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We propose an explanation of the Island and Banana states and the relation between Atoll and Z-track sources, constituting a unified model for Low Mass X-ray Binaries (LMXB). We find a dramatic transition at a luminosity of 1 - 2.10^{37} erg/s above which the high energy cut-off E_CO of the Comptonized emission in all sources is low at a few keV. There is thermal equilibrium between the neutron star at ~2 keV and the Comptonizing accretion disk corona (ADC) causing the low E_CO in the Banana state of Atolls and all states of the Z-track sources. Below this luminosity, E_CO increases towards 100 keV causing the hardness of the island state. Thermal equilibrium is lost, the ADC becoming much hotter than the neutron star via an additional coronal heating mechanism. This suggests a unified model of LMXB: the Banana state is a basic state with the mass accretion rate Mdot increasing, corresponding to the Normal Branch of Z-track sources. The Island state has high ADC temperature, this state not existing in the Z-track sources with luminosities much greater than the critical value. The Z-track sources have an additional flaring branch consistent with unstable nuclear burning on the neutron star at high Mdot. This burning regime does not exist at low Mdot so this branch is not seen in Atolls (except GX Atolls). The Horizontal Branch in Z-track sources has a strong increase in radiation pressure disrupting the inner disk and launching relativistic jets.
The aim of this review is to describe the nature, formation and evolution of the three kinds of high mass X-ray binary (HMXB) population: i. systems hosting Be stars (BeHMXBs), ii. systems accreting the stellar wind of supergiant stars (sgHMXBs), and
A recent study of a small sample of X-ray binaries (XRBs) suggests a significant softening of spectra of neutron star (NS) binaries as compared to black hole (BH) binaries in the luminosity range 10$^{34}$ - 10$^{37}$ erg/s. This softening is quantif
We report on unusually very hard spectral states in three confirmed neutron-star low-mass X-ray binaries (1RXS J180408.9-342058, EXO 1745-248, and IGR J18245-2452) at a luminosity between ~ 10^{36-37} erg s^{-1}. When fitting the Swift X-ray spectra
We analyze a flux-limited sample of persistent and bright (with 2-10 keV fluxes exceeding 1.4e-10 erg/s/cm2) low-mass X-ray binaries (LMXBs) in our Galaxy. It is demonstrated that the majority of binary systems with X-ray luminosities below logL(erg/
To investigate the physics of mass accretion onto weakly-magnetized neutron stars, 95 archival RXTE datasets of an atoll source 4U 1608-522, acquired over 1996-2004 in so-called upper-banana state, were analyzed. The object meantime exhibited 3-30 ke