Relations Between Timing Features and Colors in Accreting Millisecond Pulsars

We have studied the aperiodic X-ray timing and color behavior of the accreting millisecond pulsars SAX J1808.4-3658, XTE J1751-305, XTE J0929-314, and XTE J1814-338 using large data sets obtained with the Rossi X-ray Timing Explorer. We find that the accreting millisecond pulsars have very similar timing properties to the atoll sources and the low luminosity bursters. Based on the correlation of timing and color behavior SAX J1808.4-3658 can be classified as an atoll source, and XTE J0929-314 and XTE J1814-338 are consistent with being atoll sources, but the color behavior of XTE J1751-305 is different. Unlike in other atoll sources the hard color in the extreme island state of XTE J1751-305 is strongly correlated with soft color and intensity, and is not anti-correlated with any of the characteristic frequencies. We found previously, that the frequencies of the variability components of atoll sources follow a universal scheme of correlations. The frequency correlations of the accreting millisecond pulsars are similar, but in the case of SAX J1808.4-3658 and XTE J0929-314 shifted compared to those of the other atoll sources in a way that is most easily described as a shift in upper and lower kilohertz QPO frequency by a factor close to 1.5. Although, we note that the shift in lower kilohertz QPO frequency is based on only one observation for SAX J1808.4-3658. XTE J1751-305 and XTE J1814-338, as well as the low luminosity bursters show no or maybe small shifts.