A relativistically intense short laser pulse can
produce a large flux of X rays through the interaction
with electrons that are driven by its intense electromagnetic
fields. Apart from X rays from the high-Z matter
irradiation by an intense laser, two main processes, Larmor
and Bremsstrahlung radiation, are among the most significant
mechanisms for X-ray emission from short-pulse laser irradiation
on low-Z matter in the regime of relativistic
intensities. We evaluate the power, energy spectrum, brilliance,
polarization, and time structure of these X rays. We suggest
a few methods that significantly enhance the power of Larmor
X rays. Because of the peakedness in the energy spectrum
of Larmor X rays, Larmor X rays have important applications.