  The effect of an external magnetic field on the finite temperature transition
of QCD is studied. We measure thermodynamic observables including the quark
condensates and susceptibilities and the strange quark number susceptibility.
We generate configurations at various values of the quantized magnetic flux
with N_f=2+1 flavors of stout smeared staggered quarks at physical quark
masses. We perform the renormalization of our observables and approach the
continuum limit with N_t=6,8 and 10 lattices. We also check finite volume
effects using various lattice volumes. Our main result is that the transition
temperature significantly decreases with growing magnetic field, and that the
transition remains an analytic crossover up to our largest external field eB ~
1 GeV^2.
