We present here the results from new Very Long Baseline Array observations at 1.6 and 5 GHz of 19 galaxies of a complete sample of 21 UGC FRI radio galaxies .
New Chandra data of two sources , viz . , UGC 00408 and UGC 08433 , are combined with the Chandra archival data of 13 sources .
The 5 GHz observations of ten “ core-jet ” sources are polarization-sensitive , while the 1.6 GHz observations constitute second epoch total intensity observations of nine “ core-only ” sources .
Polarized emission is detected in the jets of seven sources at 5 GHz , but the cores are essentially unpolarized , except in M 87 .
Polarization is detected at the jet edges in several sources , and the inferred magnetic field is primarily aligned with the jet direction .
This could be indicative of magnetic field “ shearing ” due to jet-medium interaction , or the presence of helical magnetic fields .
The jet peak intensity I _ { \nu } falls with distance d from the core , following the relation , I _ { \nu } \propto d ^ { a } , where a is typically \sim - 1.5 .
Assuming that adiabatic expansion losses are primarily responsible for the jet intensity “ dimming ” , two limiting cases are considered : [ 1 ] the jet has a constant speed on parsec-scales and is expanding gradually such that the jet radius r \propto d ^ { 0.4 } ; this expansion is however unobservable in the laterally unresolved jets at 5 GHz , and [ 2 ] the jet is cylindrical and is accelerating on parsec-scales .
Accelerating parsec-scale jets are consistent with the phenomenon of “ magnetic driving ” in Poynting flux dominated jets .
While slow jet expansion as predicted by case [ 2 ] is indeed observed in a few sources from the literature that are resolved laterally , on scales of tens or hundreds of parsec , case [ 2 ] can not be ruled out in the present data , provided the jets become conical on scales larger than those probed by the VLBA . Chandra observations of 15 UGC FRIs detect X-ray jets in nine of them .
The high frequency of occurance of X-ray jets in this complete sample suggests that they are a signature of a ubiquitous process in FRI jets .
It appears that the FRI jets start out relativistically on parsec-scales but decelerate on kiloparsec scales , with the X-ray emission revealing the sites of bulk deceleration and particle reacceleration .