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Register a new userThe effects of plasma lensing on the inferred dispersion measures of Fast Radio Bursts
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Radio signals are delayed when propagating through plasma. This type of delay is frequency-dependent and is usually used for estimating the projected number density of electrons along the line of sight, called the dispersion measure. The dense and clumpy distribution of plasma can cause refractive deflections of radio signals, analogous to lensing effects. Similar to gravitational lensing, there are two contributions to the time delay effect in plasma lensing: a geometric delay, due to increased path length of the signal, and a dispersive delay due to the change of speed of light in a plasma medium. We show the delay time for two models of the plasma distribution, and point out that the estimated dispersion measure can be biased. Since the contribution of the geometric effect can be comparable to that of the dispersive delay, the bias in the measured dispersion measure can be dramatically large if plasma lensing effects are not taken into account when signals propagate through a high-density gradient clump of plasma.
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Fast Radios Bursts (FRBs) show large dispersion measures (DMs), suggesting an extragalactic location. We analyze the DMs of the 11 known FRBs in detail and identify steps as integer multiples of half the lowest DM found, 187.5cm$^{-3}$ pc, so that DMs occur in groups centered at 375, 562, 750, 937, 1125cm$^{-3}$ pc, with errors observed <5%. We estimate the likelhood of a coincidence as 5:10,000. We speculate that this could originate from a Galaxy population of FRBs, with Milky Way DM contribution as model deviations, and an underlying generator process that produces FRBs with DMs in discrete steps. However, we find that FRBs tend to arrive at close to the full integer second, like man-made perytons. If this holds, FRBs would also be man-made. This can be verified, or refuted, with new FRBs to be detected.
We focus on two repeating fast radio bursts (FRBs) recently detected by the CHIME/FRB experiment in 2018--2019 (Source 1: 180916.J0158+65, and Source 2: 181030.J1054+73). These sources have low excess dispersion measures (DMs) ($ < 100 rm , pc , cm^{-3}$ and $ < 20 rm , pc , cm^{-3}$, respectively), implying relatively small maximal distances. They were repeatedly observed by AGILE in the MeV--GeV energy range. We do not detect prompt emission simultaneously with these repeating events. This search is particularly significant for the submillisecond and millisecond integrations obtainable by AGILE. The sources are constrained to emit a MeV-fluence in the millisecond range below $F_{MeV} = 10^{-8} , rm erg , cm^{-2}$ corresponding to an isotropic energy near $E_{MeV,UL} simeq 2 times 10^{46},$erg for a distance of 150 Mpc (applicable to Source 1). We also searched for $gamma$-ray emission for time intervals up to 100 days, obtaining 3$,sigma$ upper limits (ULs) for the average isotropic luminosity above 50 MeV, $L_{gamma,UL} simeq ,$(5-10)$,times 10^{43} rm , erg , s^{-1}$. For a source distance near 100 kpc (possibly applicable to Source 2), our ULs imply $E_{MeV,UL}simeq10^{40} rm erg$, and $L_{gamma,UL} simeq ,$2$,times 10^{37} rm , erg , s^{-1}$. Our results are significant in constraining the high-energy emission of underlying sources such as magnetars, or other phenomena related to extragalactic compact objects, and show the prompt emission to be lower than the peak of the 2004 magnetar outburst of SGR 1806-20 for source distances less than about 100 Mpc.
Since the discovery of FRB 200428 associated with the Galactic SGR 1935+2154, magnetars are considered to power fast radio bursts (FRBs). It is widely believed that magnetars could form by core-collapse (CC) explosions and compact binary mergers, such as binary neutron star (BNS), binary white dwarfs (BWD), and neutron star-white dwarf (NSWD) mergers. Therefore, it is important to distinguish the various progenitors. The expansion of the merger ejecta produces a time-evolving dispersion measure (DM) and rotation measure (RM) that can probe the local environments of FRBs. In this paper, we derive the scaling laws for the DM and RM from ejecta with different dynamical structures (the mass and energy distribution) in the uniform ambient medium (merger scenario) and wind environment (CC scenario). We find that the DM and RM will increase in the early phase, while DM will continue to grow slowly but RM will decrease in the later phase in the merger scenario. We fit the DM and RM evolution of FRB 121102 simultaneously for the first time in the BNS merger scenario, and find the source age is $ sim9-10 $ yr when it was first detected in 2012, and the ambient medium density is $ sim 2.5-3.1 $ cm$ ^{-3} $. The large offsets of some FRBs are consistent with BNS/NSWD channel. The population synthesis method is used to estimate the rate of compact binary mergers. The rate of BWD mergers is close to the observed FRB rate. Therefore, the progenitors of FRBs may not be unique.
Scenario of formation of fast radio bursts (FRBs) is proposed. Just like radio pulsars, sources of FRBs are magnetized neutron stars. Appearance of strong electric field in a magnetosphere of a neutron star is associated with close passage of a dense body near hot neutron star. For the repeating source FRB 121102, which has been observed in four series of bursts, the period of orbiting of the body is about 200 days. Thermal radiation from the surface of the star (temperature is of the order of $ 10^8 , K $) causes evaporation and ionization of the matter of the dense body. Ionized gas (plasma) flows around the magnetosphere of the neutron star with the velocity $ u simeq 10^7 , cm / s $, and creates electric potential $ psi_0 simeq 10^{11} , V $ in the polar region of the magnetosphere. Electrons from the plasma flow are accelerated toward the star, and gain Lorentz factor of $ simeq 10 ^ 5 $. Thermal photons moving toward precipitating electrons are scattered by them, and produce gamma photons with energies of $ simeq 10^5 , m_e c^2 $. These gamma quanta produce electron-positron pairs in collisions with thermal photons. The multiplicity, the number of born pairs per one primary electron, is about $ 10^5 $. The electron-positron plasma, produced in the polar region of magnetosphere, accumulates in a narrow layer at a bottom of a potential well formed on one side by a blocking potential $ psi_0 $, and on the other side by pressure of thermal radiation. The density of electron-positron plasma in the layer increases with time, and after short time the layer becomes a mirror for thermal radiation of the star. The thermal radiation in the polar region under the layer is accumulated during time $ simeq 500 , s $, then the plasma layer is ejected outside. The ejection is observed as burst of radio emission formed by the flow of relativistic electron-positron plasma.
In 2007, a very bright radio pulse was identified in the archival data of the Parkes Telescope in Australia, marking the beginning of a new research branch in astrophysics. In 2013, this kind of millisecond bursts with extremely high brightness temperature takes a unified name, fast radio burst (FRB). Over the first few years, FRBs seemed very mysterious because the sample of known events was limited. With the improvement of instruments over the last five years, hundreds of new FRBs have been discovered. The field is now undergoing a revolution and understanding of FRB has rapidly increased as new observational data increasingly accumulates. In this review, we will summarize the basic physics of FRBs and discuss the current research progress in this area. We have tried to cover a wide range of FRB topics, including the observational property, propagation effect, population study, radiation mechanism, source model, and application in cosmology. A framework based on the latest observational facts is now under construction. In the near future, this exciting field is expected to make significant breakthroughs.
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