Energy Dependent Time Delays of kHz Oscillations due to Thermal Comptonization

Abstract

We study the energy dependent photon variability from a thermal Comptonizing plasma that is oscillating at kHz frequencies. In particular, we solve the linearised time dependent Kompaneets equation and consider the oscillatory perturbation to be either in the soft photon source or in the heating rate of the plasma. For each case, we self consistently consider the energy balance of the plasma and the soft photon source. The model incorporates the possibility of a fraction of the Comptonized photons impinging back into the soft photon source. We find that when the oscillation is due to the soft photon source, the variation of the fractional root mean sqaure (r.m.s) is nearly constant with energy and the time-lags are hard. However, for the case when the oscillation is due to variation in the heating rate of the corona, and when a significant fraction of the photons impinge back into the soft photon source, the r.m.s increases with energy and the time lags are soft. As an example, we compare the results with the $\sim 850$ Hz oscillation observed on March 3, 1996 for 4U 1608-52 and show that both the observed soft time-lags as well as the r.m.s versus energy can be well described by such a model where the size of the Comptonizing plasma is $\sim 1$ km. Thus, modelling of the time lags as due to Comptonization delays, can provide tight constraints on the size and geometry of the system. Detailed analysis would require well constrained spectral parameters.

8 pages, 6 figures, Accepted in MNRAS

References in corpus (8)

• X-ray Properties of Black-Hole Binaries
• X-ray time variability across the atoll source states of 4U 1636--53
• On the maximum amplitude and coherence of the kilohertz quasi-periodic oscillations in low-mass X-ray binaries
• On the kHz QPO frequency correlations in bright neutron star X-ray binaries
• RossiXTE monitoring of 4U 1636-53: I. Long-term evolution and kHz Quasi-Periodic Oscillations
• Stability of the Magnetopause of Disk-Accreting Rotating Stars
• Fractional amplitude of kilohertz quasi-periodic oscillation from 4U 1728-34: evidence of decline at higher energies
• Three Disk Oscillation Modes of Rotating Magnetized Neutron Stars

Cited by in corpus (7)

• The Comptonising medium of the neutron star in 4U 1636-53 through its lower kilohertz Quasi-Periodic Oscillations
• A two-component Comptonisation model for the type-B QPO in MAXI J1348-630
• A variable corona for GRS 1915+105
• X-ray Spectral and Timing evolution of MAXI J1727-203 with NICER
• Relation between the properties of the kilohertz quasi-periodic oscillations and spectral parameters in 4U 1636$-$53
• Spectral-Timing Analysis of the Lower kHz QPO in the Low-Mass X-ray Binary Aquila X-1
• Modeling the upper kHz QPOs of 4U 1728-34 with X-ray reverberation