Far‐Infrared Polarization of the Quasar 3C 279

We present the first FIR polarisation results of the OVV quasar 3C 279obtained with ISOPHOT for two epochs in 1996 and 1997. We describe its integralpolarisation properties at a wavelength of 170 micron where the source shows amaximum in its energy distribution. After a gamma-ray flare in January 1996, apolarisation of 23 % closely aligned with the radio jet axis was measured inJuly 1996. In June 1997, the polarisation degree had decreased to 6.5 % with aless good alignment. On the other hand, the total 170 micron flux is the samefor both epochs. Our measurements provide additional constraints for themulti-wavelength properties of synchrotron emission in radio jets and thetemporal evolution of these properties: they show that the FIR radiation of 3C279 is optically thin and that its origin is very close to the core. Thevariability of the FIR polarisation without any change of the total FIR fluxcan be explained by a disordering of the magnetic field in between the core andthe first stationary VLBI radio knot.