An extreme magneto-ionic environment associated with the fast radio burst source FRB 121102

TL;DR

This paper reports the discovery of extremely high and variable Faraday rotation measures in FRB 121102, indicating a highly magnetized, dynamic environment likely involving a neutron star near a massive black hole or within a nebula.

Contribution

It presents the first observation of highly variable, large Faraday rotation measures in a repeating FRB, revealing an extreme magneto-ionic environment around FRB 121102.

Findings

High and variable Faraday rotation measures demonstrate a dynamic magneto-ionic environment.

Short burst durations support a neutron star origin.

Properties suggest proximity to a massive black hole or a magnetized nebula.

Abstract

Fast radio bursts (FRBs) are millisecond-duration, extragalactic radio flashes of unknown physical origin. FRB 121102, the only known repeating FRB source, has been localized to a star-forming region in a dwarf galaxy at redshift z = 0.193, and is spatially coincident with a compact, persistent radio source. The origin of the bursts, the nature of the persistent source, and the properties of the local environment are still debated. Here we present bursts that show ~100% linearly polarized emission at a very high and variable Faraday rotation measure in the source frame: RM_src = +1.46 x 10^5 rad m^-2 and +1.33 x 10^5 rad m^-2 at epochs separated by 7 months, in addition to narrow (< 30 mus) temporal structure. The large and variable rotation measure demonstrates that FRB 121102 is in an extreme and dynamic magneto-ionic environment, while the short burst durations argue for a neutron star origin. Such large rotation measures have, until now, only been observed in the vicinities of massive black holes (M_BH > 10^4 MSun). Indeed, the properties of the persistent radio source are compatible with those of a low-luminosity, accreting massive black hole. The bursts may thus come from a neutron star in such an environment. However, the observed properties may also be explainable in other models, such as a highly magnetized wind nebula or supernova remnant surrounding a young neutron star.