Chandra HRC observations are investigated for evidence of proper motion and brightness changes in the X-ray jet of the nearby radio galaxy M87 .
Using images spanning 5 yr , proper motion is measured in the X-ray knot HST-1 , with a superluminal apparent speed of 6.3 \pm 0.4 c , or 24.1 \pm 1.6 mas yr ^ { -1 } , and in Knot D , with a speed of 2.4 \pm 0.6 c .
Upper limits are placed on the speeds of the remaining jet features .
The X-ray knot speeds are in excellent agreement with existing measurements in the radio , optical , and ultraviolet .
Comparing the X-ray results with images from the Hubble Space Telescope indicates that the X-ray and optical/UV emitting regions co-move .
The X-ray knots also vary by up to 73 % in brightness , whereas there is no evidence of brightness changes in the optical/UV .
Using the synchrotron cooling models , we determine lower limits on magnetic field strengths of \sim 420 ~ { } \mu G and \sim 230 ~ { } \mu G for HST-1 and Knot A , respectively , consistent with estimates of the equipartition fields .
Together , these results lend strong support to the synchrotron cooling model for Knot HST-1 , which requires that its superluminal motion reflects the speed of the relativistic bulk flow in the jet .