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 Q _ { dust } \simeq 86 ~ { } kg . s ^ { -1 } and a parameter Af \rho \simeq 10 , 000 cm ; ( iii ) no emission lines ( CN and C _ { 2 } ) can be detected in the visible range ; ( iv ) the upper limits for the CN and C _ { 2 } production rates are \simeq 3.8 \times 10 ^ { 25 } and \simeq 10 ^ { 26 } molecules.s ^ { -1 } respectively ; ( v ) we detected no Echeclus ’ satellite before the outburst up to M _ { R } \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 .

Conclusions :