  There must be electromagnetic fields created during high-energy heavy-ion
collisions. As the quark-gluon plasma (QGP) starts to evolve hydrodynamically,
although these fields may become weak comparing to the energy scales of the
strong interaction, they are potentially important to some electromagnetic
probes. In this work, we focus on the dissipative corrections in QGP due to the
presence of a weak external magnetic field, and calculate accordingly the
induced photon radiation in the framework of viscous hydrodynamics. By
event-by-event hydrodynamical simulations, the experimentally measured direct
photon elliptic flow can be well reproduced. Correspondingly, the direct photon
elliptic flow implies a magnetic field strength around 0.1$m_\pi^2 \sim
10^{16}$ G. This is indeed a weak field in heavy-ion physics that is compatible
to the theoretical predictions, however, it is still an ultra-strong magnetic
field in nature.
