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Register a new userSynthesizing the intrinsic FRB population using frbpoppy
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Fast Radio Bursts (FRBs) are radio transients of an unknown origin. Naturally, we are curious as to their nature. Enough FRBs have been detected for a statistical approach to parts of this challenge to be feasible. To understand the crucial link between detected FRBs and the underlying FRB source classes we perform FRB population synthesis, to determine how the underlying population behaves. The Python package we developed for this synthesis, frbpoppy, is open source and freely available. Our goal is to determine the current best fit FRB population model. Our secondary aim is to provide an easy-to-use tool for simulating and understanding FRB detections. It can compare surveys, or inform us of the intrinsic FRB population. frbpoppy simulates intrinsic FRB populations and the surveys that find them, to produce virtual observed populations. These resulting populations can then be compared with real data, allowing constrains to be placed on underlying physics and selection effects. We are able to replicate real Parkes and ASKAP FRB surveys, in terms of both detection rates and distributions observed. We also show the effect of beam patterns on the observed dispersion measure (DM) distributions. We compare four types of source models. The Complex model, featuring a range of luminosities, pulse widths and spectral indices, reproduces current detections best. Using frbpoppy, an open-source FRB population synthesis package, we explain current FRB detections and offer a first glimpse of what the true population must be.
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The observed Fast Radio Burst (FRB) population can be divided into one-off and repeating FRB sources. Either this division is a true dichotomy of the underlying sources, or selection effects and low activity prohibit us from observing repeat pulses from all constituents making up the FRB source population. We attempt to break this degeneracy through FRB population synthesis. With that aim we extend frbpoppy, which earlier only handled one-off FRBs, to also simulate repeaters. We next model the Canadian Hydrogen Intensity Mapping Experiment FRB survey (CHIME/FRB). Using this implementation, we investigate the impact of luminosity functions on the observed dispersion measure (DM) and distance distributions of both repeating and one-off FRBs. We show that for a single, intrinsically repeating source population with a steep luminosity function, selection effects should shape the DM distributions of one-off and repeating FRB sources differently. This difference is not yet observed. We next show how the repeater fraction over time can help in determining the repetition rate of an intrinsic source population. We simulate this fraction for CHIME/FRB, and show that a source population comprised solely of repeating FRBs can describe CHIME/FRB observations with the use of a flat luminosity function. From the outcome of these two methods we thus conclude that all FRBs originate from a single and mostly uniform population of varying repeaters. Within this population, the luminosity function cannot be steep, and there must be minor differences in physical or behaviour parameters that correlate with repeat rate.
Fast Radio Bursts (FRBs) are energetic, short, bright transients that occur frequently over the entire radio sky. The observational challenges following from their fleeting, generally one-off nature have prevented identification of the underlying sources producing the bursts. As the population of detected FRBs grows, the observed distributions of brightness, pulse width and dispersion measure now begin to take shape. Meaningful direct interpretation of these distributions is, however, made impossible by the selection effects that telescope and search pipelines invariably imprint on each FRB survey. Here we show that multi-dimensional FRB population synthesis can find a single, self-consistent population of FRB sources that can reproduce the real-life results of the major ongoing FRB surveys. This means that individual observed distributions can now be combined to derive the properties of the intrinsic FRB source population. The characteristics of our best-fit model for one-off FRBs agree with a population of magnetars. We extrapolate this model and predict the number of FRBs future surveys will find. For surveys that have commenced, the method we present here can already determine the composition of the FRB source class, and potentially even its subpopulations.
We present results of the coordinated observing campaign that made the first subarcsecond localization of a Fast Radio Burst, FRB 121102. During this campaign, we made the first simultaneous detection of an FRB burst by multiple telescopes: the VLA at 3 GHz and the Arecibo Observatory at 1.4 GHz. Of the nine bursts detected by the Very Large Array at 3 GHz, four had simultaneous observing coverage at other observatories. We use multi-observatory constraints and modeling of bursts seen only at 3 GHz to confirm earlier results showing that burst spectra are not well modeled by a power law. We find that burst spectra are characterized by a ~500 MHz envelope and apparent radio energy as high as $10^{40}$ erg. We measure significant changes in the apparent dispersion between bursts that can be attributed to frequency-dependent profiles or some other intrinsic burst structure that adds a systematic error to the estimate of DM by up to 1%. We use FRB 121102 as a prototype of the FRB class to estimate a volumetric birth rate of FRB sources $R_{FRB} approx 5x10^{-5}/N_r$ Mpc$^{-3}$ yr$^{-1}$, where $N_r$ is the number of bursts per source over its lifetime. This rate is broadly consistent with models of FRBs from young pulsars or magnetars born in superluminous supernovae or long gamma-ray bursts, if the typical FRB repeats on the order of thousands of times during its lifetime.
We examine the spectra of 23 fast radio bursts detected in a flys-eye survey with the Australian SKA Pathfinder, including those of three bursts not previously reported. The mean spectral index of $alpha = -1.6_{-0.2}^{+0.3}$ ($F_ u propto u^alpha$) is close to that of the Galactic pulsar population. The sample is dominated by bursts exhibiting a large degree of spectral modulation: 17 exhibit fine-scale spectral modulation with an rms exceeding 50% of the mean, with decorrelation bandwidths (half-maximum) ranging from $approx$ to 49 MHz. Most decorrelation bandwidths are an order of magnitude lower than the $gtrsim 30,$MHz expected of Galactic interstellar scintillation at the Galactic latitude of the survey, $|b| = 50 pm 5 deg$. A test of the amplitude distribution of the spectral fluctuations reveals only 12 bursts consistent at better than a 5% confidence level with the prediction of 100%-modulated diffractive scintillation. Moreover, five of six FRBs with a signal-to-noise ratio exceeding 18 are consistent with this prediction at less than 1% confidence. Nonetheless, there is weak evidence (88-95% confidence) that the amplitude of the fine-scale spectral modulation is anti-correlated with dispersion measure (DM) that would suggest it originates as a propagation effect. This effect appears to be corroborated by the smoothness of the higher-DM Parkes FRBs, and could arise due to quenching of diffractive scintillation (e.g. in the interstellar medium of the host galaxy) by angular broadening in the intergalactic medium.
We consider a sample of $82$ non-repeating FRBs detected at Parkes, ASKAP, CHIME and UTMOST each of which operates over a different frequency range and has a different detection criteria. Using simulations, we perform a maximum likelihood analysis to determine the FRB population model which best fits this data. Our analysis shows that models where the pulse scatter broadening increases moderately with redshift ($z$) are preferred over those where this increases very sharply or where scattering is absent. Further, models where the comoving event rate density is constant over $z$ are preferred over those where it follows the cosmological star formation rate. Two models for the host dispersion measure ($DM_{rm host}$) distribution (a fixed and a random $DM_{rm host}$) are found to predict comparable results. We obtain the best fit parameter values $alpha=-1.53^{+0.29}_{-0.19}$, $overline{E}_{33}=1.55^{+0.26}_{-0.22}$ and $gamma=0.77pm 0.24$. Here $alpha$ is the spectral index, $gamma$ is the exponent of the Schechter luminosity function and $overline{E}_{33}$ is the mean FRB energy in units of $10^{33} , {rm J}$ across $2128 - 2848; {rm MHz}$ in the FRB rest frame.
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