Discovery of a 0.02 Hz QPO feature in the transient X-ray pulsar KS 1947+300

James, Marykutty; Paul, Biswajit; Devasia, Jincy; Indulekha, K.

doi:10.1111/j.1365-2966.2010.16880.x

Cited by 37 publications

(38 citation statements)

References 29 publications

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“…Likewise, QPOs in HMXB lie typically in the milliHertz range (James et al 2010). If one assumes that QPOs are produced as a result of Keplerian motion of inhomogeneities in an accretion disc, this frequency range (0.01-1 Hz) agrees with the QPO being originated outside (but near) the magnetosphere, i.e., at length scales r m 10 8 cm.…”

Section: Correlation Of Spectral and Timing Parametersmentioning

confidence: 68%

“…Although the discovery of rapid aperiodic variability (flickering) in the Be X-ray pulsar V0332+53 dates back to EXOSAT times (Stella et al 1985), timing analysis on accreting pulsars have concentrated on the properties and variability of the coherent X-ray emission (pulsations) and the detection of quasi-periodic oscillations (James et al 2010, and references therein). In contrast, the study of the aperiodic variability and broad-band noise components is very scarce (Belloni & Hasinger 1990a;Revnivtsev et al 2009).…”

Section: Introductionmentioning

confidence: 99%

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Patterns of variability in Be/X-ray pulsars during giant outbursts

Reig

Nespoli

2013

A&A

124

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Context. The discovery of source states in the X-ray emission of black-hole binaries and neutron-star low-mass X-ray binaries constituted a major step forward in the understanding of the physics of accretion onto compact objects. While there are numerous studies on the correlated timing and spectral variability of these systems, very little work has been done on high-mass X-ray binaries, the third major type of X-ray binaries. Accretion-powered pulsars with Be companions represent the most numerous group of high-mass X-ray binaries. When active, they are amongst the brightest extra-solar objects in the X-ray sky and are characterised by dramatic variability in brightness on timescales of days. Aims. The main goal of this work is to investigate whether Be accreting X-ray pulsars display source states and characterise those states through their spectral and timing properties. Methods. We have made a systematic study of the power spectra, energy spectra and X-ray hardness-intensity diagrams of nine Be/X-ray pulsars. Energy spectra were fitted with an absorbed power-law modified by an exponential cutoff. Discrete components such as iron emission lines and cyclotron lines were represented by Gaussian and pseudo-Lorentzian profiles, respectively. Power spectra were fitted by a combination of Lorentzian functions. The evolution of the timing and spectral parameters were monitored through changes over two orders of magnitude in luminosity. Results. We find that Be/X-ray pulsars trace two different branches in the hardness-intensity diagram: the horizontal branch corresponds to a low-intensity state of the source and it is characterised by fast colour and spectral changes and high X-ray variability. The diagonal branch is a high-intensity state that emerges when the X-ray luminosity exceeds a critical limit. The photon index anticorrelates with X-ray flux in the horizontal branch but correlates with it in the diagonal branch. The correlation between quasi-periodic oscillation frequency and X-ray flux reported in some pulsars is also observed if the peak frequency of the broad-band noise that accounts for the aperiodic variability is used. In some sources, a significant correlation between spectral and timing parameters is seen, implying and interplay between the accretion column and the inner accretion disc. Conclusions. The two branches may reflect two different accretion modes, depending on whether the luminosity of the source is above or below a critical value. This critical luminosity is mainly determined by the magnetic field strength, hence it differs for different sources. In this work, the systems that display the two branches have critical luminosities in the range (1−4) × 10 37 erg s −1 .

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Section: Correlation Of Spectral and Timing Parametersmentioning

confidence: 68%

Section: Introductionmentioning

confidence: 99%

Patterns of variability in Be/X-ray pulsars during giant outbursts

Reig

Nespoli

2013

A&A

124

142

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“…This peaked component has a characteristic frequency of 0.08 Hz, and it is only observable in high-flux observations, down to L X ∼ 40% of the maximum observed luminosity. Including 1A 1118-615, QPOs have been detected in 17 accretion-powered high-magnetic field pulsars, comprising many HMXBs and a few LMXBs, both transient and persistent (see James et al 2010, and references therein). Quasiperiodic oscillations in X-ray pulsars provide strong evidence for the presence of an accretion disk during giant outbursts because A7, page 6 of 9 they are believed to be related to inhomogeneities in the inner disk (Paul & Rao 1998).…”

Section: Aperiodic Variability Analysismentioning

confidence: 99%

“…4). No general trend has been identified for QPOs in HMXBs, with cases of correlation (as V0332+53, see Qu et al 2005), anti-correlation (XTE J1858+034, KS 1947+300, see Mukherjee et al 2006;James et al 2010), or no dependence (EXO 2030+375, see Angelini et al 1989).…”

Section: Aperiodic Variability Analysismentioning

confidence: 99%

“…The formation of an accretion disk during type II outbursts is still under debate. However, the discovery of quasi-periodic oscillations in some systems (see James et al 2010, and references therein) would support this scenario. The presence of an accretion disk also helps explain the large and steady spin-up rates seen during the giant outbursts, which are difficult to account for by means of direct accretion.…”

Section: Introductionmentioning

confidence: 98%

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Discovery of a quasi-periodic oscillation in the X-ray pulsar 1A 1118-615: correlated spectral and aperiodic variability

Reig

2010

A&A

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Aims. Our goal is to investigate the X-ray timing and spectral variability of the high-mass X-ray binary 1A 1118-615 during a type-II outburst.Methods. We performed a detailed color, spectral, and timing analysis of a giant outburst from 1A 1118-615 using RXTE data. Results. We report the discovery of a variable quasi-periodic oscillation (QPO) in the power spectral density of 1A 1118-615, with a centroid frequency of ∼0.08 Hz. The centroid frequency of the QPO correlates with the X-ray flux, as expected according to the most accredited models for QPO production. For energies above ∼4 keV, the QPO rms variability decreases as the energy increases. Pulse profiles display an energy dependence with a two-peak profile at lower energies and a single peak at higher energies. From the spectral analysis we confirm the presence of a cyclotron absorption feature at ∼60 keV, the highest value measured for an X-ray pulsar. We find that the spectral parameters (photon index, cutoff energy, iron fluorescence line strength) display a marked dependence with flux. We detect two different levels of neutral hydrogen column density, possibly owing to to the Be companion activity. We report for the first time a correlation between the timing and spectral parameters in an X-ray pulsar. All correlations found between spectral/timing parameters and X-ray flux are present up to a flux of ∼6 × 10 −9 erg cm −2 s −1 , when a saturation level is reached. We propose that the observed saturation corresponds to the minimum extent of the neutron star magnetosphere. We estimate the magnetic field of the neutron star from two independent ways, using results from spectral (cyclotron line energy) and timing (QPO frequency) analysis, obtaining consistent values of ∼7−8 × 10 12 G. Results from the comprehensive spectral and timing analysis are discussed in comparison with other X-ray pulsars.

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