Confronting X‐Ray Emission Models with the Highest Redshift Kiloparsec‐Scale Jets: Thez = 3.89 Jet in Quasar 1745+624

A newly identified kiloparsec-scale X-ray jet in the high-redshift z=3.89quasar 1745+624 is studied with multi-frequency Very Large Array, Hubble SpaceTelescope, and Chandra X-ray imaging data. This is only the third large-scaleX-ray jet beyond z>3 known and is further distinguished as being the mostluminous relativistic jet observed at any redshift, exceeding 10^45 erg/s inboth the radio and X-ray bands. Apart from the jet's extreme redshift,luminosity, and high inferred equipartition magnetic field (in comparison tolocal analogues), its basic properties such as X-ray/radio morphology and radiopolarization are similar to lower-redshift examples. Its resolved linearstructure and the convex broad-band spectral energy distributions of threedistinct knots are also a common feature among known powerful X-ray jets atlower-redshift. Relativistically beamed inverse Compton and `non-standard'synchrotron models have been considered to account for such excess X-rayemission in other jets; both models are applicable to this high-redshiftexample but with differing requirements for the underlying jet physicalproperties, such as velocity, energetics, and electron acceleration processes.One potentially very important distinguishing characteristic between the twomodels is their strongly diverging predictions for the X-ray/radio emissionwith increasing redshift. This is considered, though with the limited sample ofthree z>3 jets it is apparent that future studies targeted at veryhigh-redshift jets are required for further elucidation of this issue. Finally,from the broad-band jet emission we estimate the jet kinetic power to be noless than 10^46 erg/s, which is about 10% of the Eddington luminositycorresponding to this galaxy's central [abridged]Comment: ApJ, accepted. No changes from version 1. 17 pages emulateapj style, 7 figure