  Emission of two short hard X-ray bursts on 2009 June 5 disclosed the
existence of a new soft gamma-ray repeater, now catalogued as SGR 0418+5729.
After a few days, X-ray pulsations at a period of 9.1 s were discovered in its
persistent emission. SGR 0418+5729 was monitored almost since its discovery
with the Rossi X-ray Timing Explorer (2-10 keV energy range) and observed many
times with Swift (0.2-10 keV). The source persistent X-ray emission faded by a
factor 10 in about 160 days, with a steepening in the decay about 19 days after
the activation. The X-ray spectrum is well described by a simple absorbed
blackbody, with a temperature decreasing in time. A phase-coherent timing
solution over the 160 day time span yielded no evidence for any significant
evolution of the spin period, implying a 3-sigma upper limit of 1.1E-13 s/s on
the period derivative and of 3E+13 G on the surface dipole magnetic field.
Phase-resolved spectroscopy provided evidence for a significant variation of
the spectrum as a function of the stellar rotation, pointing to the presence of
two emitting caps, one of which became hotter during the outburst. Finally, a
deep observation of the field of SGR 0418+5729 with the new Gran Telescopio
Canarias 10.4-m telescope allowed us to set an upper limit on the source
optical flux of i'>25.1 mag, corresponding to an X-ray-to-optical flux ratio
exceeding 10000, consistent with the characteristics of other magnetars.
