A Persistently Active Fast Radio Burst source Embedded in an Expanding Supernova Remnant

TL;DR

This work presents the first long-term study of a persistently active FRB, FRB 20190520B, using ~4 years of FAST observations to measure dispersion-measure evolution with high precision. The authors document a substantial DM decline of $d\rm DM/dt = -12.4\pm0.3$ pc cm$^{-3}$ yr$^{-1}$ and show that the DM evolution, supported by a large host DM and a power-law relation $\rm DM \propto t^{-\alpha}$, is naturally explained if the source resides in an expanding supernova remnant surrounding a young magnetar. A two-screen propagation model, with near-source scattering and Milky Way scintillation, along with a detailed structure-function analysis of DM, reinforces the interpretation of a dense, evolving local environment rather than turbulence alone. These results place the SNR age in the ~10–100 year range and constrain ejecta parameters, offering strong evidence for the magnetar-in-SNR origin of at least this class of FRBs and highlighting long-term DM monitoring as a powerful probe of FRB environments.

Abstract

Fast radio bursts (FRBs) remain one of the most puzzling astrophysical phenomena. While most FRBs are detected only once or sporadically, we present the identification of FRB 20190520B as the first persistently active source over a continuous span of ~ four years. This rare long-term activity enabled a detailed investigation of its dispersion measure (DM) evolution. We also report that FRB 20190520B exhibits a substantial decrease in DM at a global rate of minus 12.4 plus or minus 0.3 pc cm^-3 yr^-1, exceeding previous FRB DM variation measurements by a factor of three and surpassing those observed in pulsars by orders of magnitude. The magnitude and consistency of the DM evolution, along with a high host DM contribution, strongly indicate that the source resides in a dense, expanding ionized medium, likely a young supernova remnant (SNR).

Figures (5)

• Figure 1: The evolution of the propagation effect for FRB 20190520B. The top 3 panels (a), (b) and (c) depict the evolution of DM, scattering timescale, and scintillation bandwidth. The horizontal dashed lines and shaded areas in these panels denote the mean values and 3$\sigma$ intervals of the measured data. The black dashed vertical line at 59111.37098 marks the boundary, with data before this point coming from previous studies. The red diamonds in panel (a) represent 72-day averaged DM values, and the green hexagram denote the DM values for individual burst, with marker size scaled by burst energy. Triangles, squares, and hexagons in panel (b) represent $\tau_{\rm{s}}$ measured in 72-day averaged on the top (1300–1500 MHz), middle (1150–1300 MHz), and bottom (1000–1150 MHz) bandpass, respectively. The blue vertical solid line in panel (c) indicates the scintillation bandwidth resulting from the Milky Way’s DM contribution. Panel (d) shows how the event rate varies with date revealing this source's long-term activity. The vertical cyan regions represent the total observation windows.
• Figure 2: The structure function of DMs for FRB 20190520B. The structure function $D_{\rm{DM}}(\tau)$ is derived from the DM values of all the 435 detected bursts. The brown diamonds ($D_{\rm DM}$) represent the structure function calculated from the measured DM values. At large time lags, the SF exhibits a steep rise that follows the purple dashed line, corresponding to a power-law fit with an index of $2.08 \pm 0.19$, consistent with a dominant linear trend. The pink hexagons ($D_{\rm \Delta DM}$) show the SF of the de-trended data, obtained after subtracting the best-fit linear model. This de-trended SF remains flat, indicating no significant turbulence signal above the noise level. The shaded regions denote $1\sigma$ uncertainties.
• Figure 3: The waterfall plots corresponds to yearly duration. The top panel displays the burst profile, while the bottom panel shows the corresponding waterfall plot. The left plots presents data from the 95th burst observed on 2020-12-25, whereas the right panel depicts data from the 430th burst observed on 2023-02-02. In the top panel, the black line represents the profile dedispersed with a DM of 1200.9 pc cm$^{-3}$ , and the blue line corresponds to the profile dedispersed with a DM of 1170.8 pc cm$^{-3}$ , the measured value for the 430th burst. The right waterfall figure combines dedispersed data with DMs of 1200.9 and 1170.8 pc cm$^{-3}$ , highlighting the over-dedispersion effect at 1200.9 pc cm$^{-3}$ .
• Figure 4: The distribution of central frequencies and bandwidth for FRB 20190520B. The central frequencies of most bursts from FRB 20190520B are concentrated in the higher end of FAST's frequency band, suggesting that the intrinsic central frequencies of these events may lie at relatively higher frequencies. The bandwidths of individual bursts also exhibit significant diversity.
• Figure 5: The scattering timescale, $\tau_s$, was obtained by fitting the stacked profile spectrum of bursts from FRB 20190520B. Following the method proposed by Ocker2022Ocker2023, we analyzed burst profiles in the Fourier domain across three sub-bands and performed fits to derive the scattering timescales, $\tau_s$.