The new X-ray transient SAX J1711.6-3808: decoupling between its 3-20 keV luminosity and its state transitions

Abstract

We present a study of the correlated spectral and timing behavior of the new X-ray transient SAX J1711.6-3808 during its 2001 outburst using data obtained with the RXTE. We also investigate the correlations between those source properties and the 3-20 keV X-ray luminosity. The behavior of the source during the observations can be divided into two distinct state types. During the hard state, the energy spectra are relatively hard and can be described by only a power-law component, and the characteristic frequencies (i.e., the frequency of the 1-7 Hz QPOs observed for the first time in this source) in the power spectra are low. However, during the ``soft'' state, the spectra are considerably softer (in addition to the power-law component, a soft component is necessary to fit the spectra) and the frequencies are the highest observed. Remarkably, this distinction into two separate states cannot be extrapolated to also include the 3-20 keV X-ray luminosity. Except for one observation, this luminosity steadily decreased but the hard state was observed both at the highest and lowest observed luminosities. In contrast, the soft state occurred only at intermediate luminosities. This clearly demonstrates that the state behavior of SAX J1711.6-3808 is decoupled from its X-ray luminosity and that if the X-ray luminosity traces the accretion rate in SAX J1711.6-3808, then the state transitions are not good accretion rate indicators, or vice versa. The data of SAX J1711.6-3808 does not allow us to conclusively determine its exact nature. The source resembles both neutron star and black hole systems when they have low luminosities. We discuss our results with respect to the correlated timing and spectral behavior observed in other LMXBs and the implications of our results on the modeling of the outburst light curves of X-ray transients.