High Energy Emission Processes in OJ 287 during 2009 Flare

TL;DR

This study analyzes the high energy emission mechanisms in OJ 287 during the 2009 flare, concluding that a combination of SSC and EC processes best explains the observed spectrum and suggesting the emission region is located about 9 parsecs from the central engine.

Contribution

It demonstrates that a combined SSC and EC model explains the high energy emission in OJ 287, and estimates the emission region's distance from the central black hole.

Findings

High energy emission can be explained by combined SSC and EC mechanisms.

The emission region is approximately 9 parsecs from the central engine.

The surrounding IR region has a temperature of about 250 K.

Abstract

The broadband spectrum of a BL Lac object, OJ 287, from radio to $\gamma$-rays obtained during a major $\gamma$-ray flare detected by \emph{Fermi} in 2009 are studied to understand the high energy emission mechanism during this episode. Using a simple one-zone leptonic model, incorporating synchrotron and inverse Compton emission processes, we show that the explanation of high energy emission from X-rays to $\gamma$-rays, by considering a single emission mechanism, namely, synchrotron self-Compton (SSC) or external Compton (EC) requires unlikely physical conditions. However, a combination of both SSC and EC mechanisms can reproduce the observed high energy spectrum satisfactorily. Using these emission mechanisms we extract the physical parameters governing the source and its environment. Our study suggests that the emission region of OJ 287 is surrounded by a warm infrared (IR) emitting region of $\sim 250 \, K$. Assuming this region as a spherical cloud illuminated by an accretion disk, we obtain the location of the emission region to be $\sim 9 pc$. This supports the claim that the $\gamma$-ray emission from OJ 287 during the 2009 flare arises from a location far away from the central engine as deduced from millimeter-gamma ray correlation study and very long baseline array images.