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The dispersive sweep of fast radio bursts (FRBs) has been used to probe the ionized baryon content of the intergalactic medium1, which is assumed to dominate the total extragalactic dispersion. Although the host-galaxy contributions to the dispersion measure appear to be small for most FRBs2, in at least one case there is evidence for an extreme magneto-ionic local environment3,4 and a compact persistent radio source5. Here we report the detection and localization of the repeating FRB 20190520B, which is co-located with a compact, persistent radio source and associated with a dwarf host galaxy of high specific-star-formation rate at a redshift of 0.241 ± 0.001. The estimated host-galaxy dispersion measure of approximately $${903}_{-111}^{+72}$$ 903 − 111 + 72 parsecs per cubic centimetre, which is nearly an order of magnitude higher than the average of FRB host galaxies2,6, far exceeds the dispersion-measure contribution of the intergalactic medium. Caution is thus warranted in inferring redshifts for FRBs without accurate host-galaxy identifications.

Fast radio bursts (FRBs) are millisecond-timescale radio transients, the origins of which are predominantly extragalactic and likely involve highly magnetized compact objects. FRBs undergo multipath propagation, or scattering, from electron density fluctuations on sub-parsec scales in ionized gas along the line-of-sight. Scattering observations have located plasma structures within FRB host galaxies, probed Galactic and extragalactic turbulence, and constrained FRB redshifts. Scattering also inhibits FRB detection and biases the observed FRB population. We report the detection of scattering times from the repeating FRB 20190520B that vary by up to a factor of two or more on minutes to days-long timescales. In one notable case, the scattering time varied from 7.9 ± 0.4 ms to less than 3.1 ms ($95{{\ \rm per\ cent}}$ confidence) over 2.9 minutes at 1.45 GHz. The scattering times appear to be uncorrelated between bursts or with dispersion and rotation measure variations. Scattering variations are attributable to dynamic, inhomogeneous plasma in the circumsource medium, and analogous variations have been observed from the Crab pulsar. Under such circumstances, the frequency dependence of scattering can deviate from the typical power-law used to measure scattering. Similar variations may therefore be detectable from other FRBs, even those with inconspicuous scattering, providing a unique probe of small-scale processes within FRB environments.

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In the version of this article initially published, an equal-contributor footnote was missing for authors H. Xu, J. R. Niu and P. Chen. The Author contributions section has been amended to read "H.X., J.R.N. and P.C. contributed equally and led the data analysis". The changes have been made to the HTML and PDF versions of the article.

The central engine of fast radio bursts (FRB) is not yet understood. Due to the interaction between the pulse and the intervening plasma, the dispersion sweep of FRBs provides a unique probe of its environment and the ionized baryon content of the intergalactic medium. Active repeaters has been shown to be associated with persistent radio source (PRS), and dense, energetic, magnetized plasmas. Here we report the discovery and localization of a new, extremely active repeater, FRB 190520, which is co-located with a compact PRS and identified with a dwarf host galaxy of high star formation at a redshift z=0.241. The estimated host galaxy contribution DMhost≈912+69−108pc cm−3 is nearly an order of magnitude higher than the average of FRB host galaxies and much larger than the contribution from the intergalactic medium, suggesting caution in inferring redshifts for FRBs without accurate host galaxy identifications. This represents the second source after FRB 121102 with confirmed association between FRB and compact PRS. The dense, complex host galaxy environment and the association with PRS may point to a distinctive origin or an earlier evolutionary stage for highly active repeating FRBs.