Multi-wavelength observations of blazar AO 0235+164 in the 2008-2009 flaring state

TL;DR

This study presents comprehensive multi-wavelength observations of blazar AO 0235+164 during a flaring state, revealing correlated gamma-ray and optical activity, and suggests a bulk Compton emission component in the X-ray spectrum.

Contribution

It provides the first detailed multi-wavelength campaign analysis of AO 0235+164 during a flare, including interpretation of X-ray features as bulk Compton emission.

Findings

Gamma-ray activity correlates with near-IR/optical flares and increased polarization.

X-ray shows a 20-day high state with an unusually soft spectrum.

Spectral modeling supports a leptonic emission scenario with external IR radiation.

Abstract

The blazar AO 0235+164 (z = 0.94) has been one of the most active objects observed by Fermi Large Area Telescope (LAT) since its launch in Summer 2008. In addition to the continuous coverage by Fermi, contemporaneous observations were carried out from the radio to {\gamma} -ray bands between 2008 September and 2009 February. In this paper, we summarize the rich multi-wavelength data collected during the campaign (including F-GAMMA, GASP- WEBT, Kanata, OVRO, RXTE, SMARTS, Swift, and other instruments), examine the cross-correlation between the light curves measured in the different energy bands, and interpret the resulting spectral energy distributions in the context of well-known blazar emission models. We find that the {\gamma} -ray activity is well correlated with a series of near-IR/optical flares, accompanied by an increase in the optical polarization degree. On the other hand, the X-ray light curve shows a distinct 20 day high state of unusually soft spectrum, which does not match the extrapolation of the optical/UV synchrotron spectrum. We tentatively interpret this feature as the bulk Compton emission by cold electrons contained in the jet, which requires an accretion disk corona with an effective covering factor of 19% at a distance of 100 Rg . We model the broadband spectra with a leptonic model with external radiation dominated by the infrared emission from the dusty torus.