Multi-Wavelength Observations of Blazar Ao 0235+164 in the 2008-2009 Flaring State

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 γ-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 γ-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 R g. We model the broadband spectra with a leptonic model with external radiation dominated by the infrared emission from the dusty torus.

The Optical Variability of the BL Lac AO 0235+164

“…Although it was classified as a BL Lac object, the equivalent width (EW) of emission lines varies from one observational epoch to another. It is violently variable in all wavebands from radio through high-energy γ-rays [23].…”

“…BL Lac AO 0235+164 shows variability timescales from less than one hour to several years [9,[18][19][20][21][22][23][24][25]. In our previous paper, variations in the UBVRI bands are ∆U = 4.26, ∆B = 5.47, ∆V = 4.74, ∆R = 4.18, and ∆I = 3.85 mag [26].…”

Multiwavelength Probes of the Environs of Relativistic Shocks in Blazar Jets

“…This appears in Fig. 7 of Ackermann et al 41 , wherein it can be compared with a low state spectrum obtained about six weeks later that does not contain a Swift XRT detection.…”

“…The BL Lac object AO 0235+164 was observed to be flaring in the period during September -November 2008. We focus here on modeling data collected during the multi wavelength campaign discussed in Ackermann et al 41 ; see also Agudo et al…”

“…The list of observatories and missions participating in this campaign is extensive 41 -it spans radio and optical, with Swift XRT and RXTE providing X-ray monitoring, and Fermi-LAT providing the principal gamma-ray observations. The LAT gamma-ray spectrum displays a break or possible turnover at energies around 3 − 4 GeV, with a power-law index of about 2.1 below this feature; the source is not detected in the TeV band 43 .…”