THE LARGEST GRAVITATIONAL LENS: MACS J0717.5+3745 (z=0.546)

We identify 13 sets of multiply-lensed galaxies around MACS J0717.5+3745 (z = 0.546), outlining a very large tangential critical curve of major axis similar to 2.'8, filling the field of the Hubble Space Telescope/Advanced Camera for Surveys. The equivalent circular Einstein radius is theta(e) = 55 '' +/- 3 '' ( at an estimated source redshift of z(s) similar to 2.5), corresponding to r(e) similar or equal to 350 +/- 20 kpc at the cluster redshift, nearly three times greater than that of A1689 (r(e) similar or equal to 140 kpc for zs = 2.5). The mass enclosed by this critical curve is very large, 7.4 +/- 0.5 x 10(14) M-circle dot and only weakly model dependent, with a relatively shallow mass profile within r < 250 kpc, reflecting the unrelaxed appearance of this cluster. This shallow profile generates a much higher level of magnification than the well-known relaxed lensing clusters of higher concentration, so that the area of sky exceeding a magnification of > 10 times, is similar or equal to 3.5 square' for sources with z similar or equal to 8, making MACS J0717.5+3745 a compelling target for accessing faint objects at high redshift. We calculate that only one such cluster, with theta(e) >= 55 '', is predicted within similar to 10(7) Universes with z >= 0.55, corresponding to a virial mass >= 3 x 10(15) M-circle dot, for the standard Lambda CDM (WMAP5 parameters with 2 sigma uncertainties).