Very-high energy gamma-rays from extragalactic sources pair-produce off of the extragalactic background light , yielding an electron-positron pair beam .
This pair beam is unstable to various plasma instabilities , especially the “ oblique ” instability , which can be the dominant cooling mechanism for the beam .
However , recently , it has been claimed that nonlinear Landau damping renders it physically irrelevant by reducing the effective damping rate to a low level .
Here , we show with numerical calculations that the effective damping rate is 8 \times 10 ^ { -4 } of the growth rate of the linear instability , which is sufficient for the “ oblique ” instability to be the dominant cooling mechanism of these pair beams .
In particular , we show that previous estimates of this rate ignored the exponential cutoff in the scattering amplitude at large wavenumber and assumed that the damping of scattered waves entirely depends on collisions , ignoring collisionless processes .
We find that the total wave energy eventually grows to approximate equipartition with the beam by increasingly depositing energy into long wavelength modes .
As we have not included the effect of nonlinear wave-wave interactions on these long wavelength modes , this scenario represents the “ worst-case ” scenario for the oblique instability .
As it continues to drain energy from the beam at a faster rate than other processes , we conclude that the “ oblique ” instability is sufficiently strong to make it the physically dominant cooling mechanism for high-energy pair beams in the intergalactic medium .