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

A newly identified kiloparsec-scale X-ray jet in the high-redshift z = 3.89 quasar 1745+624 is studied with multifrequency radio, HST, and Chandra X-ray imaging data. This is only the third large-scale X-ray jet beyond z > 3 known and is further distinguished as being the most luminous relativistic jet observed at any redshift, exceeding 10⁴⁵ ergs s^-1 in both the radio and X-ray bands. Apart from the jet's extreme redshift and luminosity, its basic properties, such as X-ray/radio morphology, radio polarization, and the convex broadband spectral energy distributions of three distinct knots are also similar to lower z examples. Relativistically beamed inverse Compton and "nonstandard" synchrotron models have been considered to account for such excess X-ray emission in other jets; both models are applicable here, but with differing requirements for the underlying jet physical properties, such as velocity, energetics, and electron acceleration processes. One potentially very important distinguishing characteristic between the two models is their strongly diverging predictions for the X-ray/radio emission with increasing redshift. This is considered, although with the limited sample of three z > 3 jets it is apparent that future studies targeted at very high-redshift jets are required for further elucidation. Finally, from the broadband jet emission we estimate the jet kinetic power to be no less than 10⁴⁶ ergs s^-1, which is about 10% of the Eddington luminosity corresponding to this galaxy's central supermassive black hole mass M_BH ≳ 10⁹ M_☉ estimated here via the virial relation. The optical luminosity of the quasar core is about 10 times over Eddington, hence the inferred jet power seems to be much less than that available from mass accretion. The apparent super-Eddington accretion rate may, however, suggest contribution of an unresolved jet in the observed optical nucleus.