Escaping of Fast Radio Bursts

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.

One widely discussed mechanism to produce highly coherent radio emission of fast radio bursts (FRBs) is coherent emission by bunches, either via curvature radiation or inverse Compton scattering (ICS). It has been suggested that the plasma oscillation effect can significantly suppress coherent emission power by bunches. We examine this criticism in this paper. The suppression factor formalism was derived within the context of radio pulsars in which radio waves are in the low-amplitude, linear regime and cannot directly be applied to the large-amplitude, non-linear regime relevant for FRBs. Even if one applies this linear treatment, plasma suppression is not important for two physical reasons. First, for an efficient radiation mechanism, such as ICS, the required plasma density is not high so that a high-density plasma may not exist. Secondly, both bunched coherent mechanisms demand that a large global parallel electric field (E∥) must exist in the emission region in order to continuously inject energy to the bunches to power an FRB. In order to produce typical FRB duration via coherent curvature or ICS radiation, a parallel electric field must be present to balance the acceleration and radiation back reaction. The plasma suppression factor should be modified with the existence of E∥. We show that the correction factor for curvature radiation, fcur, increases with E∥ and becomes 1 when E∥ reaches the radiation-reaction-limited regime. We conclude that the plasma suppression effect can be ignored for realistic FRB emission models invoking bunched coherent radio emission.

The mechanism that produces fast radio burst (FRB) emission is poorly understood. Targeted monitoring of repeating FRB sources provides the opportunity to fully characterize the emission properties in a manner impossible with one-off bursts. Here, we report observations of the source of FRB 20201124A, with the Australian Square Kilometre Array Pathfinder (ASKAP) and the ultra-wideband low (UWL) receiver at the Parkes 64-m radio telescope (Murriyang). The source entered a period of emitting bright bursts during early 2021 April. We have detected 16 bursts from this source. One of the bursts detected with ASKAP is the brightest burst ever observed from a repeating FRB source with an inferred fluence of 640 ± 70 Jy ms. Of the five bursts detected with the Parkes UWL, none display any emission in the range 1.1–4 GHz. All UWL bursts are highly polarized, with their Faraday rotation measures (RMs) showing apparent variations. We obtain an average RM of −614 rad m−2 for this FRB source with a standard deviation of 16 rad m−2 in the UWL bursts. In one of the UWL bursts, we see evidence of significant circularly polarized emission with a fractional extent of 47 ± 1 per cent. Such a high degree of circular polarization has never been seen before in bursts from repeating FRB sources. We also see evidence for significant variation in the linear polarization position angle in the pulse profile of this UWL repeat burst. Models for repeat burst emission will need to account for the increasing diversity in the burst polarization properties.

Recently a repeating fast radio burst (FRB) 121102 has been confirmed to be an extragalactic event and a persistent radio counterpart has been identified. While other possibilities are not ruled out, the emission properties are broadly consistent with Murase et al. (2016) that theoretically proposed quasi-steady radio emission as a counterpart of both FRBs and pulsar-driven supernovae. Here we constrain the model parameters of such a young neutron star scenario for FRB 121102. If the associated supernova has a conventional ejecta mass of $M_{\rm ej}\gtrsim{\rm a \ few}\ M_\odot$, a neutron star with an age of $t_{\rm age} \sim 10-100 \ \rm yrs$, an initial spin period of $P_{i} \lesssim$ a few ms, and a dipole magnetic field of $B_{\rm dip} \lesssim {\rm a \ few} \times 10^{13} \ \rm G$ can be compatible with the observations. However, in this case, the magnetically-powered scenario may be favored as an FRB energy source because of the efficiency problem in the rotation-powered scenario. On the other hand, if the associated supernova is an ultra-stripped one or the neutron star is born by the accretion-induced collapse with $M_{\rm ej} \sim 0.1 \ M_\odot$, a younger neutron star with $t_{\rm age} \sim 1-10$ yrs can be the persistent radio source and might produce FRBs with the spin-down power. These possibilities can be distinguished by the decline rate of the quasi-steady radio counterpart.

An approximate analytical solution for the rotating twisted magnetosphere of magnetars is presented. The poloidal flux is approximated by the self-similar twisted dipole field. The toroidal field is obtained by the minimum torque model. Under this approximation, it is found that: (1) the Y-point radius decreases with the increase of twist of the magnetic field. (2) The polar cap is larger for larger twist. (3) The particle outflow luminosity is larger for larger twist. (4) The maximum acceleration potential, pulse width of magnetar radio emission both increase with the twist. (5) For an untwisting magnetosphere, the physical properties evolve towards that of the normal pulsars. The above findings are consistent with previous analytical and numerical results. The larger polar cap may correspond to the hot spot during magnetar outburst. In general, a rotating twisted magnetosphere has larger open field line regions. The radio emission of magnetars and fast radio bursts may both originate in the larger and evolving open field line regions of magnetars.

The seminal strong-field tunnelling theory introduced by L V Keldysh plays a pivotal role. It has shaped our understanding of atomic strong-field processes, where it represents the first step in complex ionisation dynamics and provides reliable tunnelling rates. Tunnelling rates, however, cannot be necessarily equated with ionisation rates. Taking into account the electron dynamics in the Coulomb potential following the tunnelling process, the process of frustrated tunnelling ionisation has been found to lead to excited Rydberg atoms. Here, we excite He atoms in the strong-field tunnelling regime into Rydberg states. A high percentage of these Rydberg atoms survive in high intensity laser fields. We exploit this fact together with their high polarisability to kinematically manipulate the Rydberg atoms with a second elliptically polarised focused strong laser field. By varying the spatial overlap of the two laser foci, we are able to selectively control the deflection of the Rydberg atoms. The results of semi-classical calculations, which are based on the frustrated tunnelling model and on the ponderomotive acceleration, are in accord with our experimental data.

: The objective of this ONR program is to develop fundamental understanding on the material requirement of dielectric polymers in order to achieve high energy density with low loss and high dielectric strength. On the material front, the objective is to design and develop dielectric polymers of high energy density and low losses for pulse power applications. In the past three years, we have developed and investigated dielectric polymers with strongly coupled dipoles (modified PVDF based polymers) and their blends to achieve these objectives. Built upon the success of the polar-fluoropolymers, we have investigated a series of high temperature dielectrics with molecular units of high dipole moment and developed new class of polymer dielectrics based on aromatic polythiourea (ArPTU), which is capable of maintaining its low dielectric loss (high electric charge-discharge energy efficiency 90%) up to an ultra high operation fields ( 1 GV/m) with a very high electric energy density ( 24 J/cm3). To develop understanding of the molecular and nano-structure mechanisms, a systematic dielectric studies, including the thermal stimulated discharge current (TSDC) measurement, have been carried out. It was shown that it is the deep traps with high activation energy and large trap density in the PTU thin films that are responsible for the observed low conduction loss and high dielectric strength. Trap-limited high field conduction was studied by the electron hopping theory in the amorphous material.

SGR 1935+2154 has emerged as the most active magnetar in recent years, exhibiting X-ray burst activity nearly annually and emitting a fast radio burst (FRB 200428 (FRB1)) in its 2020 April activity, for the first time from a Milky Way source, accompanied by an X-ray burst (XFRB1). The source was active again in 2021 September and 2022 January, but without emitting radio bursts. In 2022 October, the magnetar entered a new activity that included radio bursts, three of them reported to be associated with X-ray bursts. We present a comprehensive study of the activity, as observed by the Fermi Gamma-Ray Burst Monitor, and offer a comparison with the previous activities. The mean burst duration of the observed 113 X-ray bursts (∼122 ms) is ∼30% shorter than in 2020 April but longer than most other activities. The mean spectral parameters (ΓCPL ∼ 0.82, E p CPL ∼ 26 keV and kT BBs ∼ 5.10 keV, kT BBh ∼ 8.96 keV) are generally comparable to the previous activities but with a softer ΓCPL and larger R BBh than in 2020 April. The R BB 2 – kT BB parameter space shows a marked contrast where two distinctive, nonoverlapping, and smaller soft and hard blackbody regions are manifested in 2020 April but not in 2022 October and the other episodes. The fluence–duration power-law correlation is ∼60% steeper in 2022 October and the other episodes than in 2020 April. The three new X-ray bursts associated with radio bursts stand out in being ∼2–10-fold longer than the mean duration, with X-ray-to-radio duration, flux, and fluence ratios of ∼102–3, 103–4, and 105–7, respectively, comparable to the XFRB1/FRB1 ratio for duration but larger for flux and fluence due to fainter radio bursts in 2022. We discuss the results in the context of the new emission models that propose physical mechanisms for the X-ray and radio burst emission from magnetars.

Electromagnetic precursor waves generated by the synchrotron maser instability at relativistic magnetized shocks have been recently invoked to explain the coherent radio emission of fast radio bursts. By means of 2D particle-in-cell simulations, we explore the properties of the precursor waves in relativistic electron–positron perpendicular shocks as a function of the pre-shock magnetization σ ≳ 1 (i.e. the ratio of incoming Poynting flux to particle energy flux) and thermal spread Δγ ≡ kT/mc2 = 10−5−10−1. We measure the fraction fξ of total incoming energy that is converted into precursor waves, as computed in the post-shock frame. At fixed magnetization, we find that fξ is nearly independent of temperature as long as Δγ ≲ 10−1.5 (with only a modest decrease of a factor of 3 from Δγ = 10−5 to Δγ = 10−1.5), but it drops by nearly two orders of magnitude for Δγ ≳ 10−1. At fixed temperature, the scaling with magnetization $f_\xi \sim 10^{-3}\, \sigma ^{-1}$ is consistent with our earlier 1D results. For our reference σ = 1, the power spectrum of precursor waves is relatively broad (fractional width ∼1 − 3) for cold temperatures, whereas it shows pronounced line-like features with fractional width ∼0.2 for 10−3 ≲ Δγ ≲ 10−1.5. For σ ≳ 1, the precursor waves are beamed within an angle ≃σ−1/2 from the shock normal (as measured in the post-shock frame), as required so they can outrun the shock. Our results can provide physically grounded inputs for FRB emission models based on maser emission from relativistic shocks.

Stabilization effect of a strong HF electrical field on beam–plasma interaction in a relativistic plasma waveguide

We present a time-dependent model of the high energy emission of blazars based on a pair creation instability that can develop spontaneously in the two-flow model. In this scenario, a relativistic pair plasma is generated inside a mildly relativistic jet emitted by an accretion disk. Pairs cool by Synchrotron Self Compton (SSC) and External Compton (EC) process and produce gamma-rays that are self absorbed to produced new pairs. Thanks to continuous reacceleration by the MHD turbulence, a pair runaway instability can develop, which can saturate only through the damping of turbulence. We adopt a simple prescription to describe this effect. In some regime, a limiting cycle can develop, with oscillations of pair densities and turbulence level along the jet, producing flares in high energy emission and at other wavelengths. Although calculations have been carried out assuming a power law distribution of particles, it seems that a pile-up distribution could better account for TeV BL Lacs.

This paper presents a novel miniature 2 × 2 optical switch with double vertical micromirrors driven by single piezoelectric cantilever using PZT (Lead Zirconium Titanate) films. Double vertical micromirrors, piezoelectric cantilever and optical fiber V-grooves are integrated on a single silicon chip in compact and low-cost form. The optical switch switches light beams by moving perpendicularly double vertical micromirrors with smooth side surfaces simultaneously which are driven by single piezoelectric cantilever using PZT films. The fabrication process of the optical switch is described. Simulation and analysis have been carried out to evaluate mechanical performance of the optical switch. Due to low insertion loss, small switch size and low power dissipation, this novel optical switch should be promising for optical network applications.

Active galactic nuclei, X-ray binaries, pulsars and gamma-ray bursts are all believed to be powered by compact objects surrounded by relativistic plasma flows driving phenomena such as accretion, winds and jets. These flows are often accurately modelled by the relativistic magnetohydrodynamic (MHD) approximation. Time-dependent numerical MHD simulations have proven to be especially insightful, but one regime that remains difficult to simulate is when the energy scales (kinetic, thermal, magnetic) within the plasma become disparate. We develop a numerical scheme that significantly improves the accuracy and robustness of the solution in this regime. We use a modified form of the weighted essentially non-oscillatory (WENO) method to construct a finite-volume general relativistic hydrodynamics code called WHAM that converges at fifth order. We avoid (1) field-by-field decomposition by adaptively reducing down to two-point stencils near discontinuities for a more accurate treatment of shocks and (2) excessive reduction to low-order stencils, as in the standard WENO formalism, by maintaining high-order accuracy in smooth monotonic flows. Our scheme performs the proper surface integral of the fluxes, converts cell-averaged conserved quantities to point-conserved quantities before performing the reconstruction step, and correctly averages all source terms. We demonstrate that the scheme is robust in strong shocks, very accurate in smooth flows and maintains accuracy even when the energy scales in the flow are highly disparate.

Strong plasma outflows can be driven by plasma injected into the magnetosphere of rapidly rotating compact objects. The rapid rotation also builds up a strong toroidal component of the magnetic field, which efficiently pinches the outflowing plasma leading to the formation of collimated relativistic jets. The appearance of the light cylinder due to this rapid rotation requires a fully relativistic treatment of the entire problem. We present the most general theory of axisymmetric relativistic plasma flows including the effects of the gravitational background, the structure of the magnetic flux tube and the finite pressure of the plasma. With this reduction of the relativistic MHD we end up with a nonlinearly coupled system consisting of the hot wind equation and the relativistic Grad-Schluter-Shafranov equation for the poloidal magnetic flux. We show self-consistent computer solutions of this system resulting in the formation of supermagnetosonic jets flowing along the rotational axis of the central object.

The flux-rope theory of solar magnetic fields is reviewed briefly and, together with the dynamo theory, compared with various observational results. Dynamo and related theories are based on fields controlled by the plasma, and it is shown that such fields cannot account for the strong surface fields or even emerge without becoming tangled. Observations which appear uniquely explicable in terms of powerful (⪝4000 G), helically twisted flux ropes and their many twisted flux fibres (≈3×1018 Mx) are listed as follows. (i) Emerging magnetic flux is seen first as pairs of small, closely spaced flux concentrations whose motions suggest magnetic control to provide bipolar regions of extent⪞105 km. The associated system of arch filaments rotates on the disk as would a series of emerging flux fibres twisted into a rope. (ii) Sunspots form by the accretion of pores and magnetic knots of like polarity, sometimes moving along curved paths between stationary elements of opposite polarity. (iii) Fluxes of⪞1022 Mx in large sunspots must have been concentrated to strengths of⪞4000 G before emerging, and also strongly helically twisted to avoid the flute instability. (iv) The trumpet-shaped flux-rope-fibre sunspot model (Figure 6) accounts readily for the phenomena of the moat convection, the sunspot energy deficit, the complex Evershed flow, penumbral filaments (flux ≈3×1018 Mx) and temporary light bridges. (v) Asymmetries in sunspot groups (in spot size, lifetime and proper motion) show that the spot fields are extensions of two submerged magnetic structures comprising strong fields. (vi) Sunspots decay by the loss of magnetic knots with strong fields and flux ≈5×1018 Mx. These must be isolated flux tubes, twisted to account for their stability. (vii) Flux fibres leaving a spot are prone to the kink instability, thus accounting for their sudden appearance in pairs, the transport of total flux several times that of the spot and net flux equal to that of the spot. (viii) Ephemeral active regions and X-ray bright points are explained similarly without invoking improbably huge quantities of new flux. (ix) Atmospheric structures show a high prevalence of helical twists (force-free fields) and rotary motions on all scales from spicules to large prominences. It is difficult to account for these twists unless they are present in emerging flux. (x) In and above the photosphere the flux fibres (≈3×1018 Mx) fray into loose associations of flux threads (≈3×1017 Mx) to provide a simple, selfconsistent model of the solar filigree and the chromospheric rosette (bush) with its group of mottles (spicules). (xi) Global patterns of surface and coronal magnetic fields reveal puzzling features such as the migration of large unipolar regions and the freedom from differential rotation of some structures. Submerged flux ropes ‘peeling’ out of the Sun provide a starting point for explaining these effects. These results provide a strong case for the flux-rope theory against the entrenched dynamo theory, and suggest that more observations should be made of the above ten phenomena. Where possible, simultaneous observations should be made of Zeeman effects and of plasma distributions and velocity field seen in white light and spectral lines.