Search

The Fermi γ -ray telescope has detected 6658 sources, with 1845 of them remaining unidentified. We show that polarimetry of γ -ray fields is a powerful asset in the hunt of active galactic nuclei (AGN) as potential optical counterparts for γ -ray sources. We have studied an unidentified Fermi field (3FGL J0221.2+2518) and found a previously-unknown highly-polarized extragalactic object as a potential optical counterpart within the 1-sigma error ellipse of the corresponding γ -ray source. Based on a collection of data, we find that it most probably is a composite object: a star-forming galaxy accompanied by AGN. PASIPHAE is a large polarimetric experiment which will measure the polarisation of sources away from the galactic plane. This will provide an excellent opportunity to study hundreds of unidentified γ -ray sources and unveil potential optical counterparts, using polarimetry. [ABSTRACT FROM AUTHOR]