  Using and comparing kinetic theory and second-order Chapman-Enskog
hydrodynamics, we study the non-conformal dynamics of a system undergoing
Bjorken expansion. We use the concept of `free-streaming fixed lines' for
scaled shear and bulk stresses in non-conformal kinetic theory and
hydrodynamics, and show that these `fixed lines' behave as early-time
attractors and repellors of the evolution. In the conformal limit, the
free-streaming fixed lines reduce to the well-known fixed points of conformal
Bjorken dynamics. A new fixed point in the free streaming regime is identified
which lies at the intersection of these fixed lines. Contrary to the conformal
scenario, both kinetic theory and hydrodynamics predict the absence of
attractor behavior in the normalised shear stress channel. In kinetic theory a
far-off-equilibrium attractor is found for the normalised effective
longitudinal pressure, driven by rapid longitudinal expansion. Second-order
viscous hydrodynamics fails to accurately describe this attractor. From a
thorough analysis of the free-streaming dynamics in Chapman-Enskog
hydrodynamics we conclude that this failure results from an inaccurate
approximation of the fixed lines and a related incorrect description of the
nature of the fixed point. A modified anisotropic hydrodynamic description is
presented that provides excellent agreement with kinetic theory results and
reproduces the far-from-equilibrium attractor for the scaled longitudinal
pressure.
