A steadily declining dispersion measure for the repeating fast radio burst FRB 20220529A: Evidence for an FRB engine embedded in an expanding supernova remnant

Abstract

We present the discovery and subsequent 3.2 year monitoring campaign of the repeating fast radio burst FRB 20220529A with CHIME/FRB. We observe a gradual dispersion measure (DM) decline of $-0.881\pm0.001~\mathrm{pc}~\mathrm{cm}^{-3}~\mathrm{year}^{-1}$ ($-1.235\pm0.001~\mathrm{pc}~\mathrm{cm}^{-3}~\mathrm{year}^{-1}$ in the rest frame), implying a $\geq3.5\pm0.2$% decrease of the total electron column in the source environment, and we see scattering timescale variations over weeks to years. We observe a short-lived excursion in which the DM rises by $\sim 1~\mathrm{pc}~\mathrm{cm}^{-3}$, immediately preceding a transient $\sim 2000~\mathrm{rad}~\mathrm{m}^{-2}$ Faraday rotation measure (RM) increase previously reported for this source, before returning to its gradual DM decline. We identify a local line-of-sight magnetic field around FRB 20220529A during this DM/RM excursion of $3.4 \pm 0.2~\mathrm{mG}$, corresponding to one of the most strongly magnetized FRB environments. We measure a decrease in the linear polarization fraction of FRB 20220529A bursts with decreasing frequency that we attribute to depolarization from multi-path propagation in the source environment. We also place a $5σ$ upper limit on the spectral luminosity of an associated persistent radio source of $\leq 5\times10^{28}~\mathrm{erg}~\mathrm{s}^{-1}~\mathrm{Hz}^{-1}$ at 1.5 GHz. These observations are consistent with FRB 20220529A originating from a young ($\sim$ years to centuries old) expanding supernova remnant, with short-lived DM and RM variability arising from interactions with the supernova remnant or with a binary companion.

Figures (6)

• Figure 1: Total intensity dynamic spectra, dedispersed to their best-fit DMs, for the 16 FRB 20220529A bursts with baseband data detected by CHIME/FRB. All dynamic spectra have a frequency resolution of $390.625$ kHz and have been downsampled to a common time resolution of $163.84~\mu\mathrm{s}$ to ensure sufficient S/N in fainter bursts. Frequency-averaged total intensity (black) and linearly polarized intensity (red) profiles are plotted above the dynamic spectra. Individual burst components are highlighted by shaded regions (in chronological order for multiple components: red, blue, purple, yellow, and green) overlaid on the intensity profiles. Channels masked out due to radio frequency interference are highlighted in red on the left of the dynamic spectra.
• Figure 2: Temporal evolution of the DM, $\tau$, RM, and $L/I$ in FRB 20220529A as seen by CHIME/FRB (squares). Uncertainties in these parameters are smaller than the markers. The "RM excursion" 2026Sci...391..280L is shaded in gray. In the top panel, the best-fit linear DM decline is plotted as a dashed line and the slope of the rise in DM preceding the RM excursion is depicted with a dotted line. The DM of a burst detected by the RT-1 Westerbork radio telescope is plotted as an open purple circle. In the second and the bottom panels, upper limits on $\tau$ and $L/I$, respectively, are plotted as downward arrows. To show how the CHIME/FRB RMs fit into the broader RM evolution of FRB 20220529A, we overplot the FAST RM measurements by 2026Sci...391..280L as blue open circles in the third panel; the RM excursion peak falls outside the y-axis range in this panel.
• Figure 3: Linear polarization fraction versus central frequency of FRB 20220529A bursts reported by CHIME/FRB and FAST. Polarized bursts observed by CHIME/FRB are plotted as gold squares and $L/I$ upper limits are shown as downward arrows. The large quantity of FAST observations are depicted as a gray density map with increasing opacity at discrete contour levels of 5%, 33%, 67%, and 95%, for visual clarity. Bursts during the RM excursion have been removed from the FAST data. The best-fit model is overplotted as a black dashed line and the fit parameters are presented in the bottom right of the main panel. The binned $L/I$ data on which the depolarization model was fit are plotted as vertical black bars centered on the mean binned values and spanning their standard deviations. Upper limits on $L/I$ are not used in the depolarization model fitting. Smoothed kernel density estimates of the central frequency and $L/I$ distributions for polarized bursts observed by each instrument are plotted in gold and gray, respectively, above and to the right of the main panel. Gold and gray dotted lines in these panels represent the medians of the corresponding distributions. Median $L/I$ for polarized CHIME/FRB repeaters 2025ApJ...982..154N and non-repeaters 2024ApJ...968...50P are overplotted as blue and red dotted lines, respectively, in the right-most panel.
• Figure 4: FRB 20220529A sky position as measured by the CHIME/FRB Outriggers in this work (white), corresponding to the combined localization of bursts 12 and 16, compared to the position reported by the VLA (green; 2026Sci...391..280L). Solid and dashed ellipses represent the $1\sigma$ and $3\sigma$ localization uncertainties, respectively. The localization ellipses are overplotted on an archival Dark Energy Camera Legacy Survey $gri$-filter image of the field. The faint host galaxy identified by 2026Sci...391..280L can be seen consistent with both localization ellipses.
• Figure 5: (Left) L-band dynamic spectrum of the FRB 20220529A burst detected by Westerbork. A frequency-averaged burst profile is presented in the top panel. (Right) Peak S/N of the burst at a range of DM trials with the peak DM represented by a red vertical line and the $1\sigma$ uncertainty region around the peak, where the S/N drops by 1, is highlighted in orange.