We present nine epochs of Hubble Space Telescope optical imaging of the bipolar outflow from the pre-main sequence binary XZ Tauri .
Our data monitors the system from 1995 – 2005 and includes emission line images of the flow .
The northern lobe appears to be a succession of bubbles , the outermost of which expanded ballistically from 1995 – 1999 but in 2000 began to deform and decelerate along its forward edge .
It reached an extent of 6 ^ { \prime \prime } from the binary in 2005 .
A larger and fainter southern counterbubble was detected for the first time in deep ACS images from 2004 .
Traces of shocked emission are seen as far as 20 ^ { \prime \prime } south of the binary .
The bubble emission nebulosity has a low excitation overall , as traced by the [ S II ] /H \alpha line ratio , requiring a nearly comoving surrounding medium that has been accelerated by previous ejections or stellar winds .

Within the broad bubbles there are compact emission knots whose alignments and proper motions indicate that collimated jets are ejected from each binary component .
The jet from the southern component , XZ Tau A , is aligned with the outflow axis of the bubbles and has tangential knot velocities of 70 – 200 km s ^ { -1 } .
Knots in the northern flow are seen to slow and brighten as they approach the forward edge of the outermost bubble .
The knots in the jet from the other star , XZ Tau B , have lower velocities of \sim 100 km s ^ { -1 } .

To explain the observations of the outer bubble , we propose that the XZ Tau A stellar jet underwent a large velocity pulse circa 1980 .
This ejection quickly overtook older , slower-moving ejecta very near the star , producing a \sim 70 km s ^ { -1 } shock in a hot ( T \sim 80,000 K ) , compact “ fireball ” .
The initial thermal pressure of this gas parcel drove the expansion of a spherical bubble .
Subsequent cooling caused the bubble to transition to ballistic expansion , followed by slowing of its forward edge by mass-loading from the pre-shock medium .
Repeated pulses may explain the multiple bubbles seen in the data .
Collimated jets continue to flow through the bubble ’ s interior , and with the fading of the original fireball they are becoming the primary energizing mechanism for the emission line structures .
Future evolution of the flow should see the outer bubble structures fade from view , and the emergence of a more typical Herbig-Haro jet/bowshock morphology .
We present a preliminary numerical model of a pulsed jet to illustrate this scenario .