Low-frequency high-resolution radio observations of the TeV-emitting blazar SHBL J001355.9−185406
Context. In the framework of the unification scheme of radio-loud active galactic nuclei, BL Lac objects and quasars are the beamed end-on counterparts of low-power (FR I) and high-power (FR II) radio galaxies, respectively. Aims. Some BL Lacs have been found to possess the FR II-type, large-scale radio morphology, suggesting that the parent population of BL Lacs is a mixture of low- and high-power radio galaxies. This seems to apply only to “low frequency-peaked” BL Lacs, since all the “high frequency-peaked” BL Lacs studied so far were shown to host the FR I-type radio jets, exclusively. While analyzing the NRAO VLA Sky Survey (NVSS) maps of the TeV detected BL Lacs, we have discovered, however, that the high frequency-peaked object SHBL J001355.9 185406 is associated uniquely with the one-sided, arcmin-scale, and edge-brightened jet/lobe-like feature extending to the south-west from the blazar core. Methods. In order to investigate the large-scale morphology of SHBL J001355.9 185406 in detail, we have performed low-frequency and high-resolution observations of the source at 156, 259, and 629 MHz using the Giant Metrewave Radio Telescope (GMRT). Results. Our analysis indicates that no di use arcmin-scale emission is present around the unresolved blazar core, and that the arcmin-scale structure seen on the NVSS map breaks into three distinct features unrelated to the blazar, but instead associated with background AGN. Conclusions. The upper limits for the extended radio halo around the TeV-emitting BL Lac object SHBL J001355.9 185406 read as <10% 1% at 156 629 MHz. The fact that the integrated radio spectrum of the unresolved blazar core is flat down to 156 MHz indicates that a self-similar character of the jet in the source holds up to relatively large distances from the jet base.