  We present updated constraints on the free-streaming of warm dark matter
(WDM) particles derived from an analysis of the Lya flux power spectrum
measured from high-resolution spectra of 25 z > 4 quasars obtained with the
Keck High Resolution Echelle Spectrometer (HIRES) and the Magellan Inamori
Kyocera Echelle (MIKE) spectrograph. We utilize a new suite of high-resolution
hydrodynamical simulations that explore WDM masses of 1, 2 and 4 keV (assuming
the WDM consists of thermal relics), along with different physically motivated
thermal histories. We carefully address different sources of systematic error
that may affect our final results and perform an analysis of the Lya flux power
with conservative error estimates. By using a method that samples the
multi-dimensional astrophysical and cosmological parameter space, we obtain a
lower limit mwdm > 3.3 keV (2sigma) for warm dark matter particles in the form
of early decoupled thermal relics. Adding the Sloan Digital Sky Survey (SDSS)
Lya flux power spectrum does not improve this limit. Thermal relics of masses 1
keV, 2 keV and 2.5 keV are disfavoured by the data at about the 9sigma, 4sigma
and 3sigma C.L., respectively. Our analysis disfavours WDM models where there
is a suppression in the linear matter power spectrum at (non-linear) scales
corresponding to k=10h/Mpc which deviates more than 10% from a LCDM model.
Given this limit, the corresponding "free-streaming mass" below which the mass
function may be suppressed is 2x10^8 Msun/h. There is thus very little room for
a contribution of the free-streaming of WDM to the solution of what has been
termed the small scale crisis of cold dark matter.
