An extreme magneto-ionic environment associated with the fast radio burst source FRB 121102 (1801.03965v1)

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.

• The paper establishes that FRB 121102 is embedded in a dynamic magneto-ionic environment through detailed Faraday rotation measurements.
• Using data from Arecibo and Green Bank Telescopes, the study reports RM values over 10^5 rad m⁻² with nearly 100% linear polarization and about 10% temporal variability.
• The findings support models involving neutron stars or magnetars near compact sources, thereby refining our understanding of FRB progenitors and their environments.

An Analysis of Magneto-Ionic Environments in Fast Radio Bursts

The paper of fast radio bursts (FRBs), and specifically the observations detailed in this paper, represents a significant move toward understanding these enigmatic cosmological phenomena. The authors focus on an FRB source, denoted in prior literature as a repeating entity, localized to a star-forming region within a dwarf galaxy, strongly linked to a persistent radio source. This paper documents particular observational results, presenting substantial evidence that the FRB source resides within a highly dynamic magneto-ionic environment.

The researchers achieved significant results by using data from bursts detected with Arecibo and Green Bank Telescopes, noting their high Faraday rotation measures (RMs). Specifically, RM values of +1.46 and +1.33 × 10^5 rad m^-2 have been reported over two different epochs. This observation indicates an extraordinary magnetized environment, which is notable given that such magnitudes have historically been associated almost exclusively with regions proximate to massive black holes exceeding 10^4 solar masses.

Notably, the bursts demonstrated approximately 100% linear polarization with the analysis showing a high degree of RM variability, approximately 10% over several months. The characterized Faraday rotation could only plausibly originate from within the host galaxy, given the minimal potential contribution from the Milky Way or the intergalactic medium. Comprehensive analyses suggested that the region inducing such high RMs might be compact and possess a dense magnetic field circa milligauss.

The paper scrutinizes various models to rationalize the inherent physical environment of FRBs. Among these, scenarios featuring neutron stars, particularly magnetars, within such magnetized regions, were highlighted. It also entertains the possibility that these bursts could be emerging from a neutron star located near an accreting black hole, contributing to the persistent radio emissions observed. On the other hand, alternatives such as a wind nebula enveloping a magnetar or a nearby supernova remnant are considered feasible under the current model framework.

This research posits significant implications for the field. The extreme conditions observed suggest that intrinsic variability in the originating environment of the FRB contributes massively to the detected polarized emissions — insights which challenge broader arguments about the progenitors of FRBs, as well as the interactions between extreme magnetic environments and neutron stars. If validated further, this new understanding could fundamentally enhance gravitational wave and electromagnetic studies.

In sum, the findings add substantial depth to the ongoing discourse surrounding FRBs, advocating for a multitude of astrophysical models. While the unique magneto-ionic characteristics of the examined FRB source provide valuable insights, they prompt further detailed observational campaigns and theoretical modeling. Continued assessments of RM and polarization with time via technological advancements in radio observatories will be instrumental in exploring the potential correlation between FRB repetition rates and RM values. The results reported herein represent a foundational step in understanding the conditions under which these astrophysical transients occur and foster a more nuanced appreciation of the diverse environments these profound cosmic signals traverse.