A sudden dramatic change and recovery of magneto-environment of a repeating fast radio burst

Technical Summary: A Sudden Dramatic Change and Recovery of Magneto-Environment of a Repeating Fast Radio Burst

Problem and Context
Fast Radio Bursts (FRBs) are millisecond-duration, extragalactic radio bursts of unknown origin, though associations with magnetars (highly magnetized neutron stars) have been established. While some repeating FRBs exhibit mild variations in their magneto-ionic environments—often attributed to plasma turbulence or binary configurations—no "smoking-gun" signature has definitively confirmed the existence of a companion star to the FRB engine. The Rotation Measure (RM), which quantifies the Faraday rotation of polarized light by magnetized plasma along the line of sight, serves as a critical probe of these local environments. Previous studies have shown diverse RM behaviors, including long-term decays (e.g., FRB 20121102A) and sign reversals (e.g., FRB 20190520B), yet a sudden, dramatic, and rapid recovery of RM had not been previously reported.

Methodology
The study focuses on FRB 20220529, a repeating FRB discovered by CHIME. The authors conducted a comprehensive monitoring campaign using the Five-hundred-meter Aperture Spherical radio Telescope (FAST) and the Parkes telescope's Ultra-Wideband Low (UWL) receiver.

• Observations: Between June 2022 and September 2024, the team performed 112 FAST observations (53.9 hours) and 59 Parkes observations (132.0 hours), detecting a total of 1,156 bursts from FAST and 56 from Parkes.
• Localization and Host Identification: Using the Karl G. Jansky Very Large Array (VLA) with the realfast system, the source was localized to a positional uncertainty of 0.3 arcseconds. Optical spectroscopy of the host galaxy candidate was conducted using the Gran Telescopio Canarias (GTC) to determine redshift and physical properties.
• Data Analysis: Polarization properties, including the Rotation Measure (RM) and linear polarization fraction ($f_L$), were derived for individual bursts. The authors calculated structure functions to characterize turbulence and employed Bayesian inference (MCMC techniques) to fit physical models (magnetar outflows, turbulence in supernova remnants/pulsar wind nebulae, binary orbital motion, and Coronal Mass Ejections) to the observed RM evolution.

Key Results

1. Host Galaxy Properties: FRB 20220529 is localized to a disk galaxy at a redshift of $z = 0.1839 \pm 0.0001$. The host has a stellar mass of $(2.7 \pm 0.7) \times 10^9 M_\odot$ and a star formation rate of $0.13 M_\odot \text{yr}^{-1}$. The FRB offset is $4.4 \pm 0.6$ kpc from the host center, consistent with the birth sites of young magnetars.
2. Long-term Activity: The source exhibited an unusually long active episode (over 2.2 years of monitoring), with peak burst rates reaching 204 bursts per hour, distinguishing it from other repeaters like FRB 20121102A and FRB 20201124A.
3. The "RM Flare":
   • Baseline State: Prior to December 2023, the RM fluctuated with a median of $17 \text{ rad m}^{-2}$ and a scatter of $101 \text{ rad m}^{-2}$, consistent with a turbulent environment.
   • The Event: In December 2023, the RM underwent a sudden, dramatic increase, jumping from $-139 \pm 19 \text{ rad m}^{-2}$ (last recorded in October 2023) to a peak of $1977 \pm 84 \text{ rad m}^{-2}$ on December 14, 2023.
   • Recovery: Within approximately 14 days (by December 28, 2023), the RM returned monotonically to the baseline range of $\pm 300 \text{ rad m}^{-2}$.
   • Significance: This variation represents a signal-to-noise ratio ($SNR_{RM}$) of 20.4 relative to the pre-flare scatter, significantly higher than RM variations observed in other active repeaters.
4. Correlated Polarization Drop: During the RM flare, the linear polarization fraction dropped from a typical $\sim 90\%$ to $27 \pm 3\%$, recovering as the RM returned to baseline.
5. Model Constraints:
   • Magnetar Outflows: Ruled out because the observed RM evolution (negative second derivative, $d^2RM/dt^2 < 0$) contradicts the power-law decay expected from expanding magnetar ejecta.
   • Turbulence (SNR/PWN): Unlikely, as the structure function including the flare deviates significantly from the baseline turbulence, and the required clump size and extreme physical conditions are improbable.
   • Binary Orbital Motion: Possible but requires an extremely high orbital eccentricity ($e > 0.95$) and a long period ($P_{orb} > 1.5$ years). Statistical analysis suggests such configurations are rare ($\sim 0.6\%$) among neutron star binaries.
   • Coronal Mass Ejection (CME): The most plausible scenario. The data fits a model where a magnetized plasma clump (CME) from a companion star enters and exits the line of sight. The best-fit parameters suggest a CME mass of $\sim 10^{17}$ g from an M dwarf or $\sim 10^{21}$ g from a giant star/mass-transfer binary. The observed DM variation is also consistent with this model.

Significance and Claims
The paper claims that the observation of FRB 20220529 provides the first report of an abrupt, substantial variance in the Faraday rotation measure over a timescale of weeks, followed by a rapid recovery. The authors assert that this "RM flare" is unprecedented in its magnitude and speed compared to known repeating FRBs.

The primary significance of this work lies in the strong implication that FRB 20220529 resides in a binary system. The CME scenario, which best explains the data, necessitates a companion star capable of ejecting massive, magnetized plasma. While the authors acknowledge that binary orbital motion is a possible alternative, they emphasize that the CME hypothesis offers a more natural explanation for the specific temporal evolution and magnitude of the event. Consequently, this observation serves as a potential indirect signature of a companion star to an FRB engine, a long-sought piece of evidence in FRB astrophysics. The study suggests that similar abrupt RM variations may occur in other repeating FRBs, though their detection rate is likely low due to geometric constraints (line-of-sight alignment).