VLBI Observations of Some Unusual BL Lac Objects

BL Lac objects constitute a subclass of extreme Active Galactic Nuclei (AGN). Based on their synchrotron peak frequencies, they can be classified as low-frequency peaked BL Lac objects (LBLs), intermediate-frequency peaked BL Lac objects (IBLs), high-frequency peaked BL Lac objects (HBLs), and Ultra-high-energy peak BL Lac objects (UHBLs). The properties of BL Lacs usually vary along the peak frequency. From radio, optical, X-ray, and Gamma-ray observations of BL Lac objects, more and more unusual BL Lac objects will be found. The VLBI is currently the technique to achieve the highest resolution, and is the only effective measure for observing compact nucleus regions of BL Lac objects. This paper mainly covers the research progress of BL Lac objects with the VLBI technique, and our recent results of the VLBI observations of some unusual BL Lac objects.