Ultra-Wideband Polarimetry of the April 2021 Profile Change Event in PSR J1713+0747

TL;DR

This study presents a comprehensive wideband spectro-polarimetric analysis of the April 2021 profile change in PSR J1713+0747 using Parkes UWL data spanning 704–4032 MHz. By constructing sub-band templates, applying RM-corrected polarimetry, and employing PCA to quantify long-term recovery, the authors demonstrate broad, frequency-dependent changes in Stokes I and linear polarisation, with distinct, persistent OPM rearrangements and a stable leading-edge PA. The results strongly favor a magnetospheric origin for the event over interstellar propagation, and reveal a complex evolution that increasingly challenges conventional timing models for pulsar timing arrays. These findings have direct implications for improving timing accuracy in PTA datasets and for planning rapid, wideband follow-up observations with next-generation radio telescopes.

Abstract

The millisecond pulsar PSR J1713+0747 is a high-priority target for pulsar timing array experiments due to its long-term timing stability, and bright, narrow pulse profile. In April 2021, PSR~J1713$+$0747 underwent a significant profile change event, observed by several telescopes worldwide. Using the broad-bandwidth and polarimetric fidelity of the Ultra-Wideband Low-frequency receiver on Murriyang, CSIRO's Parkes radio telescope, we investigated the long-term spectro-polarimetric behaviour of this profile change in detail. We highlight the broad-bandwidth nature of the event, which exhibits frequency dependence that is inconsistent with cold-plasma propagation effects. We also find that spectral and temporal variations are stronger in one of the orthogonal polarisation modes than the other, and observe mild variations ($\sim 3$ - $5\,σ$ significance) in circular polarisation above 1400 MHz following the event. However, the linear polarisation position angle remained remarkably stable in the profile leading edge throughout the event. With over three years of data post-event, we find that the profile has not yet recovered back to its original state, indicating a long-term asymptotic recovery, or a potential reconfiguration of the pulsar's magnetic field. These findings favour a magnetospheric origin of the profile change event over a line-of-sight propagation effect in the interstellar medium.

Figures (11)

• Figure 1: Stokes $I$, PA, Linear polarisation and Stokes $V$ profiles colour-mapped as a function of frequency per row. The first column represents the template profiles, the second column represents MJD 59368 (47 days post-event) and the last column represents MJD 60049 (roughly two years post-event). Annotated are five profile components: (A) the leading peak, (B) the profile shoulder, (C) the main pulse peak, (D) the descending gradient, and (E) the trailing peak. The four OPM sub-components are also annotated. In addition to the normalisation by the integrated Stokes $I$ flux density, we additionally normalise the Stokes $I$, $L$, and Stokes $V$ profiles for each epoch in this plot by the maximum of normalised Stokes $I$ flux densities across all sub-bands, scaling the normalised intensities to a maximum of 1.
• Figure 2: Stokes $I$, $L$ and Stokes $V$ profile residuals as a function of time, where pulse phase at each epoch is predicted from the timing ephemeris. The top row shows Stokes $I$, the middle row shows linearly polarised intensity ("$L$"), and the bottom row shows Stokes $V$. The red dashed line indicates the time of the profile change event. The colour bar indicates the intensity of the profile residual. As can be seen, Stokes $I$ and $L$ were affected, whereas Stokes $V$ was minimally affected in the lower sub-bands A--E, but shows variations in the higher-frequency sub-bands sbF -- sbH. Additionally, the profile has not yet returned to its pre-event state, indicating either a long recovery timescale or potential reconfiguration of the profile.
• Figure 3: As in Figure \ref{['fig:waterfalls_Eph']}, but for the case where the profiles have been aligned to the template.
• Figure 4: Results of PCA analysis showing the first principal component scores in Stokes $I$(red) and linear polarisation (blue) as a function of time. The top subpanel of each plot shows the fit to the PC score data, and the bottom subpanel shows the fit residuals. A solid line indicates that the power-law model is preferred, where a dashed line indicates the exponential model. The orange dashed line indicates the profile change event, and the surrounding shaded grey region indicates the period of no observations. The $\chi^{2}_{\mathrm{r}}$ for both models are also shown in each sub-plot.
• Figure 5: Difference in BIC scores indicating model preference for either the exponential (Exp) or power law (PL) fit to the PC scores for the first principle component of the Stokes $I$ and linear polarisation residuals. A negative $\Delta\mathrm{BIC}$ value indicates that the exponential model is favoured, whereas a positive value indicates the power-law model is favoured. The full dataset encompasses both pre and post-event epochs. The tail dataset is a restricted subset that commences at MJD 59800 where the fitted curve appears to flatten. In the Stokes $I$tail dataset, the power-law model is dominant, whereas support for either model is mixed for linear polarisation. Here, $\Delta\mathrm{BIC}=\mathrm{BIC}_{\mathrm{Exp}}- \mathrm{BIC}_{\mathrm{PL}}$. For clarity, we indicate $\Delta \mathrm{BIC}$ for each data sub-set on each cell.