The WEBT Campaign on the Blazar 3C279 in 2006

TL;DR

This paper reports on a detailed multiwavelength monitoring of blazar 3C279 in 2006, revealing correlated optical variability, wavelength-dependent time delays, and spectral changes that suggest complex jet magnetic field structures or slow acceleration processes.

Contribution

It provides new insights into the variability patterns, spectral behavior, and possible magnetic field configurations of 3C279 during the 2006 WEBT campaign.

Findings

Optical flux and spectral shape varied substantially with a few-day timescale.

Shorter-wavelength variability lagged behind longer wavelengths, increasing with frequency.

Spectral hardening during flares was delayed relative to optical flux rise.

Abstract

The quasar 3C279 was the target of an extensive multiwavelength monitoring campaign from January through April 2006, including an optical-IR-radio monitoring campaign by the Whole Earth Blazar Telescope (WEBT) collaboration. In this paper we focus on the results of the WEBT campaign. The source exhibited substantial variability of optical flux and spectral shape, with a characteristic time scale of a few days. The variability patterns throughout the optical BVRI bands were very closely correlated with each other. In intriguing contrast to other (in particular, BL Lac type) blazars, we find a lag of shorter- behind longer-wavelength variability throughout the RVB ranges, with a time delay increasing with increasing frequency. Spectral hardening during flares appears delayed with respect to a rising optical flux. This, in combination with the very steep IR-optical continuum spectral index of ~ 1.5 - 2.0, may indicate a highly oblique magnetic field configuration near the base of the jet. An alternative explanation through a slow (time scale of several days) acceleration mechanism would require an unusually low magnetic field of < 0.2 G, about an order of magnitude lower than inferred from previous analyses of simultaneous SEDs of 3C279 and other FSRQs with similar properties.