Terzan 5 transient IGR J17480-2446: variation of burst and spectral
  properties with spectral states
  We study the spectral state evolution of the Terzan 5 transient neutron star
low-mass X-ray binary IGR J17480-2446, and how the best-fit spectral parameters
and burst properties evolved with these states, using the Rossi X-ray Timing
Explorer data. As reported by other authors, this is the second source which
showed transitions between atoll state and `Z' state. We find large scale
hysteresis in the almost `C'-like hardness-intensity track of the source in the
atoll state. This discovery is likely to provide a missing piece of the jigsaw
puzzle involving various types of hardness-intensity tracks from `q'-shaped for
Aquila X-1, 4U 1608-52, and many black holes to `C'-shaped for many atoll
sources. Furthermore, the regular pulsations, a diagonal transition between
soft and hard states, and the large scale hysteresis observed from IGR
J17480-2446 argue against some of the previous suggestions involving magnetic
field about atolls and millisecond pulsars. Our results also suggest that the
nature of spectral evolution throughout an outburst does not, at least
entirely, depend on the peak luminosity of the outburst. Besides, the source
took at least a month to trace the softer banana state, as opposed to a few
hours to a day, which is typical for an atoll source. In addition, while the
soft colour usually increases with intensity in the softer portion of an atoll
source, IGR J17480-2446 showed an opposite behaviour. From the detailed
spectral fitting we conclude that a blackbody+powerlaw model is the simplest
one, which describes the source continuum spectra well throughout the outburst.
We find that these two spectral components were plausibly connected with each
other, and they worked together to cause the source state evolution.
(Truncated).
