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Register a new userFast radio bursts: superpulsars, magnetars, or something else?
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Authors
S.B. Popov
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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 $sim$~1~Jy~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.
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We discuss coherent free electron laser (FEL) operating during explosive reconnection events in magnetized pair plasma of magnetar magnetospheres. The model explains many salient features of Fast Radio Bursts/magnetars radio emission: temporal coincidence of radio and high energy bursts, high efficiency of conversion of plasma kinetic energy into coherent radiation, presence of variable, narrow-band emission features drifting down in frequency, high degree of linear polarization. The model relies on magnetar-specific drifting $e^pm$ plasma components (which generate wiggler field due to the development of the firehose instability) and the presence of reconnection-generated particle beam with mild Lorentz factor of $gamma_b sim$ few hundred.
We consider constraints on primordial black holes (PBHs) in the mass range $( 10^{-18}text{-}10^{15} ),M_{odot}$ if the dark matter (DM) comprises weakly interacting massive particles (WIMPs) which form halos around them and generate $gamma$-rays by annihilations. We first study the formation of the halos and find that their density profile prior to WIMP annihilations evolves to a characteristic power-law form. Because of the wide range of PBH masses considered, our analysis forges an interesting link between previous approaches to this problem. We then consider the effect of the WIMP annihilations on the halo profile and the associated generation of $gamma$-rays. The observed extragalactic $gamma$-ray background implies that the PBH DM fraction is $f^{}_{rm PBH} lesssim 2 times 10^{-9},( m_{chi} / {rm TeV} )^{1.1}$ in the mass range $2 times 10^{-12},M_{odot},( m_{chi} / {rm TeV} )^{-3.2} lesssim M lesssim 5 times 10^{12},M_{odot},( m_{chi} / {rm TeV} )^{1.1}$, where $m_{chi}$ and $M$ are the WIMP and PBH masses, respectively. This limit is independent of $M$ and therefore applies for any PBH mass function. For $M lesssim 2times 10^{-12},M_{odot},( m_{chi}/ {rm TeV} )^{-3.2}$, the constraint on $f^{}_{rm PBH}$ is a decreasing function of $M$ and PBHs could still make a significant DM contribution at very low masses. We also consider constraints on WIMPs if the DM is mostly PBHs. If the merging black holes recently discovered by LIGO/Virgo are of primordial origin, this would rule out the standard WIMP DM scenario. More generally, the WIMP DM fraction cannot exceed $10^{-4}$ for $M > 10^{-9},M_{odot}$ and $m_{chi} > 10,$GeV. There is a region of parameter space, with $M lesssim 10^{-11},M_{odot}$ and $m_{chi} lesssim 100,$GeV, in which WIMPs and PBHs can both provide some but not all of the DM, so that one requires a third DM candidate.
The repeating FRBs 180916.J0158 and 121102 are visible during periodically-occuring windows in time. We consider the constraints on internal magnetic fields and geometry if the cyclical behavior observed for FRB~180916.J0158 and FRB 121102 is due to precession of magnetars. In order to frustrate vortex line pinning we argue that internal magnetic fields must be stronger than about $10^{16}$ Gauss, which is large enough to prevent superconductivity in the core and destroy the crustal lattice structure. We conjecture that the magnetic field inside precessing magnetars has three components, (1) a dipole component with characteristic strength $sim 10^{14}$ Gauss; (2) a toroidal component with characteristic strength $sim 10^{15}-10^{16}$ Gauss which only occupies a modest fraction of the stellar volume; and (3) a disordered field with characteristic strength $sim 10^{16}$ Gauss. The disordered field is primarily responsible for permitting precession, which stops once this field component decays away, which we conjecture happens after $sim 1000$ years. Conceivably, as the disordered component damps bursting activity diminishes and eventually ceases. We model the quadrupolar magnetic distortion of the star, which is due to its ordered components primarily, as triaxial and very likely prolate. We address the question of whether or not the spin frequency ought to be detectable for precessing, bursting magnetars by constructing a specific model in which bursts happen randomly in time with random directions distributed in or between cones relative to a single symmetry axis. Within the context of these specific models, we find that there are precession geometries for which detecting the spin frequency is very unlikely.
Recently, one fast radio burst, FRB 200428, was detected from the Galactic magnetar SGR J1935+2154 during one X-ray burst. This suggests that magnetars can make FRBs. On the other hand, the majority of X-ray bursts from SGR J1935+2154 are not associated with FRBs. One possible reason for such rarity of FRB-SGR-burst associations is that the FRB emission is much more narrowly beamed than the SGR burst emission. If such an interpretation is correct, one would expect to detect radio bursts with viewing angles somewhat outside the narrow emission beam. These slow radio bursts (SRBs) would have broader widths and lower flux densities due to the smaller Doppler factor involved. We derive two closure relations to judge whether a long, less luminous radio burst could be an SRB. The 2.2-s, 308 Jy ms, 111 MHz radio burst detected from SGR J1935+2154 by the BSA LPI radio telescope may be such an SRB. The 2-ms, 60 mJy ms faint burst detected by FAST from the same source could be also an SRB if the corresponding FRB has a narrow spectrum. If the FRB beam is narrow, there should be many more SRBs than FRBs from Galactic magnetars. The lack of detection of abundant SRBs from magnetars would disfavor the hypothesis that all SGR bursts are associated with narrow-beam FRBs.
It is widely believed that magnetars could be born in core-collapse supernovae (SNe), binary neutron star (BNS) or binary white dwarf (BWD) mergers, or accretion-induced collapse (AIC) of white dwarfs. In this paper, we investigate whether magnetars could also be produced from neutron star--white dwarf (NSWD) mergers, motivated by FRB 180924-like fast radio bursts (FRBs) possibly from magnetars born in BNS/BWD/AIC channels suggested by cite{mar19}. By a preliminary calculation, we find that NSWD mergers with unstable mass transfer could result in the NS acquiring an ultra-strong magnetic field via the dynamo mechanism due to differential rotation and convection or possibly via the magnetic flux conservation scenario of a fossil field. If NSWD mergers can indeed create magnetars, then such objects could produce at least a subset of FRB 180924-like FRBs within the framework of flaring magnetars, since the ejecta, local environments, and host galaxies of the final remnants from NSWD mergers resemble those of BNS/BWD/AIC channels. This NSWD channel is also able to well explain both the observational properties of FRB 180924-like and FRB 180916.J0158+65-like FRBs within a large range in local environments and host galaxies.
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