  Laser-plasma electron accelerators can be used to produce high-intensity
X-rays, as electrons accelerated in wakefields emit radiation due to betatron
oscillations.Such X-ray sources inherit the features of the electron beam;
sub-femtosecond electron bunches produce betatron sources of the same duration,
which in turn allow probing matter on ultrashort time scales. In this paper we
show, via Particle-in-Cell simulations, that attosecond electron bunches can be
obtained using low-energy, ultra-short laser beams both in the self-injection
and the controlled injection regimes at low plasma densities. However, only in
the controlled regime does the electron injection lead to a stable, isolated
attosecond electron bunch. Such ultrashort electron bunches are shown to emit
attosecond X-ray bursts with high brilliance
