A free-electron laser in the pulsar magnetosphere

Free access article

Issue		A&A Volume 422, Number 3, August II 2004


Page(s)		817 - 830
Section		Astrophysical processes
DOI		http://dx.doi.org/10.1051/0004-6361:20040295

A&A 422, 817-830 (2004)
DOI: 10.1051/0004-6361:20040295

A free-electron laser in the pulsar magnetosphere

P. K. Fung¹ and J. Kuijpers²

¹ Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands
e-mail: fung@astro.uu.nl
² Department of Astrophysics, University of Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands

(Received 18 February 2004 / Accepted 2 May 2004)

Abstract
We have studied systematically the free-electron laser in the context of high brightness pulsar radio emission. In this paper, we have numerically examined the case where a transverse electromagnetic wave is distorting the motion of a relativistic electron beam while travelling over one stellar radius ( $\approx$ $10\:\mbox{km}$ ). For different sets of parameters, coherent emission is generated by bunches of beam electrons in the radio domain, with bandwidths of 3 GHz. Pulse power often reached $10^{13}\:\mbox{W}$ , which corresponds with brightness temperature of $10^{30}\:\mbox{K}$ . The duration of these pulses is of the order of nanoseconds. In the context of pulsar radio emission, our results indicate that the laser can produce elementary bursts of radiation which build up the observed microstructures of a few tens of microseconds duration. The process is sensitive mostly to the beam particles energy, number density and the background magnetic field, but much less so to the transverse wave parameters. We demonstrate that the operation of a free-electron laser with a transverse electromagnetic wiggler in the pulsar magnetosphere occurs preferably at moderate Lorentz factors $\gamma \geq 100$ , high beam density $n \gtrsim 0.1\;n_{{\rm GJ}}(r_\ast)$ where $n_{{\rm GJ}}(r_\ast)$ is the Goldreich-Julian density at a stellar radius $r_\ast$ , and finally, at large altitude where the background magnetic field is low $B_0 \leq 10^{-2}\;{\rm T}$ .

Key words: masers -- radiation mechanisms: non-thermal -- plasmas

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