Propeller-driven Spectral State Transition in the Low-Mass X-Ray Binary 4U 1608-52

Spectral state transitions in neutron star-low-mass X-ray binary (LMXB) systems have been widely observed, but are not well understood. Here we report an abrupt spectral change in 4U 1608-52, a typical atoll source, during the decay phase of its 2004 outburst. The source is found to undergo sudden changes in spectral hardness and other properties. The transition occurred when its luminosity was between 3.3×10³⁶ and 5.3×10³⁶ ergs s^-1, assuming a distance of 3.6 kpc. Interpreting this event in terms of the ``propeller effect,'' we derive a neutron star surface magnetic field of 1.4-1.8×10⁸ G. We also briefly discuss similarities and differences between the spectral states of neutron star and black hole binary systems.