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Register a new userHigh time resolution and polarisation properties of ASKAP-localised fast radio bursts
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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.
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Fast spinning (e.g., sub-second) neutron star with ultra-strong magnetic fields (or so-called magnetar) is one of the promising origins of repeating fast radio bursts (FRBs). Here we discuss circularly polarised emissions produced by propagation effects in the magnetosphere of fast spinning magnetars. We argue that the polarisation-limiting region is well beyond the light cylinder, suggesting that wave mode coupling effects are unlikely to produce strong circular polarisation for fast spinning magnetars. Cyclotron absorption could be significant if the secondary plasma density is high. However, high degrees of circular polarisation can only be produced with large asymmetries in electrons and positrons. We draw attention to the non-detection of circular polarisation in current observations of known repeating FRBs. We suggest that the circular polarisation of FRBs could provide key information on their origins and help distinguish different radiation mechanisms.
The pulse morphology of fast radio bursts (FRBs) provides key information in both understanding progenitor physics and the plasma medium through which the burst propagates. We present a study of the profiles of 33 bright FRBs detected by the Australian Square Kilometre Array Pathfinder. We identify seven FRBs with measureable intrinsic pulse widths, including two FRBs that have been seen to repeat. In our modest sample we see no evidence for bimodality in the pulse width distribution. We also identify five FRBs with evidence of millisecond timescale pulse broadening caused by scattering in inhomogeneous plasma. We find no evidence for a relationship between pulse broadening and extragalactic dispersion measure. The scattering could be either caused by extreme turbulence in the host galaxy or chance propagation through foreground galaxies. With future high time resolution observations and detailed study of host galaxy properties we may be able to probe line-of-sight turbulence on gigaparsec scales.
In this paper we develop a model for fast radio bursts (FRBs) based on triggered superradiance (SR) and apply it to previously published data of FRB 110220 and FRB 121102. We show how a young pulsar located at ~100 pc or more from an SR/FRB system could initiate the onset of a powerful burst of radiation detectable over cosmological distances. Our models using the OH$^2Pi_{3/2}$ $left(J=3/2right)$ 1612 MHz and $^2Pi_{3/2}$ $left(J=5/2right)$ 6030 MHz spectral lines match the light curves well and suggest the entanglement of more than $10^{30}$ initially inverted molecules over lengths of approximately 300 au for a single SR sample. SR also accounts for the observed temporal narrowing of FRB pulses with increasing frequency for FRB 121102, and predicts a scaling of the FRB spectral bandwidth with the frequency of observation, which we found to be consistent with the existing data.
We present results of a search for late-time radio emission and Fast Radio Bursts (FRBs) from a sample of type-I superluminous supernovae (SLSNe-I). We used the Karl G. Jansky Very Large Array to observe ten SLSN-I more than 5 years old at a frequency of 3 GHz. We searched fast-sampled visibilities for FRBs and used the same data to perform a deep imaging search for late-time radio emission expected in models of magnetar-powered supernovae. No FRBs were found. One SLSN-I, PTF10hgi, is detected in deep imaging, corresponding to a luminosity of $1.2times10^{28}$ erg s$^{-1}$. This luminosity, considered with the recent 6 GHz detection of PTF10hgi in Eftekhari et al (2019), supports the interpretation that it is powered by a young, fast-spinning ($sim$ ms spin period) magnetar with $sim$ 15 Msun of partially ionized ejecta. Broadly, our observations are most consistent with SLSNe-I being powered by neutron stars with fast spin periods, although most require more free-free absorption than is inferred for PTF10hgi. We predict that radio observations at higher frequencies or in the near future will detect these systems and begin constraining properties of the young pulsars and their birth environments.
We detail a new fast radio burst (FRB) survey with the Molonglo Radio Telescope, in which six FRBs were detected between June 2017 and December 2018. By using a real-time FRB detection system, we captured raw voltages for five of the six events, which allowed for coherent dedispersion and very high time resolution (10.24 $mu$s) studies of the bursts. Five of the FRBs show temporal broadening consistent with interstellar and/or intergalactic scattering, with scattering timescales ranging from 0.16 to 29.1 ms. One burst, FRB181017, shows remarkable temporal structure, with 3 peaks each separated by 1 ms. We searched for phase-coherence between the leading and trailing peaks and found none, ruling out lensing scenarios. Based on this survey, we calculate an all-sky rate at 843 MHz of $98^{+59}_{-39}$ events sky$^{-1}$ day$^{-1}$ to a fluence limit of 8 Jy-ms: a factor of 7 below the rates estimated from the Parkes and ASKAP telescopes at 1.4 GHz assuming the ASKAP-derived spectral index $alpha=-1.6$ ($F_{ u}propto u^{alpha}$). Our results suggest that FRB spectra may turn over below 1 GHz. Optical, radio and X-ray followup has been made for most of the reported bursts, with no associated transients found. No repeat bursts were found in the survey.
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