Testing the blazar sequence with the least luminous BL Lacs

TL;DR

This study investigates the spectral energy distributions of low-power BL Lac objects, revealing a broad variety of shapes and challenging the traditional blazar sequence, especially at low radio luminosities.

Contribution

The paper presents a new method for selecting low-power BL Lacs and provides the first comprehensive SED analysis of these sources, questioning the role of radio power in their properties.

Findings

Wide range of synchrotron peak frequencies (log(nu_peak)~13.5 to ~20 Hz).

Majority are high-energy peaked BL Lacs, but a significant fraction are LBLs.

Disagreement with the blazar sequence at low radio luminosities.

Abstract

In a previous paper, we proposed a new method to select low-power BL Lacs (LPBLs) based on mid-infrared emission and flux contrast through the Ca II spectral break; that study led to the selection of a complete sample formed by 34 LPBLs with 0.05<z<=0.15 and radio luminosities spanning the range log(L_r) = 39.2-41.5 [erg/s]. We now assemble the broadband spectral energy distributions (SEDs) of these sources to investigate their nature and compare them with brighter BL Lacs. We find that the ratios between the X-ray and radio luminosities range from ~20 to ~30000 and that the synchrotron peak frequencies span a wide energy interval, from log(nu_peak)~13.5 to ~20 [Hz]. This indicates a broad variety of SED shapes and a mixture of BL Lac flavors. Indeed, although the majority of our LPBLs are high-energy peaked BL Lacs (HBLs), we find that a quarter of them are low-energy peaked BL Lacs (LBLs), despite the fact that the sample is biased against the selection of LBLs. The analysis of the median LPBL SED confirms disagreement with the blazar sequence at low radio luminosities. Furthermore, if we limit the sample to the LBLs subsample, we find that their median SED shape is essentially indistinguishable from that of the most luminous BL Lacs. We conclude that the observed radio power is not the main driving parameter of the multiwavelength properties of BL Lacs.