Constraining the origin of the long term periodicity of FRB 20180916B with Polarization Position Angle

TL;DR

This study uses polarization position angle (PA) measurements of bursts from FRB 20180916B, collected with the uGMRT at 650 MHz, to constrain the physical origin of its 16.34-day periodic activity. By calibrating PA across observations, estimating a single Rotation Measure (RM) per observing session, and transforming PA to infinite frequency, the authors quantify intra- and inter-cycle PA variability and compare them with predictions of dynamical progenitor models. They find tight short-term PA stability (ΔPA ≤ 7° over about 4 hours) but significant long-term modulation tied to activity phase and cycle, allowing them to rule out fast precession and to place nontrivial constraints on slow-rotation and slow-precession geometries with future tests. The work draws parallels to Her X-1 and emphasizes the need for stable PA calibration and long-baseline monitoring to robustly discriminate among dynamical FRB models with broad implications for FRB engine geometries and environments.

Abstract

FRB 20180916B is a repeating Fast Radio Burst (FRB) which produces bursts in a 5.1 day active window which repeats with a 16.34 day period. Models have been proposed to explain the periodicity using dynamical phenomena such as rotation, precession or orbital motion. Polarization Position Angle (PA) of the bursts can be used to distinguish and constraint the origin of the long term periodicity of the FRB. We aim to study the PA variability on short (within an observation) and long timescales (from observation to observation). We aim to compare the observed PA variability with the predictions of various dynamical progenitor models for the FRB. We use the calibrated burst dataset detected by uGMRT in Band 4 (650 MHz) which have been published in arXiv:2409.12584 . We transform the PA measured at 650 MHz to infinite frequency such that PAs measured in different observations are consistent, and finally measure the changes within and across active windows. We find that PA of the bursts vary according to the periodicity of the source. We constrain the PA variability to be within seven degrees on timescales less than four hours for all MJDs. In addition, we also tentatively note the PA measured at the same phase in the active window varies from one cycle to another. Using the findings, we constrain rotational, precession and binary progenitor models. Rotational model partially agrees with observed PA variability but requires further study to fully constrain. We robustly rule out all flavors of precessional models where either precession explains the periodicity of the FRB or the variability from one cycle to another. Lastly, we draw similarities between FRB 20180916B and a X-ray binary system, Her X 1, and explicitly note that both the sources exhibit a similar form of PA variability.

Figures (18)

• Figure 1: Applying one RM to all the bursts detected during the observation of MJD 59243 and measuring standard deviation of all the PA measurements from every timebin of every burst. Repeating this procedure for multiple RMs yields the black dots. The blue line is the fitted parabola. The vertical black line is the center of the fitted parabola. The red dashed vertical line is the RM measured after fitting using all the bursts. See Table \ref{['tab:ppdata']} and Sect. \ref{['ssec:mrm']}.
• Figure 2: PA$_\infty$ values of each burst plotted one after the another in individual subplots for every MJD. Each PA$_\infty$ has been rotated by $\langle \mathrm{PA}_\infty \rangle$ taken from Table \ref{['tab:ppdata']}. The different colors indicate different bursts. The shaded region around it is $\Delta$PA, as reported in Tab. \ref{['tab:ppdata']}. The top-left text in each subplot is the corresponding MJD and the number of bursts in parenthesis.
• Figure 3: $\langle \mathrm{PA}_\infty \rangle$ versus MJD. The points are color coded according to their corresponding Activity Cycle.
• Figure 4: $\langle \mathrm{PA}_\infty \rangle$ versus Activity Phase of FRB 20180916B plotted with markers with black border. PA$_\infty$ versus Activity Phase of the bursts plotted with faint markers. Both PA$_\infty$ and $\langle \mathrm{PA}_\infty \rangle$ have been rotated by 45 deg.
• Figure 5: $\langle \mathrm{PA}_\infty \rangle$ versus Activity Cycle for the two phase clusters - $\mathbf{G}_1$ and $\mathbf{G}_2$. $\mathbf{G}_1$ refers to the $\langle \mathrm{PA}_\infty \rangle$ measurements made at around Phase 0.359 (MJD 59894, 60303 and 60352) and $\mathbf{G}_2$ at around Phase 0.400 (MJD 59274, 59568, 59944, 59993). See Sect. \ref{['ssec:long']}. The black markers are used to dote $\langle \mathrm{PA}_\infty \rangle$ measurements with one burst. The top x-axis is in MJD since the reference MJD of the periodicity model of FRB 20180916B.