Propeller driven spectral state transition in LMXB 4U 1608-52

Spectral state transitions in neutron star LMXB systems have been widely observed yet not well understood. Here we report an abrupt spectral change in 4U 1608-52, a typical atoll source, during its decay phase of the 2004 outburst. The source is found to undergo sudden changes in its spectral hardness and other properties. The transition occurred when its luminosity is between (3.3-5.3) E36 ergs/s, assuming a distance of 3.6 kpc. Interpreting this event in terms of the propeller effect, we infer the neutron star surface magnetic field as (1.4-1.8) E8 Gauss. We also briefly discuss similarities and differences between the spectral states of neutron star and black hole binary systems.