Context :

Aims : GRB 050502B is well known for the very bright flare displayed in its X-ray light curve .
Despite extensive studies , however , the optical light curve has never been discussed and its redshift is unconstrained .
Possible correlations between optical and X-ray data are analysed .

Methods : Photometric data from TNG in the R and I bands were used to compare the optical afterglow with the X-ray light curve .
The HyperZ package and a late time VLT host observation were used to derive redshift estimates .

Results : The I -band afterglow decay followed a power-law of index \alpha = 2.1 \pm 0.6 , after a late break at \sim 1.3 \times 10 ^ { 5 } s. The R - I color is remarkably red and the broadband spectral index \beta _ { OX } = 0.9 \pm 0.1 is consistent with the X-ray spectral slope \beta _ { X } .
Although a photometric redshift of z > 4 is the most conservative result to consider , a photometric redshift of z = 5.2 \pm 0.3 is suggested with no extinction in the host , based on which an isotropic energy E _ { \gamma, iso } = ( 3.8 \pm { 0.7 } ) \times 10 ^ { 52 } erg and a jet opening angle \theta \sim 3.7 \degr are subsequently derived .

Conclusions : The combined X-ray and optical data suggest an achromatic break , which we interpret as a jet break .
The post jet break slope obeys roughly the closure relation for the jet slow cooling model .
Because of the afterglow ’ s very red color , in order for the redshift to be low ( z < 1 ) , extinction , if present in the host , must be significantly high .
Since the optical-to-X-ray index is consistent with the X-ray spectrum , and there is no XRT evidence for excess N _ { H } , GRB 050502B was likely at high redshift .