Fast Radio Bursts Research Articles

A Statistical Analysis of Crab Pulsar Giant Pulse Rates

A small number of pulsars are known to emit giant pulses (GPs), single pulses much brighter than average. Among these is PSR J0534+2200, also known as the Crab pulsar, a young pulsar with high GP rates. Long-term monitoring of the Crab pulsar presents an excellent opportunity to perform statistical studies of its GPs and the processes affecting them, potentially providing insight into the behavior of other neutron stars that emit bright single pulses. Here, we present an analysis of a set of 24,985 Crab GPs obtained from 88 hr of daily observations at a center frequency of 1.55 GHz by the 20 m telescope at the Green Bank Observatory, spread over 461 days. We study the effects of refractive scintillation at higher frequencies than previous studies and compare methods of correcting for this effect. We also search for deterministic patterns seen in other single-pulse sources, possible periodicities seen in several rotating radio transients and fast radio bursts, and clustering of GPs like that seen in the repeating fast radio burst FRB 121102.

Towards solving the origin of circular polarisation in FRB 20180301A

Abstract Fast Radio Bursts (FRBs) are short-timescale transients of extragalactic origin. The number of detected FRBs has grown dramatically since their serendipitous discovery from archival data. Some FRBs have also been seen to repeat. The polarimetric properties of repeating FRBs show diverse behaviour and, at times, extreme polarimetric morphology, suggesting a complex magneto-ionic circumburst environment for this class of FRB. The polarimetric properties such as circular polarisation behaviour of FRBs are crucial for understanding their surrounding magnetic-ionic environment. The circular polarisation previously observed in some of the repeating FRB sources has been attributed to propagation effects such as generalised Faraday rotation (GFR), where conversion from linear to circular polarisation occurs due to the non-circular modes of transmission in relativistic plasma. The discovery burst from the repeating FRB 20180301A showed significant frequency-dependent circular polarisation behaviour, which was initially speculated to be instrumental due to a sidelobe detection. Here we revisit the properties given the subsequent interferometric localisation of the burst, which indicates that the burst was detected in the primary beam of the Parkes/Murriyang 20-cm multibeam receiver. We develop a Bayesian Stokes-Q, U, and V fit method to model the GFR effect, which is independent of the total polarised flux parameter. Using the GFR model we show that the rotation measure (RM) estimated is two orders of magnitude smaller and opposite sign (∼28 rad m−2) than the previously reported value. We interpret the implication of the circular polarisation on its local magnetic environment and reinterpret its long-term temporal evolution in RM.

Refractive lensing of scintillating FRBs by subparsec cloudlets in the multiphase CGM

We consider the refractive lensing effects of ionized cool ([Formula: see text]) gas cloudlets in the circumgalactic medium (CGM) of galaxies. In particular, we discuss the combined effects of lensing from these cloudlets and scintillation from plasma screens in the Milky Way interstellar medium (ISM). We show that, if the CGM comprises a mist of subparsec cloudlets with column densities of order [Formula: see text] (as predicted by [M. McCourt, S. P. Oh, R. O'Leary, A. M. Madigan, MNRAS 473, 5407-5431 (2018)]), then fast radio bursts (FRBs) whose sightlines pass within a virial radius of a CGM halo will may be lensed into tens of refractive images with a ∼10 ms scattering timescale. When these images are formed, they will be resolved by scintillating screens in the Milky Way ISM and will suppress the observed scintillation. We illustrate this effect in refractive lensing and argue that positive detections of FRB scintillation may constrain the properties of these cool-gas cloudlets, with current scintillation observation weakly disfavoring the cloudlet model. We propose that sheet-like geometries for the cool gas in the CGM can reconcile quasar absorption measurements (from which we infer the presence of the cool gas with structure on subparsec scales) and the unexpected lack of lensing signals from this gas thus far observed.

Design of Broadband Phased Array Feed Using Stereolithographic Additive Manufacturing Heterogeneous Integrating Technology for Fast Radio Burst Observation

Herein, a stereolithography appearance 3‐D printed phased array feed (PAF) on radio telescopes based on broadband Vivaldi antenna elements covering 4–10 GHz, which meets the requirements of low cost, lightweight, and compact volume, is designed and proposed. The prototype is integrated with two orthogonal linear polarized 4 × 5 arrays, each of which is composed of 20 Vivaldi antenna elements. The distance between adjacent antenna elements is 1/2 λ in both E‐plane and H‐plane. By increasing thickness of the slice elements compared with traditional end‐fire arrays on printed circuit board, the wide‐band impedance matching is realized. The mutual impedance matrix of the dual‐polarization array is calculated and the equivalent circuit of the active and parasitic elements is demonstrated. Due to the orthogonal parasitic elements inserted between the adjacent active antennas, equivalent capacitance is introduced in the tightly coupled array. The measured active voltage standing wave ratio of the manufactured PAF exhibits 4–10 GHz operating bandwidth for both broadside radiation and 45° scan in both E‐ and H‐plane polarization. The proposed wideband PAF has a promising potential application prospect in fast radio burst and other astronomic observation.

An Image-based Blind Search for Fast Radio Bursts in 88 hr of Data from the EoR0 Field, with the Murchison Widefield Array

This work is part of ongoing efforts to detect fast radio bursts using the Murchison Widefield Array (MWA) in a spectral window below 300 MHz. We used an image-based method based on the pilot study of Tingay et al., scaled up via massively parallel processing using a commercial supercomputer. We searched 87.6 hr of 2 s snapshot images, each covering 1165 square degrees of the EoR0 field, over a dispersion measure range of 170–1035 pc cm−3. The large amount of data necessitated the construction of a series of filters to classify and reject the large number of false positives. Our search was more sensitive than any previous blind search using the MWA telescope, but we report no fast radio bursts detections, a result which is consistent with the extrapolation into the low-frequency domain of the results of M. Sokolowski et al. We obtain upper bounds on the event rate ranging from <1783 sky−1 day−1 at a fluence of 392 Jy ms, to <31 sky−1 day−1 at 8400 Jy ms, for our spectral window of 167–198 MHz. Our method was shown to be computationally efficient and scalable by the two or three orders of magnitude required to seriously test the model of Sokolowski et al. Our process is especially sensitive to detections of satellites and meteor trails and may find applications in the identification of these transients. We comment on future surveys using this method, with both the MWA and the SKA.

Time-integrated constraint on neutrino flux of CHIME fast radio burst sources with 10-yr IceCube point-source data

ABSTRACT Despite numerous studies, the sources of IceCube cosmic neutrinos are mostly unidentified. Utilizing recently released IceCube neutrino and CHIME fast radio burst (FRB) catalogues, we examine the possibility of an association between neutrinos and CHIME/FRB catalogue 1 FRBs for both the entire FRB population and individual FRBs using the unbinned maximum likelihood method. Our results do not directly support the possibility of the above-mentioned association with three weighting schemes: equal, total radio fluence, and event rate. We then attempt to constrain the diffuse muon neutrino flux upper limit from CHIME/FRB catalogue 1 FRBs. After considering a completeness correction, we find the 95 per cent diffuse muon neutrino flux upper limit at $100 \,\mathrm{T}\mathrm{eV}$ for all FRB sources in the universe to be ${\sim} 1.01 \times 10^{-18} \,\mathrm{G}^{-1}\mathrm{eV}\mathrm{/}\,\mathrm{c}\mathrm{m}^{2}\,{\rm s}^{-1}\rm {sr}^{-1}$, or ${\sim} 70.3~{{\ \rm per\ cent}}$ of the 10-year diffuse neutrino flux observed by IceCube. Our results match the non-detection results of other studies, but we do not rule out FRBs being a significant contributor to the diffuse neutrino flux measured by IceCube.

Sudden Polarization Angle Jumps of the Repeating Fast Radio Burst FRB 20201124A

We report the first detection of polarization angle orthogonal jumps, a phenomenon previously only observed from radio pulsars, from a fast radio burst (FRB) source FRB 20201124A. We find three cases of orthogonal jumps in over 2000 bursts, all resembling those observed in pulsar single pulses. We propose that the jumps are due to the superposition of two orthogonal emission modes that could only be produced in a highly magnetized plasma, and they are caused by the line of sight sweeping across a rotating magnetosphere. The shortest jump timescale is of the order of 1 millisecond, which hints that the emission modes come from regions smaller than the light cylinder of most pulsars or magnetars. This discovery provides convincing evidence that FRB emission originates from the complex magnetosphere of a magnetar, suggesting an FRB emission mechanism that is analogous to radio pulsars despite a huge luminosity difference between two types of objects.

Constraining the Earth-mass Primordial Black Hole Mergers Model of the Non-repeating FRBs Using the First CHIME/FRB Catalog

In this paper, we upgrade the constraints for the Earth-mass primordial black hole mergers model based on the first Canadian Hydrogen Intensity Mapping Experiment (CHIME)/fast radio burst (FRB) catalog. Assuming the null hypothesis that the observed non-repeating FRBs originate from Earth-mass primordial black hole mergers, we find that how the charges were distributed in the primordial black hole population is well described by a double power-law function with typical charge value of qc/10−5=1.60−0.28+0.28 , where the power-law index α1=2.33−0.18+0.15 for q < q c and α2=4.56−0.26+0.30 for q ≥ q c. Here, q represents the charge of the black hole in units of GM , where M is the mass of the black hole. Furthermore, we infer the local event rate of the bursts is 8.8−2.1+5.7×104Gpc−3yr−1 , which indicates that an abundance of the primordial black hole population f ≳ 10−4 is needed to account for the observed FRBs by CHIME. The results of this paper lay the basis for further research on the electromagnetic radiation background generated by the merger of primordial black hole mergers.

Objects May Be Closer than They Appear: Significant Host Galaxy Dispersion Measures of Fast Radio Bursts in Zoom-in Simulations

We investigate the contribution of host galaxies to the overall dispersion measures (DMs) for fast radio bursts (FRBs) using the Feedback in Realistic Environments (FIRE-2) cosmological zoom-in simulation suite. We calculate DMs from every star particle in the simulated L* galaxies by ray-tracing through their multiphase interstellar medium, summing the line-of-sight free thermal electron column for all gas elements within ±20 kpc of the galactic midplane. At z = 0, we find average (median) host-galaxy DMs of 74 (43) and 210 (94) pc cm−3 for older (≳10 Myr) and younger (≲10 Myr) stellar populations, respectively. Inclination raises the median DM measured for older populations (≳10 Myr) in the simulations by a factor of ∼2 but generally does not affect the younger stars deeply embedded in H ii regions except in extreme edge-on cases (inclination ≳85°). In kinematically disturbed snapshots (z = 1 in FIRE), the average (median) host-galaxy DMs are higher: 80 (107) and 266 (795) pc cm−3 for older (≳10 Myr) and younger (≲10 Myr) stellar populations, respectively. FIRE galaxies tend to have higher DM values than cosmological simulations such as IllustrisTNG, with larger tails in their distributions to high DMs. As a result, FRB host galaxies may be closer (lower redshift) than previously inferred. Furthermore, constraining host-galaxy DM distributions may help significantly constrain FRB progenitor models.

The Environments of Fast Radio Bursts Viewed Using Adaptive Optics

We present observations from the Gemini Multi-Conjugate Adaptive Optics System/Gemini South Adaptive Optics Imager at Gemini South of five fast radio burst (FRB) host galaxies of FRBs with subarcsecond localizations. We examine and quantify the spatial distributions and locations of the FRBs with respect to their host galaxy light distributions, finding a median host-normalized offset of 2.09 half-light radii (r e ) and the trend that these FRBs occur in fainter regions of their host galaxies. When combined with the FRB host galaxy sample from Mannings et al., we find that FRBs are statistically distinct from Ca-rich transients in terms of light at the source location and from SGRBs and LGRBs in terms of host-normalized offset. We further find that most FRBs are in regions of elevated local stellar mass surface densities in comparison to the mean global values of their hosts. This, along with the finding that the FRB locations trace the distribution of stellar mass, points toward a possible similarity of the environments of CCSNe and FRBs. We also find that four out of five FRB hosts exhibit distinct spiral arm features, and the bursts originating from such hosts tend to appear on or close to their host’s spiral structure, with a median distance of 0.53 ± 0.27 kpc. With many well-localized FRB detections looming on the horizon, we will be able to better characterize the properties of FRB environments relative to their host galaxies and other transient classes. Such insights may only require us to double the number of FRBs with subarcsecond localizations.

Force-free Wave Interaction in Magnetar Magnetospheres: Computational Modeling in Axisymmetry

Crustal quakes of highly magnetized neutron stars can disrupt their magnetospheres, triggering energetic phenomena like X-ray and fast radio bursts. Understanding plasma wave dynamics in these extreme environments is vital for predicting energy transport across scales to the radiation length. This study models relativistic plasma wave interaction in magnetar magnetospheres with force-free electrodynamics simulations. For propagation along curved magnetic field lines, we observe the continuous conversion of Alfvén waves to fast magnetosonic (FMS) waves. The conversion efficiency can be up to three times higher when counter-propagating Alfvén waves interact in the equatorial region. Alfvén waves generate FMS waves of twice their frequency during their first crossing of the magnetosphere. After the initial transient burst of FMS waves, Alfvén waves convert to FMS waves periodically, generating variations on timescales of the magnetospheric Alfvén wave crossing time. This decaying FMS wave tail carries a significant portion (half) of the total energy emitted. Plastic damping of “bouncing” Alfvén waves by the magnetar crust has minimal impact on the FMS efficiency. We discuss the implications of the identified wave phenomena for magnetar observations. Outgoing FMS waves can develop electric zones, potential sources of coherent radiation. Long wavelength FMS waves could generate FRBs through reconnection beyond the light cylinder.

Prospects of fast radio bursts and large-scale 21 cm power spectra in constraining the Epoch of Reionization

The Epoch of Reionization (EoR) is a crucial link through which the complete evolutionary history of the Universe can be grasped. Several attempts with a variety of observables have been used to understand the thermal and ionization evolution of the intergalactic medium during EoR. In this study, we explore the simultaneous prospects of two important observables that are expected to be available in the near future, that is, the dispersion measure (DM) of high-redshift fast radio bursts (FRBs) and large-scale 21 cm power spectra. For this purpose, we used a previously developed explicitly photon conserving seminumerical model, SCRIPT, which includes realistic recombination and radiative feedback effects. We verified that the DM evolution of 100 mock FRBs at high redshift (7.0 ≤ ɀ ≤ 15.0) is sufficient to recover the underlying reionization model, while 1000 FRB mocks at this redshift range can constrain the reionization timeline within the percentage level uncertainties at the 68% confidence limit. Furthermore, we studied the effect of including large-scale 21 cm power spectra (using only a single bin, k ~ 0.14 h/cMpc) at three redshifts along with the FRB DM distribution. The joint exploration using these two observables can significantly improve the constraints on the various parameters ( an uncertainty of ≲8% for the reionization interval and midpoint at the 95% confidence), alleviate the degeneracies, and can narrow down the thermal history of the Universe by discarding some of the extreme heating models.

Probing the magnetized gas distribution in galaxy groups and the cosmic web with POSSUM Faraday rotation measures

ABSTRACT We present initial results from the Polarization Sky Survey of the Universe’s Magnetism (POSSUM), analysing 22 817 Faraday rotation measures (RMs) with median uncertainties of 1.2 rad m$^{-2}$ across 1520 deg2 to study magnetized gas associated with 55 nearby galaxy groups ($z\lesssim 0.025$) with halo masses between $10^{12.5}$ and $10^{14.0}$ M$_\odot$. We identify two distinct gas phases: the intragroup medium (IGrM) within 0–2 splashback radii and the warm-hot intergalactic medium (WHIM) extending from 2 to 7 splashback radii. These phases enhance the standard deviation of residual (i.e. Galactic foreground RM-subtracted) RMs by $6.9\pm 1.8$ rad m$^{-2}$ and $4.2 \pm 1.2$ rad m$^{-2}$, respectively. Estimated magnetic field strengths are several μG within the IGrM and 0.1–1 μG in the WHIM. We estimate the plasma $\beta$ in both phases, and show that magnetic pressure might be more dynamically important than in the ICM of more massive clusters or sparse cosmic web filaments. Our findings indicate that ‘missing baryons’ in the WHIM likely extend beyond the gravitational radii of group-mass haloes to Mpc scales, consistent with large-scale, outflow-driven ‘magnetized bubbles’ seen in cosmological simulations. We demonstrate that RM grids are an effective method for detecting magnetized thermal gas at galaxy group interfaces and within the cosmic web. This approach complements X-ray and Sunyaev-Zel’dovich effect methods, and when combined with fast radio burst dispersion measures, data from the full POSSUM survey – comprising approximately a million RMs – will allow direct magnetic field measurements to further our understanding of baryon circulation in these environments and the magnetized universe.

Coherent Inverse Compton Scattering in Fast Radio Bursts Revisited

Growing observations of temporal, spectral, and polarization properties of fast radio bursts (FRBs) indicate that the radio emission of the majority of bursts is likely produced inside the magnetosphere of its central engine, likely a magnetar. We revisit the idea that FRBs are generated via coherent inverse Compton scattering (ICS) off low-frequency X-mode electromagnetic waves (fast magnetosonic waves) by bunches at a distance of a few hundred times the magnetar radius. The following findings are revealed: (1) Crustal oscillations during a flaring event would excite kHz Alfvén waves. Fast magnetosonic waves with essentially the same frequency can be generated directly or be converted from Alfvén waves at a large radius, with an amplitude large enough to power FRBs via the ICS process. (2) The cross section increases rapidly with radius and significant ICS can occur at r ≳ 100R ⋆ with emission power much greater than the curvature radiation power but still in the linear scattering regime. (3) The low-frequency fast magnetosonic waves naturally redistribute a fluctuating relativistic plasma in the charge-depleted region to form bunches with the right size to power FRBs. (4) The required bunch net charge density can be sub-Goldreich–Julian, which allows a strong parallel electric field to accelerate the charges, maintain the bunches, and continuously power FRB emission. (5) This model can account for a wide range of observed properties of repeating FRB bursts, including high degrees of linear and circular polarization and narrow spectra as observed in many bursts from repeating FRB sources.

Constraining Near-simultaneous Radio Emission from Short Gamma-Ray Bursts Using CHIME/FRB

We use the Canadian Hydrogen Intensity Mapping Experiment (CHIME) Fast Radio Burst (FRB) Project to search for FRBs that are temporally and spatially coincident with gamma-ray bursts (GRBs) occurring between 2018 July 7 and 2023 August 3. We do not find any temporal (within 1 week) and spatial (within overlapping 3σ localization regions) coincidences between any CHIME/FRB candidates and all GRBs with 1σ localization uncertainties <1°. As such, we use CHIME/FRB to constrain the possible FRB-like radio emission for 27 short gamma-ray bursts (SGRBs) that were within 17° of CHIME/FRB’s meridian at a point either 6 hr prior up to 12 hr after the high-energy emission. Two SGRBs, GRB 210909A and GRB 230208A, were above the horizon at CHIME at the time of their high-energy emission and we place some of the first constraints on simultaneous FRB-like radio emission from SGRBs. While neither of these two SGRBs have known redshifts, we construct a redshift range for each GRB based on their high-energy fluence and a derived SGRB energy distribution. For GRB 210909A, this redshift range corresponds to z = [0.009, 1.64] with a mean of z = 0.13. Thus, for GRB 210909A, we constrain the radio luminosity at the time of the high-energy emission to L < 2 × 1046 erg s−1, L < 5 × 1044 erg s−1, and L < 3 × 1042 erg s−1 assuming redshifts of z = 0.85, z = 0.16, and z = 0.013, respectively. We compare these constraints with the predicted simultaneous radio luminosities from different compact object merger models.

Stokes Phenomena in Lensing

As lensing of coherent astrophysical sources, e.g., pulsars, fast radio bursts, and gravitational waves, becomes observationally relevant, the mathematical framework of Picard–Lefschetz theory has recently been introduced to fully account for wave optics effects. Accordingly, the concept of lensing images has been generalized to include complex solutions of the lens equation referred to as “imaginary images,” and more radically, to the Lefschetz thimbles, which are a sum of the steepest descent contours connecting the real and imaginary images in the complex domain. In this wave-optics-based theoretical framework of lensing, we study the “Stokes phenomena” as the change of the topology of the Lefschetz thimbles. Similar to the well-known caustics at which the number of geometric images changes abruptly, the corresponding Stokes lines are the boundaries in the parameter space where the number of effective imaginary images changes. We map the Stokes lines for a few lens models. The resulting Stokes line-caustics network represents a unique feature of the lens models. The observable signature of the Stokes phenomena is the change of interference behavior, in particular the onset of frequency oscillation for some Stokes lines. We also demonstrate high-order Stokes phenomena where the system has a continuous number of effective images but with an abrupt change in the way they are connected to each other by the Lefschetz thimbles. Their full characterization calls for an analogy of the catastrophe theory for caustics.

A bright burst from FRB 20200120E in a globular cluster of the nearby galaxy M81

Fast radio bursts (FRBs) are immensely energetic millisecond-duration radio pulses. Observations indicate that nearby FRBs can be produced by old stellar populations, as suggested by the localization of the repeating source FRB 20200120E in a globular cluster of M81. Nevertheless, the burst energies of FRB 20200120E are significantly smaller than those of other cosmological FRBs. Here, we report the detection of a bright burst from FRB 20200120E in 1.1 – 1.7 GHz, with a fluence of approximately 30 Jy ms, which is more than 42 times larger than the previously detected bursts near 1.4 GHz frequency. It reaches one-third of the energy of the weakest burst from FRB 20121102A and is detectable at a distance exceeding 200 Mpc. Our finding bridges the gap between nearby and cosmological FRBs and indicates that FRBs hosted in globular clusters can be bright enough to be observable at cosmological distances.

RRAT J1913+1330: An Extremely Variable and Puzzling Pulsar

Rotating radio transients (RRATs) are neutron stars that emit sporadic radio bursts. We detected 1955 single pulses from RRAT J1913+1330 using the 19 beam receiver of the Five-hundred-meter Aperture Spherical radio Telescope. These pulses were detected in 19 distinct clusters, with 49.4% of them occurring with a waiting time of one rotation period. The energy distribution of these individual pulses exhibited a wide range, spanning 3 orders of magnitude, reminiscent of repeating fast radio bursts (FRBs). Furthermore, we observed abrupt variations in pulse profile, width, peak flux, and fluence between adjacent sequential pulses. These findings suggest that this RRAT could be interpreted as a pulsar with extreme pulse-to-pulse modulation. The presence of sequential pulse trains during active phases, along with significant pulse variations in profile, fluence, flux, and width, should be intrinsic to a subset of RRATs. Our results indicate that J1913+1330 represents a peculiar source that shares certain properties with populations of nulling pulsars, giant pulses, and FRBs from different perspectives. The dramatic pulse-to-pulse variation observed in J1913+1330 could be attributed to unstable pair creation above the polar cap region and the variation of the site where streaming pairs emit coherently. Exploring a larger sample of RRATs exhibiting similar properties to J1913+1330 has the potential to significantly advance our understanding of pulsars, RRATs, and FRBs.

Recent progresses in strange quark stars

According to the hypothesis that strange quark matter may be the true ground state of matter at extremely high densities, strange quark stars should be stable and could exist in the Universe. It is possible that pulsars may actually be strange stars, but not neutron stars. Here we present a short review on recent progresses in the field of strange quark stars. First, three popular phenomenological models widely used to describe strange quark matter are introduced, with special attention being paid on the corresponding equation of state in each model. Combining the equation of state with the Tolman-Oppenheimer-Volkov equations, the inner structure and mass-radius relation can be obtained for the whole sequence of strange stars. Tidal deformability and oscillations (both radial and non-radial oscillations), which are sensitive to the composition and the equations of state, are then described. Hybrid stars as a special kind of quark stars are discussed. Several other interesting aspects of strange stars are also included. For example, strong gravitational wave emissions may be generated by strange stars through various mechanisms, which may help identify strange stars via observations. Especially, close-in strange quark planets with respect to their hosts may provide a unique test for the existence of strange quark objects. Fierce electromagnetic bursts could also be generated by strange stars. The energy may come from the phase transition of neutron stars to strange stars, or from the merger of binary strange stars. The collapse of the strange star crust can also release a huge amount of energy. It is shown that strange quark stars may be involved in short gamma-ray bursts and fast radio bursts.

Positive and unlabelled machine learning reveals new fast radio burst repeater candidates

ABSTRACT Fast radio bursts (FRBs) are astronomical radio transients of unknown origin. A minority of FRBs have been observed to originate from repeating sources, and it is unknown which apparent one-off bursts are hidden repeaters. Recent studies increasingly suggest that there are intrinsic physical differences between repeating and non-repeating FRBs. Previous research has used machine learning classification techniques to identify apparent non-repeaters with repeater characteristics, whose sky positions would be ideal targets for future observation campaigns. However, these methods have not sufficiently accounted for the positive and unlabelled (PU) nature of the data, wherein true labels are only available for repeaters. Modified techniques that do not inadvertently learn properties of hidden repeaters as characteristic of non-repeaters are likely to identify additional repeater candidates with greater accuracy. We present in this paper the first known attempt at applying PU-specific machine learning techniques to study FRBs. We train an ensemble of five PU-specific classifiers on the available data and use them to identify 66 repeater candidates in burst data from the CHIME/FRB collaboration, 18 of which were not identified with the use of machine learning classifiers in past research. Our results additionally support repeaters and non-repeaters having intrinsically different physical properties, particularly spectral index, frequency width, and burst width. This work additionally opens new possibilities to study repeating and non-repeating FRBs using the framework of PU learning.