The alternating 'changing-look' blazar OQ 334 (B2 1420+32): New observational clues to the blazar state transitions

TL;DR

This study investigates the state transitions of the blazar OQ 334 between FSRQ and BLL states, using gamma-ray spectral slope and optical observations over five years, revealing a correlation between optical flares and state changes.

Contribution

It introduces a gamma-ray spectral slope diagnostic for monitoring blazar state transitions and provides new observational evidence linking optical flares to FSRQ to BLL state changes.

Findings

All four observed FSRQ to BLL transitions coincided with optical flares.

Gamma-ray spectral slope < 2.0 reliably indicates BLL state.

Optical flares are associated with blazar state transitions.

Abstract

The high-luminosity blazar OQ 334 is a leading exponent of the intriguing rare phenomenon of alternating between a flat-spectrum radio quasar (FSRQ) and a BL Lac (BLL) states. Its two optical continuum outbursts observed during the $\sim$ 1.5-year long time span, starting Jan 2018, had been shown to coincide with transition from the FSRQ to BLL state, manifested by a sharp drop in the equivalent width of MgII broad emission-line. Recently, a continuous monitoring of its blazar state, over a much longer duration (past $\sim$ 5 years) has become possible by deploying the observed $\gamma-ray$ spectral slope ($\Gamma_{\gamma}$) as a diagnostic. This opens prospects of making a much less biased and statistically more robust check on the association of optical flaring with FSRQ $\rightarrow$ BLL transition. We find that all 4 such transitions ($\Gamma_{\gamma}$ becoming < 2.0), observed during the past $\sim$ 5 years, were accompanied by an optical flare. While this appears consistent with the transition to BL Lac state happening purely due to an enhanced optical continuum (flaring) swamping out the broad emission-lines, this simple scheme may need additional ingredients, considering the hint found for a day-like offset between the flaring and the state transition.