Multiband Optical Photometric and Spectroscopic Monitoring of the 2024 Flare Event in Transition Blazar OP313

TL;DR

This study investigates a dramatic transitional event in the blazar OP313, examining whether accretion-rate-driven changes can move the source along a continuum between FSRQ-like and BL Lac-like states. By combining 100 days of optical spectroscopy and multi-band photometry with continuous Fermi/LAT gamma-ray monitoring, the authors track the evolution of the synchrotron peak frequency $\nu_s$, the peak flux $F_s$, and the powers of synchrotron and inverse-Compton components $P_s$ and $P_{IC}$ around a flare that boosts the gamma-ray flux by about $60\times$. They find a pre-transitional phase with rising $P_s$ and $P_{IC}$, a brief transitional spike where $\nu_s$ surges to $>1.46\times10^{15}$ Hz and the spectrum becomes BL Lac-like (Mg II effectively drowned out), followed by a post-transitional state where IC activity decouples from synchrotron emission and the magnetic field weakens. These results support SSC-driven gamma-ray emission during the flare and argue for a continuum-based blazar classification, driven by accretion-rate variations that modulate the jet’s particle density and magnetic field, with implications for understanding high-energy jet physics in transitional blazars.

Abstract

Blazars are active galactic nuclei known for their extreme variability, offering unique opportunities to study jet physics and high-energy emission mechanisms. In 2024, the Flat Spectrum Radio Quasar (FSRQ) OP313 underwent a remarkable flare event, during which the gamma-ray flux observed by the Fermi Large Area Telescope (Fermi/LAT) increased by a factor of 60 over its average value. The flare peak lasted less than two days. Using optical telescopes, we conducted 100-day time-scale observations. Multi-wavelength data revealed that OP313 entered an active state 50 days prior to the flare and remained active for at least 50 days afterward. We propose that this prolonged activity results from variations in electron density within the shock front due to changes in the accretion rate. Concurrently, OP313's spectrum transitioned from an FSRQ-like state to a BL Lac-like state, characterized by a significant increase in the synchrotron peak frequency and the disappearance of broad-line region emission lines. In the post-flare phase, we observed a decoupling between synchrotron radiation and inverse Compton scattering, along with a possible decrease in the magnetic field strength within the shock front.

Figures (5)

• Figure 1: (A): The KOOLS-IFU and SDSS spectra of OP313, The position of the unresolved doublet of singly-ionized magnesium (Mg II 2796 & 2803) is labeled; (B): The MITSuME 3-band light curve of OP313, error bars are omitted for clarity; (C): MITSuME g-Rc color index curve of of OP313; (D): Tomo-e Gozen (see zhang2024optical) and HONIR light curve of OP313. The red dashed line represents the average magnitude of OP313 during the entire Tomo-e Gozen monitoring period; (E): Fermi/LAT energy flux (left axis) and $\gamma$-ray photon index (right axis) curve of OP313.
• Figure 2: The fitting results of the Mg II emission line in the OP313 spectra, after removing contributions from the continuum and Fe II emission.
• Figure 3: Spectral energy distribution of OP313 after the flare event. Except for the SED fitting at T+2.5, which exhibits a distinct trend, all other SEDs are fitted with a second-order polynomial, represented by solid lines. Non-physical absorption lines affecting the fits have been corrected in this process.
• Figure 4: Order estimate of $\gamma'$ and $B'$ of OP313 between T+5.5 and T+38. In the figure, the rate of change of the magnetic field is represented by the left axis, while the rate of change of the particle Lorentz factor is represented by the right axis. The primed quantities correspond to values in the "post-transition state" (T + 5.5 to T + 38), while the unprimed quantities represent the "normal state", as listed in the 4LAC catalog.
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