PMC

The dependence of the amplitude and quality factor of the lower kHz QPO on its frequency for the neutron-star source 4U 1636–56 []. The drop of the QPO amplitude and coherence at high frequencies have been discussed as signatures of the innermost stable circular orbit.

Reversible inhibition of the QPO reaction by NO was monitored spectrophotometrically. After addition of 6 μM NO, the reaction is inhibited promptly but resumes due to disappearance of NO. The QPO reaction was started by addition of 200 μM dQH₂ and 10 mM H₂O₂ to 9 nM of cytochrome bd at room temperature.

The dependence of the twin QPO frequencies on the X-ray count rate observed by the PCA instrument onboard RXTE, for the neutron-star source 4U 1820-30 []. The flattening of the correlation at high frequencies has been discussed as a signature of the innermost stable circular orbit.

(A) The proportional relation between the initial rate of quinol-peroxide reduction and cytochrome bd concentration. The QPO initial rates were measured in standard buffer in the presence of 120 μM dQH₂ and 1 mM H₂O₂ at room temperature. (B) The QPO activity of cytochrome bd as function of the H₂O₂ concentration showing saturation kinetics. Initial rates are expressed as turnover number (mol dQH₂/mol enzyme/s). The data were fitted to the Michaelis-Menten equation (lines). The fitting parameters (apparent V_max and K_M values) were V_max = 75 ± 4.5 s⁻¹ and K_M = 6.6 ± 1.1 mM. The inset shows the pH-dependence of the QPO reactions at 1 mM H₂O₂. The assays were performed in the presence of 120 μM dQH₂ and 23 nM cytochrome bd. The results are presented as means ± SD of duplicates (n = 2). The inset shows single measurements.