Constraints on the engines of fast radio bursts

Abstract

We model the sample of fast radio bursts (FRB), including the newly discovered CHIME repeaters, using the decelerating synchrotron blast wave model of Metzger, Margalit & Sironi (2019), which built on earlier work by Lyubarsky (2014), Beloborodov (2017). This model postulates that FRBs are precursor radiation from ultra-relativistic magnetized shocks generated as flare ejecta from a central engine collides with an effectively stationary external medium. Downward drifting of the burst frequency structure naturally arises from the deceleration of the blast-wave coupled with the dependence of the maser spectral energy distribution, and induced Compton scattering depth, on the upstream medium. The data are consistent with FRBs being produced by flares of energy E flare ∼ 10 43 − 10 46 ( f ξ /10 −3 ) −4/5 erg, where f ξ is the maser efficiency, and minimum bulk Lorentz factors Γ ≈ 10 2 − 10 3 , which generate the observed FRBs at shock radii r sh ∼ 10 12 − 10 13 cm. We infer upstream densities n ext (r sh ) ∼ 10 2 − 10 4 cm −3 and radial profiles n ext ∝ r −k showing a range of slopes k ≈ [−2, 1] (which are seen to evolve between bursts), both broadly consistent with the upstream medium being the inner edge of an ion-loaded shell released by a recent energetic flare. The burst timescales, energetics, rates, and external medium properties are consistent with repeating FRBs arising from young, hyper-active flaring magnetars, but the methodology presented is generally applicable to any central engine which injects energy impulsively into a dense magnetized medium. Several uncertainties and variations of the model regarding the composition and magnetization of the upstream medium, and the effects of the strong electric field of the FRB wave (strength parameter a 1) on the upstream medium and its scattering properties, are discussed. One-dimensional particle-in-cell simulations of magnetized shocks into a pair plasma are presented which demonstrate that high maser efficiency can be preserved, even in the limit a 1 in which the FRB wave accelerates the upstream electrons to ultra-relativistic speeds.

What does FRB light-curve variability tell us about the emission mechanism?

“…Indeed, some FRB sub-pulses exhibit downward frequency drifts (Gajjar et al 2018;Hessels et al 2019;CHIME/FRB Collaboration et al 2019a), in which the peak frequency of the spectrum decreases over time. Within the framework of the baryonic shock model for FRBs, these drifts can be directly related to the external density profile (Margalit et al 2020b). That being said, the recently discovered FRB 200428, exhibits a significant lack of emission at frequencies below ∼ 550 MHz for the second pulse (The CHIME/FRB Collaboration et al 2020), and a lack of frequency down-drift, which are not expected according to the shock model.…”

Implications of a Fast Radio Burst from a Galactic Magnetar

“…The fact that the second radio burst detected by CHIME appears to have a rising spectrum is also in tension with this model (since n~10 MHz pk , as discussed above), although scintillation may affect the observed spectrum (Simard & Ravi 2020). (Metzger et al 2019;Margalit et al 2020) In this version of the synchrotron maser model, first proposed by Beloborodov (2017), the ultrarelativistic head of the magnetar flare collides not with the magnetar wind nebula but rather with matter ejected from a recent, earlier flare. Motivated by the inference from the radio afterglow of the 2004 giant flare from SGR 1806-20 of a slow ejecta shell generated by the burst (Gelfand et al 2005;Granot et al 2006), Metzger et al (2019) considered the upstream medium to be a subrelativistic baryon-loaded shell with an electron-ion composition.…”

“…x -f 10 10 2 3 (for moderate magnetization; Plotnikov & Sironi 2019) and a further reduction by a factor of~-10 2 due to the effects of induced Compton scattering suppressing the low-frequency portion of the maser's intrinsic SED (see Metzger et al 2019, their Section 3.2). 18 Following the methodology of Margalit et al (2020), we can use the energy, frequency, and duration of the observed radio burst to derive the intrinsic parameters of the flare demanded by the synchrotron maser model. Adopting the observed quantities from Section 2, we find that the energy of the relativistic flare, E ;…”

“…Extending the same model to the shock properties derived for the observed populations of cosmological FRBs predicts that the afterglow emission for these more energetic bursts will occur at much higher energies,  E peak MeV-GeV, in the gamma-ray band (Figure 8). Unfortunately, gamma-ray satellites like Swift and Fermi are generally not sensitive enough to detect this emission to the cosmological distances of most FRB sources (Metzger et al 2019;Chen et al 2020;Margalit et al 2020). We furthermore emphasize that this predicted short (∼milliseconds duration) gamma-ray signal from the shocks is distinct from the longer-lasting and typically softer gamma-ray emission observed from giant Galactic magnetar flares (e.g., Hurley et al 2005;Palmer et al 2005), which is instead well explained as a pair fireball generated by dissipation very close to the NS surface.…”

The Multiwavelength Counterparts of Fast Radio Bursts

“…5.1.1. Metzger et al (2019) ModelSynchrotron masers have been widely discussed as an astrophysical coherent emission process (e.g.,Hoshino & Arons 1991;Long & Pe'er 2018), and one common variation is coherent emission from synchrotron masers produced by ultrarelativistic shock in magnetized plasmas (e.g.,Langdon et al 1988;Lyubarsky 2014;Beloborodov 2017Beloborodov , 2019Margalit et al 2020;Metzger et al 2019).Metzger et al …”