  The sub-atomic experimental exploration of physical processes on extremely
short time scales has become possible by the generation of high quality
electron bunches and x-ray pulses with sub-femtosecond durations. Temporal
coherence is crucial for achieving high resolution in diffraction-based
diagnostics and if the transverse spatial coherence is also provided, then
phenomena with very small cross sections could also be observed. Increasing the
photon energy from x-ray to gamma-ray regime makes probing of extremely small
space-time domains accessible. Here, a mechanism for generating attosecond
gamma photon and positron bunches with small divergence using laser intensities
below $10^{23}$ W/cm$^2$ is proposed. Numerical simulations are used to
formulate the conditions for coherent radiation and to characterize the
generated photon and positron bunches.
