The quasar 3CÂ 279 was the target of an extensive multiwavelength monitoring campaign from January through April 2006 .
An optical-IR-radio monitoring campaign by the Whole Earth Blazar Telescope ( WEBT ) collaboration was organized around Target of Opportunity X-ray and soft \gamma -ray observations with Chandra and INTEGRAL in mid-January 2006 , with additional X-ray coverage by RXTE and Swift XRT .
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 , while there was no obvious correlation between the optical and radio variability .
After the ToO trigger , the optical flux underwent a remarkably clean quasi-exponential decay by about one magnitude , with a decay time scale of \tau _ { d } \sim 12.8 Â d .

In intriguing contrast to other ( in particular , BLÂ Lac type ) blazars , we find a lag of shorter-wavelength behind longer-wavelength variability throughout the RVB wavelength 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 \alpha _ { o } \sim 1.5 – 2.0 , may indicate a highly oblique magnetic field configuration near the base of the jet , leading to inefficient particle acceleration and a very steep electron injection spectrum .

An alternative explanation through a slow ( time scale of several days ) acceleration mechanism would require an unusually low magnetic field of B \lesssim 0.2 Â G , about an order of magnitude lower than inferred from previous analyses of simultaneous SEDs of 3CÂ 279 and other FSRQs with similar properties .