[Paper Review] Gamma-ray Flaring Emission in Blazar OJ287 Located in the Jet >14 pc from the Black Hole

I. Agudo, S. G. Jorstad+9 more|arXiv (Cornell University)|Oct 28, 2011

Astrophysics and Cosmic Phenomena3 citations

TL;DR

The paper proposes that the γ-ray flares in the blazar OJ287 originate via synchrotron self-Compton (SSC) emission from a plasma blob interacting with a standing shock in the jet, located >14 pc from the black hole. Multi-wavelength monitoring shows simultaneous peaks in γ-ray, millimeter-wave, and optical flares, along with high linear polarization, confirming the emission site in a second jet feature (C1) beyond the acceleration and collimation zone.

ABSTRACT

We combine the Fermi-LAT light curve of the BL Lacertae type blazar OJ287 with time-dependent multi-waveband flux and linear polarization observations and submilliarcsecond-scale polarimetric images at lambda=7mm to locate the gamma-ray emission in prominent flares in the jet of the source >14pc from the central engine. We demonstrate a highly significant correlation between the strongest gamma-ray and millimeter-wave flares through Monte Carlo simulations. The two reported gamma-ray peaks occurred near the beginning of two major millimeter-wave outbursts, each of which is associated with a linear polarization maximum at millimeter wavelengths. Our very long baseline array observations indicate that the two millimeter-wave flares originated in the second of two features in the jet that are separated by >14pc. The simultaneity of the peak of the higher-amplitude gamma-ray flare and the maximum in polarization of the second jet feature implies that the gamma-ray and millimeter-wave flares are cospatial and occur >14pc from the central engine. We also associate two optical flares, accompanied by sharp polarization peaks, with the two gamma-ray events. The multi-waveband behavior is most easily explained if the gamma-rays arise from synchrotron self Compton scattering of optical photons from the flares. We propose that flares are triggered by interaction of moving plasma blobs with a standing shock.

Motivation & Objective

• To determine the spatial location of γ-ray flaring emission in the jet of the BL Lac object OJ287.
• To investigate the physical mechanism responsible for the γ-ray flares by correlating multi-wavelength light curves and polarization data.
• To test whether the flares originate via synchrotron self-Compton (SSC) scattering or external Compton processes.
• To identify the role of shock structures and plasma blob dynamics in triggering flares in the jet.

Proposed method

• Combined Fermi-LAT γ-ray light curves with multi-wavelength observations across radio, millimeter, optical, and X-ray bands.
• Used very long baseline array (VLBA) 7 mm polarimetric imaging to resolve jet structure and track evolution of components C0 and C1.
• Applied Monte Carlo simulations to assess the significance of cross-correlations between γ-ray and millimeter-wave light curves.
• Modeled brightness distribution in VLBA images using circular Gaussian components to identify flaring regions.
• Analyzed time-averaged and time-resolved polarization data to infer magnetic field geometry and emission region properties.
• Used cosmological parameters (H₀ = 71 km s⁻¹ Mpc⁻¹, Ωₘ = 0.27, ΩΛ = 0.73) to convert angular separations to physical distances (1 mas ≈ 4.48 pc).

Experimental results

Research questions

• RQ1 Where in the jet is the γ-ray emission site located, specifically in relation to the central engine?
• RQ2 What physical mechanism produces the γ-ray flares—SSC or external Compton scattering?
• RQ3 How do the timing and polarization behavior of millimeter and optical flares relate to the γ-ray flares?
• RQ4 What role do standing shocks and moving plasma blobs play in triggering the flares?
• RQ5 Can the observed polarization peaks and flux variations be explained by shock-induced particle acceleration and relativistic beaming?

Key findings

• The two strongest γ-ray flares in OJ287 occurred simultaneously with the rising phases of two major 1 mm flares in jet component C1, located >14 pc from the central engine.
• The γ-ray flares coincided with peak linear polarization of ~14% and ~22% in the millimeter-wave emission from C1, indicating a highly ordered magnetic field at the emission site.
• Optical flares with polarization peaks of ~35% also coincided with the γ-ray flares, supporting a common origin in the same emission region.
• The polarization position angle remained stable at χ ≈ 160°–170° during and between flares, consistent with a stable jet structure and shock geometry.
• Monte Carlo simulations confirmed a highly significant correlation between γ-ray and 1 mm flares, with a 99.7% confidence level for true correlation over stochastic variability.
• The observed multi-wavelength behavior is best explained by synchrotron self-Compton (SSC) emission from a plasma blob crossing a standing shock in the jet, with the emission site located in the second feature (C1) beyond the acceleration and collimation zone (ACZ).