RS Ophiuchi began its latest outburst on 2006 February 12 .
Previous outbursts have indicated that high velocity ejecta interact with a pre-existing red giant wind , setting up shock systems analogous to those seen in Supernova Remnants .
However , in the previous outburst in 1985 , X-ray observations did not commence until 55 days after the initial explosion .
Here we report on Swift observations covering the first month of the 2006 outburst with the Burst Alert ( BAT ) and X-ray Telescope ( XRT ) instruments .
RS Oph was clearly detected in the BAT 14-25 keV band from t = 0 to t \sim 6 days .
XRT observationsfrom 0.3-10 keV , started at 3.17 days after outburst .
The rapidly evolving XRT spectra clearly show the presence of both line and continuum emission which can be fitted by thermal emission from hot gas whose characteristic temperature , overlying absorbing column , [ N _ { H } ] _ { W } , and resulting unabsorbed total flux decline monotonically after the first few days .
Derived shock velocities are in good agreement with those found from observations at other wavelengths .
Similarly , [ N _ { H } ] _ { W } is in accord with that expected from the red giant wind ahead of the forward shock .
We confirm the basic models of the 1985 outburst and conclude that standard Phase I remnant evolution terminated by t \sim 10 days and the remnant then rapidly evolved to display behaviour characteristic of Phase III .
Around t = 26 days however , a new , luminous and highly variable soft X-ray source began to appear whose origin will be explored in a subsequent paper .