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Register a new userRepeating fast radio bursts caused by small bodies orbiting a pulsar or a magnetar
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Authors
Fabrice Mottez
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Asteroids orbiting into the highly magnetized and highly relativistic wind of a pulsar offer a favourable configuration for repeating fast radio bursts (FRB). The body in direct contact with the wind develops a trail formed of a stationary Alfven wave, called an textit{Alfven wing}. When an element of wind crosses the Alfven wing, it sees a rotation of the ambient magnetic field that can cause radio-wave instabilities. In the observers reference frame, the waves are collimated in a very narrow range of directions, and they have an extremely high intensity. A previous work, published in 2014, showed that planets orbiting a pulsar can cause FRB when they pass in our line of sight. We predicted periodic FRB. Since then random FRB repeaters have been discovered. We present an upgrade of this theory where repeaters can be explained by the interaction of smaller bodies with a pulsar wind. Considering the properties of relativistic Alfven wings attached to a body in the pulsar wind, and taking thermal consideration into account we conduct a parametric study. We find that FRBs, including the Lorimer burst (30 Jy), can be explained by small size pulsar companions (1 to 10 km) between 0.03 and 1 AU from a highly magnetized millisecond pulsar. Some sets of parameters are also compatible with a magnetar. Our model is compatible with the high rotation measure of FRB121102. The bunched timing of the FRBs is the consequence of a moderate wind turbulence. As asteroid belt composed of less than 200 bodies would suffice for the FRB occurrence rate measured with FRB121102. This model, after the present upgrade, is compatible with the properties discovered since its first publication in 2014, when repeating FRB were still unknown. It is based on standard physics, and on common astrophysical objects that can be found in any kind of galaxy. It requires $10^{10}$ times less power than (common) isotropic-emission FRB models.
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We investigate the electromagnetic interaction of a relativistic stellar wind with small bodies in orbit around the star. Based on our work on the theory of Alfven wings to relativistic winds presented in a companion paper, we estimate the force exerted by the associated current system on orbiting bodies and evaluate the resulting orbital drift. This Alfvenic structure is found to have no significant influence on planets or smaller bodies orbiting a millisecond pulsar. %influence on the orbit of bodies around a millisecond pulsar. On the timescale of millions of years, it can however affect the orbit of bodies with a diameter of 100 kilometres around standard pulsars with a period $P sim $1 s and a magnetic field $B sim 10^{8}$ T. Kilometer-sized bodies experience drastic orbital changes on a timescale of $10^4$ years.
Repeating fast radio bursts (FRBs) present excellent opportunities to identify FRB progenitors and host environments, as well as decipher the underlying emission mechanism. Detailed studies of repeating FRBs might also hold clues to the origin of FRBs as a population. We aim to detect the first two repeating FRBs: FRB 121102 (R1) and FRB 180814.J0422+73 (R2), and characterise their repeat statistics. We also want to significantly improve the sky localisation of R2. We use the Westerbork Synthesis Radio Telescope to conduct extensive follow-up of these two repeating FRBs. The new phased-array feed system, Apertif, allows covering the entire sky position uncertainty of R2 with fine spatial resolution in one pointing. We characterise the energy distribution and the clustering of detected R1 bursts. We detected 30 bursts from R1. Our measurements indicate a dispersion measure of 563.5(2) pc cm$^{-3}$, suggesting a significant increase in DM over the past few years. We place an upper limit of 8% on the linear polarisation fraction of the brightest burst. We did not detect any bursts from R2. 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 that is not observed at higher frequencies. The non-detection of any bursts from R2 implies either a highly clustered nature of the bursts, a steep spectral index, or a combination of both. Alternatively, R2 has turned off completely, either permanently or for an extended period of time.
Fast Radio Bursts are millisecond-duration astronomical radio pulses of unknown physical origin that appear to come from extragalactic distances. Previous follow-up observations have failed to find additional bursts at the same dispersion measures (i.e. integrated column density of free electrons between source and telescope) and sky position as the original detections. The apparent non-repeating nature of the fast radio bursts has led several authors to hypothesise that they originate in cataclysmic astrophysical events. Here we report the detection of ten additional bursts from the direction of FRB121102, using the 305-m Arecibo telescope. These new bursts have dispersion measures and sky positions consistent with the original burst. This unambiguously identifies FRB121102 as repeating and demonstrates that its source survives the energetic events that cause the bursts. Additionally, the bursts from FRB121102 show a wide range of spectral shapes that appear to be predominantly intrinsic to the source and which vary on timescales of minutes or shorter. While there may be multiple physical origins for the population of fast radio bursts, the repeat bursts with high dispersion measure and variable spectra specifically seen from FRB121102 support models that propose an origin in a young, highly magnetised, extragalactic neutron star.
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.
A bright radio burst was newly discovered in SGR 1935+2154, which exhibit some FRB-like temporal- and frequency-properties, suggesting a neutron star (NS)/magnetar magnetospheric origin of FRBs. We propose an explanation of the temporal- and frequency-properties of sub-pulses of repeating FRBs based on the generic geometry within the framework of charged-bunching coherent curvature radiation in the magnetosphere of an NS. The sub-pulses in a radio burst come from bunches of charged particles moving along different magnetic field lines. Their radiation beam sweep across the line of sight at slightly different time, and those radiating at the more curved part tend to be seen earlier and at higher frequency. However, by considering bunches generated at slightly different times, we find there is also a small probability that the emission from the less curved part be seen earlier. We simulate the time--frequency structures by deriving various forms of the electric acceleration field in the magnetosphere. Such structure of sub-pulses is a natural consequence of coherent curvature radiation from an NS magnetosphere with suddenly and violently triggered sparks. We apply this model to explain the time--frequency structure within specific dipolar configuration by invoking the transient pulsar-like sparking from the inner gap of a slowly rotating NS, and have also developed in more generic configurations.
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