A Large-scale Magneto-ionic Fluctuation in the Local Environment of Periodic Fast Radio Burst Source FRB 20180916B

Abstract

Fast radio burst (FRB) source FRB 20180916B exhibits a 16.33-day periodicity in its burst activity. It is as of yet unclear what proposed mechanism produces the activity, but polarization information is a key diagnostic. Here we report on the polarization properties of 44 bursts from FRB 20180916B detected between 2018 December and 2021 December by CHIME/FRB, the FRB project on the Canadian Hydrogen Intensity Mapping Experiment. In contrast to previous observations, we find significant variations in the Faraday rotation measure (RM) of FRB 20180916B. Over the 9-month period 2021 April and 2021 December we observe an apparent secular increase in RM of ∼50 rad m−2 (a fractional change of over 40%) that is accompanied by a possible drift of the emitting band to lower frequencies. This interval displays very little variation in the dispersion measure (ΔDM ≲ 0.8 pc cm−3), which indicates that the observed RM evolution is likely produced from coherent changes in the Faraday-active medium’s magnetic field. Burst-to-burst RM variations appear unrelated to the activity cycle phase. The degree of linear polarization of our burst sample (≳80%) is consistent with the negligible depolarization expected for this source in the 400–800 MHz bandpass of CHIME. FRB 20180916B joins other repeating FRBs in displaying substantial RM evolution. This is consistent with the notion that repeater progenitors may be associated with young stellar populations by their preferential occupation of dynamic magnetized environments commonly found in supernova remnants, in pulsar wind nebulae, or near high-mass stellar companions.

A CHIME/FRB Study of Burst Rate and Morphological Evolution of the Periodically Repeating FRB 20180916B

“…Only the bursts from the baseband sample were used in this analysis, as their spectral and temporal properties are better constrained. Mckinven et al (2023) reported a systematic linear increase in RM from this source (see Figure 6, third panel). However, interestingly, we do not find corresponding changes in any other source parameter.…”

“…But the CHIME/FRB transit duration is ∼15 minutes; extended exposure with a sensitive telescope might provide more insights into whether FRB 20180916B is prone to burst storms. In addition, the RM change observed in the source (Mckinven et al 2023) does not coincide with any change in the activity of the source, suggesting that the secular rise observed likely has more to do with evolution in the propagation environment rather than variations in the intrinsic nature of the source.…”

“…The RM of FRB 20180916B is 4 orders of magnitude less than that of FRB 20121102A and was fairly stable until 2021 April. Mckinven et al (2023) reported a linear increase in the RM of FRB 20180916B in their recent CHIME/FRB polarimetric data analysis. No significant DM changes have been observed for the source so far.…”

“…We also performed a structure-function analysis similar to the one presented by Mckinven et al (2023) for scattering, in order to search for preferred timescales for variation. We find no measurable trend at any timescale, suggesting that the variations are random and occur due to fluctuations in the propagation environment.…”

“…Here we present a morphology analysis of 45 bursts from FRB 20180916B detected by the CHIME/FRB baseband system from its first detection in 2018 September 16 to 2021 December 31. We use the same baseband burst sample published by Mckinven et al (2023), where they report the polarization properties. In addition, we present burst properties for 15 bursts that have been detected only with intensity data since the last published results for the source .…”