U. Klaas 1, Th. Müller 1, R.J. Laureijs 2, J. Clavel 2, A. Moneti 2, J.L.V. Lagerros 3, R.J. Tuffs 4, & E. Kreysa 5
1 Max-Planck-Institut für Astronomie Heidelberg, Germany
2 ISO Science Operations Centre Astrophysics Division, Space Science Department of ESA Villafranca, Spain
3 Astronomiska observatoriet, Uppsala, Sweden
4 Max-Planck-Institut für Kernphysik Heidelberg, Germany
5 Max-Planck-Institut für Radioastronomie, Bonn, Germany
The ISOPHOT polarisation modes were successfully commissioned by the end of 1997, so that they became available to the ISO observers community during the last few months of the ISO mission.
We report here about the measurement strategies and performances of both the 25 and 170 micron modes, including the determination of the instrumental polarisation. For both modes a 1 sigma polarisation degree uncertainty of 1-2% can be achieved for source-to-background flux ratios of 2:1. This means that polarimetry with this accuracy can be obtained for sources down to 2 Jy at 170 micron and down to 10 Jy at 25 micron. Sources with polarisation degrees above 10% can be reliably measured to fainter levels.
We present also scientific results from 3 programs: a) FIR polarisation variability of the quasar 3C 279 The polarisation of a variable quasar has been measured for the first time in the FIR. It turned out to be variable over a time interval of nearly one year with 23% polarisation at the first epoch and only 6.5% at the second epoch. The interpretation of the results, the comparison with measurements at other wavelengths and the implications on models are presented.
b) Search for polarised mid-IR emission from elongated asteroids The polarisation degree to be expected depends on the assumed form and surface structure. Polarisation measurements provide further constraints for the thermophysical modelling of asteroids which has been successfully done for ISOPHOT photometric calibration.
c) Polarisation measurements of the Crab Nebula We present the first results about polarisation at both 25 and 170 micron and a comparison with model expectations.