  High-order harmonic generation from atomic systems is considered in the
crossed fields of a relativistically strong infrared laser and a weak
attosecond-pulse train of soft x-rays. Due to one-photon ionization by the
x-ray pulse, the ionized electron obtains a starting momentum that compensates
the relativistic drift which is induced by the laser magnetic field, and allows
the electron to efficiently emit harmonic radiation upon recombination with the
atomic core in the relativistic regime. In this way, short pulses of coherent
hard x-rays of up to 40 keV energy and 10 as duration can be brought about.
