New class of low frequency QPOs: Signature of nuclear burning or accretion disk instabilities?

Revnivtsev, M.; Churazov, E.; Gilfanov, M.; Sunyaev, R.

doi:10.1051/0004-6361:20010434

Cited by 100 publications

(175 citation statements)

References 20 publications

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149

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“…The latter rules out a thermonuclear origin, because then the flux would not undercut the decaying trend of the initial flash. An example of thermonuclear variability is millihertz oscillatory behavior in the non-burst emission of some bursters (4U 1608-52 and 4U 1636-536; Revnivtsev et al 2001;Heger et al 2007;Altamirano et al 2008). The characteristics of these oscillations are different from the achromatic variability reported here, both in time scales and spectral behavior.…”

Section: Origin Of Fluctuationscontrasting

confidence: 63%

Achromatic late-time variability in thermonuclear X-ray bursts

Zand

Ballantyne

2010

A&A

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An unusual Eddington-limited thermonuclear X-ray burst was detected from the accreting neutron star in 2S 0918-549 with the Rossi X-ray Timing Explorer. The burst commenced with a brief (40 ms) precursor and maintained near-Eddington fluxes during the initial 77 s. These characteristics are indicative of a nova-like expulsion of a shell from the neutron star surface. Starting 122 s into the burst, the burst shows strong (87 ± 1% peak-to-peak amplitude) achromatic fluctuations for 60 s. We speculate that the fluctuations are caused by Thompson scattering by fully-ionized inhomogeneities in a resettling accretion disk that was disrupted by the effects of super-Eddington fluxes. An expanding shell may be the necessary prerequisite for the fluctuations.

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Section: Origin Of Fluctuationscontrasting

confidence: 63%

Achromatic late-time variability in thermonuclear X-ray bursts

Zand

Ballantyne

2010

A&A

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“…In actuality, the centrifugal force may reduce L crit even further for fast-spinning neutron stars. Revnivtsev et al (2001) observe from the LMXBs 4U 1636-53, 4U 1608-52 and Aql X-1 millihertz quasi-periodic oscillations (mHz QPOs) and speculate that this phenomenon may be related to a special mode of nuclear burning. This behavior is thought to occur at the boundary of stable and unstable burning, since it is observed in a narrow range of accretion rates, where type I X-ray bursts were observed at lower accretion rates, while they were absent at higher accretion rates.…”

Section: Centrifugal Forcementioning

confidence: 85%

The effect of rotation on the stability of nuclear burning in accreting neutron stars

Keek

Langer²,

Zand

2009

A&A

107

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Hydrogen and/or helium accreted by a neutron star from a binary companion may undergo thermonuclear fusion. Different burning regimes are discerned at different mass accretion rates. Theoretical models predict helium fusion to proceed as a thermonuclear runaway for accretion rates below the Eddington limit and as stable burning above this limit. Observations, however, place the boundary close to 10% of the Eddington limit. We study the effect of rotationally induced transport processes on the stability of helium burning. For the first time, detailed calculations of thin-shell helium burning on neutron stars are performed using a hydrodynamic stellar evolution code including rotation and rotationally induced magnetic fields. We find that in most cases the instabilities from the magnetic field provide the dominant contribution to the chemical mixing, while Eddington-Sweet circulations become important at high rotation rates. As helium is diffused to greater depths, the stability of the burning is increased, such that the critical accretion rate for stable helium burning is found to be lower. Combined with a higher heat flux from the crust, as suggested by recent studies, turbulent mixing could explain the observed critical accretion rate. Furthermore, close to this boundary we find oscillatory burning, which previous studies have linked to mHz QPOs. In models where we continuously lower the heat flux from the crust, the period of the oscillations increases by up to several tens of percents, similar to the observed frequency drift, suggesting that this drift could be caused by the cooling of deeper layers.

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“…For example, mHz quasi-periodic oscillations (QPOs) resulting from marginally-stable thermonuclear burning are observed at an inferred mass-accretion rate that is a factor ≃10 lower than the theoretical predictions (e.g., Revnivtsev et al 2001;Altamirano et al 2008;Keek et al 2009;Linares et al 2012). This can be reconciled if there is an additional heat flux coming from the crust.…”

Section: Introductionmentioning

confidence: 90%

“…Aql X-1 is a transient LMXB that exhibits (normal) thermonuclear X-ray bursts and mHz QPOs (e.g., Koyama et al 1981; Revnivtsev et al 2001; Altamirano et al 2008). It is located at a distance of D≃5 kpc (e.g., Rutledge et al 2001), and spins at a frequency of νs≃550 Hz (as inferred from coherent X-ray pulsations detected once during a ≃150-s episode; Casella et al 2008).…”

Section: Aql X-1mentioning

confidence: 99%

Constraining the properties of neutron star crusts with the transient low-mass X-ray binary Aql X-1

Waterhouse

Degenaar

Wijnands

et al. 2016

Mon. Not. R. Astron. Soc.

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Aql X-1 is a prolific transient neutron star low-mass X-ray binary that exhibits an accretion outburst approximately once every year. Whether the thermal X-rays detected in intervening quiescent episodes are the result of cooling of the neutron star or due to continued low-level accretion remains unclear. In this work we use Swift data obtained after the long and bright 2011 and 2013 outbursts, as well as the short and faint 2015 outburst, to investigate the hypothesis that cooling of the accretion-heated neutron star crust dominates the quiescent thermal emission in Aql X-1. We demonstrate that the X-ray light curves and measured neutron star surface temperatures are consistent with the expectations of the crust cooling paradigm. By using a thermal evolution code, we find that ≃1.2 − 3.2 MeV nucleon −1 of shallow heat release describes the observational data well, depending on the assumed mass-accretion rate and temperature of the stellar core. We find no evidence for varying strengths of this shallow heating after different outbursts, but this could be due to limitations of the data. We argue that monitoring Aql X-1 for up to ≃1 year after future outbursts can be a powerful tool to break model degeneracies and solve open questions about the magnitude, depth and origin of shallow heating in neutron star crusts.

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New class of low frequency QPOs: Signature of nuclear burning or accretion disk instabilities?

Cited by 100 publications

References 20 publications

Achromatic late-time variability in thermonuclear X-ray bursts

Achromatic late-time variability in thermonuclear X-ray bursts

The effect of rotation on the stability of nuclear burning in accreting neutron stars

Constraining the properties of neutron star crusts with the transient low-mass X-ray binary Aql X-1

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