Properties Of Fast Radio Bursts Research Articles

Gravitational Self-lensing of Fast Radio Bursts in Neutron Star Magnetospheres. I. The Model

Fast radio bursts (FRBs) are cosmological subsecond bursts of coherent radio emission, whose source is still unknown. To date, the Galactic magnetar SGR 1935 + 2154 is the only astrophysical object known to emit radio bursts akin to FRBs, albeit less powerful, supporting suggestions that FRBs originate from magnetars. Many remarkable properties of FRBs—e.g., the dichotomy between repeaters and one-off sources, and their power-law energy distributions (with typical index ∼2–3)—are not well understood yet. Moreover, the huge radio power released by the most active repeaters is challenging even for the magnetic energy reservoir of magnetars. Here, we assume that FRBs originate from corotating hotspots anchored in neutron star (NS) magnetospheres and occasionally get amplified by large factors via gravitational self-lensing in the strong NS field. We evaluate the probability of amplification and show that: (i) a power-law energy distribution of events ∝E −(2−3) is generally expected; (ii) all FRB sources may be regarded as repeating, their appearance as one-off sources or repeaters being determined by the critical dependence of the amplification probability on the emission geometry and source orientation relative to Earth; and (iii) the most active repeaters, in particular, correspond to extremely rare and finely tuned orientations (∼1 in 106), leading to large probabilities of amplification that make their bursts frequently detectable. At the same time, their power release appears enhanced, typically by factors ≳10, easing their energy budget problem.

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.

Validating the sub-burst slope law: a comprehensive multisource spectro-temporal analysis of repeating fast radio bursts

ABSTRACT We conduct a comprehensive spectro-temporal analysis of repeating fast radio bursts (FRBs) utilizing nine distinct sources, the largest sample to date. Our data set includes 175 sub-bursts and 31 multicomponent bursts from 11 data sets, with centre frequencies ranging from 149 to 7144 MHz and durations spanning from 73 µs to 13 ms. Our findings are consistent with the predictions of the triggered relativistic dynamical model (TRDM) of FRB emission. We affirm the predicted quadratic relationship between sub-burst slope and central frequency, as well as a linear dependence of the sub-burst bandwidth on central frequency that is consistent with mildly relativistic Doppler broadening of narrow-band emission. Most importantly, we confirm the sub-burst slope law, a predicted inverse relationship between sub-burst slope and duration, to hold consistently across different sources. Remarkably, we also discover that the drift rates of multicomponent bursts follow the same law as the sub-burst slopes, an unexplained result that warrants further investigation. These findings not only support the TRDM as a viable framework for explaining several aspects of FRB emission, but also provide new insights into the complex spectro-temporal properties of FRBs.

A broad survey of spectro-temporal properties from FRB 20121102A

ABSTRACT We survey the spectro-temporal properties of fast radio bursts from FRB 20121102A observed by earlier studies across a wide range of frequencies. We investigate 167 bursts from FRB 20121102A spanning frequencies 1–7.5 GHz, durations of less than 1 to ∼10 ms, with low and high energies, and with different wait-times. We find from this sample of bursts a strong agreement with the inverse relationship between sub-burst slope and duration and with other predictions made by the triggered relativistic dynamical model. Earlier results found agreement with those predictions across three different repeating FRB sources. For this sample of bursts, we find that the sub-burst slope as well as the ‘sad trombone’ drift rate are consistent with being in a quadratic relationship with frequency and that both these quantities are inversely proportional to the duration. We also find that the duration decreases with increasing frequency as well as a statistically significant correlation between the sub-burst duration and bandwidth (proportional to t−1/2) that is unexpected. No distinct group of bursts in this sample deviated from these relationships, however significant scatter can be seen in measurements. This study demonstrates the consistent existence of relationships between the spectro-temporal properties of bursts from a repeating FRB source. A simple explanation for the inverse relation between the sub-burst slope and duration is an inherently narrowband emission process. We make all measurements available as well as a graphical user interface called Frbgui developed and used to perform measurements of burst waterfalls.

Stellar prospects for FRB gravitational lensing

ABSTRACTGravitational lensing of fast radio bursts (FRBs) offers an exciting avenue for several cosmological applications. However, it is not yet clear how many such events future surveys will detect nor how to optimally find them. We use the known properties of FRBs to forecast detection rates of gravitational lensing on delay time-scales from microseconds to years, corresponding to lens masses spanning 15 orders of magnitude. We highlight the role of the FRB redshift distribution on our ability to observe gravitational lensing. We consider cosmological lensing of FRBs by stars in foreground galaxies and show that strong stellar lensing will dominate on microsecond time-scales. Upcoming surveys such as DSA-2000 and CHORD will constrain the fraction of dark matter in compact objects (e.g. primordial black holes) and may detect millilensing events from intermediate mass black holes (IMBHs) or small dark matter halos. Coherent all-sky monitors will be able to detect longer-duration lensing events from massive galaxies, in addition to short time-scale lensing. Finally, we propose a new application of FRB gravitational lensing that will measure directly the circumgalactic medium of intervening galaxies.

Continuous gravitational wave detection to understand the generation mechanism of fast radio bursts

ABSTRACT Since the unexpected discovery of fast radio bursts (FRBs), researchers have proposed varied theories and models to explain these phenomena. One such model that has recently been developed incorporates the so-called Gertsenshtein–Zel’dovich (GZ) effect, which states that when gravitational waves traverse a pulsar magnetosphere, a portion of the gravitational radiation is transformed into electromagnetic (EM) radiation. The observed properties of FRBs are consistent with the properties of this EM radiation, implying, remarkably, that the GZ effect can account for both repeating and non-repeating FRBs. If this model is correct, the pulsar’s properties should not change over time, and it would continue to emit both EM dipole and gravitational quadrupole radiation for a long period of time. This article targets the gravitational radiation produced by the pulsar mechanism and shows that several proposed gravitational wave detectors can detect these gravitational waves. If such detections are performed in the future from the location of FRBs, it might validate the GZ process for FRB production and potentially rule out several other theories of FRB generation.

Statistical properties of fast radio bursts elucidate their origins: magnetars are favored over gamma-ray bursts

Fast radio bursts (FRBs) are extremely strong radio flares lasting several milliseconds, most of which come from unidentified objects at a cosmological distance. They can be apparently repeating or not. In this paper, we analyzed 18 repeaters and 12 non-repeating FRBs observed in the frequency bands of 400–800 MHz from Canadian Hydrogen Intensity Mapping Experiment (CHIME). We investigated the distributions of FRB isotropic-equivalent radio luminosity, considering the K correction. Statistically, the luminosity distribution can be better fitted by Gaussian form than by power-law. Based on the above results, together with the observed FRB event rate, pulse duration, and radio luminosity, FRB origin models are evaluated and constrained such that the gamma-ray bursts (GRBs) may be excluded for the non-repeaters while magnetars or neutron stars (NSs) emitting the supergiant pulses are preferred for the repeaters. We also found the necessity of a small FRB emission beaming solid angle (about 0.1 sr) from magnetars that should be considered, and/or the FRB association with soft gamma-ray repeaters (SGRs) may lie at a low probability of about 10%. Finally, we discussed the uncertainty of FRB luminosity caused by the estimation of the distance that is inferred by the simple relation between the redshift and dispersion measure (DM).

Comparative Analysis of the Observational Properties of Fast Radio Bursts at the Frequencies of 111 and 1400 MHz

A comparative analysis of the observational characteristics of fast radio bursts at the frequencies 111 and 1400 MHz is carried out. The distributions of radio bursts by the dispersion measure are constructed. At both frequencies, they are described by a lognormal distribution with the parameters $\mu =6.2$ $\sigma = 0.7$. The dependence $\tau_{sc}(DM)$ of the scattering value on the dispersion measure at 111 MHz and 1400 MHz is also constructed. This dependence is fundamentally different from the dependence for pulsars. A comparative analysis of the relationship between the scattering of pulses and the dispersion measure at 1400 MHz and 111 MHz showed that for both frequencies it has the form $\tau_{sc}(DM)\sim DM^k$, where $k = 0.49 \pm 0.18$ and $k = 0.43 \pm 0.15$ for the frequencies 111 and 1400 MHz, respectively. The obtained dependence is explained within the framework of the assumption of the extragalactic occurrence of fast radio bursts and an almost uniform distribution of matter in intergalactic space. From the dependence $\tau_{sc}(DM)$ a total estimate of the contribution to the matter of the halo of our and the host galaxy to $DM$ is obtained $DM_{halo} + \frac{DM_{host}}{1+z}\approx 60\;{\rm pc/cm}^3$. Based on the LogN - LogS dependence, the average spectral index of radio bursts is derived $\alpha = - 0.63 \pm 0.20$ provided that the statistical properties of these samples at 111 and 1400 MHz are the same.

Quark-Novae in the outskirts of galaxies: an explanation of the fast radio burst phenomenon

ABSTRACT We show that old isolated neutron stars in groups and clusters of galaxies experiencing a Quark-Nova phase (QN: an explosive transition to a quark star) may be the source of fast radio bursts (FRBs). Each of the millions of fragments of the ultrarelativistic QN ejecta provides a collisionless plasma for which the ambient medium (galactic/halo, the intragroup/intracluster medium) acts as a relativistic plasma beam. The Buneman and the Weibel instabilities, successively induced by the beam in the fragment, generate particle bunching and observed coherent emission at GHz frequency with a corresponding fluence in the Jy ms range. The duration, frequency drift, and the rate are in agreement with observed properties of FRBs. Repeats (on time-scales of minutes to months) are due to seeing multiple fragments each beaming at a different direction and coming in at different times. Single (non-repeating) FRBs occur when only emission from the primary fragment is within the detector’s sensitivity. Key properties of FRB 121102 (its years of activity) and of FRB 180916.J0158+65 (its ∼16 d period) are recovered. The spatial and temporal coincidence between SGR 1935+2154 and FRB 200428 finds an explanation in our model. We give testable predictions.

Population Modeling of Fast Radio Bursts from Source Properties

Abstract We present a method to estimate the source properties of Fast Radio Bursts (FRBs) from observations by assuming a fixed dispersion measure contribution from a Milky Way–like host galaxy, pulse temporal broadening models for turbulent plasma, and a flat FRB energy spectrum. Then we perform Monte Carlo simulations to constrain the properties of the FRB source, its host galaxy, and scattering in the intervening plasma from the observational data of FRBs detected with Parkes. The typical scatter broadening of the intrinsic pulse is found to be considerably small, ≲ 10−2 − 1 ms, from physical models, with the interstellar medium contribution suppressed significantly relative to that of the intergalactic medium. The intrinsic width for nonrepeating FRBs is broadened by a factor of ∼2–3 on average, primarily due to dispersive smearing. From the simulations, we find that the host galaxy dispersion measure contribution is likely to be comparable to the Galactic contribution and the FRB energy decreases significantly at high frequencies with a negative spectral index. The FRB spatial density is found to increase up to redshift ∼2.0 and then drops significantly at larger distances. We obtain the energy distribution for FRB 121102 with repetition rate ∼0.1–0.3 hr−1 and exponential energy cutoff that is significantly smaller compared to typical FRB energies. We find that the probability of observing none of the other FRBs to be repeating at Parkes is ∼0.8–0.9 with the current follow-up data insufficient to suggest more than one class of FRB progenitors.

Gravitational microlensing time delays at high optical depth: image parities and the temporal properties of fast radio bursts

ABSTRACT Due to differing gravitational potentials and path lengths, gravitational lensing induces time delays between multiple images of a source that, for solar mass objects, are of the order of ∼10−5 s. If an astrophysically compact source, such as a fast radio burst (FRB), is observed through a region with a high optical depth of such microlensing masses, this gravitational lensing time delay can be imprinted on short time-scale transient signals. In this paper, we consider the impact of the parity of the macroimage on the resultant microlensing time delays. It is found that this parity is directly imprinted on the microlensing signal, with macroimages formed at minima of the time arrival surface beginning with the most highly magnified microimages and then progressing to the fainter microimages. For macroimages at the maxima of the time arrival surface, this situation is reversed, with fainter images observed first and finishing with the brightest microimages. For macroimages at saddle points, the signal again begins with fainter images, followed by brighter images before again fading through the fainter microimages. The growing populations of cosmologically distant bursty transient sources will undoubtedly result in the discovery of strong lensed, multiply imaged FRBs, which will be susceptible to microlensing by compact masses. With the temporal resolution being offered by modern and future facilities, the detection of microlensing-induced time delays will reveal the parities of the gravitational lens macroimages, providing additional constraints on macrolensing mass models and improving the efficacy of these transient sources as cosmological probes.

Which bright fast radio bursts repeat?

ABSTRACT A handful of fast radio bursts (FRBs) are now known to repeat. However, the question remains – do they all? We report on an extensive observational campaign with the Australian Square Kilometre Array Pathfinder (ASKAP), Parkes, and Robert C. Byrd Green Bank Telescope, searching for repeat bursts from FRBs detected by the Commensal Real-time ASKAP Fast Transients survey. In 383.2 h of follow-up observations covering 27 FRBs initially detected as single bursts, only two repeat bursts from a single FRB, FRB 171019, were detected, which have been previously reported by Kumar et al. We use simulations of repeating FRBs that allow for clustering in burst arrival times to calculate new estimates for the repetition rate of FRB 171019, finding only slight evidence for incompatibility with the properties of FRB 121102. Our lack of repeat bursts from the remaining FRBs set limits on the model of all bursts being attributable to repeating FRBs. Assuming a reasonable range of repetition behaviour, at most 60 per cent (90 per cent confidence limit) of these FRBs have an intrinsic burst distribution similar to FRB 121102. This result is shown to be robust against different assumptions on the nature of repeating FRB behaviour, and indicates that if indeed all FRBs repeat, the majority must do so very rarely.

Fast Radio Bursts from Activity of Neutron Stars Newborn in BNS Mergers: Offset, Birth Rate, and Observational Properties

Abstract Young neutron stars (NSs) born in core-collapse explosions are promising candidates for the central engines of fast radio bursts (FRBs), since the first localized repeating burst FRB 121102 occurs in a star-forming dwarf galaxy similar to the host galaxies of superluminous supernovae and long gamma-ray bursts. However, FRB 180924 and FRB 190523 are localized to massive galaxies with low rates of star formation, compared with the host of FRB 121102. The offsets between the bursts and host centers are about 4 and 29 kpc for FRB 180924 and FRB 190523, respectively. These host properties are similar to those of short gamma-ray bursts (GRBs), which are produced by binary neutron star (BNS) or NS–black hole mergers. Therefore, the NSs powering FRBs may be formed in BNS mergers. In this paper, we study BNS merger rates and merger times, and predict the most likely merger locations for different types of host galaxies using the population synthesis method. We find that the BNS merger channel is consistent with the recently reported offsets of FRB 180924 and FRB 190523. The offset distribution of short GRBs is well reproduced by population synthesis using a galaxy model similar to that of GRB hosts. The event rate of FRBs (including non-repeating and repeating), is larger than those of BNS mergers and short GRBs, and requires a large fraction of observed FRBs emitting several bursts. Using curvature radiation by bunches in NS magnetospheres, we also predict the observational properties of FRBs from BNS mergers, including the dispersion measure and rotation measure. At late times (t ≥ 1 yr), the contribution to dispersion measure and rotation measure from BNS merger ejecta can be neglected.

Explanation of detailed spectral properties of fast radio bursts by the axion star model

Abstract We have proposed a generation mechanism for non-repeating (repeating) fast radio bursts (FRBs): They arise by axion star collisions with neutron stars (accretion disks of galactic black holes). The axion star, as a coherent state of axions with mass $m_a$, generates a homogeneous electric field oscillating with frequency $m_a/2\pi$ under strong magnetic fields. The field makes electrons oscillate coherently and emit coherent dipole radiation (FRBs). The radiation stops when the oscillations are disturbed by the thermalization of the oscillating energies. Thus, the duration of the FRB is determined by the time scale of the thermalization. We show that it can be shorter than $1$ ms. The line spectra of the dipole radiation are broadened by the thermal effects. The thermally broadened spectra have the feature that the bandwidths $\delta\nu$ are proportional to their center frequencies $\nu_{\rm c}$: $\delta\nu \propto \nu_{\rm c}$. Because the accretion disks can orbit with relativistic velocities, the radiation is Doppler shifted. This leads to the presence of various center frequencies ($\nu_{\rm c}=600 \, \text{MHz} \sim 7$ GHz) in repeating FRB 121102. On the other hand, non-repeating FRBs do not show such a variety in the center frequencies. They come from the surfaces of neutron stars whose motions are non-relativistic. The Doppler shift also makes the duration of bursts with higher frequencies become shorter. Because the magnetic fields of the neutron stars are stronger than those of the accretion disks, the peak flux densities of non-repeating FRBs are larger than those of repeating FRB 121102. They also lead to wide bandwidths of non-repeating FRBs, which cover the full extent of the receiver frequency range. The spectral features of the recently discovered new repeating FRB 180814.J0422+75 are coincident with our general analyses of the repeating FRB 121102.

Emission from a Pulsar Wind Nebula: Application to the Persistent Radio Counterpart of FRB 121102

Abstract The properties of fast radio bursts (FRBs) indicate that the physical origin of this type of astrophysical phenomenon is related to neutron stars. The first detected repeating source, FRB 121102, is associated with a persistent radio counterpart. In this paper, we propose that this radio counterpart could arise from a pulsar wind nebula powered by a magnetar without surrounding supernova ejecta. Its medium is a stratified structure produced by a progenitor wind. The model parameters are constrained by the spectrum of the counterpart emission, the size of the nebula, and the large but decreasing rotation measure (RM) of the repeating bursts. In addition, the observed dispersion measure is consistent with the assumption that all of the RM comes from the shocked medium.

Explaining the Statistical Properties of Fast Radio Bursts with Suppressed Low-frequency Emission

Abstract The possibility of fast radio burst (FRB) emission being suppressed at low frequencies, resulting in a cutoff of the average rest-frame spectrum, has been raised as an explanation for the lack of detections at meter wavelengths. We examine propagation effects that could cause this suppression, and find that a low-frequency spectral cutoff may be generic regardless of the specific FRB emission mechanism. We then illustrate the effects of a low-frequency spectral cutoff on the statistics of FRBs, given a cosmological source population. The observed FRB rate peaks at a specific frequency under a variety of assumptions. Observations at lower frequencies are more sensitive to high-redshift events than observations above the maximal-rate frequency, and therefore result in more sharply broken fluence distributions. Our results suggest that the absence of low-frequency FRBs, and the differences between the Parkes and the Australian Square Kilometre Array FRB samples, can be fully explained by suppressed low-frequency FRB emission.

Fast radio bursts: Superpulsars, magnetars, or something else?

We briefly review main observational properties of fast radio bursts (FRBs) and discuss two most popular hypothesis for the explanation of these enigmatic intense millisecond radio flashes. FRBs most probably originate on extragalactic distances, and their rate on the sky is about a few thousand per day with fluences above [Formula: see text]1[Formula: see text]Jy[Formula: see text]ms (or with fluxes larger than few tenths of Jy). Two leading scenarios describing these events include strong flares of magnetars and supergiant pulses of young radio pulsars with large rotational energy losses, correspondingly. At the moment, it is impossible to choose between these models. However, new telescopes can help to solve the puzzle of FRBs in near future.

The observed properties of fast radio bursts

I present an empirical study of the properties of fast radio bursts (FRBs): Gigahertz-frequency, dispersed pulses of extragalactic origin. I focus my investigation on the sample of seventeen FRBs detected at the Parkes radio telescope with largely self-consistent instrumentation. Of this sample, six are temporally unresolved, eight exhibit evidence for scattering in inhomogeneous plasma, and five display potentially intrinsic temporal structure. The characteristic scattering timescales at a frequency of 1 GHz range between 0.005 ms and 32 ms; moderate evidence exists for a relation between FRB scattering timescales and dispersion measures. Additionally, I present constraints on the fluences of Parkes FRBs, accounting for their uncertain sky-positions, and use the multiple-beam detection of FRB 010724 (the Lorimer burst) to measure its fluence to be $800\pm400$ Jy ms. FRBs, including the repeating FRB 121102, appear to manifest with a plethora of characteristics, and it is uncertain at present whether they share a common class of progenitor object, or arise from a selection of independent progenitors.

The SUrvey for Pulsars and Extragalactic Radio Bursts – III. Polarization properties of FRBs 160102 and 151230

We report on the polarization properties of two fast radio bursts (FRBs): 151230 and 160102 discovered in the SUrvey for Pulsars and Extragalactic Radio Bursts (SUPERB) at the Parkes radio telescope. FRB 151230 is observed to be 6 $\pm$ 11% circularly polarized and 35 $\pm$ 13 % linearly polarized with a rotation measure (RM) consistent with zero. Conversely, FRB 160102 is observed to have a circular polarization fraction of 30 $\pm$ 11 %, linear polarization fraction of 84 $\pm$ 15 % for RM = -221(6) rad m$^{-2}$ and the highest measured DM (2596.1 $\pm$ 0.3 pc cm$^{-3}$) for an FRB to date. We examine possible progenitor models for FRB 160102 in extragalactic, non-cosmological and cosmological scenarios. After accounting for the Galactic foreground contribution, we estimate the intrinsic RM to be -256(9) rad m$^{-2}$ in the low-redshift case and $\sim 2.4 \times 10^{2}$ rad m$^{-2}$ in the high-redshift case. We assess the relative likeliness of these scenarios and how each can be tested. We also place constraints on the scattering measure and study the impact of scattering on the signal's polarization position angle.

FRB 121102: A Repeatedly Combed Neutron Star by a Nearby Low-luminosity Accreting Supermassive Black Hole

Abstract The origin of fast radio bursts (FRBs) remains mysterious. Recently, the only repeating FRB source, FRB 121102, was reported to possess an extremely large and variable rotation measure (RM). The inferred magnetic field strength in the burst environment is comparable to that in the vicinity of the supermassive black hole Sagittarius A* of our Galaxy. Here, we show that all of the observational properties of FRB 121102 (including the high RM and its evolution, the high linear polarization degree, an invariant polarization angle across each burst and other properties previously known) can be interpreted within the “cosmic comb” model, which invokes a neutron star with typical spin and magnetic field parameters whose magnetosphere is repeatedly and marginally combed by a variable outflow from a nearby low-luminosity accreting supermassive black hole in the host galaxy. We propose three falsifiable predictions (periodic “on/off” states, and periodic/correlated variation of RM and polarization angle) of the model and discuss other FRBs within the context of the cosmic comb model as well as the challenges encountered by other repeating FRB models in light of the new observations.