Characterising the extended morphologies of BL lacertae objects at 144MHz with LOFAR

Abstract

We present a morphological and spectral study of a sample of 99 BL Lac objects using the LOFAR Two-MetreSky Survey Second Data Release (LDR2). Extended emission has been identified at gigahertz frequencies aroundBL Lac objects, but with LDR2 it is now possible to systematically study their morphologies at 144 MHz, wheremore diffuse emission is expected. LDR2 reveals the presence of extended radio structures around 66/99 of the BLLac nuclei, with angular extents ranging up to 115″, corresponding to spatial extents of 410 kpc. The extendedemission is likely to be both unbeamed diffuse emission and beamed emission associated with relativistic bulkmotion in jets. The spatial extents and luminosities of the extended emission are consistent with the unificationscheme for active galactic nuclei, where BL Lac objects correspond to low-excitation radio galaxies with the jetaxis aligned along the line of sight. While extended emission is detected around the majority of BL Lac objects, the median 144–1400 MHz spectral index and core dominance at 144 MHz indicate that the core component contributes ∼42% on average to the total low-frequency flux density. A stronger correlation was found between the144 MHz core flux density and the γ-ray photon flux (r = 0.69) than between the 144 MHz extended flux density and the γ-ray photon flux (r = 0.42). This suggests that the radio-to-γ-ray connection weakens at low radiofrequencies because the population of particles that give rise to the γ-ray flux are distinct from the electrons producing the diffuse synchrotron emission associated with spatially extended features.

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