We report on the analysis of two XMM-Newton observations of the recently discovered soft gamma repeater Swift J1834.9–0846 , taken in September 2005 and one month after the source went into outburst on 2011 August 7 .
We performed timing and spectral analyses on the point source as well as on the extended emission .
We find that the source period is consistent with an extrapolation of the Chandra ephemeris reported earlier and the spectral properties remained constant .
The source luminosity decreased to a level of 1.6 \times 10 ^ { 34 } erg s ^ { -1 } following a decay trend of \propto t ^ { -0.5 } .
Our spatial analysis of the source environment revealed the presence of two extended emission regions around the source .
The first ( Region A ) is a symmetric ring around the point source , starting at 25 ^ { \prime \prime } and extending to \sim 50 ^ { \prime \prime } .
We argue that Region A is a dust scattering halo .
The second ( Region B ) has an asymmetrical shape extending between 50 ^ { \prime \prime } and 150 ^ { \prime \prime } , and is detected both in the pre- and post-outburst data .
We argue that this region is a possible magnetar wind nebula ( MWN ) .
The X-ray efficiency of the MWN with respect to the rotation energy loss is substantially higher than those of rotation powered pulsars : \eta _ { X } \equiv L _ { MWN, 0.5 - 8 ~ { } keV } / \dot { E } _ { rot } \approx 0.7 .
The higher efficiency points to a different energy source for the MWN of Swift J1834.9–0846 , most likely bursting activity of the magnetar , powered by its high magnetic field , B = 1.4 \times 10 ^ { 14 } G .