A Concordance Picture of FRB 121102 as a Flaring Magnetar Embedded in a Magnetized Ion–Electron Wind Nebula

Abstract

The fast radio burst FRB 121102 has repeated multiple times, enabling the identification of its host galaxy and of a spatially-coincident, compact, steady ('persistent') radio synchrotron source. It was proposed that FRB 121102 is powered by a young flaring magnetar, embedded within a decadesold supernova remnant. Using a time-dependent one-zone model, we show that a single expanding magnetized electron-ion nebula (created by the same outbursts likely responsible for the FRBs) can explain all the basic properties of the persistent source (size, flux, self-absorption constraints) and the large but decreasing rotation measure (RM) of the bursts. The persistent emission is powered by relativistic thermal electrons heated at the termination shock of the magnetar wind, while the RM originates from non-relativistic electrons injected earlier in the nebula's evolution and cooled through expansion and radiative losses. The model contains few free parameters, which are tightly constrained by observations: the total energy injected into the nebula over its history, ∼ 10 50 −10 51 erg, agrees with the magnetic energy of a millisecond magnetar; the baryon loading of the magnetar outflow (driven by intermittent flares) is close to the neutron star escape speed; the predicted source age ∼ 10 − 40 years is consistent with other constraints on the nebula size. For an energy input rateĖ ∝ t −α following the onset of magnetar activity, we predict secular decay of the RM and persistent source flux, which approximately follow RM ∝ t −(6+α)/2 and F ν ∝ t −(α 2 +7α−2)/4 , respectively.

Limits on absorption from a 332-MHz survey for fast radio bursts

“…Values of physical characteristics of the absorbing medium. For the UC HII and Ionized SNR scenario, we use the upper limit on the linear size as reported in Sokolowski et al (2018) and for the SLSNe, we use the upper limit on the linear size of the absorber from Margalit & Metzger (2018). The nominal values for T e and EM are taken from Churchwell (2002) (1) veys combined with detections of FRBs in the 400-800 MHz band might suggest that absorption of radio emission at low frequency could explain the observations.…”

“…If we consider various examples of absorbers that are found in our Galaxy, we can use the reported upper limit on the linear size to constrain the most plausible circum-burst medium that would be applicable to FRBs. To do that, we considered three cases; 1) Ultra-Compact (UC) HII regions where electron densities can be of the order of 10 4 cm −3 (Churchwell 2002); 2) dense ionized filaments that are found in Supernova Remnants (SNR) (Koo et al 2007); and 3) Post-shock region of Super-Luminous Supernovae (SLSNe) where the electron temperature can reach 10 6−7 K with electron densities of 10 5−6 cm −3 (Margalit & Metzger 2018). Using the expected values of electron temperature in these regions, we constrain the electron density of the absorber for lower limits of A reported in Table 1 for different γs.…”

“…The obtained values of electron density for the absorber are larger than the densest filaments that have been observed in Type-II SNRs like SNR G11.2−0.3 (Koo et al 2007) and the Crab Nebula (Sankrit et al 1998). Margalit & Metzger (2018) have proposed a model for FRBs where the pulse originates from a millisecond magnetar that has formed in a SLSN. Immediately following the supernova explosion, extremely high densities and temperatures are reached in the post-shock region of the outflowing ejecta that can cause significant absorption of radio emission from the magnetar over a timescale of a few hundred years at which point, the ejecta have expanded enough to become transparent to lowfrequency emission (τ 1).…”

Emission from a Pulsar Wind Nebula: Application to the Persistent Radio Counterpart of FRB 121102

“…Our model with an electron-positron component pulsar wind is more general than the model that assumes that pulsar wind contains a large number of ions (Margalit & Metzger 2018). In addition, our model is very simple.…”

A repeating fast radio burst source localized to a nearby spiral galaxy

“…The absence of a similarly bright radio nebula associated with FRB 180916.J0158+65 could be explained, in this model, as an older system whose emission has al-ready faded. Assuming a similar model 22 Gemini data at r (a) and g bands (b). The position of FRB 180916.J0158+65 is highlighted by the white cross.…”