X-ray jets in AGN are commonly observed in FRII and FRI radiogalaxies , but rarely in BL Lacs , most probably due to their orientation close to the line of sight and the ensuing foreshortening effects .
Only three BL Lacs are known so far to contain a kpc-scale X-ray jet .
In this paper , we present the evidence for the existence of a fourth extended X-ray jet in the classical radio-selected source S5 2007+777 , which for its hybrid FRI/II radio morphology has been classified as a HYMOR ( HYbrid MOrphology Radio source ) .
Our Chandra ACIS-S observations of this source revealed an X-ray counterpart to the 19 ^ { \prime \prime } -long radio jet .
Interestingly , the X-ray properties of the kpc-scale jet in S5 2007+777 are very similar to those observed in FRII jets .
First , the X-ray morphology closely mirrors the radio one , with the X-rays being concentrated in the discrete radio knots .
Second , the X-ray continuum of the jet/brightest knot is described by a very hard power law , with photon index \Gamma _ { x } \sim 1 , although the uncertainties are large .
Third , the optical upper limit from archival HST data implies a concave radio-to-X-ray SED .
If the X-ray emission is attributed to IC/CMB with equipartition , strong beaming ( \delta =13 ) is required , implying a very large scale ( Mpc ) jet .
The beaming requirement can be somewhat relaxed assuming a magnetic field lower than equipartition .
Alternatively , synchrotron emission from a second population of very high-energy electrons is viable .
Comparison to other HYMOR jets detected with Chandra is discussed , as well as general implications for the origin of the FRI/II division .