Coherent Emission from Magnetars

It is proposed that magnetospheric currents above the surfaces of magnetars radiate coherent emission in analogy to pulsars. Scaling the magnetospheric parameters suggests that the coherent emission from magnetars would emerge in the infra-red or optical. Pulsar radio emission and molecular masers are two examples of naturally occurring coherent emission. But there is no accepted analogue in the optical to our knowledge. A brief optical flash was detected from gamma ray burst GRB 990123, but it can be explained as being incoherent if the bulk Lorentz factor of the expanding fireball thought to generate is above 50 (Akerlof et al, 1999). In this letter we consider the possibility of coherent optical and IR emission in certain astrophysical situations, such as magnetars. The basis for such a hypothesis is quite simple: Magnetars, it has been proposed (Duncan and Thompson, 1992, Thompson and Duncan, 1995, 1996), have twisted magnetic loops in their magnetospheres, and, most of the time, the thermal scale height of their atmospheres is too low to populate the magnetosphere with thermal plasma. On the other hand, magnetospheric currents can easily be drawn out of the surface of the star from at least one of the footpoints. This current can be estimated. The rate of dissipation dB/dt of the magnetic field can also be estimated; it is presumably the value of dB/dt that yields a sufficient potential drop across the length of the loop to create enough plasma to short out any larger potential drop. The density of plasma so estimated is many orders of magnitude larger than that in pulsar magnetospheres. If pulsars can radiate coherently in the radio, this frequency being ultimately determined by the plasma