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

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).

💡 Research Summary

The authors present a comprehensive four‑year monitoring campaign of FRB 20190520B using the Five‑hundred‑meter Aperture Spherical radio Telescope (FAST). Unlike most repeating fast radio bursts, which exhibit intermittent activity, FRB 20190520B was detected in every FAST observing session, establishing it as the first persistently active FRB over a continuous multi‑year baseline. A total of 435 bursts were identified (including 75 previously reported events) with a time resolution of 49 µs and a frequency resolution of 0.122 MHz across the 1.0–1.5 GHz band. Burst candidates were extracted with the HEIMDALL pipeline, RFI mitigated via a zero‑DM filter, and only events with S/N > 7 were retained for further analysis.

To measure dispersion measures (DMs) with high fidelity, the authors employed the DM_PHASE software, which maximizes coherent signal power and preserves intrinsic temporal structure, making it robust against complex burst morphologies. An energy‑weighted daily average of the DM values was computed to reduce statistical scatter from faint bursts. The resulting DM series exhibits a clear linear decline at a rate of –12.4 ± 0.3 pc cm⁻³ yr⁻¹, a factor of three larger than the most extreme DM variations reported for other repeating FRBs (e.g., FRB 20121102A, FRB 20180301A) and orders of magnitude above the typical yearly DM fluctuations seen in pulsars (10⁻³–10⁻¹ pc cm⁻³ yr⁻¹). The authors fit a simple linear model to the full dataset, confirming the trend’s statistical significance.

A structure‑function (SF) analysis of the DM time series reveals two distinct regimes. On timescales longer than ~350 days the SF follows a power‑law with an index close to unity, reflecting the dominant linear trend. On shorter timescales (<100 days) the SF flattens, indicating that the residual variations are consistent with measurement noise and that no additional turbulent component is detectable. After subtracting the linear trend, the SF remains flat across all lags, reinforcing the interpretation that the large‑scale DM evolution is driven by a coherent physical process rather than stochastic ISM turbulence.

Scattering properties were also examined. The mean scattering timescale at 1 GHz is ≈10 ms, but individual bursts show a wide range (3.3–24 ms) and rapid variability on minute‑scale intervals, far exceeding the modest scattering variations expected from the Milky Way interstellar medium. The frequency dependence follows τ ∝ ν⁻⁴, consistent with a thin scattering screen near the source. Scintillation bandwidths measured for 25 bursts cluster around 1 MHz at 1.4 GHz, matching predictions for Galactic turbulence and supporting a two‑screen model: one screen in the Milky Way and a second, more variable screen associated with the local environment of the FRB. No significant correlation was found between scattering timescale and DM, suggesting that the two propagation effects arise from distinct plasma regions.

The host galaxy contribution to the DM is estimated to be 300–900 pc cm⁻³ after accounting for foreground contributions from two intersecting galaxy clusters. This large host DM, together with the monotonic decline, points to a dense, evolving ionized medium surrounding the source. The authors argue that an expanding supernova remnant (SNR) provides a natural explanation. In the free‑expansion phase of an SNR, the electron column density scales as DM ∝ t⁻², while in the Sedov‑Taylor phase it scales as DM ∝ t⁻³⁄⁵ (for partially ionized ejecta). Applying these scalings to the observed –12.4 pc cm⁻³ yr⁻¹ trend yields an inferred SNR age of roughly 10–100 years, implying that FRB 20190520B is powered by a young magnetar embedded within a freshly formed SNR.

In summary, this work establishes FRB 20190520B as the first continuously active repeating FRB, provides the most pronounced long‑term DM variation measured for any extragalactic radio transient, and presents compelling evidence that the source resides in a young, expanding supernova remnant. These findings place strong constraints on FRB progenitor models, favoring scenarios involving young magnetars in dense, rapidly evolving environments, and they highlight the diagnostic power of long‑baseline DM monitoring for probing the local conditions of FRB sources.