Fast Radio Bursts Progenitors Research Articles

Repeating fast radio bursts with WSRT/Apertif

Context. Repeating fast radio bursts (FRBs) present excellent opportunities to identify FRB progenitors and host environments as well as to decipher the underlying emission mechanism. Detailed studies of repeating FRBs might also hold clues as to the origin of FRBs as a population. Aims. We aim to detect bursts from the first two repeating FRBs, FRB 121102 (R1) and FRB 180814.J0422+73 (R2), and to characterise their repeat statistics. We also want to significantly improve the sky localisation of R2 and identify its host galaxy. Methods. We used the Westerbork Synthesis Radio Telescope to conduct extensive follow-up of these two repeating FRBs. The new phased-array feed system, Apertif, allows one to cover the entire sky position uncertainty of R2 with fine spatial resolution in a single pointing. The data were searched for bursts around the known dispersion measures of the two sources. We characterise the energy distribution and the clustering of detected R1 bursts. Results. We detected 30 bursts from R1. The non-Poissonian nature is clearly evident from the burst arrival times, which is consistent with earlier claims. Our measurements indicate a dispersion measure (DM) of 563.5(2) pc cm−3, suggesting a significant increase in DM over the past few years. Assuming a constant position angle across the burst, we place an upper limit of 8% on the linear polarisation fraction for the brightest burst in our sample. We did not detect any bursts from R2. Conclusions. A single power-law might not fit the R1 burst energy distribution across the full energy range or widely separated detections. Our observations provide improved constraints on the clustering of R1 bursts. Our stringent upper limits on the linear polarisation fraction imply a significant depolarisation, either intrinsic to the emission mechanism or caused by the intervening medium at 1400 MHz, which is not observed at higher frequencies. The non-detection of any bursts from R2, despite nearly 300 h of observations, implies either a highly clustered nature of the bursts, a steep spectral index, or a combination of the two assuming that the source is still active. Another possibility is that R2 has turned off completely, either permanently or for an extended period of time.

The Remnant of Neutron Star-White Dwarf Merger and the Repeating Fast Radio Bursts

Fast radio bursts (FRBs) at cosmological distances still hold concealed physical origins. Previously Liu (2018) proposes a scenario that the collision between a neutron star (NS) and a white dwarf (WD) can be one of the progenitors of non-repeating FRBs and notices that the repeating FRBs can also be explained if a magnetar formed after such NS-WD merger. In this paper, we investigate this channel of magnetar formation in more detail. We propose that the NS-WD post-merger, after cooling and angular momentum redistribution, may collapse to either a black hole or a new NS or even remains as a hybrid WDNS, depending on the total mass of the NS and WD. In particular, the newly formed NS can be a magnetar if the core of the WD collapsed into the NS while large quantities of degenerate electrons of the WD compressed to the outer layers of the new NS. A strong magnetic field can be formed by the electrons and positive charges with different angular velocities induced by the differential rotation of the newborn magnetar. Such a magnetar can power the repeating FRBs by the magnetic reconnections due to the crustal movements or starquakes.

The prevalence of repeating fast radio bursts

Fast radio bursts are extragalactic, sub-millisecond radio impulses of unknown origin [1,2]. Their dispersion measures, which quantify the observed frequency-dependent dispersive delays in terms of free-electron column densities, significantly exceed predictions from models [3] of the Milky Way interstellar medium. The excess dispersions are likely accrued as fast radio bursts propagate through their host galaxies, gaseous galactic halos and the intergalactic medium [4,5]. Despite extensive follow-up observations of the published sample of 72 burst sources [6], only two are observed to repeat [7,8], and it is unknown whether or not the remainder are truly one-off events. Here I show that the volumetric occurrence rate of so far non-repeating fast radio bursts likely exceeds the rates of candidate cataclysmic progenitor events, and also likely exceeds the birth rates of candidate compact-object sources. This analysis is based on the high detection rate of bursts with low dispersion measures by the Canadian Hydrogen Intensity Mapping Experiment [9]. Within the existing suite of astrophysical scenarios for fast radio burst progenitors, I conclude that most observed cases originate from sources that emit several bursts over their lifetimes.

The luminosity–volume test for cosmological fast radio bursts

We have applied the luminosity–volume test, also known as ⟨V/Vmax⟩, to fast radio bursts (FRBs). We compare the 23 FRBs, recently discovered by ASKAP, with 20 of the FRBs found by Parkes. These samples have different flux limits and correspond to different explored volumes. We put constrains on their redshifts with probability distributions (PDFs) and applied the appropriate cosmological corrections to the spectrum and rate in order to compute the ⟨V/Vmax⟩ for the ASKAP and Parkes samples. For a radio spectrum of FRBs ℱν ∝ ν−1.6, we found ⟨V/Vmax⟩ = 0.68 ± 0.05 for the ASKAP sample, that includes FRBs up to z = 0.72+0.42−0.26, and 0.54 ± 0.04 for Parkes, that extends up to z = 2.1+0.47−0.38. The ASKAP value suggests that the population of FRB progenitors evolves faster than the star formation rate, while the Parkes value is consistent with it. Even a delayed (as a power law or Gaussian) star formation rate cannot reproduce the ⟨V/Vmax⟩ of both samples. If FRBs do not evolve in luminosity, the ⟨V/Vmax⟩ values of ASKAP and Parkes sample are consistent with a population of progenitors whose density strongly evolves with redshift as ∼z2.8 up to z ∼ 0.7.

The slope of the source-count distribution for fast radio bursts

The slope of the source-count distribution of fast radio burst (FRB) fluences, $\alpha$, has been estimated using a variety of methods. Hampering all attempts have been the low number of detected FRBs, and the difficulty of defining a completeness threshold for FRB surveys. In this work, we extend maximum-likelihood methods for estimating $\alpha$, using detected and threshold signal-to-noise ratios applied to all FRBs in a sample without regard to a completeness threshold. Using this method with FRBs detected by the Parkes radio telescope, we find $\alpha=-1.18 \pm 0.24$ (68\% confidence interval, C.I.), i.e.\ consistent with a non-evolving Euclidean distribution ($\alpha=-1.5$). Applying these methods to the Australian Square Kilometre Array Pathfinder (ASKAP) Commensal Real-time ASKAP Fast Transients (CRAFT) FRB survey finds $\alpha=-2.2 \pm 0.47$ (68\% C.I.). A full maximum-likelihood estimate finds an inconsistency with the Parkes rate with a p-value of 0.86\% ($2.6\, \sigma$). If not due to statistical fluctuations or biases in Parkes data, this is the first evidence for deviations from a pure power law in the integral source-count distribution of FRBs. It is consistent with a steepening of the integral source-count distribution in the fluence range 5--40\,Jy\,ms, for instance due to a cosmological population of FRB progenitors evolving more rapidly than the star-formation rate, and peaking in the redshift range 1--3.

The future of fast radio burst science

The field of Fast Radio Burst (FRB) science is currently thriving and growing rapidly. The lines of active investigation include theoretical and observational aspects of these enigmatic millisecond radio signals. These pursuits are for the most part intertwined so that each keeps the other in check, characteristic of the healthy state of the field. The immediate future for FRB science is full of promise --- we will in the next few years see two orders of magnitude more FRBs discovered by the now diverse group of instruments spread across the globe involved in these efforts. This increased crop, and the increased information obtained per event, will allow a number of fundamental questions to be answered, and FRBs' potential as astrophysical and cosmological tools to be exploited. Questions as to the exact detailed nature of FRB progenitors and whether or not there are one or more types of progenitor will be answered. Questions as to source counts, the luminosity distribution and cosmological density of FRBs will also be addressed. Looking further ahead, applications involving FRBs at the highest redshifts look set to be a major focus of the field. The potential exists to evolve to a point where statistically robust cosmological tests, orthogonal to those already undertaken in other ways, will be achieved. Related work into FRB foregrounds, as well as how to identify new events in ever more challenging radio-frequency interference environments, also appear likely avenues for extensive investigations in the coming years.

Fast radio burst source properties from polarization measurements

Recent polarization measurements of fast radio bursts (FRBs) provide new insights on these enigmatic sources. We show that the nearly 100% linear polarization and small variation of the polarization position angles (PAs) of multiple bursts from the same source suggest that the radiation is produced near the surface of a strongly magnetized neutron star. As the emitted radiation travels through the magnetosphere, the electric vector of the X-mode wave adiabatically rotates and stays perpendicular to the local magnetic field direction. The PA freezes at a radius where the plasma density becomes too small to be able to turn the electric vector. At the freeze-out radius, the electric field is perpendicular to the magnetic dipole moment of the neutron star projected in the plane of the sky, independent of the radiation mechanism or the orientation of the magnetic field in the emission region. We discuss a number of predictions of the model. The variation of PAs from repeating FRBs should follow the rotational period of the underlying neutron star (but the burst occurrence may not be periodic). Measuring this period will provide crucial support for the neutron star nature of the progenitors of FRBs. For FRB 121102, the small range of PA variation means that the magnetic inclination angle is less than about 20 degrees and that the observer's line of sight is outside the magnetic inclination cone. Other repeating FRBs may have a different range of PA variation from that of FRB 121102, depending on the magnetic inclination and the observer's viewing angle.

Synchrotron Maser from Weakly Magnetized Neutron Stars as the Emission Mechanism of Fast Radio Bursts

Abstract The origin of fast radio bursts (FRBs) is still mysterious. All FRBs to date show extremely high brightness temperatures, requiring a coherent emission mechanism. Using constraints derived from the physics of one of these mechanisms, the synchrotron maser, as well as observations, we show that accretion-induced explosions of neutron stars with surface magnetic fields of B * ≲ 1011 G are favored as FRB progenitors.

The Dispersion and Rotation Measure of Supernova Remnants and Magnetized Stellar Winds: Application to Fast Radio Bursts

Abstract Recent studies of fast radio bursts (FRBs) have led to many theories associating them with young neutron stars. If this is the case, then the presence of supernova ejecta and stellar winds provides a changing dispersion measure (DM) and rotation measure (RM) that can potentially be probes of the environments of FRB progenitors. Here we summarize the scalings for the DM and RM in the cases of a constant density ambient medium and of a progenitor stellar wind. Since the amount of ionized material is controlled by the dynamics of the reverse shock, we find that the DM changes more slowly than in previous simpler work, which assumed a constant ionization fraction. Furthermore, the DM can be constant or even increasing as the supernova remnant sweeps up material, arguing that a young neutron star hypothesis for FRBs is not ruled out if the DM is not decreasing over repeated bursts. The combined DM and RM measurements for the repeating FRB 121102 are consistent with supernova ejecta with an age of ∼102–103 years expanding into a high density (∼100 cm−3) interstellar medium. This naturally explains its relatively constant DM over many years as well. Other FRBs with much lower RMs may indicate that they are especially young supernovae in wind environments or that their DMs are largely from the intergalactic medium. We therefore caution about inferring magnetic fields simply by dividing an RM by DM, because these quantities could originate from distinct regions along the path an FRB propagates.

Associating Fast Radio Bursts with Their Host Galaxies

Abstract The first precise localization of a fast radio burst (FRB) sheds light on the nature of these mysterious bursts and the physical mechanisms that power them. Increasing the sample of FRBs with robust host galaxy associations is the key impetus behind ongoing and upcoming searches and facilities. Here, we quantify the robustness of FRB host galaxy associations as a function of localization area and galaxy apparent magnitude. We also explore the use of FRB dispersion measures to constrain the source redshift, thereby reducing the number of candidate hosts. We use these results to demonstrate that even in the absence of a unique association, a constraint can be placed on the maximum luminosity of a host galaxy as a function of localization and dispersion measure (DM). We find that localizations of are required for a chance coincidence probability of for dwarf galaxies at if some hosts have luminosities of , then localizations of up to may suffice at . Constraints on the redshift from the DM only marginally improve the association probability unless the DM is low, pc cm−3. This approach also relies on the determination of galaxy redshifts, which is challenging at if the hosts are dwarf galaxies. Finally, interesting limits on the maximum host luminosity require localizations of at . Even a few such localizations will explain the nature of FRB progenitors, their possible diversity, and their use as cosmological tools.

Fast Radio Bursts

We summarize our current state of knowledge of fast radio bursts (FRBs) which were first discovered a decade ago. Following an introduction to radio transients in general, including pulsars and rotating radio transients, we discuss the discovery of FRBs. We then discuss FRB follow-up observations in the context of repeat bursts before moving on to review propagation effects on FRB signals, FRB progenitor models and an outlook on FRBs as potential cosmological tools.

GRAVITATIONAL-WAVE CONSTRAINTS ON THE PROGENITORS OF FAST RADIO BURSTS

ABSTRACT The nature of fast radio bursts (FRBs) remains enigmatic. Highly energetic radio pulses of millisecond duration, FRBs are observed with dispersion measures consistent with an extragalactic source. A variety of models have been proposed to explain their origin. One popular class of theorized FRB progenitor is the coalescence of compact binaries composed of neutron stars and/or black holes. Such coalescence events are strong gravitational-wave emitters. We demonstrate that measurements made by the LIGO and Virgo gravitational-wave observatories can be leveraged to severely constrain the validity of FRB binary coalescence models. Existing measurements constrain the binary black hole rate to approximately 5% of the FRB rate, and results from Advanced LIGO’s O1 and O2 observing runs may place similarly strong constraints on the fraction of FRBs due to binary neutron star and neutron star–black hole progenitors.

A survey of FRB fields: limits on repeatability

Several theories exist to explain the source of the bright, millisecond duration pulses known as fast radio bursts (FRBs). If the progenitors of FRBs are non-cataclysmic, such as giant pulses from pulsars, pulsar–planet binaries, or magnetar flares, FRB emission may be seen to repeat. We have undertaken a survey of the fields of eight known FRBs from the High Time Resolution Universe survey to search for repeating pulses. Although no repeat pulses were detected the survey yielded the detection of a new FRB, described in Petroff et al. (2015a). From our observations we rule out periodic repeating sources with periods P ≤ 8.6 h and rule out sources with periods 8.6 < P < 21 h at the 90 per cent confidence level. At P ≥ 21 h our limits fall off as ∼1/P. Dedicated and persistent observations of FRB source fields are needed to rule out repetition on longer time-scales, a task well-suited to next generation wide-field transient detectors.

The birth of black holes: neutron star collapse times, gamma-ray bursts and fast radio bursts

Recent observations of short gamma-ray bursts (SGRBs) suggest that binary neutron star (NS) mergers can create highly magnetised, millisecond NSs. Sharp cut-offs in X-ray afterglow plateaus of some SGRBs hint at the gravitational collapse of these remnant NSs to black holes. The collapse of such `supramassive' NSs also describes the blitzar model, a leading candidate for the progenitors of fast radio bursts (FRBs). The observation of an FRB associated with an SGRB would provide compelling evidence for the blitzar model and the binary NS merger scenario of SGRBs, and lead to interesting constraints on the NS equation of state. We predict the collapse times of supramassive NSs created in binary NS mergers, finding that such stars collapse $\sim10\,{\rm s}$ -- $4.4\times10^{4}\,{\rm s}$ (95% confidence) after the merger. This directly impacts observations targeting NS remnants of binary NS mergers, providing the optimal window for high time resolution radio and X-ray follow-up of SGRBs and gravitational wave bursts.

Constraining fast radio burst progenitors with gravitational lensing

Fast Radio Bursts (FRBs) are new transient radio sources discovered recently. Because of the angular resolution restriction in radio surveys, no optical counter part has been identified yet so it is hard to determine the progenitor of FRBs. In this paper we propose to use radio lensing survey to constrain FRB progenitors. We show that, different types of progenitors lead to different probabilities for a FRB to be gravitationally lensed by dark matter halos in foreground galaxies, since different type progenitors result in different redshift distributions of FRBs. For example, the redshift distribution of FRBs arising from double stars shifts toward lower redshift than of the FRBs arising from single stars, because double stars and single stars have different evolution timescales. With detailed calculations, we predict that the FRB sample size for producing one lensing event varies significantly for different FRB progenitor models. We argue that this fact can be used to distinguish different FRB models and also discuss the practical possibility of using lensing observation in radio surveys to constrain FRB progenitors.