  Protostars are actively accreting matter and they drive spectacular, dynamic
outflows, which evolve on timescales of years. X-ray emission from these jets
has been detected only in a few cases and little is known about its time
evolution. We present a new Chandra observation of L1551 IRS 5's jet in the
context of all available X-ray data of this object. Specifically, we perform a
spatially resolved spectral analysis of the X-ray emission and find that (a)
the total X-ray luminosity is constant over almost one decade, (b) the majority
of the X-rays appear to be always located close to the driving source, (c)
there is a clear trend in the photon energy as a function of the distance to
the driving source indicating that the plasma is cooler at larger distances and
(d) the X-ray emission is located in a small volume which is unresolved
perpendicular to the jet axis by Chandra. A comparison of our X-ray data of the
L1551 IRS 5 jet both with models as well as X-ray observations of other
protostellar jets shows that a base/standing shock is a likely and plausible
explanation for the apparent constancy of the observed X-ray emission. Internal
shocks are also consistent with the observed morphology if the supply of jet
material by the ejection of new blobs is sufficiently constant. We conclude
that the study of the X-ray emission of protostellar jet sources allows us to
diagnose the innermost regions close to the acceleration region of the
outflows.
