High time resolution and polarization properties of ASKAP-localized fast radio bursts

Abstract

Combining high time and frequency resolution full-polarisation spectra of Fast Radio Bursts (FRBs) with knowledge of their host galaxy properties provides an opportunity to study both the emission mechanism generating them and the impact of their propagation through their local environment, host galaxy, and the intergalactic medium. The Australian Square Kilometre Array Pathfinder (ASKAP) telescope has provided the first ensemble of bursts with this information. In this paper, we present the high time and spectral resolution, full polarisation observations of five localised FRBs to complement the results published for the previously studied ASKAP FRB 181112. We find that every FRB is highly polarised, with polarisation fractions ranging from 80 – 100%, and that they are generally dominated by linear polarisation. While some FRBs in our sample exhibit properties associated with an emerging archetype (i.e., repeating or apparently non-repeating), others exhibit characteristic features of both, implying the existence of a continuum of FRB properties. When examined at high time resolution, we find that all FRBs in our sample have evidence for multiple sub-components and for scattering at a level greater than expected from the Milky Way. We find no correlation between the diverse range of FRB properties (e.g., scattering time, intrinsic width, and rotation measure) and any global property of their host galaxy. The most heavily scattered bursts reside in the outskirts of their host galaxies, suggesting that the source-local environment rather than the host interstellar medium is likely the dominant origin of the scattering in our sample.

Scintillation Can Explain the Spectral Structure of the Bright Radio Burst from SGR 1935+2154

“…FRB 190611 shows time-varying polarization properties as well as an apparent change in dispersion measure between the two components (Day et al 2020). The apparent variations in dispersion measure, Faraday rotation measure, and polarization properties between the components of some FRBs (e.g., Cho et al 2020;Day et al 2020) may be interpreted as emission observed along different sightlines through a dense, magnetized, trans-relativistic plasma (Vedantham & Ravi 2019), possibly with several radial magnetic-field reversals (Gruzinov & Levin 2019). This scenario is consistent with our picture of significant spatial separations between emission sites for multiple-component FRBs.…”

“…However, FRB 190611, a two-component burst detected by ASKAP and tentatively localized to a host galaxy at z=0.378 (Macquart et al 2020), shows qualitative similarities with the picture described here (Day et al 2020). The two components separated by ∼1 ms show evidence of scintillation from the Milky Way ISM (consistent between the two components) in addition to an overall envelope that has a central frequency 48 MHz higher for the latter component (Day et al 2020). Estimating Δν=100 MHz at 1250 MHz, assuming that the screen, as in the case of SGR 1935+2154, is 16 pc from the source, 4 and using the Planck 2015 cosmological parameters (Planck Collaboration et al 2016) to calculate the angular diameter distance to the source, we find l d =3.2×10 13 km and V src,app >160c, implying a highly relativistic emission region, with ΓV src,app /c or a separation between two emission regions of 4.7×10 7 m. Such high Lorentz factors are predicted by Beloborodov (2020) and Margalit et al (2020b) for extragalactic FRBs.…”

“…Some FRBs show no evidence for scintillation (which may be masked by the instrumental resolution) (e.g., Ravi 2019) and others show scintillation that is constant across multiple components (e.g., Farah et al 2018Farah et al , 2019. However, FRB 190611, a two-component burst detected by ASKAP and tentatively localized to a host galaxy at z=0.378 (Macquart et al 2020), shows qualitative similarities with the picture described here (Day et al 2020). The two components separated by ∼1 ms show evidence of scintillation from the Milky Way ISM (consistent between the two components) in addition to an overall envelope that has a central frequency 48 MHz higher for the latter component (Day et al 2020).…”

“…Two-screen scattering models have recently gained traction in the study of propagation effects for extragalactic FRBs (Masui et al 2015;Ravi et al 2016;Farah et al 2018;CHIME/FRB Collaboration et al 2019b;Macquart et al 2019;Day et al 2020); typically these are motivated by scattering inconsistent with expectations from the Milky Way or by incongruous temporal scatter-broadening timescales and scintillation bandwidths. In this letter, "scintillation" refers specifically to spectral modulations caused by the coherent combination of radiation propagating along different paths, and "scattering" refers to the general occurrence of multi-path propagation, which may manifest as scintillation or temporal broadening of pulses, among several phenomena (Rickett 1990).…”

Host Galaxy Properties and Offset Distributions of Fast Radio Bursts: Implications for Their Progenitors

“…On UT 2019 July 11 at 01:53:41.1, the ASKAP telescope recorded FRB 190711 as reported by Macquart et al (2020), who also provided a brief description of its host galaxy. The FRB position is at α, δ=21 h 57 m 40 68, −80 d 21 m 28 8 (J2000), with an uncertainty of σ α,δ =0 4, 0 3 (Day et al 2020). This FRB has subsequently been found to repeat (Kumar et al 2020).…”