Correlated Timing and Spectral Variations of the Soft X-ray Transient Aquila X-1: Evidence for an Atoll classification

Abstract

Based on Rossi X-ray Timing Explorer data, we discuss the classification of the soft X-ray transient Aquila X-1 in the Z/atoll scheme, and the relation of its kilohertz quasi-periodic oscillations (kHz QPO) properties to the X-ray colors. The color-color diagram shows one elongated ("banana") structure and several "islands" of data points. The power spectra of the island are best represented by a broken power-law, whereas those of the banana by a power-law below ~ 1 Hz plus an exponentially cut-off component at intermediate frequencies (30-60 Hz). The parameters of these two components change in correlation with the position of the source in the color-color diagram. Based on the pattern that the source shows in the color-color diagram and its aperiodic variability we conclude that Aquila X-1 is an atoll source. We have also investigated the possible correlation between the frequency of the kHz QPO and the position of the source in the color-color diagram. The complexity seen in the frequency versus count rate diagram is reduced to a single track when the frequency is plotted against hard or soft color.

arXiv comment9 pages, 5 figures. Accepted for publication in ApJ

References in corpus (9)

• Lense-Thirring Precession and QPOs in Low Mass X-Ray Binaries
• Dependence of the Frequency of the Kilohertz Quasi-Periodic Oscillations on X-ray Count Rate and Colors in 4U 1608-52
• Aquila X-1 from outburst to quiescence: the onset of the propeller effect and signs of a turned-on rotation-powered pulsar
• Milli-second Oscillations in the Persistent and Bursting Flux of Aql X-1 During an Outburst
• Strong Correlation Between Noise Features at Low Frequency and the Kilohertz QPOs in the X-Ray Binary 4U 1728-34
• Spectral State Transitions in Aql X-1: Evidence for `Propeller' Effects
• Energy Spectra and High Frequency Oscillations in 4U 0614+091
• Evolution of AQL X-1 During the Rising Phase of its 1998 Outburst
• Kilohertz QPO Frequency and Flux Decrease in AQL X-1 and Effect of Soft X-ray Spectral Components

Cited by in corpus (30)

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• The atoll source states of 4U 1608-52
• Relations Between Timing Features and Colors in the X-Ray Binary 4U 0614+09
• How Do Z and Atoll X-ray Binaries Differ?
• The Amplitude of the Kilohertz Quasi-periodic Oscillations in 4U 1728-34, 4U 1608-52, and Aql X-1, as a Function of X-ray Intensity
• Relations Between Timing Features and Colors in Accreting Millisecond Pulsars
• Study of the Temporal Behavior of 4U 1728-34 as a Function of its Position in the Color-Color Diagram
• Spectral Fits to the 1999 Aql X-1 Outburst Data
• Simultaneous Measurements of X-Ray Luminosity and Kilohertz Quasi-Periodic Oscillations in Low-Mass X-Ray Binaries
• Evolution of the radio - X-ray coupling throughout an entire outburst of Aquila X-1
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• The disc-jet coupling in the neutron star X-ray binary Aquila X-1
• A comment on the colour-colour diagrams of Low Mass X-ray Binaries
• Evolution of Spectral States of Aql X-1 during the 2000 Outburst
• Timing properties and spectral states in Aquila X-1
• Discovery of the upper kilo-Hz QPO from the X-ray transient Aql X-1
• Hard X-ray Flares Preceding Soft X-ray Outbursts in Aquila X-1: A Link between Neutron Star and Black Hole State Transitions
• The hard X-ray behavior of Aql X-1 during type-I bursts
• Kilohertz quasi-periodic oscillations in low mass X-ray binary sources and their relation with the neutron star magnetic field
• A NICER look at the Aql X-1 hard state
• Possible twin kHz Quasi Periodic Oscillations in the accreting millisecond X-ray Pulsar IGR J17511-3057
• Evolution of broad-band SED during outburst rise in NS X-ray Nova Aql X-1
• Drifts of the marginally stable burning frequency in the X-ray binaries 4U 1608--52 and Aql X--1
• AstroSat view of `Clocked` burster GS 1826-238:broad-band spectral nature of persistent and burst emission during soft state
• Parallel tracks as quasi-steady states for the magnetic boundary layers in neutron-star low-mass X-ray binaries
• Evaluating the jet/accretion coupling of Aql X-1: probing the contribution of accretion flow spectral components
• Low frequency QPOs and possible change in the accretion geometry during the outbursts of Aquila X$-$1