Magnetars and rotation-powered pulsars (RPPs) historically represented
two distinct subclasses of neutron stars. Magnetars are slowly-rotating
(~2-12 s), isolated neutron stars (NSs) with super-strong magnetic
fields, B~10^13-10^15 G. RPPs, on the other hand, are rapidly-rotating
(~0.01-0.3~s), isolated NSs with surface dipole magnetic field in the
range ~10^11-10^13 G. Most pulsars possess a large rotational energy
loss rate that powers a relativistic magnetized particle wind, often
seen as a pulsar wind nebula (PWN; the Crab PWN being the most famous).
There has not yet been convincing evidence for a wind nebula around
magnetars, most likely due to their low rotational energy loss rate.
Here, we report the study of new deep X-ray observations of the peculiar
extended emission around the magnetar Swift J1834.9-0846. Our new
results strongly support a wind nebula as the nature of the extended
emission, thus, establishing Swift J1834.9-0846 as the first magnetar to
possess a surrounding nebula. This implies that wind nebulae are no
longer exclusive to RPPs and, along with recent discoveries in the
field, further narrow the gaps between these two sub-populations of
isolated NSs. The physical properties of this wind nebula, however, show
peculiarities, especially its high radiative efficiency of about 10%,
only shared with two other known very young RPPs, the Crab and its twin.