[Paper Review] Free electron laser in magnetars/Fast Radio Bursts

Maxim Lyutikov|arXiv (Cornell University)|Jun 29, 2020

Pulsars and Gravitational Waves Research13 references4 citations

TL;DR

This paper proposes a free electron laser (FEL) mechanism in magnetar magnetospheres as the origin of Fast Radio Bursts (FRBs), where relativistic electron-positron beams interact with a self-generated wiggler field from firehose instability in twisted, counter-streaming pair plasma. The model explains key FRB features—coincident high-energy bursts, high radiation efficiency, frequency drift, polarization, and intermittency—via coherent emission from beam-plasma interaction in ultra-strong magnetic fields.

ABSTRACT

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 $γ_b \sim$ few hundred.

Motivation & Objective

• To explain the origin of coherent radio emission in Fast Radio Bursts (FRBs) and magnetar radio bursts.
• To address the long-standing problem of how relativistic plasma in neutron star magnetospheres can produce highly efficient, coherent radio emission despite low radiative efficiency in typical astrophysical systems.
• To demonstrate that magnetar-specific plasma instabilities and reconnection events naturally produce the conditions for a self-sustained free electron laser (FEL).
• To account for observed FRB features such as frequency drift, high linear polarization, and intermittency through a physically self-consistent FEL model.
• To provide a mechanism that explains the temporal coincidence of radio and high-energy bursts, resolving a key observational puzzle in FRB physics.

Proposed method

• The model employs a free electron laser (FEL) mechanism in which a relativistic electron-positron beam (γb ~ few hundred) propagates through a pre-existing wiggler magnetic field.
• The wiggler field is self-generated via the firehose instability in counter-streaming e± plasma components on twisted magnetic field lines in the magnetar magnetosphere.
• The FEL operates in the ultra-strong guide field regime (ωB ≫ ωp, ω), where the cyclotron frequency greatly exceeds plasma and radiation frequencies.
• Coherent radiation arises from collective backscattering of the wiggler field by density modulations created by the ponderomotive force of the wiggler, leading to phase-locked emission from all electrons.
• The emission frequency is determined by the beam Lorentz factor and the wiggler wavelength, as per the FEL resonance condition (Eq. 13).
• The model relies on plasma parameters such as twist angle Δφ and pair multiplicity ℳ to determine the growth rate of the firehose instability and the resulting wiggler field strength.

Experimental results

Research questions

• RQ1 Can coherent radio emission in FRBs and magnetar bursts be explained by a free electron laser mechanism in magnetar magnetospheres?
• RQ2 What physical conditions in magnetar magnetospheres can naturally generate the required wiggler field and relativistic beam for FEL operation?
• RQ3 How does the FEL model account for the observed frequency drift in FRBs?
• RQ4 Why is the radio emission highly polarized and intermittent in some bursts but not others?
• RQ5 What role does the firehose instability in counter-streaming pair plasma play in enabling FEL operation in ultra-strong magnetic fields?

Key findings

• The firehose instability in counter-streaming e± plasma on twisted magnetar field lines generates a self-consistent wiggler field with spatial periodicity, enabling FEL operation.
• The FEL mechanism efficiently converts kinetic energy of the beam into coherent radiation, with conversion efficiency potentially reaching a large fraction of the beam’s energy.
• The observed frequency drift in FRBs arises from the decreasing central frequency of the FEL emission as the beam propagates and its effective Lorentz factor changes, consistent with Eq. (13).
• High linear polarization (up to 100%) is naturally explained by the linearly polarized wiggler field and one-dimensional electron motion in the FEL regime.
• The model explains the intermittency of radio emission: only bursts with sufficiently high beam Lorentz factor (γb ≥ threshold) and strong firehose growth rates (requiring high Δφ and ℳ) produce observable FEL emission.
• The model accounts for the temporal coincidence of radio and high-energy bursts, as the same reconnection event that produces the beam also triggers the instability that generates the wiggler field.