Cyclotron Maser Emission from Blazar Jets?

We consider the production of electron cyclotron maser emission by low-density, highly magnetized plasmas in relativistic jets. The population inversion required to drive cyclotron maser instability could occur in localized, transient sites where hydromagnetic instabilities, shocks, and/or turbulence lead to magnetic mirroring along current-carrying flux tubes. The maser is pumped by the conversion of kinetic and magnetic energy into j b dot E work as electrons are accelerated into the maser by the parallel electric field that develops as a result of the mirror. We estimate the maximum brightness temperatures T_B that can be obtained in a single maser site and in an array of many masers operating simultaneously, under conditions likely to apply in blazar jets. Synchrotron absorption, by relativistic electrons within the jet, presents the largest obstacle to the escape of the maser radiation and may render most of it invisible. However, we argue that a high brightness temperature could be produced in a thin boundary layer outside the synchrotron photosphere, perhaps in the shear layer along the wall of the jet. Induced Compton scattering provides additional constraints on the maximum brightness temperature of a masing jet. We suggest that recent observations of diffractive scintillation in the blazar J1819+3845, indicating intrinsic brightness temperatures greater than 10¹⁴ K at 5 GHz, may be explained in terms of cyclotron maser emission. High-T_B maser emission from blazar jets may extend to frequencies as high as ~100 GHz, with the maximum possible T_B scaling with frequency roughly ~ν^-1. Less massive relativistic jet sources, such as microquasars, are even better candidates for producing cyclotron maser emission, primarily in the infrared and optical bands.