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We aim to obtain new insights into the internal dynamics of the cluster Abell1240, showing the presence of two roughly symmetric radio relics, separated by~2 h_70^-1 Mpc. Our analysis is mainly based on redshift data for 145 galaxiesand on new photometric data. We also use X-ray data from the Chandra archiveand photometric data from the SDSS (DR7). We combine galaxy velocities andpositions to select 89 cluster galaxies and analyze the internal dynamics ofthe Abell 1237 + Abell 1240 cluster complex. We estimate similar redshifts forAbell 1237 and Abell 1240, <z>=0.1935 and <z>=0.1948, respectively. For Abell1237 we estimate velocity dispersion sigma_v~740 km/s and a mass M~6 10^14h_70^-1 M_sun. For Abell 1240 we estimate a LOS sigma_v~870 km/s and a massrange M~0.9-1.9 10^15 h_70^-1 M_sun, which takes into account its complexdynamics. Abell 1240 is shown to have a bimodal structure with two galaxyclumps roughly defining the N-S direction, the same one defined by theelongation of its X-ray surface brightness and by the axis of symmetry of therelics. The two--body model agrees with the hypothesis that we are looking at acluster merger occurred largely in the plane of the sky, with the two galaxyclumps separated by a rest-frame velocity difference V_rf~2000 km/s at a timeof 0.3 Gyrs after the crossing core, while Abell 1237 is still infalling ontoAbell 1240. Chandra archive data confirm the complex structure of Abell 1240and allow us to estimate a global X-ray temperature T_X=6.0+-0.5 keV. Inagreement with the findings from radio data, our results for Abell 1240strongly support the "outgoing merger shocks" model to explain the presence ofthe relics.

Internal dynamics of Abell 1240: a galaxy cluster with symmetric double radio relics