Kilohertz QPO Frequency and Flux Decrease in AQL X-1 and Effect of Soft X-ray Spectral Components

Abstract

We report on an RXTE/PCA observation of Aql X-1 during its outburst in March 1997 in which, immediately following a Type-I burst, the broad-band 2-10 keV flux decreased by about 10% and the kilohertz QPO frequency decreased from 813+-3 Hz to 776+-4 Hz. This change in kHz QPO frequency is much larger than expected from a simple extrapolation of a frequency-flux correlation established using data before the burst. Meanwhile a very low frequency noise (VLFN) component in the broad-band FFT power spectra with a fractional root-mean-square (rms) amplitude of 1.2% before the burst ceased to exist after the burst. All these changes were accompanied by a change in the energy spectral shape. If we characterize the energy spectra with a model composed of two blackbody (BB) components and a power law component, almost all the decrease in flux was in the two BB components. We attribute the two BB components to the contributions from a region very near the neutron star or even the neutron star itself and from the accretion disk, respectively.

12 pages with 4 figures, accepted for publication in ApJ Letters, typos corrected and references updated

References in corpus (9)

• Correlation Between Energy Spectral States and Fast Time Variability and Further Evidence for the Marginally Stable Orbit in 4U 1820-30
• Bounds on the Compactness of Neutron Stars from Brightness Oscillations
• Milli-second Oscillations in the Persistent and Bursting Flux of Aql X-1 During an Outburst
• Discovery of a second kHz QPO peak in 4U 1608-52
• Spectral State Transitions in Aql X-1: Evidence for `Propeller' Effects
• Strong-field general relativity and quasi-periodic oscillations in x-ray binaries
• Energy Spectra and High Frequency Oscillations in 4U 0614+091
• Evolution of AQL X-1 During the Rising Phase of its 1998 Outburst
• Constraints on Neutron Star Masses and Radii from Kilohertz QPOs

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