  The low energy (<10 keV) X-ray emission of the Soft Gamma-ray Repeater
SGR1806-20 has been studied by means of four XMM-Newton observations carried
out in the last two years, the latter performed in response to a strong
sequence of hard X-ray bursts observed on 2004 October 5. The source was caught
in different states of activity: over the 2003-2004 period the 2-10 keV flux
doubled with respect to the historical level observed previously. The long term
raise in luminosity was accompanied by a gradual hardening of the spectrum,
with the power law photon index decreasing from 2.2 to 1.5, and by a growth of
the bursting activity. The pulse period measurements obtained in the four
observations are consistent with an average spin-down rate of 5.5x10e-10 s/s,
higher than the values observed in the previous years. The long-term behavior
of SGR1806-20 exhibits the correlation between spectral hardness and spin-down
rate previously found only by comparing the properties of different sources
(both SGRs and Anomalous X-ray Pulsars). The best quality spectrum (obtained on
6 September 2004) cannot be fitted by a single power law, but it requires an
additional blackbody component (kT=0.79 keV, R_BB = 1.9 (d/15 kpc)^2 km),
similar to the spectra observed in other SGRs and in Anomalous X-ray Pulsars.
No spectral lines were found in the persistent emission, with equivalent width
upper limits in the range 30-110 eV. Marginal evidence for an absorption
feature at 4.2 keV is present in the cumulative spectrum of 69 bursts detected
in September-October 2004.
