FAST Polarization Catalog of FRB 20240114A

Abstract

Polarization measurements of fast radio bursts (FRBs) probe the magnetized plasma surrounding their central engines. FRB~20240114A is an exceptionally active repeating source, with 17,356 bursts detected between 2024 January 28 and 2025 May 30 by FAST, enabling time-resolved polarimetric studies. In this work, we present a polarimetric catalog of 6,131 bright bursts (with a signal-to-noise ratio S/N $\geq$ 20, 35.3% of the total sample), including arrival time (MJD$_{\text{topo}}$), dispersion measure (DM), burst width (W$_{\text{eff}}$), bandwidth, Faraday rotation measure (RM), linear and circular polarization degrees (DOL, DOC), and intrinsic polarization angle (PA$_0$). We detect a clear temporal evolution of RM: after an initial stable phase, it decreases linearly by $\sim$200 $\rm rad\ m^{-2}$ over 200 days, forming a bimodal distribution, whereas DM remains stable at 528.9 $\rm pc\ cm^{-3}$. The linear polarization fraction is generally high, with the 3$σ$ lower bound around 76%, while circular polarization is low, with 1,157 of 17,356 bursts (6.67%) having DOC $\geq$10%. We perform a power-law fit between $|\textrm{V}|$/I and $|\textrm{RM}|$, which yields an index of $-2.98 \pm 0.80$. It is found that the combined 2D distribution of L/I versus V/I remains stable, implying that the emission mechanism is largely invariant. Our PA$_0$ measurements show a broad, non-uniform distribution, implying a complex emission geometry. These results suggest that FRB~20240114A resides in a dynamically evolving magneto-ionic environment. This catalog provides a foundation for studies of repeating FRB progenitors and their environments.

Figures (10)

• Figure 1: Distributions of key parameters for the 6,131 bursts from FRB 20240114A. (a) DM. The distribution is narrow and peaks near 528.9 $\rm pc\ cm^{-3}$, indicating a stable integrated electron density along the line of sight. (b) RM. The distribution shows a complex, non-Gaussian shape with a primary peak near 362.2 $\rm rad\ m^{-2}$ and a secondary peak near 229.8 $\rm rad\ m^{-2}$, reflecting the temporal evolution of the local magnetic field. (c) Linear (L/I, green hist and dashed line) and circular (V/I, purple hist and solid line) polarization fractions. A significant population exhibits high linear polarization ($>$ 76%), while circular polarization is generally low. (d) Intrinsic polarization position angle (PA$_0$), corrected for Faraday rotation. The distribution is centered around 10$^\circ$ and broad with a noticeable, non-Gaussian excess, indicating a lack of a single, stable global magnetic field orientation.
• Figure 2: Temporal evolution of burst properties. Panels (a) to (e) share a common horizontal axis (Topocentric MJD). (a) RM as a function of time. The blue contour is the 2D kernel density estimation (KDE) of the RM. The result shows a distinct two-phase evolution: an initial period of relative stability followed by a rapid, near-linear decay of approximately 200 $\rm rad\ m^{-2}$ over $\sim$ 200 days. (b) DM over time. The blue contour is the 2D KDE of the DM. The DM remains stable around 529.3 $\rm pc\ cm^{-3}$ throughout the campaign, with no correlated long-term trend with the RM evolution. (c) Total polarization fraction (P/I) over time. (d) Linear polarization fraction (L/I) over time. (e) Circular polarization fraction (V/I) over time. Panel (f) (right) shows the relationship between the absolute circular polarization fraction ($|\textrm{V}|$/I) and the absolute value of the RM ($|\textrm{RM}|$). The red solid line is the fitting for 5 repeating FRBs including FRB 20240114A, while the blue dashed line is the fitting for 4 repeating FRBs not including FRB20240114A. Plotted in gray are the results of the allowable values from the Markov Chain Monte Carlo (MCMC) run.
• Figure 3: Temporal evolution of linear and circular polarization fraction distributions. (Top) Histogram of the number of bursts with signal-to-noise ratio $\geq$ 20 per observing session. (Middle) Energy Distance (a statistical measure of similarity between distributions) calculated between the polarization fraction distributions of cumulative bursts in consecutive observing sessions with different time-scale cumulative windows. Some peaks in energy distance correspond to sessions with a sudden, large change in the number of data points (bursts) rather than a genuine shift in the underlying distribution shape. (Bottom) Temporal evolution of the polarization fraction distributions visualized using a semicircle diagram. Each colored semicircle represents one observing session, positioned horizontally according to its MJD. Within each semicircle, the horizontal axis corresponds to the circular polarization fraction (L/I) and the vertical axis to the linear polarization fraction (V/I). All bursts from that session are plotted as points in the same color within its semicircle. This visualization confirms that the core shapes of the L/I vs. V/I distributions remain remarkably stable over time, despite the changing RM.
• Figure 4: Rotation measure plotted versus dispersion measure and polarization position angle. (Left) RM vs. DM, with points color-coded by topocentric MJD. Two clusters are visible: a larger one at higher RM (earlier epochs) and a smaller one at lower RM (later epochs), separated primarily along the RM axis. (Right) RM vs. intrinsic polarization position angle (PA$_0$), similarly color-coded. While also separable into two groups chronologically, the distribution in PA$_0$ is broader for a given RM, resulting in two vertical columns rather than compact clusters. This indicates that while the RM (probing the line-of-sight magnetic field) changed systematically, the intrinsic PA (related to the emitting region magnetic geometry) retained a wide range of orientations throughout the evolution.
• Figure 5: Relationships involving polarization fractions and signal-to-noise ratio (S/N). (a) Linear polarization fraction (L/I) vs. S/N. (b) Circular polarization fraction (V/I) vs. S/N. In both (a) and (b), measurement scatter increases at low S/N as expected, confirming robust calibration at high S/N. (c) Relationship between linear (L/I) and circular (V/I) polarization fractions. Points are color-coded by S/N, and number-density contours (gray lines) illustrate the distribution of the large dataset. The solid red semi-circle represents the theoretical physical limit for 100% polarized emission. The majority of points, particularly those in high-density regions, cluster near the high-linear-polarization end of this limit. Points lying outside the semi-circle are predominantly those with low S/N, consistent with noise pushing measurements beyond the physical boundary and validating our error estimates.