The radio structure of ultra-high-energy synchrotron peak BL Lacs
  We present the results of EVN and MERLIN 5 GHz observations of nine
ultra-high-energy synchrotron peak BL Lacs (UHBLs) selected as all BL Lacs with
\textbf{log ($\nu_{\rm peak}/ \rm Hz)>20$} from Nieppola et al.. The radio
structure was investigated for these sources, in combination with the available
VLBA archive data. We found that the core-jet structure is detected in five
sources, while four sources only have a compact core on pc scale. The core of
all sources shows high brightness temperature (with mean and median values
\textbf{log ($T_{\rm b} / {\rm K}) \sim11$}, which implies that the beaming
effect likely present in all sources. When the multi-epoch VLBI data are
available, we found no significant variations either for core or total flux
density in two sources (2E 0414+0057 and EXO 0706.1+5913), and no evident
proper motion in 2E 0414+0057, while the superluminal motion is likely detected
in EXO 0706.1+5913. Our sources are found to be less compact than the typical
HBLs in Giroletti et al, by comparing the ratio of the VLBI total flux to the
core flux at arcsec scale. Combining all our results, we propose that the
beaming effect might be present in the jets of UHBLs, however, it is likely
weaker than that of typical HBLs. Moreover, we found that UHBLs could be less
Doppler beamed versions of HBLs with similar jet power, by comparing the
distribution of redshift, and radio luminosities. The results are in good
consistence with the expectations from our previous work.
