X - Ray Time Variability across the Atoll Source States of 4 U 1636 – 53

We have studied the rapid X-ray time variability in 149 pointed observations with the Rossi X-ray Timing Explorer (RXTE)'s Proportional Counter Array of the atoll source 4U 1636–53 in the banana state and, for the first time with RXTE, in the island state. We compare the frequencies of the variability components of 4U 1636–53 with those in other atoll and Z-sources and find that 4U 1636– 53 follows the universal scheme of correlations previously found for other atoll sources at (sometimes much) lower luminosities. Our results on the hectohertz QPO suggest that the mechanism that sets its frequency differs from that for the other components, while the amplitude setting mechanism is common. A previously proposed interpretation of the narrow low-frequency QPO frequencies in different sources in terms of harmonic mode switching is not supported by our data, nor by some previous data on other sources and the frequency range that this QPO covers is found not to be related to spin, angular momentum or luminosity. Low-mass X-ray binaries (LMXBs) can be divided into systems containing a black hole candidate (BHC) and those containing a neutron star (NS). The accretion process onto these compact objects can be studied through the timing properties of the associated X-ray emission (see, e.g., van der Klis 2006, for a review). Hasinger & van der Klis (1989) classified the NS LMXBs based on the correlated variations of the X-ray spectral and rapid X-ray variability properties. They distinguished two sub-types of NS LMXBs, the Z sources and the atoll sources, whose names were inspired by the shapes of the tracks that they trace out in an X-ray color-color diagram on time scales of hours to days. The Z sources are the most luminous; the atoll sources cover a much wider range in luminosities (e.g. , Ford et al. 2000, and references therein). For each type of source, several spectral/timing states are identified which are thought to arise from qualitatively different inner flow configurations. In the case of atoll sources, the main three states are the extreme island state (EIS), the island state (IS) and the banana branch, the latter subdivided into lower-left banana (LLB), lower banana (LB) and upper banana (UB) states. Each state is characterized by a unique combination of color color diagram and timing behavior. The EIS and the IS occupy the spectrally harder parts of the color color diagram (CD) corresponding to lower X-ray …