2025A&A...694A..75B

Context. Fast Radio Burst 20180916B is a repeating FRB whose activity window has a 16.34-day periodicity that also shifts and varies in duration with the observing frequency. Recent observations report that the FRB has started to show an increasing trend in secular Rotation Measure (RM) after only showing stochastic variability around a constant value of -114.6 rad m^–2 since its discovery. RM studies let us directly probe the magnetic field structure in the local environment of the FRB. The trend of the variability can be used to constrain progenitor models of the FRB. Hence, further study of the RM variability forms the basis of this work.
Aims. We studied the local environment of FRB 20180916B. We did so by focusing on polarization properties, namely RM, and studied how it varies with time. The data comes from the ongoing campaigns of FRB 20180916B using the upgraded Giant Metrewave Radio Telescope (uGMRT). The majority of the observations are in Band 4, which is centered at 650 MHz with 200 MHz bandwidth. Additionally, we used a few observations where we had simultaneous coverage in Band 4 and Band 5 (centered at 1100 MHz).
Methods. We applied a standard single-pulse search pipeline to search for bursts. In total, we detected 116 bursts with ∼36 hours of on-source time spanning 1200 days from December 2020 to February 2024, with two bursts detected during simultaneous frequency coverage observations. We developed and applied a polarization calibration strategy suited for our dataset. On the calibrated bursts, we used QU-fitting to measure RM. We verified the veracity of calibration solution and RM measurement by performing RM measurements on single pulses of PSR J0139+5814. We also measured various other properties such as rate, linear polarization fraction, and fluence distribution.
Results. Of the 116 detected bursts, we could calibrate 79 of them. We observed in our early observations that the RM continued to follow a secular linear trend, as already seen in past observations. However, our later observations suggest that the source switched from the linear trend to stochastic variations around a constant value of -58.75 rad m^–2. It has ceased any secular variability and is only showing stochastic variability. Using the predicted Milky Way RM contribution, we report a tentative detection of a sign flip in the RM in the host galaxy host-frame. We also studied a cumulative rate against fluence and note that the rate at higher fluences (1.2 Jy ms) scales as γ = -1.09(7), whereas that at lower fluences (between 0.2 and 1.2 Jy ms) only scales as γ = -0.51(1), meaning the rate at the higher fluence regime is steeper than at the lower fluence regime. Finally, we qualitatively assess the two extremely large bandwidth bursts that we detected in our simultaneous multi-band observations.
Conclusions. Future measurements of RM variations would help place stronger constraints on the local environment. Moreover, any periodic behavior in the RM measurements would directly test progenitor models. Therefore, we motivate such endeavors.