What powers the persistent radio sources around some repeating FRBs?
In plain words
A few repeating FRB sources lie inside a compact, steady radio glow and very strongly magnetized gas, which could be a nebula inflated by a young magnetar or the outflow of an accreting black hole.
Precise statement
FRB 20121102A and FRB 20190520B are co-located with compact persistent radio sources (spectral luminosity $\sim 1e29\,\mathrm{erg}\,\mathrm{s}^{-1}\,\mathrm{Hz}^{-1}$, size below $\sim 1\,\mathrm{pc}$) and show rotation measures up to $\sim 1e5\,\mathrm{rad}\,\mathrm{m}^{-2}$ that change by tens of percent over years. Determine whether these sources are magnetar wind nebulae, nebulae inflated by hyper-accreting compact binaries, or accreting intermediate-mass or massive black holes, and test the predicted relation between persistent-source luminosity and $\mid \mathrm{RM} \mid$. Answer: an identification whose predicted decline of persistent flux and RM over decades matches monitoring.
What would settle it
Decade-long monitoring of persistent-source flux, size and RM for several repeaters, compared with the evolution predicted by each model.
Status in the literature
Unverified note
2024: a persistent radio source was reported for FRB 20201124A, a third repeater with this feature.