The first-known tidal disruption event ( TDE ) with strong evidence for a relativistic jet – based on extensive multi-wavelength campaigns – is Swift J1644 + 5734 .
In order to directly measure the apparent speed of the radio jet , we performed very long baseline interferometry ( VLBI ) observations with the European VLBI network ( EVN ) at 5 GHz .
Our observing strategy was to identify a very nearby and compact radio source with the real-time e-EVN , and then utilise this source as a stationary astrometry reference point in the later five deep EVN observations .
With respect to the in-beam source FIRST J1644 + 5736 , we have achieved a statistical astrometric precision about 12 \umu as ( 68 % confidence level ) per epoch .
This is one of the best phase-referencing measurements available to date .
No proper motion has been detected in the Swift J1644 + 5734 radio ejecta .
We conclude that the apparent average ejection speed between 2012.2 and 2015.2 was less than 0.3 c with a confidence level of 99 % .
This tight limit is direct observational evidence for either a very small viewing angle or a strong jet deceleration due to interactions with a dense circum-nuclear medium , in agreement with some recent theoretical studies .