A long-term multiwavelength study of the flat spectrum radio quasar OP 313

TL;DR

This study presents a comprehensive 15-year, multiwavelength analysis of OP 313, a high-redshift FSRQ, focusing on a prominent gamma-ray active phase from 2023 to 2024. By combining Fermi-LAT, Swift, optical surveys, radio monitoring, and VLBI kinematics from MOJAVE and VLBA-BU-BLAZAR, the authors identify seven major gamma-ray flares and link several of them to the emergence and interaction of new jet components with standing shocks. Cross-band correlations reveal a zero-lag optical–gamma connection consistent with a leptonic emission scenario, while SED modeling with JetSeT favors external Compton scattering off dusty torus photons from a region outside the BLR, supporting a far-from-core emission site. The results underscore the role of jet dynamics in driving high-energy flares and provide constraints on particle acceleration, jet magnetization, and the location of the gamma-ray emission zone, with implications for VHE detections and future VLBI campaigns.

Abstract

The Flat Spectrum Radio Quasar OP 313 is a high-redshift (z = 0.997) blazar that entered an intense gamma-ray active phase from November 2023 to March 2024, as observed by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. We present a multiwavelength analysis covering 15 years of data, from August 2008 to March 2024, to contextualize this period of extreme gamma-ray activity within the long-term emission of the source. We analyzed a long-term, comprehensive, multiwavelength dataset from different facilities and projects from radio to gamma-rays. We identified the 7 most intense gamma-ray flaring periods and performed a kinematic analysis of Very Long Baseline Array (VLBA) data to determine whether new jet components emerged before or during these flares. For 2 of these flaring periods, we performed the modeling of the spectral energy distribution (SED). The VLBA-BU-BLAZAR and MOJAVE datasets reveal a new jet component appearing in both visibility datasets prior to the onset of one of the strongest gamma-ray flares. By comparing the timing of the VLBA-BU-BLAZAR knots ejection with the gamma-ray flaring periods, we constrained the setup of the SED modeling. We also found that the first gamma-ray flaring period is less Compton-dominated than the others. Our results suggest that the recent activity of OP 313 is triggered by new jet components emerging from the core and interacting with a standing shock. The γ-ray emission likely arises from dusty torus photons upscattered via Inverse Compton (IC) by relativistic jet electrons. The SED modeling indicates that this component is less dominant during the first γ-ray flaring period than the later ones.

Figures (15)

• Figure 1: Top: 15-year Fermi-LAT lightcurve of OP 313, divided into quiescent (blue) and flaring (yellow) states. The green line shows the time-weighted average flux of $4.93\times 10^{-8}\, \unit{photons/cm^{2}s}$ computed between the 9th of December 2014 and the 9th of August 2018. Bottom: Fermi-LAT photon index over the same 15-year period. The green line shows the average photon index computed between the 9th of December 2014 and 9th of August 2018 ($\Gamma = 2.39$), with the green band indicating its corresponding $1\sigma$ uncertainty. The red vertical bands highlight the period in which LST-1 detected OP 313.
• Figure 2: Multiwavelength lightcurve of OP 313. The red vertical band indicates the period when LST-1 detected OP 313. a: Fermi-LAT lightcurve at $E>100\,$MeV. b: Swift-XRT X-ray lightcurve from 0.3$\, \mathrm{keV}$ to 10$\, \mathrm{keV}$. c: Swift-UVOT lightcurve in the optical/UV bands. d: Optical lightcurve from several facilities and filters: CRTS V-filter (cV), KAIT R-filter (kR), Tuorla R-filter (tR), ATLAS o (Ao) and c-filters (Ac), and Palomar ZTF g, r, and i-filters (zG, zR, and zI). The label zGzR refers to ZTF observations using both g and r filters. e: Radio Single Dish F-GAMMA lightcurves above 15$\, \mathrm{GHz}$, SMA data at 225, 273 and 350$\, \mathrm{GHz}$, including SMAPOL data at 225$\, \mathrm{GHz}$ lightcurves, Mets$\ddot{a}$hovi at 37$\, \mathrm{GHz}$ and VLBA-BU-BLAZAR at 43$\, \mathrm{GHz}$ lightcurves. f: Radio Single Dish F-GAMMA lightcurves below 15$\, \mathrm{GHz}$ and MOJAVE VLBA lightcurve.
• Figure 3: Fermi-LAT (top) and Optical (bottom) lightcurves from the 1st of January 2022 to 9th of March 2024. The dashed black lines indicate the brightest $\gamma$-ray flaring periods. The fourth and sixth flaring periods are not between the brightest but are located in time between the ATels bartolini_atel_2023 and bartolini_atel_2024. The red vertical bands indicate the period when LST-1 detected OP 313.
• Figure 4: a) Hysteresis pattern of the first flaring period (2nd of July 2022 - 15th of July 2022). The grey dashed line is included to guide the eye in identifying the temporal sequence of the points. b) Hysteresis pattern of the fifth flaring period (30th of January 2024 - 8th of February 2024). c) Hysteresis pattern of the second flaring period (20th of November 2023 - 29th of November 2023).
• Figure 5: SED of the 1st (2022), in orange, the 5th flaring periods (January 2024), in blue, and a quiescent period that goes from the 1st of March 2016 to the 1st of January 2018, in purple. The X-ray, UV, optical, and radio SED points correspond to the average flux values within each period at their respective frequencies. No Swift-XRT and UVOT data were available during the quiescent period.