# A repeating fast radio burst source localized to a nearby spiral galaxy

## Abstract

Fast radio bursts (FRBs) are brief, bright, extragalactic radio flashes1,2. Their physical origin remains unknown, but dozens of possible models have been postulated3. Some FRB sources exhibit repeat bursts4,5,6,7. Although over a hundred FRB sources have been discovered8, only four have been localized and associated with a host galaxy9,10,11,12, and just one of these four is known to emit repeating FRBs9. The properties of the host galaxies, and the local environments of FRBs, could provide important clues about their physical origins. The first known repeating FRB, however, was localized to a low-metallicity, irregular dwarf galaxy, and the apparently non-repeating sources were localized to higher-metallicity, massive elliptical or star-forming galaxies, suggesting that perhaps the repeating and apparently non-repeating sources could have distinct physical origins. Here we report the precise localization of a second repeating FRB source6, FRB 180916.J0158+65, to a star-forming region in a nearby (redshift 0.0337 ± 0.0002) massive spiral galaxy, whose properties and proximity distinguish it from all known hosts. The lack of both a comparably luminous persistent radio counterpart and a high Faraday rotation measure6 further distinguish the local environment of FRB 180916.J0158+65 from that of the single previously localized repeating FRB source, FRB 121102. This suggests that repeating FRBs may have a wide range of luminosities, and originate from diverse host galaxies and local environments.

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## Data availability

The datasets generated from the EVN observations and analysed in this study are available at the Public EVN Data Archive (http://www.jive.eu/select-experiment) under the experiment code EM135C. The datasets generated from the Gemini observations are available at the Public Gemini Observatory Archive (https://archive.gemini.edu) with Program ID GN-2019A-DD-110.

## Code availability

The codes used to analyse the data are available at the following sites: AIPS (http://www.aips.nrao.edu/index.shtml), CASA (https://casa.nrao.edu), Difmap (ftp://ftp.astro.caltech.edu/pub/difmap/difmap.html), IRAF (http://ast.noao.edu/data/software), PRESTO (https://github.com/scottransom/presto), and PSRCHIVE (http://psrchive.sourceforge.net). This research made use of APLpy, an open-source plotting package for Python hosted at http://aplpy.github.com, Astropy, a community-developed core Python package for Astronomy77, and Matplotlib78.

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## Author information

B.M. is the Principal Investigator of the EVN observing programme and led the radio imaging analysis of those data. K.N. discovered the radio bursts and led the time-domain analysis. J.W.T.H. guided the time-domain analysis and made major contributions to the writing of the manuscript. S.P.T. and C.G.B. led the optical imaging and spectroscopic analyses. Z.P. performed an independent analysis of the EVN data, and in previous years pioneered the development of the EVN’s capabilities for fast transient research. A.K. created the software used to correlate the EVN baseband data. M.B. analysed the chance coincidence probability. R.K. arranged the Effelsberg PSRIX observations. C.J.L. analysed the VLA imaging data. D.M. determined the CHIME/FRB baseband position. V.M.K. had an important coordination role, enabling these results to be gathered. All other co-authors contributed to the CHIME/FRB discovery of the source or the interpretation of the analysis results and the final version of the manuscript.

Correspondence to J. W. T. Hessels.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature thanks Matthew Bailes and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Extended data figures and tables

### Extended Data Fig. 1 Burst detections in Effelsberg PSRIX data.

Band-averaged profiles and dynamic spectra of the four bursts, as detected in the PSRIX data (a, b, c and d). A 20-ms time window is shown surrounding the burst centre. Each burst was fitted with a Gaussian distribution to determine the FWHM duration, which is represented by the cyan bars. The lighter cyan encloses the 2σ region. The solid white lines are frequency channels that have been removed from the data due to either RFI or subband edges, indicated by the red and blue markers, respectively. For visual clarity, bursts B1, B2 and B3 (a, b and c) are downsampled in both time and frequency by a factor of two. The RFI excision was done before downsampling. The time and frequency resolution used for plotting is shown in the top right of each panel.

### Extended Data Fig. 2 VLA field image.

Field of the continuum radio emission around FRB 180916.J0158+65 as seen by the VLA at 1.6 GHz with a bandwidth of 0.6 GHz. The position of FRB 180916.J0158+65 is marked by the red cross at the centre of the image. Contours start at the 3σ r.m.s. noise level of 18 μJy per beam and increase by factors of $$\sqrt{2}$$. The synthesized beam is represented by the grey ellipse in the bottom-left corner. Note that a faint source is detected at around 6 arcsec north of FRB 180916.J0158+65, but its separation is significant (>3σ confidence level) and we thus conclude that it is not associated with FRB 180916.J0158+65.

### Extended Data Fig. 3 Full field of view of the Gemini r′ filter.

The position of FRB 180916.J0158+65 is highlighted by the red cross. Note that the spiral galaxy associated with FRB 180916.J0158+65 is the only clearly visible galaxy in the field.

### Extended Data Fig. 4 Zoomed-in images at the position of FRB 180916.J0158+65.

Gemini data at r′ (a) and g′ bands (b). The position of FRB 180916.J0158+65 is highlighted by the white cross. The uncertainty in its position is smaller than the resolution of these images. The dashed lines represent the orientation and placement of the 1.5-arcsec spectroscopic slit used to obtain the optical spectra. Note that the slit does not cover the full star-forming region but the region centred on FRB 180916.J0158+65, and that the whole region is strongly affected by extinction (E(gr) = 1.73 ± 0.09).

### Extended Data Fig. 5 Host galaxy source of ionization.

Emission line flux ratios of [N ii]/Hα (left) and [S ii]/Hα (right) plotted against [O iii]/Hβ. The greyscale distribution represents the SDSS DR12 sample15 of 240,000 galaxies that display significant emission lines (>5σ), where the solid and dotted grey lines denote the demarcations between star-forming and AGN-dominated galaxies73,74,75. The host galaxies of FRB 121102 and FRB 180924 (shown by the green and red circles, respectively, where error bars show the 1σ uncertainty) are consistent with star-forming and AGN-dominated galaxies, respectively11,17. Though the Gemini-North spectrum of FRB 180916.J0158+65 does not cover the [O iii] and Hβ lines, its [N ii]/Hα and [S ii]/Hα line ratios are broadly consistent with a star-formation-dominated galaxy (represented by the vertical lines and the 1σ region as linewidth).

### Extended Data Fig. 6 Burst brightness and arrival times.

Burst S/N as a function of time during our 2019 June 19 observation of FRB 180916.J0158+65. The grey bars represent scans of the FRB 180916.J0158+65 field. The red crosses represent the four bursts (from left to right: B1, B2, B3, B4). The black dashed line indicates the detection threshold of our search in the pulsar-backend data (S/N = 7).

### Extended Data Fig. 7 Auto-correlation function and scintillation bandwidth of brightest burst, B4.

a, The ACF of the spectrum of the bright, narrow burst component of burst B4. b, The ACF for lags between −1.016 MHz and +1.016 MHz. The zero-lag noise spike has been removed from the ACF. A Lorentzian fit is shown in green in b. The black vertical dashed line represents the scintillation bandwidth, defined as the half width at half maximum of the Lorentzian fit.

### Extended Data Fig. 8 Redshift-cumulated probability of chance alignment coincidence.

Probability of a chance alignment between FRB 180916.J0158+65 and twice the median half-light radius of any galaxy with magnitude MB ≤ −16 (orange region) or with a dwarf galaxy like the host of FRB 121102 (blue region) as a function of redshift. The horizontal grey line represents the 1% probability threshold. At the redshift of the host galaxy, z = 0.0337 (vertical dashed red line), the chance coincidence probability is P 0.1%, and at the maximum possible redshift of about 0.11 derived from the observed DM the probability is <1% (vertical dashed black line).

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Marcote, B., Nimmo, K., Hessels, J.W.T. et al. A repeating fast radio burst source localized to a nearby spiral galaxy. Nature 577, 190–194 (2020). https://doi.org/10.1038/s41586-019-1866-z