Population Of Fast Radio Bursts Research Articles

A cumulative search for hard X/γ-ray emission associated with fast radio bursts in Fermi/GBM data

Context. Fast radio bursts (FRBs) are millisecond-long bursts uniquely detected at radio frequencies. FRB 131104 is the only case for which a γ-ray transient positionally and temporally consistent was claimed. This high-energy transient had a duration of ∼400 s and a 15–150 keV fluence Sγ ∼ 4 × 10−6 erg cm−2. However, the association with the FRB is still debated. Aims. We aim at testing the systematic presence of an associated transient high-energy counterpart throughout a sample of the FRB population. Methods. We used an approach like that used in machine learning methodologies to accurately model the highly-variable Fermi/GBM instrumental background on a time interval comparable to the duration of the proposed γ-ray counterpart of FRB 131104. A possible γ-ray signal is then constrained considering sample average lightcurves. Results. We constrain the fluence of the possible γ-ray signal in the 8–1000 keV band down to 6.4 × 10−7 (7.1 × 10−8) erg cm−2 for a 200-s (1-s) integration time. Furthermore, we found the radio-to-gamma fluence ratio to be η > 108 Jy ms erg−1 cm2. Conclusions. Our fluence limits exclude ∼94% of Fermi/GBM detected long gamma-ray bursts and ∼96% of Fermi/GBM detected short gamma-ray bursts. In addition, our limits on the radio-to-gamma fluence ratio point to a different emission mechanism from that of magnetar giant flares. Finally, we exclude a γ-ray counterpart as fluent as the one possibly associated with FRB 131104 to be a common feature of FRBs.

Constraints on the low frequency spectrum of FRB 121102

While repeating fast radio bursts (FRBs) remain scarce in number, they provide a unique opportunity for follow-up observations that enhance our knowledge of their sources and potentially of the FRB population as a whole. Attaining more burst spectra could lead to a better understanding of the origin of these bright, millisecond-duration radio pulses. We therefore performed ∼20 h of simultaneous observations on FRB 121102 with the Effelsberg 100 m radio telescope and the low frequency array (LOFAR) to constrain the spectral behaviour of bursts from FRB 121102 at 1.4 GHz and 150 MHz. This campaign resulted in the detection of nine new bursts at 1.4 GHz but no simultaneous detections with LOFAR. Assuming that the ratio of the fluence at two frequencies scales as a power law, we placed a lower limit of α > −1.2 ± 0.4 on the spectral index for the fluence of the instantaneous broad band emission of FRB 121102. For the derivation of this limit, a realistic fluence detection threshold for LOFAR was determined empirically assuming a burst would be scattered as predicted by the NE2001 model. A significant variation was observed in the burst repeat rate R at L-band. During observations in September 2016, nine bursts were detected, giving R = 1.1 ± 0.4 h−1, while in November no bursts were detected, yielding R < 0.3 h−1 (95% confidence limit). This variation is consistent with earlier seen episodic emission of FRB 121102. In a blind and targeted search, no bursts were found with LOFAR at 150 MHz, resulting in a repeat rate limit of R < 0.16 h−1 (95% confidence limit). Burst repeat rate ratios of FRB 121102 at 3, 2, 1.4, and 0.15 GHz are consistent within the uncertainties with a flattening of its spectrum below 1 GHz.

Are all fast radio bursts repeating sources?

We present Monte-Carlo simulations of a cosmological population of repeating fast radio burst (FRB) sources whose comoving density follows the cosmic star formation rate history. We assume a power-law model for the intrinsic energy distribution for each repeating FRB source located at a randomly chosen position in the sky and simulate their dispersion measures (DMs) and propagation effects along the chosen lines-of-sight to various telescopes. In one scenario, an exponential distribution for the intrinsic wait times between pulses is chosen, and in a second scenario we model the observed pulse arrival times to follow a Weibull distribution. For both models we determine whether the FRB source would be deemed a repeater based on the telescope sensitivity and time spent on follow-up observations. We are unable to rule out the existence of a single FRB population based on comparisons of our simulations with the longest FRB follow-up observations performed. We however rule out the possibility of FRBs 171020 and 010724 repeating with the same rate statistics as FRB 121102 and also constrain the slope of a power-law fit to the FRB energy distribution to be $-2.0 < \gamma <-1.0$. All-sky simulations of repeating FRB sources imply that the detection of singular events correspond to the bright tail-end of the adopted energy distribution due to the combination of the increase in volume probed with distance, and the position of the burst in the telescope beam.

Probing Galactic Halos with Fast Radio Bursts

The precise localization (<1'') of multiple fast radio bursts (FRBs) to z>0.1 galaxies has confirmed that the dispersion measures (DMs) of these enigmatic sources afford a new opportunity to probe the diffuse ionized gas around and in between galaxies. In this manuscript, we examine the signatures of gas in dark matter halos (aka halo gas) on DM observations in current and forthcoming FRB surveys. Combining constraints from observations of the high velocity clouds, OVII absorption, and the DM to the Large Magellanic Cloud with hydrostatic models of halo gas, we estimate that our Galactic halo will contribute ${\rm DM}_{\rm MW,halo} \approx 50-80 \rm pc/cm^{-3}$ from the Sun to 200 kpc independent of any contribution from the Galactic ISM. Extending analysis to the Local Group, we demonstrate that M31's halo will be easily detected by high-sample FRB surveys (e.g. CHIME) although signatures from a putative Local Group medium may compete. We then review current empirical constraints on halo gas in distant galaxies and discuss the implications for their DM contributions. We further examine the DM probability distribution function of a population of FRBs at z >> 0 using an updated halo mass function and new models for the halo density profile. Lastly, we illustrate the potential of FRB experiments for resolving the baryonic fraction of halos by analyzing simulated sightlines through the CASBaH survey. All of the code and data products of our analysis are available at https://github.com/FRBs.

A second source of repeating fast radio bursts.

The discovery of a repeating fast radio burst (FRB) source1,2, FRB121102, eliminated models involving cataclysmic events for this source. No other repeating FRB has hitherto been detected despite many recent discoveries and follow-ups3-5, suggesting that repeaters may be rare in the FRB population. Here we report the detection of six repeat bursts from FRB180814.J0422+73, one of the 13 FRBs detected6 by the Canadian Hydrogen Intensity Mapping Experiment (CHIME) FRB project7 during its pre-commissioning phase in July and August 2018. These repeat bursts are consistent with its origin from a single position on the sky, with the same dispersion measure, about 189 parsecs per cubic centimetre. This line of sight traces approximately twice the expected Milky Way column density of free electrons, which implies an upper limit on the source redshift of 0.1, showing it to be closer to Earth by a factor of at least 2 than FRB1211028. In some of the repeat bursts, we observe subpulse frequency structure, drifting and spectral variation reminiscent of that seen in FRB1211029,10, suggesting similar emission mechanisms or propagation effects. This second repeater, found among the first few CHIME/FRB discoveries, suggests that there exists-and that CHIME/FRB and other wide-field, sensitive radio telescopes will find-a substantial population of repeating FRBs.

PALFA Single-pulse Pipeline: New Pulsars, Rotating Radio Transients, and a Candidate Fast Radio Burst

Abstract We present a new single-pulse pipeline for the PALFA survey to efficiently identify single radio pulses from pulsars, rotating radio transients (RRATs), and fast radio bursts (FRBs). We conducted a sensitivity analysis of this new pipeline in which many single pulses were injected into PALFA data and run through the pipeline. We find that for single pulse widths &lt;5 ms, the sensitivity of our new pipeline is at most a factor of ∼2 less sensitive than theoretically predicted. For pulse widths &gt;10 ms, as the DM decreases, the degradation in sensitivity gets worse and can increase up to a factor of ∼4.5. Using this pipeline, we have discovered seven pulsars and two RRATs, and identified three candidate RRATs and one candidate FRB. The confirmed pulsars and RRATs have DMs ranging from 133 to 386 pc cm−3 and flux densities ranging from 20 to 160 mJy. The pulsar periods range from 0.4 to 2.1 s. We report on candidate FRB 141113, which is likely astrophysical and extragalactic, having DM ≃ 400 pc cm−3, which is over the Galactic maximum along this line of sight by ∼100–200 pc cm−3. We consider implications for the FRB population and show via simulations that if FRB 141113 is real and extragalactic, the slope α of the distribution of integral source counts as a function of flux density (N(&gt;S) ∝ S −α ) is 1.4 ± 0.5 (95% confidence range). However, this conclusion is dependent on assumptions that require verification.

The maximum luminosity of fast radio bursts

ABSTRACT Under the assumption that fast radio bursts (FRBs) are from coherent curvature emission powered by the dissipation of magnetic energy in the magnetosphere of neutron stars, we show that there is a maximum isotropic equivalent luminosity $L_{\rm max}\sim (2\times 10^{47}\, \mathrm{erg\, s^{-1}})\, \mathrm{min}(\rho _6^2, B_{16}\rho _6^{4/3}\nu _9^{-2/3})$, where $\rho _6=\rho /10\,$ km is the curvature radius of the magnetic field lines near the source region, $B_{16} = B/10^{16}\,$ G is the local magnetic field strength, and ν9 = ν/GHz is the FRB wave frequency. This is because the electric field responsible for accelerating the emitting particles becomes close to the quantum critical strength and is then quickly shielded by Schwinger pairs within a nano-second. Future observations should be able to measure this cut-off luminosity and hence provide a unique probe of the source location and magnetic field strength. We discuss the effects of Lmax on the observed flux distributions for repeating bursts from the same object and for the entire FRB population.

The dispersion-brightness relation for fast radio bursts from a wide-field survey.

Despite considerable efforts over the past decade, only 34 fast radio bursts-intense bursts of radio emission from beyond our Galaxy-have been reported1,2. Attempts to understand the population as a whole have been hindered by the highly heterogeneous nature of the searches, which have been conducted with telescopes of different sensitivities, at a range of radio frequencies, and in environments corrupted by different levels of radio-frequency interference from human activity. Searches have been further complicated by uncertain burst positions and brightnesses-a consequence of the transient nature of the sources and the poor angular resolution of the detecting instruments. The discovery of repeating bursts from one source3, and its subsequent localization4 to a dwarf galaxy at a distance of 3.7 billion light years, confirmed that the population of fast radio bursts is located at cosmological distances. However, the nature of the emission remains elusive. Here we report a well controlled, wide-field radio survey for these bursts. We found 20, none of which repeated during follow-up observations between 185-1,097hours after the initial detections. The sample includes both the nearest and the most energetic bursts detected so far. The survey demonstrates that there is a relationship between burst dispersionand brightness and thatthe high-fluence bursts are the nearby analogues of the more distant events found in higher-sensitivity, narrower-field surveys5.

Fast radio bursts

First discovered in 2007, fast radio bursts (FRBs) are highly luminous (10−1−102 Jy), millisecond-scale, highly dispersive single radio pulses whose record high brightness temperatures suggest a nonthermal emission mechanism. As of March 2018, a total of 32 FRBs have been recorded. There is also one repeating source, from which hundreds of bursts have already been detected. The rate of events is estimated to be several thousand per day per sky (disregarding bursts from the repeater), and their isotropic distribution in the sky suggests a likely cosmological origin. While numerous hypotheses have been proposed for FRBs since their discovery, the origin of these transients is not yet known. The most promising models either relate them to burst-type radiation from magnetars (neutron stars powered by the dissipation of their magnetic energy) or consider them analogous to giant pulses from some radio pulsars (strongly magnetized rotating neutron stars). The increasing statistics on the observed bursts and improvements in characterizing the FRB population will allow FRBs to become another tool for probing the intergalactic medium, estimating the cosmological parameters, and testing fundamental physical theories.

FRB event rate counts – II. Fluence, redshift, and dispersion measure distributions

We examine how the various observable statistical properties of the FRB population relate back to their fundamental physical properties in a model independent manner. We analyse the flux density and fluence distributions of Fast Radio Bursts (FRBs) as a tool to investigate their luminosity distance distribution and the evolution of their prevalence throughout cosmic history. We examine in detail particular scenarios in which the burst population follows some power of the cosmic star formation rate. FRBs present an important additional measurable over source counts of existing cosmological populations, namely the dispersion measure. Based on the known redshift of FRB121102 (the repeater) we expect at least 50% of the dispersion measure to be attributable to the inter-galactic medium and hence it can be used as a proxy for distance. We develop the framework to interpret the dispersion measure distribution, and investigate how the effect of Helium reionization in the intergalactic medium is evident in this distribution. Examination of existing data suggests that the FRB luminosity function is flatter than a critical slope, making FRBs easily detectable to large distances; in this regime the reduction in flux density with distance is outweighed by the increase in the number of bright bursts within the search volume.

FRB microstructure revealed by the real-time detection of FRB170827

We report a new Fast Radio Burst (FRB) discovered in real-time as part of the UTMOST project at the Molonglo Observatory Synthesis Radio Telescope (MOST). FRB170827 is the first detected with our low-latency ($< 24$ s), machine-learning-based FRB detection system. The FRB discovery was accompanied by the capture of voltage data at the native time and frequency resolution of the observing system, enabling coherent dedispersion and detailed off-line analysis, which have unveiled fine temporal and frequency structure. The dispersion measure (DM) of 176.80 $\pm$ 0.04 pc cm$^{-3}$, is the lowest of the FRB population. The Milky Way contribution along the line of sight is $\sim$ 40 pc cm$^{-3}$, leaving an excess DM of $\sim$ 145 pc cm$^{-3}$. The FRB has a fluence $>$ 20 $\pm$ 7 Jy ms, and is narrow, with a width of $\sim$ 400 $\mu$s at 10$\%$ of its maximum amplitude. However, the burst shows three temporal components, the narrowest of which is $\sim$ 30 $\mu$s, and a scattering timescale of $4.1 \pm 2.7$ $\mu$s. The FRB shows spectral modulations on frequency scales of 1.5 MHz and 0.1 MHz. Both are prominent in the dynamic spectrum, which shows a very bright region of emission between 841 and 843 MHz, and weaker, patchy emission across the entire band. We show the fine spectral structure could arise in the FRB host galaxy, or its immediate vicinity.

Are There Multiple Populations of Fast Radio Bursts?

Abstract The repeating FRB 121102 (the “repeater”) shows repetitive bursting activities and was localized in a host galaxy at z = 0.193. On the other hand, despite dozens of hours of telescope time spent on follow-up observations, no other fast radio bursts (FRBs) have been observed to repeat. Yet, it has been speculated that the repeater is the prototype of FRBs, and that other FRBs should show similar repeating patterns. Using the published data, we compare the repeater with other FRBs in the observed time interval (Δt)–flux ratio (S i /S i+1) plane. We find that whereas other FRBs occupy the upper (large S i /S i+1) and right (large Δt) regions of the plane due to the non-detections of other bursts, some of the repeater bursts fall into the lower left region of the plot (short interval and small flux ratio) excluded by the non-detection data of other FRBs. The trend also exists even if one only selects those bursts detectable by the Parkes radio telescope. If other FRBs were similar to the repeater, our simulations suggest that the probability that none of them have been detected to repeat with the current searches would be ∼(10−4–10−3). We suggest that the repeater is not representative of the entire FRB population, and that there is strong evidence of more than one population of FRBs.

The astrophysical consequences of intervening galaxy gas on fast radio bursts

We adopt and analyze results on the incidence and physical properties of damped Ly$\alpha$ systems (DLAs) to predict the astrophysical impact of gas in galaxies on observations of Fast Radio Bursts (FRBs). Three DLA measures form the basis of this analysis: (i) the HI column density distribution, parameterized as a double power-law; (ii) the incidence of DLAs with redshift (derived here), $\ell(z)=A+B \arctan(z-C)$ with $A=0.236_{-0.021}^{+0.016}, B=0.168_{-0.017}^{+0.010}, C=2.87_{-0.13}^{+0.17}$ and (iii) the electron density, parameterized as a log-normal deviate with mean $10^{-2.6} cm^{-3}$ and dispersion 0.3dex. Synthesizing these results, we estimate that the average rest-frame dispersion measure from the neutral medium of a single, intersecting galaxy is DM$^{NM}_{DLA}=0.25$ pc/cm^3. Analysis of AlIII and CII* absorption limits the putative warm ionized medium to contribute DM$^{WIM}_{DLA}<20$pc/cm^3. Given the low incidence of DLAs, we find that a population of FRBs at z=2 will incur DM(z=2)=0.01 pc/cm^3 on average, with a 99% c.l. upper bound of 0.22 pc/cm^3. Assuming that turbulence of the ISM in external galaxies is qualitatively similar to our Galaxy, we estimate that the angular broadening of an FRB by intersecting galaxies is negligible ($\theta<0.1$mas). The temporal broadening is also predicted to be small, $\tau \approx 0.3$ms for a z=1 galaxy intersecting a z=2 FRB for an observing frequency of $\nu=1$GHz. Even with $\nu=600$MHz, the fraction of sightlines broadened beyond 25ms is only approximately 0.1%. We conclude that gas within the ISM of intervening galaxies has a minor effect on the detection of FRBs and their resultant DM distributions. Download the repository at https://github.com/FRBs/FRB to repeat and extend the calculations presented here.

Fast radio burst event rate counts – I. Interpreting the observations

The fluence distribution of the Fast Radio Burst (FRB) population (the "source counts" distribution, $N(>F) \propto F^\alpha$), is a crucial diagnostic of its distance distribution, and hence the progenitor evolutionary history. We critically reanalyse current estimates of the FRB source counts distribution. We demonstrate that the Lorimer burst (FRB 010724) is subject to discovery bias, and should be excluded from all statistical studies of the population. We re-examine the evidence for flat, $\alpha >-1$, source counts estimates based on the ratio of single-beam to multiple-beam detections with the Parkes multi-beam receiver, and show that current data implies only a very weak constraint of $\alpha \lesssim -1.3$. A maximum likelihood analysis applied to the portion of the Parkes FRB population detected above the observational completeness fluence of 2 Jy ms yields $\alpha = -2.6_{-1.3}^{+0.7}$. Uncertainties in the location of each FRB within the Parkes beam render estimates of the Parkes event rate uncertain in both normalising survey area and the estimated post-beam-corrected completeness fluence; this uncertainty needs to be accounted for when comparing the event rate against event rates measured at other telescopes.

A Fast Radio Burst Occurs Every Second throughout the Observable Universe

Abstract Recent multi-telescope observations of the repeating fast radio burst (FRB) FRB 121102 reveal a Gaussian-like spectral profile and associate the event with a dwarf metal-poor galaxy at a cosmological redshift of 0.19. Assuming that this event represents the entire FRB population, we make predictions for the expected number counts of FRBs observable by future radio telescopes between 50 MHz and 3.5 GHz. We vary our model assumptions to bracket the expected rate of FRBs and find that it exceeds one FRB per second per sky when accounting for faint sources. We show that future low-frequency radio telescopes, such as the Square Kilometre Array, could detect more than one FRB per minute over the entire sky originating from the epoch of reionization.

Detecting fast radio bursts at decametric wavelengths

Fast radio bursts (FRBs) are highly dispersed, sporadic radio pulses that are likely extragalactic in nature. Here we investigate the constraints on the source population from surveys carried out at frequencies $<1$~GHz. All but one FRB has so far been discovered in the 1--2~GHz band, but new and emerging instruments look set to become valuable probes of the FRB population at sub-GHz frequencies in the near future. In this paper, we consider the impacts of free-free absorption and multi-path scattering in our analysis via a number of different assumptions about the intervening medium. We consider previous low frequency surveys alongwith an ongoing survey with the University of Technology digital backend for the Molonglo Observatory Synthesis Telescope (UTMOST) as well as future observations with the Canadian Hydrogen Intensity Mapping Experiment (CHIME) and the Hydrogen Intensity and Real-Time Analysis Experiment (HIRAX). We predict that CHIME and HIRAX will be able to observe $\sim$ 30 or more FRBs per day, even in the most extreme scenarios where free-free absorption and scattering can significantly impact the fluxes below 1~GHz. We also show that UTMOST will detect 1--2 FRBs per month of observations. For CHIME and HIRAX, the detection rates also depend greatly on the assumed FRB distance scale. Some of the models we investigated predict an increase in the FRB flux as a function of redshift at low frequencies. If FRBs are truly cosmological sources, this effect may impact future surveys in this band, particularly if the FRB population traces the cosmic star formation rate.

Five new fast radio bursts from the HTRU high-latitude survey at Parkes: first evidence for two-component bursts

Abstract The detection of five new fast radio bursts (FRBs) found in the 1.4-GHz High Time Resolution Universe high-latitude survey at Parkes, is presented. The rate implied is 7$^{+5}_{-3}\times 10^3$ (95 per cent) FRBs sky−1 d−1 above a fluence of 0.13 Jy ms for an FRB of 0.128 ms duration to 1.5 Jy ms for 16 ms duration. One of these FRBs has a two-component profile, in which each component is similar to the known population of single component FRBs and the two components are separated by 2.4 ± 0.4 ms. All the FRB components appear to be unresolved following deconvolution with a scattering tail and accounting for intrachannel smearing. The two-component burst, FRB 121002, also has the highest dispersion measure (1629 pc cm−3) of any FRB to-date. Many of the proposed models to explain FRBs use a single high-energy event involving compact objects (such as neutron-star mergers) and therefore cannot easily explain a two-component FRB. Models that are based on extreme versions of flaring, pulsing, or orbital events, however, could produce multiple component profiles. The compatibility of these models and the FRB rate implied by these detections is discussed.

Are the distributions of fast radio burst properties consistent with a cosmological population?

High time resolution radio surveys over the last few years have discovered a population of millisecond-duration transient bursts called Fast Radio Bursts (FRBs), which remain of unknown origin. FRBs exhibit dispersion consistent with propagation through a cold plasma and dispersion measures indicative of an origin at cosmological distances. In this paper we perform Monte Carlo simulations of a cosmological population of FRBs, based on assumptions consistent with observations of their energy distribution, their spatial density as a function of redshift and the properties of the interstellar and intergalactic media. We examine whether the dispersion measures, fluences, inferred redshifts, signal-to-noises and effective widths of known FRBs are consistent with a cosmological population. Statistical analyses indicate that at least 50 events at Parkes are required to distinguish between a constant co-moving FRB density, and a FRB density that evolves with redshift like the cosmological star formation rate density.

A repeating fast radio burst.

Fast radio bursts are millisecond-duration astronomical radio pulses of unknown physical origin that appear to come from extragalactic distances. Previous follow-up observations have failed to find additional bursts at the same dispersion measure (that is, the integrated column density of free electrons between source and telescope) and sky position as the original detections. The apparent non-repeating nature of these bursts has led to the suggestion that they originate in cataclysmic events. Here we report observations of ten additional bursts from the direction of the fast radio burst FRB 121102. These bursts have dispersion measures and sky positions consistent with the original burst. This unambiguously identifies FRB 121102 as repeating and demonstrates that its source survives the energetic events that cause the bursts. Additionally, the bursts from FRB 121102 show a wide range of spectral shapes that appear to be predominantly intrinsic to the source and which vary on timescales of minutes or less. Although there may be multiple physical origins for the population of fast radio bursts, these repeat bursts with high dispersion measure and variable spectra specifically seen from the direction of FRB 121102 support an origin in a young, highly magnetized, extragalactic neutron star.

On the paucity of fast radio bursts at low Galactic latitudes

We examine the effect of Galactic diffractive interstellar scintillation as a means of explaining the reported deficit of fast radio burst (FRB) detections at low Galactic latitude. We model the unknown underlying FRBflux density distribution as a power lawwith a rate scaling as S−5/2+δ ν and account for the fact that the FRBs are detected at unknown positions within the telescope beam. We find that the event rate of FRBs located off the Galactic plane may be enhanced by a factor of ∼30–300 per cent relative to objects near the Galactic plane without necessarily affecting the slope of the distribution. For FRBs whose flux densities are subject to relatively weak diffractive scintillation, as is typical for events detected at high Galactic latitudes, we demonstrate that an effect associated with Eddington bias is responsible for the enhancement. The magnitude of the enhancement increases with the steepness of the underlying flux density distribution, so that existing limits on the disparity in event rates between high and lowGalactic latitudes suggest that the FRB population has a steep differential flux density distribution, scaling as S−3.5 ν or steeper. Existing estimates of the event rate in the flux density range probed by the High Time Resolution Universe survey overestimate the true rate by a factor of ∼3.