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Electromagnetic Fireworks: Fast Radio Bursts from Rapid Reconnection in the Compressed Magnetar Wind
Abstract One scenario for the generation of fast radio bursts (FRBs) is magnetic reconnection in a current sheet of the magnetar wind. Compressed by a strong magnetic pulse induced by a magnetar flare, the current sheet fragments into a self-similar chain of magnetic islands. Time-dependent plasma currents at their interfaces produce coherent radiation during their hierarchical coalescence. We investigate this scenario using 2D radiative relativistic particle-in-cell simulations to compute the efficiency of the coherent emission and to obtain frequency scalings. Consistent with expectations, a fraction of the reconnected magnetic field energy, f ∼ 0.002, is converted to packets of high-frequency fast magnetosonic waves, which can escape from the magnetar wind as radio emission. In agreement with analytical estimates, we find that magnetic pulses of 10 47 erg s −1 can trigger relatively narrowband GHz emission with luminosities of approximately 10 42 erg s −1 , sufficient to explain bright extragalactic FRBs. The mechanism provides a natural explanation for a downward frequency drift of burst signals, as well as the ∼100 ns substructure recently detected in FRB 20200120E .
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Award ID(s):
Publication Date:
NSF-PAR ID:
10340757
Journal Name:
The Astrophysical Journal Letters
Volume:
932
Issue:
2
Page Range or eLocation-ID:
L20
ISSN:
2041-8205
The emission process of Fast Radio Bursts (FRBs) remains unknown. We investigate whether the synchrotron maser emission from relativistic shocks in a magnetar wind can explain the observed FRB properties. We perform particle-in-cell (PIC) simulations of perpendicular shocks in cold pair plasmas, checking our results for consistency among three PIC codes. We confirm that a linearly polarized X-mode wave is self-consistently generated by the shock and propagates back upstream as a precursor wave. We find that at magnetizations σ ≳ 1 (i.e. ratio of Poynting flux to particle energy flux of the pre-shock flow) the shock converts a fraction $f_\xi ^{\prime } \approx 7 \times 10^{-4}/\sigma ^2$ of the total incoming energy into the precursor wave, as measured in the shock frame. The wave spectrum is narrow-band (fractional width ≲1−3), with apparent but not dominant line-like features as many resonances concurrently contribute. The peak frequency in the pre-shock (observer) frame is $\omega ^{\prime \prime }_{\rm peak} \approx 3 \gamma _{\rm s | u} \omega _{\rm p}$, where γs|u is the shock Lorentz factor in the upstream frame and ωp the plasma frequency. At σ ≳ 1, where our estimated $\omega ^{\prime \prime }_{\rm peak}$ differs from previous works, themore »
3. ABSTRACT Magnetars are the most promising progenitors of fast radio bursts (FRBs). Strong radio waves propagating through the magnetar wind are subject to non-linear effects, including modulation/filamentation instabilities. We derive the dispersion relation for modulations of strong waves propagating in magnetically dominated pair plasmas focusing on dimensionless strength parameters a0 ≲ 1, and discuss implications for FRBs. As an effect of the instability, the FRB-radiation intensity develops sheets perpendicular to the direction of the wind magnetic field. When the FRB front expands outside the radius where the instability ends, the radiation sheets are scattered due to diffraction. The FRB-scattering time-scale depends on the properties of the magnetar wind. In a cold wind, the typical scattering time-scale is τsc ∼  $\mu$s–ms at the frequency $\nu \sim 1\, {\rm GHz}$. The scattering time-scale increases at low frequencies, with the scaling τsc ∝ ν−2. The frequency-dependent broadening of the brightest pulse of FRB 181112 is consistent with this scaling. From the scattering time-scale of the pulse, one can estimate that the wind Lorentz factor is larger than a few tens. In a warm wind, the scattering time-scale can approach $\tau _{\rm sc}\sim \, {\rm ns}$. Then scattering produces a frequency modulation of the observed intensitymore »
5. ABSTRACT The origin of fast radio bursts (FRBs) still remains a mystery, even with the increased number of discoveries in the last 3 yr. Growing evidence suggests that some FRBs may originate from magnetars. Large, single-dish telescopes such as Arecibo Observatory (AO) and Green Bank Telescope (GBT) have the sensitivity to detect FRB 121102-like bursts at gigaparsec distances. Here, we present searches using AO and GBT that aimed to find potential radio bursts at 11 sites of past gamma-ray bursts that show evidence for the birth of a magnetar. We also performed a search towards GW170817, which has a merger remnant whose nature remains uncertain. We place $10\sigma$ fluence upper limits of ≈0.036 Jy ms at 1.4 GHz and ≈0.063 Jy ms at 4.5 GHz for the AO data and fluence upper limits of ≈0.085 Jy ms at 1.4 GHz and ≈0.098 Jy ms at 1.9 GHz for the GBT data, for a maximum pulse width of ≈42 ms. The AO observations had sufficient sensitivity to detect any FRB of similar luminosity to the one recently detected from the Galactic magnetar SGR 1935+2154. Assuming a Schechter function for the luminosity function of FRBs, we find that our non-detections favour a steep power-law index (α ≲ −1.1) and a large cut-off luminositymore »