The spectral states of the NS LMXB 4U 1728‐34: pictures from a wider scenario

Abstract

We present the broad‐band spectrum of the atoll source 4U 1728‐34 along its hardness intensity diagram performed with INTEGRAL. During its motion along the hardness‐intensity diagram, the source passed trough two well distinct spectral states, i.e. from a hard to a soft state. In the hard state the source spectrum is well described by a simple thermal Comptonization model characterized by an electron temperature and opacity of kTe ∼ 35 keV and τ ∼ 0.5, respectively. In the soft state there is observed an additional soft thermal emission, modeled by a multicolour disk blackbody with a temperature kTbb ∼ 0.6 keV. In this state the electron temperature drops to kTe ∼ 3 keV, while the opacity increases to ∼ 5. The hard spectral states observed for 4U 1728‐34 can be considered as a part of a wider scenario of the spectral states experienced by LMXBs in which the relative importance of pure thermal and bulk (dynamic) Comptonization could be the main driving parameter.