The Nature of Transition Blazars

TL;DR

This study systematically identifies transition blazars that switch between subclasses due to jet variability, revealing they are likely FSRQs with efficient accretion, impacting our understanding of blazar populations and evolution.

Contribution

First systematic search for transition blazars using repeat spectra, showing they are likely FSRQs with variable jet emission rather than true subclass switches.

Findings

Six transition blazars identified out of 602 pairs of spectra.

Transition blazars exhibit strong EW variability due to jet continuum swamping.

Transition blazars are likely FSRQs with efficient accretion and strong jets.

Abstract

Blazars are classically divided into the BL Lac (BLL) and Flat-Spectrum Radio Quasar (FSRQ) subclasses, corresponding to radiatively inefficient and efficient accretion regimes, respectively, largely based on the equivalent width (EW) of their optical broad emission lines (BEL). However, EW-based classification criteria are not physically motivated, and a few blazars have previously 'transitioned' from one subclass to the other. We present the first systematic search for these transition blazars in a sample of 602 unique pairs of repeat spectra of 354 blazars in SDSS, finding six clear cases. These transition blazars have bolometric Eddington ratios of ~0.3 and low-frequency synchrotron peaks, and are thus FSRQ-like. We show that the strong EW variability (up to an unprecedented factor of >60) is due to swamping of the BELs from variability in jet continuum emission, which is stronger in amplitude and shorter in timescale than typical blazars. Although these transition blazars appear to switch between FSRQ and BLL according to the phenomenologically-based EW scheme, we show that they are most likely rare cases of FSRQs with radiatively efficient accretion flows and especially strongly-beamed jets. These results have implications for the decrease of the apparent BLL population at high-redshifts, and may add credence to claims of a negative BLL redshift evolution.