The alternating ‘changing-look’ blazar OQ 334 (B2 1420+32): new observational clues to the 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 yr long time span, starting 2018 Jan, had been shown to coincide with transition from the FSRQ to BLL state, manifested by a sharp drop in the equivalent width of Mg ii broad emission line. Recently, a continuous monitoring of its blazar state, over a much longer duration (past $\sim$5 yr) has become possible by deploying the observed $\gamma {-}\mathrm{ ray}$ 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 four such transitions ($\Gamma _{\gamma }$ becoming $<$2.0), observed during the past $\sim$5 yr, were accompanied by an optical flare. While this appears consistent with the transition to BLL 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.