The discovery of a 7-14 Hz quasi-periodic oscillation in the X-ray transient XTE J1806-246

We have studied the correlated X-ray spectral and X-ray timing behavior of the X-ray transient XTE J1806-246 using data obtained with the proportional counter array on board the Rossi X-Ray Timing Explorer (RXTE). In the X-ray color-color diagram two distinct patterns are traced out. The first pattern is a curved branch, which is observed during the rise and decay of the outburst. This pattern resembles the so-called banana branch of those low-luminosity neutron star low-mass X-ray binaries (LMXBs) that are referred to as atoll sources. The power spectrum of XTE J1806-246 on this curved branch consisted of a power-law and a cutoff power law component. The presence of these components and their dependence on the position of the source on the branch is also identical to the behavior of atoll sources on the banana branch. Near the end of its outburst, XTE J1806-246 formed patches in the color-color diagram, the spectrum was harder, and the power spectrum showed strong band-limited noise, characteristic of the atoll sources in the island state. A second pattern was traced out during the only observation at the peak of the outburst. It consists of a structure that we interpret as formed by two distinct branches. This pattern resembles the normal-flaring branches of the high-luminosity neutron star LMXBs (the Z sources). The discovery of a 7-14 Hz quasi-periodic oscillation (QPO) during this observation strengthens this similarity. We conclude that if XTE J1806-246 is a neutron star, it is most likely an atoll source that only at the peak of its outburst reached a luminosity level sufficiently high to show the type of QPO that Z sources show on their normal and flaring branches.