What are BL Lacs?

BL Lacs also typically show lower radio power than FSRQs. They are characterized by rapid and large-amplitude flux variability and significant optical polarization. BL Lac objects have spectra dominated by featureless non-thermal continuum. In the unified scheme of radio-loud active galactic nuclei, the observed nuclear phenomenology of BL Lacs is interpreted as being due to the effects of their relativistic jet. All known BL Lacs are associated with core dominated radio sources, many of them exhibiting super-luminal motion ^[1].

The spectral energy distribution (SED) of BL Lacs exhibits two-component structure. A low-energy component peaking between infrared (IR) and X-ray energies. The low energy component is dominated by synchrotron emission from relativistic electrons in the jet. The frequency at which this peak occurs is called synchrotron peak frequency (ν_syn). BL Lacs generally classified under three main categories according to their synchrotron peak frequency as below.

• Low energy peaked BL Lacs (LBL)
• Intermediate energy peaked BL Lacs (IBL)
• High energy peaked BL Lacs (HBL)

The class boundaries can be loosely defined as ν_syn≈ 10¹³⁻¹⁴Hz for LBLs, ν_syn ≈ 10¹⁵⁻¹⁶ Hz for IBLs and ν_syn ≈ 10¹⁷⁻¹⁸Hz for HBLs ^[2]

The high energy component of SED peaking between X and gamma rays. The frequency at which this peak happens is called the Inverse Compton peak frequency (ν_IC). The simplest scenario that explains the high energy emission of BL Lacs is the Synchrotron Self Compton (SSC) model, by the relativistic Inverse Compton (IC) scattering at soft synchrotron photons. However, there is also more complicated scenarios, single-zone SSC does not reproduce all observed properties (e.g. there are several emitting regions).