On the long-term behavior of the HZ Herculis/Hercules X-1 system

In accretion models for binary X-ray sources based on Roche-lobe overflow, mass transfer from an otherwise normal, expanding star to a collapsed companion is responsible for the production of X-rays. For a range of time-averaged rates of mass transfer, the instantaneous rate of transfer depends on the X-ray luminosity. Thus the rate may be subject to a feedback instability. Such an instability could be responsible for the long-period on-off behavior of HZ Herculis