Quadrupole moments of rotating neutron stars and strange stars

Urbanec, Martin; Miller, John C.; Stuchlík, Zdeněk

doi:10.1093/mnras/stt858

Cited by 93 publications

(117 citation statements)

References 55 publications

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“…Notice the linear behavior of the Kerr factor and its subsequent approach to the value 1 in the limit R/R S → 1.125. These results agrees with those in [65] and [101]. In their original paper Chandrasekhar & Miller [21] found that the quadrupole moment, in units of J 2 /R s , is Q = 2.002 at the Buchdahl radius R = (9/8)R S .…”

Section: Surface and Integral Properties Of Schwarzschild Stars For Rsupporting

confidence: 87%

Slowly rotating supercompact Schwarzschild stars

Posada¹

2017

Monthly Notices of the Royal Astronomical Society

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AcknowledgmentsI am grateful to the universe, for its harmony. The project is the study of integral and surface properties of slowly rotating homogeneous masses in the gravastar limit R → R s , where R s is the Schwarzschild radius. For this purpose we followed the perturbative method proposed by Hartle in 1967. In this model, the relativistic equations of structure for a slowly rotating star were derived at second order in the angular velocity Ω. An interesting, and educational, application of this model was investigated by Chandrasekhar and Miller. I am indebted to the Department of Physics andIn their approach, they solved numerically the structure equations of a homogeneous star (constant energy density) up to the Buchdahl bound (9/8)R s . Based on this work, our objective was to investigate the interesting region below the Buchdahl bound R s < R < (9/8)R s , which has not been studied previously in the literature.Our results were astonishing. We found that the surface properties and quadrupole mass moment approach the values corresponding to those of the Kerr metric when expanded at second order in angular momentum. This remarkable result provides a long sought solution to the problem of the source of rotation in the Kerr spacetime.iv

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Section: Surface and Integral Properties Of Schwarzschild Stars For Rsupporting

confidence: 87%

Slowly rotating supercompact Schwarzschild stars

Posada¹

2017

Monthly Notices of the Royal Astronomical Society

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“…Also, the power spectral fits of Ingram & Done (2012a) predict that the hot inner flow (corona) has a small scale height in the relevant spectral state. approximated fairly well by the Kerr metric (Török et al 2010(Török et al , 2012Urbanec et al 2013). Therefore, such a finding sets conditions and constraints for the applicability of our results for the case of LMXBs with neutron star.…”

Section: Introductionsupporting

confidence: 73%

Twin peak high-frequency quasi-periodic oscillations as a spectral imprint of dual oscillation modes of accretion tori

Bakala

Goluchová

Török

et al. 2015

A&A

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Context. High-frequency (millisecond) quasi-periodic oscillations (HF QPOs) are observed in the X-ray power-density spectra of several microquasars and low-mass X-ray binaries. Two distinct QPO peaks, so-called twin peak QPOs, are often detected simultaneously exhibiting their frequency ratio close or equal to 3:2. A widely discussed class of proposed QPOs models is based on oscillations of accretion toroidal structures orbiting in the close vicinity of black holes or neutron stars. Aims. Following the analytic theory and previous studies of observable spectral signatures, we aim to model the twin peak QPOs as a spectral imprint of specific dual oscillation regime defined by a combination of the lowest radial and vertical oscillation mode of slender tori. We consider the model of an optically thick slender accretion torus with constant specific angular momentum. We examined power spectra and fluorescent Kα iron line profiles for two different simulation setups with the mode frequency relations corresponding to the epicyclic resonance HF QPOs model and modified relativistic precession QPOs model. Methods. We used relativistic ray-tracing implemented in the parallel simulation code LSDplus. In the background of the Kerr spacetime geometry, we analyzed the influence of the distant observer inclination and the spin of the central compact object. Relativistic optical projection of the oscillating slender torus is illustrated by images in false colours related to the frequency shift. Results. We show that performed simulations yield power spectra with the pair of dominant peaks that correspond to the frequencies of radial and vertical oscillation modes and with the peak frequency ratio equal to the proper value 3:2 on a wide range of inclinations and spin values. We also discuss exceptional cases of a very low and very high inclination, as well as unstable high spin relativistic precession-like configurations that predict a constant frequency ratio equal to 1:2. We demonstrate a significant dependency of broadened Kα iron line profiles on the inclination of the distant observer. Conclusions. This study presents a further step towards the proper model of oscillating accretion tori producing HF QPOs. More realistic future simulations should be based on incorporating the resonant coupling of oscillation modes, the influence of torus opacity, and the pressure effects on the mode frequencies and the torus shape.

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“…We note that Kerr geometry represents an applicable approximation of NS spacetimes when the compact object mass is high Urbanec et al 2013). The above formulae valid for Kerr spacetimes well describe (epicyclic) slightly perturbed circular geodesic motion.…”

Section: Twin Peak Qpo Models Approximated In Kerr Spacetimesmentioning

confidence: 61%

“…Thus, when a given EoS is assumed, only corresponding 2D surface is relevant for fitting of datapoints by a given QPO model. Following Urbanec et al (2013), we illustrate such a surface in the right panel of Figure 3 for the SLy4 EoS. The colorcoding of the plot is the same as the one in the left panel of the same Figure. The final M − j map for the RP model and Sly4 EoS, i.e., the values of M and j implied by the common consideration of both panels of Figure 3, is shown in the left panel of Figure 4.…”

Section: Consideration Of Ns Models Given Bymentioning

confidence: 99%

Constraining Models of Twin-Peak Quasi-Periodic Oscillations With Realistic Neutron Star Equations of State

Török

Goluchová

Urbanec

et al. 2016

ApJ

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Twin-peak quasi-periodic oscillations (QPOs) are observed in the X-ray power-density spectra of several accreting low-mass neutron star (NS) binaries. In our previous work we have considered several QPO models. We have identified and explored mass-angular-momentum relations implied by individual QPO models for the atoll source 4U 1636-53. In this paper we extend our study and confront QPO models with various NS equations of state (EoS). We start with simplified calculations assuming Kerr background geometry and then present results of detailed calculations considering the influence of NS quadrupole moment (related to rotationally induced NS oblateness) assuming Hartle-Thorne spacetimes. We show that the application of concrete EoS together with a particular QPO model yields a specific mass-angular-momentum relation. However, we demonstrate that the degeneracy in mass and angular momentum can be removed when the NS spin frequency inferred from the X-ray burst observations is considered. We inspect a large set of EoS and discuss their compatibility with the considered QPO models. We conclude that when the NS spin frequency in 4U 1636-53 is close to 580Hz we can exclude 51 from 90 of the considered combinations of EoS and QPO models. We also discuss additional restrictions that may exclude even more combinations. Namely, there are 13 EOS compatible with the observed twin peak QPOs and the relativistic precession model. However, when considering the low frequency QPOs and Lense-Thirring precession, only 5 EOS are compatible with the model.

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Quadrupole moments of rotating neutron stars and strange stars

Cited by 93 publications

References 55 publications

Slowly rotating supercompact Schwarzschild stars

Slowly rotating supercompact Schwarzschild stars

Twin peak high-frequency quasi-periodic oscillations as a spectral imprint of dual oscillation modes of accretion tori

Constraining Models of Twin-Peak Quasi-Periodic Oscillations With Realistic Neutron Star Equations of State

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