In this work we address the problem of simultaneous multi-frequency detectionof extragalactic point sources in maps of the Cosmic Microwave Background. Weapply a new linear filtering technique, the so called `matched matrix filters',that incorporates full spatial information, including the cross-correlationamong channels, without making any a priori assumption about the spectralbehaviour of the sources. A substantial reduction of the background is achievedthanks to the optimal combination of filtered maps. We describe in detail thenew technique and we apply it to the detection/estimation of radio sources inrealistic all-sky Planck simulations at 30, 44, 70 and 100 GHz. Then we comparethe results with the mono-frequential approach based on the standard matchedfilter, in terms of reliability, completeness and flux accuracy of theresulting point source catalogs. The new filters outperform the standardmatched filters for all these indexes at 30, 44 and 70 GHz, whereas at 100 GHzboth kind of filters have a similar performance. We find a noticeable incrementof the number of true detections for a fixed reliability level. In particular,for a 95% reliability we practically double the number of detections at 30, 44and 70 GHz.
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