R-modes in neutron stars: Theory and observations

In this paper, I will review the theory behind the gravitational wave (GW) driven r-mode instability in rapidly rotating neutron stars (NSs) and discuss which constraints can be derived from observations of spins and temperatures in low mass X-ray binaries (LMXBs). I will discuss how a standard, 'minimal' NS model is not consistent with the data, and discuss some of the additional physical mechanisms that could reconcile theory with observations. In particular, I will focus on additional forms of damping due to exotic cores and on strong mutual friction due to superfluid vortices cutting through superconducting flux tubes, and examine the repercussions these effects could have on the saturation amplitude of the mode. Finally I will also discuss the possibility that oscillations due to r-modes may have been recently observed in the X-ray light curves of two LMXBs.

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Non-Linear Resonances in Accretion Disks and QPOs

International Astronomical Union Colloquium ◽

10.1017/s0252921100152170 ◽

2004 ◽

Vol 194 ◽

pp. 128-129

Author(s):

Włodek Kluźniak

Keyword(s):

Black Holes ◽

Neutron Stars ◽

Accretion Disk ◽

Accretion Disks ◽

High Frequency ◽

Light Curves ◽

X Ray ◽

Non Linear ◽

Low Mass ◽

X Ray Binaries

AbstractNon-linear oscillations in the accretion disk are favored as an explanation of high-frequency QPOs observed in the light curves of low-mass X-ray binaries containing neutron stars, black holes, or white dwarfs.

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Observations of Low Mass X-Ray Binaries

International Astronomical Union Colloquium ◽

10.1017/s0252921100039117 ◽

1996 ◽

Vol 158 ◽

pp. 349-358

Author(s):

Alan P. Smale

Keyword(s):

Neutron Stars ◽

Orbital Period ◽

Binary Systems ◽

Light Curves ◽

X Ray ◽

Fast Timing ◽

Low Mass ◽

And Behavior ◽

X Ray Binaries

AbstractIn this paper I review the properties and behavior of low-mass X-ray binary systems (LMXBs) that contain neutron stars (NS), concentrating on the Galactic bulge sources and bursters. I describe the observed characteristics of LMXBs including their light curves, spectra, eclipses, dips, bursts, flares, pulsations, QPO, long-term periodicities and orbital period changes, and explain how fast timing results and the distinction between ‘Z’ and ‘atoll’-type sources provide the key to a unified model of LMXB behavior.

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R-mode constraints from neutron star equation of state

HNPS Proceedings ◽

10.12681/hnps.1913 ◽

2019 ◽

Vol 23 ◽

pp. 100

Author(s):

Ch. C. Moustakidis ◽

M. C. Papazoglou

Keyword(s):

Neutron Star ◽

Equation Of State ◽

Neutron Stars ◽

Gravitational Radiation ◽

Mode Instability ◽

X Ray ◽

Long Time ◽

Low Mass ◽

X Ray Binaries

The gravitational radiation has been proposed a long time before, as an explana- tion for the observed relatively low spin frequencies of young neutron stars and of accreting neutron stars in low-mass X-ray binaries as well. In the present work we studied the effects of the neutron star equation of state on the r-mode instability window of rotating neutron stars.

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Nuclear equation of state effects on the r-mode instability of neutron stars

HNPS Proceedings ◽

10.12681/hnps.2483 ◽

2012 ◽

Vol 20 ◽

pp. 27

Author(s):

Ch. C. Moustakidis

Keyword(s):

Neutron Star ◽

Equation Of State ◽

Neutron Stars ◽

Nuclear Symmetry Energy ◽

Mode Instability ◽

X Ray ◽

Nuclear Equation Of State ◽

Theoretical Predictions ◽

Low Mass ◽

X Ray Binaries

We study the effect of nuclear equation of state on the r-mode instability of a rotating neutron star. We consider the case where the crust of the neutron star is perfectly rigid and we employ the related theory. The effects of the density dependence of the nuclear symmetry energy on r-mode instability properties are presented and analyzed. A comparison of theoretical predictions with observed neutron stars in low-mass X-ray binaries is also performed and analyzed.

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r-mode instability of neutron stars in Low-mass X-ray binaries: effects of Fermi surface depletion and superfluidity of dense matter

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3258 ◽

2020 ◽

Author(s):

J M Dong

Keyword(s):

Fermi Surface ◽

Neutron Stars ◽

Nuclear Matter ◽

Core Temperature ◽

Low Temperatures ◽

Mode Instability ◽

X Ray ◽

Surface Depletion ◽

Low Mass ◽

X Ray Binaries

Abstract The nucleon-nucleon correlation between nucleons leads to the Fermi surface depletion measured by a Z-factor in momentum distribution of dense nuclear matter. The roles of the Fermi surface depletion effect (Z-factor effect) and its quenched neutron triplet superfluidity of nuclear matter in viscosity and hence in the gravitational-wave-driven r-mode instability of neutron stars (NSs) are investigated. The bulk viscosity is reduced by both the two effects, especially the superfluid effect at low temperatures which is also able to reduce the inferred core temperature of NSs. Intriguingly, due to the neutron superfluidity, the core temperature of the NSs in known low-mass X-ray binaries (LMXBs) are found to be clearly divided into two groups: high and low temperatures which correspond to NSs with short and long recurrence times for nuclear-powered bursts respectively. Yet, a large number of NSs in these LMXBs are still located in the r-mode instability region. If the density-dependent symmetry energy is stiff enough, the occurence of direct Urca process reduces the inferred core temperature by about one order of magnitude. Accordingly, the contradiction between the predictions and observations is alleviated to some extent, but some NSs are still located inside the unstable region.

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Search for intermittent X-ray pulsations from neutron stars in low-mass X-ray binaries

Publications of the Astronomical Society of Australia ◽

10.1017/pasa.2021.6 ◽

2021 ◽

Vol 38 ◽

Author(s):

Yunus Emre Bahar ◽

Manoneeta Chakraborty ◽

Ersin Göğüş

Keyword(s):

Neutron Stars ◽

Orbital Period ◽

Oscillation Frequency ◽

Orbital Motion ◽

X Rays ◽

X Ray ◽

Second Stage ◽

Corrected Data ◽

Low Mass ◽

X Ray Binaries

Abstract We present the results of our extensive binary orbital motion corrected pulsation search for 13 low-mass X-ray binaries. These selected sources exhibit burst oscillations in X-rays with frequencies ranging from 45 to 1 122 Hz and have a binary orbital period varying from 2.1 to 18.9 h. We first determined episodes that contain weak pulsations around the burst oscillation frequency by searching all archival Rossi X-ray Timing Explorer data of these sources. Then, we applied Doppler corrections to these pulsation episodes to discard the smearing effect of the binary orbital motion and searched for recovered pulsations at the second stage. Here we report 75 pulsation episodes that contain weak but coherent pulsations around the burst oscillation frequency. Furthermore, we report eight new episodes that show relatively strong pulsations in the binary orbital motion corrected data.

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