Highly accreting quasars are quite luminous in the X-ray and optical regimes .
While , they tend to become radio quiet and have optically thin radio spectra .
Among the known quasars , IRAS F11119 + 3257 is a supercritical accretion source because it has a bolometric luminosity above the Eddington limit and extremely powerful X-ray outflows .
To probe its radio structure , we investigated its radio spectrum between 0.15 and 96.15 GHz and performed very-long-baseline interferometric ( VLBI ) observations with the European VLBI Network ( EVN ) at 1.66 and 4.93 GHz .
The deep EVN image at 1.66 GHz shows a two-sided jet with a projected separation about two hundred parsec and a very high flux density ratio of about 290 .
Together with the best-fit value of the integrated spectral index of - 1.31 \pm 0.02 in the optically thin part , we infer that the approaching jet has an intrinsic speed at least 0.57 times of the light speed .
This is a new record among the known all kinds of super-Eddington accreting sources and unlikely accelerated by the radiation pressure .
We propose a scenario in which IRAS F11119 + 3257 is an unusual compact symmetric object with a small jet viewing angle and a radio spectrum peaking at 0.53 \pm 0.06 GHz mainly due to the synchrotron self-absorption .