Cyclotron Maser Emission from Blazar Jets?

We consider the production of electron cyclotron maser emission bylow-density, highly magnetized plasmas in relativistic jets. The populationinversion required to drive cyclotron maser instability could occur inlocalized, transient sites where hydromagnetic instabilities, shocks, and/orturbulence lead to magnetic mirroring along current-carrying flux tubes. Themaser is pumped as electrons are accelerated by the parallel electric fieldthat develops as a result of the mirror. We estimate the maximum brightnesstemperatures that can be obtained in a single maser site and in an array ofmany masers operating simultaneously, under conditions likely to apply inblazar jets. Synchrotron absorption, by relativistic electrons within the jet,presents the largest obstacle to the escape of the maser radiation, and mayrender most of it invisible. However, we argue that a high brightnesstemperature could be produced in a thin boundary layer outside the synchrotronphotosphere, perhaps in the shear layer along the wall of the jet. InducedCompton scattering provides additional constraints on the maximum brightnesstemperature of a masing jet. We suggest that recent observations of diffractivescintillation in the blazar J1819+3845, indicating intrinsic brightnesstemperatures greater than 10^{14} K at 5 GHz, may be explained in terms ofcyclotron maser emission. High brightness temperature maser emission fromblazar jets may extend to frequencies as high as ~100 GHz, with the maximumpossible T_B scaling roughly as 1/frequency. Less massive relativistic jetsources, such as microquasars, are even better candidates for producingcyclotron maser emission, primarily in the infrared and optical bands.Comment: 22 pages, 1 figure, accepted for publication in The Astrophysical Journa