Hysteresis in the spectral states of the neutron star low-mass X-ray
  binary EXO 1745-248
  We study the low-frequency timing properties and the spectral state evolution
of the transient neutron star low-mass X-ray binary EXO 1745-248 using the
entire Rossi X-ray Timing Explorer Proportional Counter Array data. We
tentatively conclude that EXO 1745-248 is an atoll source, and report the
discovery of a ~ 0.45 Hz low-frequency quasi-periodic oscillation and ~ 10 Hz
peaked noises. If it is an atoll, this source is unusual because (1) instead of
a `C'-like curve, it traced a clear overall clockwise hysteresis curve in each
of the colour-colour diagram and the hardness-intensity diagram; and (2) the
source took at least 2.5 months to trace the softer banana state, as opposed to
a few hours to a day, which is typical for an atoll source. The shape of the
hysteresis track was intermediate between the characteristic `q'-like curves of
several black hole systems and `C'-like curves of atolls, implying that EXO
1745-248 is an important source for the unification of the black hole and
neutron star accretion processes.
