Revisiting FRB 20121102A: milliarcsecond localisation and a decreasing dispersion measure

TL;DR

FRB 20121102A remains a cornerstone of FRB science due to its milliarcsecond localisation and association with a luminous PRS in a dwarf host. The study combines EVN VLBI astrometry with NRT burst monitoring to tighten the FRB–PRS co-location to within $\approx$12 pc and to track DM evolution over more than a decade, revealing a ~25 pc cm$^{-3}$ drop in the local DM over ~5 years. The bursts exhibit diverse properties across Effelsberg and NRT data, while VLBI imaging shows that the FRB and PRS are spatially coincident within a few milliarcseconds, reinforcing a physical connection between the engine and its surroundings. The evolving DM and RM, together with the compact PRS and potential binary or jet scenarios, point to a dynamic magneto-ionic environment and motivate continued high-frequency VLBI and long-term DM/RM monitoring to uncover the FRB's origin and its relation to the PRS.

Abstract

FRB 20121102A is the original repeating fast radio burst (FRB) source and also the first to be localised to milliarcsecond precision using very-long-baseline interferometry (VLBI). It has been active for over 13 years and resides in an extreme magneto-ionic environment in a dwarf host galaxy at a distance of ~1 Gpc. In this work, we use the European VLBI Network (EVN) to (re-)localise FRB 20121102A and its associated persistent radio source (PRS). We confirm that the two are co-located -- improving on previous results by a factor of ~4 and constraining the FRB and PRS co-location to ~12 pc transverse offset. Over a decade, the PRS luminosity on milliarcsecond scales remains consistent with measurements on larger angular scales, showing that the PRS is still compact. We also present the detection of 18 bursts with the Nancay Radio Telescope (NRT) as part of our ÉCLAT monitoring program. These bursts, together with previously published results, show that the observed dispersion measure (DM) of FRB 20121102A has dropped by ~25 pc/cc in the past five years, highlighting a fractional decrease in the local DM contribution of >15%. We discuss potential physical scenarios and highlight possible future observations that will help reveal the nature of FRB 20121102A, which is one of only a few known FRBs with a luminous PRS.

Figures (15)

• Figure 1: Temporal profiles (top sub-panels) and dynamic spectra (bottom sub-panels) of the bursts that were detected with the Effelsberg radio telescope. Every burst is coherently de-dispersed to a DM of $551.92$ pc cm$^{-3}$ and is shown with a frequency resolution of $500$ kHz. The time resolutions used for plotting are shown in the top-right corners. Horizontal white bands are frequency channels that are flagged because of RFI or because those are the subband edges where the telescope sensitivity is significantly lower. Horizontal dashed blue lines are the manually determined frequency ranges over which the dynamic spectra are averaged to create the timeseries. The manually determined vertical regions in the timeseries indicate the start and stop times of the bursts. For visual purposes the limits of the colour map have been set to the $2^{\mathrm{nd}}$ and $98^{\mathrm{th}}$ percentile of each dynamic spectrum.
• Figure 2: Dirty maps of individual bursts and the combined visibilities of multiple bursts, as indicated by the label in the top left corner of every panel. The white ellipse in the bottom left corners show the FWHM of the synthesised beams. The colour bar is in units of mJy/beam and the limits are $20$--$100$ % of the maximum value (see Appendix \ref{['app:burst_loc_images']} for a version without limits on the colour map). The $(0,0)$ point is $\alpha=05^{\mathrm{h}}31^{\mathrm{m}}58.7016^{\mathrm{s}}$, $\delta = +33\degr08\arcmin52.5483\arcsec$ (J2000, ICRF), which is the fitted position of the combination of five bursts (A$2$, B$2$, B$3$, B$4$, and B$5$; bottom right panel). Every panel is $80\times80$ mas and was made with a cell-size of $1$ mas and Briggs weighting with a robustness parameter of $0.5$ (see Appendix \ref{['app:burst_loc_images']} for a version of this figure that shows $800\times800$ mas panels). The cyan '+' is the C-band ($\sim$$4.5$ GHz) PRS position from Marcote_2017_ApJL and the cyan '$\times$' illustrates the phase reference centre.
• Figure 3: The 'best-fit' position of the bursts depends on which, and how many, bursts are used in the imaging process. The $(0,0)$ point is $\alpha=05^{\mathrm{h}}31^{\mathrm{m}}58.7016^{\mathrm{s}}$, $\delta = +33\degr08\arcmin52.5483\arcsec$ (J2000, ICRF), which is determined using bursts A2, B2, B3, B4, and B5. The green hexagon is the PRS position from this work. We show every possible combination with $3$ and $4$ bursts as yellow diamonds and pink squares. These are filled if our brightest burst, B3, is included in the imaging. Burst localisations using a single burst are illustrated with a black circle, whose sizes scale with the peak flux density of the individual burst. The solid cyan '+' is the Radio Fundamental Catalog (RFC)-updated C-band ($\sim$$4.5$ GHz) PRS position from Marcote_2017_ApJL. The transparent cyan '+' shows the C-band PRS position if updates to the RFC are not taken into account (Section \ref{['subsec:loc_results']}). Note that precise localisations with single bursts can only be done if the 'sidelobe ambiguity' is resolved. Also, note that the panel size ($\sim$$9\times9$ mas) is much smaller than the size of the synthesised beam ($\sim$$25\times25$ mas).
• Figure 4: Left: the dirty map of the combined visibilities of bursts A$2$, B$2$, B$3$, B$4$, and B$5$ (i.e., the bottom right panel of Figure \ref{['fig:bursts_dirty_zoom']}) and the FWHM of the synthesised beam (gold ellipse). Right: The dirty map of the PRS (background colours) and the FWHM of the synthesised beam (gold ellipse). Both: we plot the contour levels of the PRS, which start at $5\sigma$, where $\sigma = 11$$\upmu$Jy beam$^{-1}$, and increase in factors of $\sqrt{2}$. In both panels the colour bar is in units of mJy/beam, the $(0,0)$ point is $\alpha=05^{\mathrm{h}}31^{\mathrm{m}}58.7016^{\mathrm{s}}$, $\delta = +33\degr08\arcmin52.5483\arcsec$ (J2000, ICRF), the cyan '+' is the C-band ($\sim$$4.5$ GHz) PRS position from Marcote_2017_ApJL, and the green hexagon is the fitted position of the PRS from this work. Both panels were made with a cell-size of $1$ mas and Briggs weighting with a robustness parameter of $0.5$. See Appendix \ref{['app:burst_loc_images']} for a zoomed-out version of the PRS without limits on the colour map.
• Figure 5: Dispersion measure as a function of time. This figure shows the DM measurements from this work: Burst B$3$ (EF, Effelsberg telescope, red square), along with various bursts from the Nançay radio telescope (NRT, green diamonds). Other previously determined DM values are also shown and include bursts from the Arecibo Observatory spitler_2014_apjlspitler_2016_naturehessels_2019_apjl, Green Bank Telescope snelders_2023_natas, Aperture Tile in Focus oostrum_2020_aa, MeerKAT platts_2021_mnras, Deep Space Network majid_2020_apjl, and the Five-hundred-meter Aperture Spherical radio Telescope li_2021_naturewang_2025_arxivzhang_2025_atel. The inferred host $+$ local DM is the observed DM minus the expected DM contributions of the Milky Way and intergalactic medium (Section \ref{['sec:dm_evolution']}).