arXiv subjects

C. M. Zhang

Publications and source records attributed to C. M. Zhang.

At least 73 records · Page 4Linked to original sources

arXiv2006

Measuring Neutron Star Mass and Radius with Three Mass-Radius Relations

We propose to determine the mass and the radius of a neutron star (NS) using three measurable mass-radius relationships, namely the ``apparent'' radius inferred from neutron star thermal emission, the gravitational redshift inferred from the absorption lines, as well as the averaged stellar mass density inferred from the orbital Keplerian frequency derived from the kilohertz quasi periodic oscillation (kHz QPO) data. We apply the method to constrain the NS mass and the radius of the X-ray sources, 1E 1207.4-5209, Aql X-1 and EXO 0748-676.

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arXiv2006

Timing Features of the Accretion--driven Millisecond X-Ray Pulsar XTE J1807--294 in 2003 March Outburst

In order to probe the activity of the inner disk flow and its effect on the neutron star surface emissions, we carried out the timing analysis of the Rossi X-Ray Timing Explorer (RXTE) observations of the millisecond X-ray pulsar XTE J1807--294, focusing on its correlated behaviors in X-ray intensities, hardness ratios, pulse profiles and power density spectra. The source was observed to have a serial of broad "puny" flares on a timescale of hours to days on the top of a decaying outburst in March 2003. In the flares, the spectra are softened and the pulse profiles become more sinusoidal. The frequency of kilohertz quasi-periodic oscillation (kHz QPO) is found to be positively related to the X-ray count rate in the flares. These features observed in the flares could be due to the accreting flow inhomogeneities. It is noticed that the fractional pulse amplitude increases with the flare intensities in a range of $\sim 2%-14%$, comparable to those observed in the thermonuclear bursts of the millisecond X-ray pulsar XTE J1814--338, whereas it remains at about 6.5% in the normal state. Such a significant variation of the pulse profile in the "puny" flares may reflect the changes of physical parameters in the inner disk accretion region. Furthermore, we noticed an overall positive correlation between the kHz QPO frequency and the fractional pulse amplitude, which could be the first evidence representing that the neutron-star surface emission properties are very sensitive to the disk flow inhomogeneities. This effect should be cautiously considered in the burst oscillation studies.

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arXiv2006

The correlations between the twin kHz QPO frequencies of LMXBs

We analyzed the recently published kHz QPO data in the neutron star low-mass X-ray binaries (LMXBs), in order to investigate the different correlations of the twin peak kilohertz quasi-eriodic oscillations (kHz QPOs) in bright Z sources and in the less luminous Atoll sources. We find that a power-law relation $\no\sim\nt^{b}$ between the upper and the lower kHz QPOs with different indices: $b\simeq$1.5 for the Atoll source 4U 1728-34 and $b\simeq$1.9 for the Z source Sco X-1. The implications of our results for the theoretical models for kHz QPOs are discussed.

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arXiv2005

The bottom magnetic field and magnetosphere evolution of neutron star in low mass X-ray binary

The accretion induced neutron star magnetic field evolution is studied through considering the accretion flow to drag the field lines aside and dilute the polar field strength, and as a result the equatorial field strength increases and is buried inside the crust. The main conclusions of model are as follows: (i) the polar field decays with increasing the accreted mass; (ii) The bottom magnetic field strength of about $10^8$ G can occur when neutron star magnetosphere radius approaches the star radius, which depends on the accretion rate as $\mdot^{1/2}$; (iii) The neutron star magnetosphere radius decreases with accretion until it reaches the star radius, and its evolution is little influenced by the initial field and the accretion rate after accreting $\sim 0.01 \ms$, which implies that the magnetosphere radii of neutron stars in LMXBs would be homogeneous for Z sources and Atoll sources if they accreted the comparable masses. As an extension, the physics effects of the possible strong magnetic zone in the X-ray neutron stars and recycled pulsars are discussed. Moreover, The strong magnetic fields in the binary pulsars PSR 1831-00 and PSR 1718-19 after accreting about half solar mass in the binary accretion phase, $8.7\times10^{10}$ G and $1.28\times10^{12}$ G, respectively, can be explained through considering the incomplete frozen flow in the polar zone. As a model's expectation, the existence of the low magnetic field ($\sim 3\times 10^{7}$ G) neutron stars or millisecond pulsars is suggested.

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arXiv2005

On The Low Frequency Quasi Periodic Oscillations of X-ray Sources

Based on the interpretation of the twin kilohertz Quasi Periodic Oscillations (kHz QPOs) of X-ray spectra of Low Mass X-Ray Binaries (LMXBs) to the Keplerian and the periastron precession frequencies at the magnetosphere-disk of X-ray neutron star (NS) respectively, we ascribe the low frequency Quasi Periodic Oscillations (LFQPO) and HBO (15-60 Hz QPO for Z sources or Atoll sources) to the periastron precession at some outer disk radius. The obtained conclusions include: all QPO frequencies increase with increasing the accretion rate. The obtained theoretical relations between HBO (LFQPO) frequency and the kHz QPO frequency are similar to the measured empirical formula. Further, the possible dynamical mechanism for QPO production is discussed.

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arXiv2004

Dirac Spin Precession in Kerr Spacetime by the parallelism description

In the framework of parallelism general relativity (PGR), the Dirac particle spin precession in the rotational gravitational field is studied. In terms of the equivalent tetrad of Kerr frame, we investigate the torsion axial-vector spin coupling in PGR. In the case of the weak field and slow rotation approximation, we obtain that the torsion axial-vector has the dipole-like structure, but different from the gravitomagnetic field, which indicates that the choice of the Kerr tetrad will influence on the physics interpretation of the axial-vector spin coupling.

gr-qc↗

arXiv2004

The MHD Alfven wave oscillation model of kHz Quasi Periodic Oscillations of Accreting X-ray Binaries

We ascribe the interpretation of the twin kilohertz Quasi Periodic Oscillations (kHz QPOs) of X-ray spectra of Low Mass X-Ray Binaries (LMXBs) to MHD Alfven wave oscillations in the different mass density regions of the accreted matter at the preferred radius, and the upper kHz QPO frequency coincides with the Keplerian frequency. The proposed model concludes that the kHz QPO frequencies depend inversely on the preferred radius, and that theoretical relation between the upper frequency ($\nt$) and the lower frequency ($\no$) is $\no \sim \nt^{2}$, which is similar to the measured empirical relation. The separation between the twin frequencies decreases (increases) with increasing kHz QPO frequency if the lower kHz QPO frequency is more (less) than $\sim$ 400 Hz.

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arXiv2003

Axial Torsion-Dirac spin Effect in Rotating Frame with Relativistic Factor

In the framework of spacetime with torsion and without curvature, the Dirac particle spin precession in the rotational system is studied. We write out the equivalent tetrad of rotating frame, in the polar coordinate system, through considering the relativistic factor, and the resultant equivalent metric is a flat Minkowski one. The obtained rotation-spin coupling formula can be applied to the high speed rotating case, which is consistent with the expectation.

gr-qc↗

arXiv2003

On the Mass Neutrino Phase calculations along the geodesic line and the null line

On the mass neutrino phase calculations along both the particle geodesic line and the photon null line, there exists a double counting error--factor of 2 when comparing the geodesic phase with the null phase. For the mass neutrino propagation in the flat spacetime, we study the neutrino interference phase calculation in the Minkowski diagram and find that the double counting effect originates from despising the velocity difference between two mass neutrinos. Moreover, we compare the phase calculations among the same energy description, the same momentum description and same velocity description by means of the Minkowski diagram, and obtain the practical equivalence of these three descriptions. Further, in the curved spacetime, we also prove the existence of the double counting of the geodesic phase to the null phase.

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arXiv2003

Two-component Fermi gas in a Harmonic Trap

We consider a mixture of two-component Fermi gases at low temperature. The density profile of this degenerate Fermi gas is calculated under the semiclassical approximation. The results show that the fermion-fermion interactions make a large correction to the density profile at low temperature. The phase separation of such a mixture is also discussed for both attractive and repulsive interatomic interactions, and the numerical calculations demonstrate the exist of a stable temperature region $T_{c1}<T<T_{c2}$ for the mixture. In addition, we give the critical temperature of the BCS-type transition in this system beyond the semiclassical approximation.

quant-ph↗

arXiv2002

ADDENDUM on the mass neutrino oscillation in a gravitational field

In the article {\it Gen. Rel. Grav.} {\bf 32}, 1633 (2000), by J. G. Pereira and C. M. Zhang, the special relativity energy-momentum tensor was used to discuss the neutrino phase-splitting in a weak gravitational field. However, it would be more appropriate to use the general relativity energy-momentum tensor. When we do that, as we are going to see, some results change, but the basic conclusion remains the same.

gr-qc↗

arXiv2001

Rotation intrinsic spin coupling--the parallelism description

For the Dirac particle in the rotational system, the rotation induced inertia effect is analogously treated as the modification of the "spin connection" on the Dirac equation in the flat spacetime, which is determined by the equivalent tetrad. From the point of view of parallelism description of spacetime, the obtained torsion axial-vector is just the rotational angular velocity, which is included in the "spin connection". Furthermore the axial-vector spin coupling induced spin precession is just the rotation-spin(1/2) interaction predicted by Mashhoon. Our derivation treatment is straightforward and simplified in the geometrical meaning and physical conception, however the obtained conclusions are consistent with that of the other previous work.

gr-qc↗

arXiv2001

The general treatment of high/low energy particle interference phase in a gravitational field

The interference phase of the high energy mass neutrinos and the low energy thermal neutrons in a gravitational field are studied. For the mass neutrinos, we obtain that the phase calculated along the null is equivalent to the half phase along the geodesic in the high energy limit, which means that the correct relative phase of the mass neutrinos is either the null phase or the half geodesic phase. Further we point out the importance of the energy condition in calculating the mass neutrino interference phase. Moreover, we apply the covariant phase to the calculation of the thermal neutron interference phase, and obtain the consistent result with that exploited in COW experiment.

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arXiv2001

Axial-Vector Torsion and the Teleparallel Kerr Spacetime

In the context of the teleparallel equivalent of general relativity, we obtain the tetrad and the torsion fields of the stationary axisymmetric Kerr spacetime. It is shown that, in the slow rotation and weak field approximations, the axial-vector torsion plays the role of the gravitomagnetic component of the gravitational field, and is thus the responsible for the Lense-Thirring effect.

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arXiv2000

Mass neutrino flavor evolution in spacetime with torsion

In the framework of the spacetime with torsion, we obtain the flavor evolution equation of the mass neutrino oscillation in vacuum. A comparison with the result of general relativity case, it shows that the flavor evolutionary equations in Riemann spacetime and Weitzenböck spacetimes are equivalent in the spherical symmetric Schwarzschild spacetime, but turns out to be different in the case of the axial symmetry.

gr-qc↗

arXiv2000

Some Remarks on the Neutrino Oscillation Phase in a Gravitational Field

The weak gravitational field expansion method to account for the gravitationally induced neutrino oscillation effect is critically examined. It is shown that the splitting of the neutrino phase into a ``kinematic'' and a ``gravitational'' phase is not always possible because the relativistic factor modifies the particle interference phase splitting condition in a gravitational field.

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arXiv2000

HBO and Periastron Precession around the X-ray Neutron Star Magnetosphere

Based on the periastron precession model to account for kHz QPO of the binary X-ray neutron star, proposed by Stella and Vietri, we ascribe the 15-60 Hz Quasi Periodic Oscillation (QPO) to the periastron precession frequency of the orbiting accreted matter at the boundary of magnetosphere-disk of X-ray neutron star (NS). The obtained conclusions include: all QPO frequencies increase with increasing the accretion rate. The theoretical relations between 15-60 Hz QPO (HBO) frequency and the twin kHz QPOs are similar to the measured empirical formula. Further, the better fitted NS mass by the proposed model is about 1.9 solar masses for the detected LMXBs.

gr-qc↗

arXiv2000

The Evolution of Magnetic Field and Spin Period of Accreting Neutron Stars

Based on the accretion induced magnetic a field decay model, in which a frozen field and an incompressible fluid are assumed, we obtain the following results. (1) An analytic relation between the magnetic field and spin period, if the fastness parameter of the accretion disk is neglected. The evolutionary tracks of accreting neutron stars in the P-B diagram in our model are different from the equilibrium period lines when the influence of the fastness parameter is taken into account. (2) The theoretical minimum spin period of an accreting neutron star is $\max (1.1{\rm ms}(\frac{ΔM}{M_{\odot}})^{-1} R^{-5/14}_6 I_{45}(\frac{M}{M_{\odot}})^{-1/2}, 1.1{\rm ms} (\frac{M}{M_{\odot}})^{-1/2} R^{17/14}_6)$, independent of the accretion rate (X-ray luminosity) but dependent on the total accretion mass $ΔM$. However, the minimum magnetic field depends on the accretion rate. (3) The magnetic field strength decreases faster with time than the period.

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