper. P. Sanghavi. led the design, construction, commissioning and data acquisition of all aspects of the TONE telescope and contributed significantly to the scientific interpretation and writing of the paper. J.M.P. designed and installed the maser hardware, characterized the clock stabilization system and made foundational contributions to the array calibration pipelines used at TONE and CHIME. S. Cary led the calibration, reduction and analysis of the optical follow-up data. All other authors from the CHIME/FRB collaboration played either leadership or significant supporting roles in one or more of the following activities: management, development, construction, commissioning and maintenance of CHIME, the CHIME/FRB instrument, the ARO10 instrument, the TONE instrument, their respective software data pipelines, or the data analysis and preparation of this paper.
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Nature Astronomy thanks Adam Deller and Zsolt Paragi for their contribution to the peer review of this work.
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Extended data
Extended Data Fig. 1 Dynamic spectra of all observations.
At each VLBI station we recorded five single pulses (including the FRB): Crab GPs which we refer to as C1-C4 in the several days surrounding FRB 20210603A. Each row corresponds to a different VLBI station (CHIME at the Dominion Radio Astrophysical Observatory, ARO10 at the Algonquin Radio Observatory, and TONE at the Green Bank Observatory). Timestamps show site-local clocks aligned to within 2.56 μs at a reference frequency of 800.0 MHz. Though the FRB is too faint to be detected at the testbeds alone, it is robustly detected in cross-correlation with CHIME at both stations. The intensity was adjusted by normalizing its standard deviation and setting the colour scale limits to the 1 and 99 percentile values of the data. Waterfall plots are shown downsampled to a frequency resolution of 390.625 kHz and a time resolution of 25.6 μs. The noisy radio frequency interference (RFI) channels in 700-750 MHz correspond to the cellular communications bands and the RFI channels at ≈ 600 MHz frequencies correspond to television transmission bands. These RFI channels were removed in our analysis and are highlighted with red strikes to the left of each waterfall plot. Symbols next to the telescope label in each waterfall plot indicate what each Crab pulse was used for. We use C2 on all baselines as a phase/delay calibrator, and C1 and C4 as rate calibrators for the CHIME-ARO10 and CHIME-TONE baselines respectively. We localized C3 as an end-to-end cross-check of our calibration solutions.
Extended Data Fig. 2 Delay residuals measured from the CHIME-ARO10 and CHIME-TONE baselines.
The graph shows the empirical uncertainty obtained by analysing earlier data sets3,4, with CHIME-ARO10 data shown in the top row and CHIME-TONE data showed in the bottom row. Each point corresponds to the residual delay after applying delay and phase corrections (CHIME-ARO10 is calibrated to 2020-10-22, and TONE is calibrated to 2021-02-18). The extracted delays have all been compensated for clock errors and for a clock rate error on the CHIME-ARO10 baseline.
Extended Data Fig. 3 Calibrated visibilities from the Crab pulsar giant pulse (C3) used to validate our calibration solutions.
We plot visibilities from the CHIME-ARO10 (left) and CHIME-TONE (right) baselines respectively. In each top panel, we plot the absolute value of the Fourier transform of the visibilities (that is the time-lag cross-correlation function ρ(τ) as a function of the delay referenced to the correlator pointing center). This shows a detection S/N exceeding 50 on each baseline. In each bottom panel we plot the phase of the calibrated visibilities V [i, k], binned to 1.6 MHz resolution, with 1σ phase errors estimated from off-pulse scans (N = 10) plotted as σ [i, k] /V [i, k] (blue points). In the bottom panels we overlay the phase model (Eq. (4)) evaluated at the parameters which maximize LΦ, where we have fit for the ionosphere and the positions simultaneously (green “full fit” curve), as well as the phase model evaluated at the Lτ position at zero ionosphere (yellow “delay only” curve). Since our correlator pointing is the Lτ position, we would then expect the yellow “delay only” curve to be flat; note that our plotting code automatically unwraps all of the phases in each bottom panel by some amount automatically chosen to reduce phase wrapping, explaining the very small deviation from zero delay.
Extended Data Fig. 4 The localization posterior of the Crab pulse (C3) as a function of RA, Dec, and ΔDMCA, and ΔDMCT.
Due to the extremely sparse sampling of the uv-plane, we bypass traditional methods of VLBI imaging, and directly fit the visibilities V [i, k]. Owing to our wide bandwidth, we see that the ionosphere parameters ΔDM are well-constrained even in the absence of external information (for example, TEC maps or ionosphere priors). In the same spirit as a MCMC corner plot, each 2D plot shows the posterior marginalized over all except two axes. Calling these projections P, we colour evenly-spaced contours between log P = 0 (the maximum value of each P is normalized to 1) and log P = − 16.
Extended Data Fig. 5 Localization of C3 as an independent, end-to-end cross check of our VLBI calibration solution used to localize the FRB.
Due to the extremely sparse sampling of the uv-plane, we avoid traditional imaging. We compare two localization methods: a delay-space χ2-minimization of the residual delays left after calibration (+), and a visibility-space fitting of the phases (×). Both methods agree to within the true position of the Crab (star) within systematic uncertainties (ellipses).
Extended Data Fig. 6 Calibrated VLBI fringes on FRB 20210603A from the CHIME- ARO10 and CHIME-TONE baselines respectively.
We plot visibilities from the CHIME- ARO10 (left) and CHIME-TONE (right) baselines respectively. In each top panel, we plot the absolute value of the Fourier transform of the visibilities (that is the time-lag cross-correlation function ρ (τ) as a function of the delay referenced to the correlator pointing center). This shows a detection S/N exceeding 50 on each baseline. In each bottom panel we plot the phase of the calibrated visibilities V [i, k], binned to 1.6 MHz resolution, with 1σ phase errors estimated from off-pulse scans (N = 10) plotted as σ [i, k] /V [i, k] (blue points). In the bottom panels we overlay the phase model (Eq. (4)) evaluated at the parameters which maximize LΦ, where we have fit for the ionosphere and the positions simultaneously (green “full fit” curve), as well as the phase model evaluated at the Lτ position at zero ionosphere (yellow “delay only” curve). Since our correlator pointing is the Lτ position, we would then expect the yellow “delay only” curve to be flat; note that our plotting code automatically unwraps all of the phases in each bottom panel by some amount automatically chosen to reduce phase wrapping, explaining the very small deviation from zero delay.
Extended Data Fig. 7 The posterior localization contour of FRB 20210603A as a function of RA, Dec, and ΔDMCA, and ΔDMCT.
The ionosphere parameters ΔDM are well- constrained even in the absence of external information (for example, TEC maps or ionosphere priors). In the same spirit as a MCMC corner plot, each 2D plot shows the posterior marginalized over all except two axes. Calling these projections P, we colour evenly-spaced contours between log P = 0 (the maximum value of each P is normalized to 1) and log P = − 16.
Extended Data Fig. 8 Spatially resolved spectroscopy of the host galaxy.
Optical image and spatially-resolved spectra of the host galaxy of FRB 20210603A acquired using CFHT MegaCAM and Gemini long-slit spectroscopy respectively. Pixel intensities are scaled linearly and normal- ized to reduce the saturation evident in Fig. 3. All spectra are given offsets in increments of 10-17 erg s-1 cm-2 A-1. One spectrum is extracted from the bulge of the galaxy (spectrum b, centered at 0). There are additional eleven spectra extracted from the FRB side of the galaxy (shown as positive offsets), and from the opposite side of the galaxy (shown as negative offsets), with offsets from the center of the galaxy in increments of 1 arcsec. All spectra are extracted using an aperture size of \(1.5{\rm{arcsec}}\times 1\) arcsec, as represented on the galaxy image. Spectrum a is extracted within the vicinity of the FRB and represented by the shaded box a in the galaxy image. The twelve spectra and Gaussian fits to the Hα and one of the NII emission lines, are plotted here after correcting for Milky-Way extinction.
Extended Data Fig. 9 Spectral energy distribution of host galaxy.
Gemini long-slit spectrum, integrated over the galaxy, with archival infrared photometry from 2MASS and WISE, plotted after correcting for extinction due to the host galaxy’s inclination angle. Plotted alongside the observations (red) are the best-fit model (blue) from Prospector, and the relative passbands for the 2MASS J, H, and Ks and WISE W1-W3 filters. Flux uncertainties are plotted by converting 1σ photometric errors reported by each catalogue.
Extended Data Fig. 10 A visualization of propagation effects due to the Milky Way’s disk, as measured via the ATNF Pulsar Catalogue.
We plot joint distributions of DM, ∣RM∣ and τscatt for Galactic pulsars for two different latitude ranges: 4∘≤∣b∣≤10∘ (blue) and ∣b∣≥20∘ (orange) taken from the ATNF Pulsar Catalogue79. Contour lines indicate 1, 2 and 3σ regions of this parameter space. Green regions/lines indicate estimates of equivalent quantities determined for the host galaxy of FRB 20210603A, namely: DMhost, ∣RMhost∣ and our upper limit on τscatt. DMhost, ∣RMhost∣ and τscatt estimates are in the source frame with τscatt referenced at 1 GHz assuming a τscatt ∝ ν−4.4 relation used by ATNF. This shows that the burst properties of FRB 20210603A (DMhost, ∣RMhost∣ and τscatt at 1 GHz), once corrected for extragalactic contributions, are similar to that of low-latitude (4∘≤∣b∣≤10∘) Galactic pulsars.
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Cassanelli, T., Leung, C., Sanghavi, P. et al. A fast radio burst localized at detection to an edge-on galaxy using very-long-baseline interferometry. Nat Astron 8, 1429–1442 (2024). https://doi.org/10.1038/s41550-024-02357-x
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DOI: https://doi.org/10.1038/s41550-024-02357-x