An extended Herschel drop-out source in the center of AS1063: a normal dusty galaxy at z=6.1 or SZ substructures?

In the course of our 870 mu m APEX/LABOCA follow-up of the Herschel Lensing Survey we have detected a source in AS1063 (RXC J2248.7-4431) that has no counterparts in any of the Herschel PACS/SPIRE bands, it is a Herschel "drop-out" with S-870/S-500 >= 0.5. The 870 mu m emission is extended and centered on the brightest cluster galaxy, suggesting either a multiply imaged background source or substructure in the Sunyaev-Zel' dovich increment due to inhomogeneities in the hot cluster gas of this merging cluster. We discuss both interpretations with emphasis on the putative lensed source. Based on the observed properties and on our lens model we find that this source may be the first submillimeter galaxy (SMG) with a moderate far-infrared (FIR) luminosity (L-FIR < 10(12) L-circle dot) detected so far at z > 4. In deep HST observations we identified a multiply imaged z similar to 6 source and measured its spectroscopic redshift to be z = 6.107 with VLT/FORS. This source may be associated with the putative SMG, but it is most likely offset spatially by 10-30 kpc and they may be interacting galaxies. With a FIR luminosity in the range [5-15] x 10(11) L-circle dot corresponding to a star formation rate in the range [80-260]M(circle dot)yr(-1), this SMG would be more representative of the z > 4 dusty galaxies than the extreme starbursts detected so far. With a total magnification of similar to 25 it would open a unique window to the normal dusty galaxies at the end of the epoch of reionization.