The radio structure of ultra-high-energy synchrotron-peak BL Lacs

We present the results of European Very Long Baseline Interferometry Network (EVN) and Multi-Element Radio Linked Interferometer Network (MERLIN) 5-GHz observations of nine ultra-high-energy synchrotron-peak BL Lacs (UHBLs), consisting of all BL Lacs with log?(?peak/Hz) > 20 from the work of Nieppola et al. The radio structure was investigated for these sources, in combination with available Very Long Baseline Array (VLBA) archive data. We found that a corejet structure is detected in five sources, while four sources only have a compact core at the pc scale. The cores of all sources show high brightness temperature (with mean and median values log?(Tb/K) 11), which implies that the beaming effect is likely present in all sources. When multi-epoch Very Long Baseline Interferometry (VLBI) data are available, we find no significant variations for either 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 superluminal motion is likely detected in EXO 0706.1+5913. Our sources are found to be less compact than the typical high-frequency peaked BL Lac objects (HBLs) of Giroletti et al., by comparing the ratio of the VLBI total flux and the core flux at the 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 for typical HBLs. Moreover, we found by comparing the distributions of redshift and radio luminosity that UHBLs could be less Doppler-beamed versions of HBLs with similar jet power. These results are in good consistence with the expectations of our previous work.