174P/Echeclus: a strange case of outburst

Context.More than ten Centaurs are now known to have cometary activity at large heliocentric distance (i.e. $\simeq$5-13 AU). Among these objects, 174P/Echeclus which showed cometary activity at 13 AU from the Sun, is a unique case, because of: (i) the amplitude of its outburst, (ii) the source of cometary activity that appears distinct from Echeclus itself. Aims.This paper aims at investigating the physical conditions that have led to this unusual outburst. The purpose is also to quantify this phenomenon and to provide observational constraints for its modeling. Methods.We use observations from different telescopes, performed before, during, and after the outburst. We performed the main observations on March 23 and 30, 2006, with the 8.2-m ESO Very Large Telescope and FORS 1 instrument. They consist of visible images and spectra. Results.Our main results are: (i) a cometary source distinct from Echeclus itself that presents a brightness distribution compatible with a diffuse source; (ii) a total dust production rate Qdust$\simeq$ 86 kg s-1 and a parameter $Af\rho\simeq$ 10 000 cm; (iii) no emission lines (CN and C2) can be detected in the visible range; (iv) the upper limits for the CN and C2 production rates are $\simeq$3.8$\times$1025 and $\simeq$1026 molecules s-1 respectively; (v) we detected no Echeclus' satellite before the outburst up to MR $\simeq$ 26; (vi) the upper limit for the object generating the coma is about 8 km in diameter; (vi) and we detected no cometary activity one year later, in March 2007