The Phenomenological Nature of Quasar-type Blazars (BZQ). I. Revisiting the Flat-Spectrum Paradigm

Abstract

We re-evaluated 610 sources classified as flat-spectrum radio quasars (FSRQs) in the 5th edition of the Roma-BZCAT. Optical spectra from SDSS DR16 confirmed broad emission lines within 0.11 ≤ 𝑧 ≤ 5.28. To assess their blazar-like behaviour, we combined ZTF optical variability with radio morphologies from FIRST, LOFAR, and VLBI, defining Confirmed, Possible, and Non-Confirmed BZQs. The rest-frame 1.4–10 GHz radio spectra were homogenized and fitted using error-weighted power laws. We show that the scheme of Park et al. (2013) often misclassifies nearly flat spectra as inverted and some prominent steep spectra as flat. Using the individual uncertainty 𝜎_𝛼,𝑖 , we classify spectra as flat if |𝛼𝑖 | ≤ 2𝜎_𝛼,𝑖 , prominently steep if 𝛼𝑖 > 2𝜎_𝛼,𝑖 , and inverted if 𝛼_𝑖 < −2𝜎_𝛼,𝑖 . This criterion, defined source by source and intended as a phenomenological classification for this sample, better reflects the observed spectral shapes and confirms that most BZQs are consistent with being flat within the measurement precision, although a non-negligible fraction departs from a strict flatness. We also classify full spectral morphologies as power-law, peaked, retriggered-peaked, or inverted-peaked, which are associated with distinct jet processes and activity cycles. Approximately 60% of the sources with at least two decades of frequency coverage exhibited retriggered-peaked spectra, suggesting recurrent jet activity. The observed diversity indicates that the label “flat-spectrum radio quasar” does not fully describe this population and that the more general term BZQ may better reflect its phenomenological diversity.