  Heavy-ion collisions at $\sqrt{s_\mathrm{NN}} \sim 10$ GeV probe the QCD
phase diagram at large baryon densities. Because the longitudinal Lorentz
contraction is small at these collision energies, understanding the dynamics
during the early phase of the collision is essential for the subsequent
modeling of the system evolution and for constraining the QGP transport
properties at finite baryon densities. Direct photons provide undistorted
information on early-stage dynamics. We model relativistic heavy-ion collisions
at RHIC Beam Energy Scan energies with a hybrid dynamical approach consisting
of a 3D-Glauber initial state followed by viscous hydrodynamics and hadronic
transport (MUSIC + UrQMD). The implemented thermal photon emission takes into
account the enhancement from finite baryon chemical potentials. We show that
direct photon spectra and their anisotropic flow coefficients have a strong
sensitivity to the early stage of heavy-ion collisions. Thus, they provide
constraints on QGP dynamics complementary to those obtained from hadronic
observables.
