Since a few years, the IRAM 30m telescope has polarimetric capabilities which are based on cross-correlations between the Observatory's single pixel heterodyne receivers. In a procedure designated XPOL, the IF signals of a pair of orthogonally polarized receivers tuned to the same frequency are cross correlated in the digital backend VESPA. Since the receivers are linearly polarized, the complex cross correlation gives Stokes and . Sum and difference of the IF powers gives the other Stokes parameters, and . Continuum observations use VESPA configured for a bandwidth of 480 MHz, using nearly all of the instantaneous bandwidth of the 3mm receivers.
Figure 4 shows a typical 3mm continuum observation of a medium strong (SJy) AGN. The four Stokes parameter are all observed simultaneously. The fractional linear and circular polarization, and , and the polarization angle are then derived in the usual way after averaging of the spectral channels. In a 4 minute observation of a 1 Jy source at 90 GHz, like the one shown here, the polarization degrees and have statistical errors of the order % and , respectively, at which level systematic errors start to dominate.
XPOL observations are now routine, and several scientific projects have been successfully completed (see, e.g. Wiesemeyer, Thum & Walmsley 2004, A&A 391, 479), and a large AGN survey was made at 90 GHz (Thum et al., in preparation). While this survey serves as a useful snapshot of the polarization properties of the AGN population as a whole, it became quite clear even during the short duration of this survey that many of the sources vary not only in their 3mm power, but also in their polarization properties. Such variations are well known at longer wavelengths where some semi-regular programs exist for monitoring their properties. No such effort is currently made at short millimeter wavelengths.
In order to contribute to a more complete characterization of these variations and help to investigate their causes, IRAM now plans to start a polarization monitoring program on the 30m telescope. Given the ease of use of XPOL and its reliability, such a program now becomes feasible. This complements the Total Power monitoring which is conducted by the Observatory since 1986 (Ungerechts et al. 1998, ASP Conf.Ser. 144, p.140 (IAU colloq. 164), and references therein). As certainly not more telescope time can be devoted to monitoring of polarization than to that of Total Power (ca. 4 hours/week), the program sources must be carefully selected.
The program is open to any interested astronomer. In fact, participation by non-IRAM astronomers working in AGN research is very welcome. Participation is conceivable at various levels, including the definition of the source sample, support of the observations, data reduction, preparation of a data base, and scientific data analysis. The acquired data are proprietary to the group of participating astronomers for a duration of 12 months and then become public.
Astronomers interested in the proposed polarization monitoring are invited to contact us.