arXiv subjects

R. X. Xu

Publications and source records attributed to R. X. Xu.

At least 91 records · Page 5Linked to original sources

arXiv2009

Lennard-Jones quark matter and massive quark stars

Quark clustering could occur in cold quark matter because of the strong coupling between quarks at realistic baryon densities of compact stars. Although one may still not be able to calculate this conjectured matter from first principles, the inter-cluster interaction might be analogized to the interaction between inert molecules. Cold quark matter would then crystallize in a solid state if the inter-cluster potential is deep enough to trap the clusters in the wells. We apply the Lennard-Jones potential to describe the inter-cluster potential, and derive the equations of state, which are stiffer than that derived in conventional models (e.g., MIT bag model). If quark stars are composed of Lennard-Jones matter, they could have high maximum masses ($>2M_{\odot}$) as well as very low masses ($<10^{-3}M_{\odot}$). These features could be tested by observations.

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arXiv2008

A Polytropic Model of Quark Stars

A polytropic quark star model is suggested in order to establish a general framework in which theoretical quark star models could be tested by observations. The key difference between polytropic quark stars and the polytropic model studied previously for normal (i.e., non-quarkian) stars is related to two issues: (i) a constant term representing the contribution of vacuum energy may be added in the energy density and the pressure for a quark star, but not for a normal star; (ii) the quark star models with non-vanishing density at the stellar surface are not avoidable due to the strong interaction between quarks. The first one implies that the vacuum inside a quark star is different from that outside, while the second one is relevant to the effect of color confinement. The polytropic equations of state are stiffer than that derived in conventional realistic models (e.g., the bag model) for quark matter, and pulsar-like stars calculated with a polytropic equation of state could then have high maximum masses (> 2 M_sun). Quark stars can also be very low massive, and be still gravitationally stable even if the polytropic index, n, is greater than 3. All these would result in different mass-radius relations, which could be tested by observations. In addition, substantial strain energy would develop in a solid quark star during its accretion/spindown phase, and could be high enough to take a star-quake. The energy released during star-quakes could be as high as ~ 10^{47} ergs if the tangential pressure is ~ 10^{-6} higher than the radial one.

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arXiv2008

Two possible approaches to form sub-millisecond pulsars

Pulsars have been recognized as normal neutron stars or quark stars. Sub-millisecond pulsars, if detected, would play an essential and important role in distinguishing quark stars from neutron stars. A key question is how sub-millisecond pulsars could form. Both sub-Keplerian (for neutron and quark stars) and super-Keplerian cases (only for quark stars, which are bound additionally by strong interaction) have been discussed in this paper in order to investigate possible ways of forming sub-millisecond pulsars. In the sub-Keplerian case, the equilibrium periods of both neutron and quark stars could be as low as ~0.4 ms when they are spun up through accretion in binary systems. In the super-Keplerian case, pulsars could very likely have an initial period of ~0.1 ms if quark stars with different mass could be formed from accretion-induced collapse (AIC) of white dwarfs. The timescale for a sub-millisecond pulsar to keep its period <1 ms is restricted due to gravitational wave radiation. We found that the timescales of neutron stars within sub-millisecond periods are approximately ~10 yr, but the timescales estimated (>10 Myr) for low-mass quark stars could be long enough for us to detect.

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arXiv2007

Pulsar slow glitches in a solid quark star model

A series of five unusual slow glitches of the radio pulsar B1822-09 (PSR J1825-0935) were observed over the 1995-2005 interval. This phenomenon is understood in a solid quark star model, where the reasonable parameters for slow glitches are presented in the paper. It is proposed that, because of increasing shear stress as a pulsar spins down, a slow glitch may occur, beginning with a collapse of a superficial layer of the quark star. This layer of material turns equivalently to viscous fluid at first, the viscosity of which helps deplete the energy released from both the accumulated elastic energy and the gravitation potential. This performs then a process of slow glitch. Numerical calculations show that the observed slow glitches could be reproduced if the effective coefficient of viscosity is ~10^2 cm^{2}/s and the initial velocity of the superficial layer is order of 10^{-10} cm/s in the coordinate rotating frame of the star.

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arXiv2007

Radio and gamma-ray emissions from pulsars: possible observational tests

Many models for the pulsar radio and $γ$-ray emissions have been developed. The tests for these models using observational data are very important. Tests for the pulsar radio emission models using frequency-altitude relation are presented in this paper. In the radio band, the mean pulse profiles evolve with observing frequencies. There are various styles of pulsar profile - frequency evolutions (which we call as "beam evolution" figure), e.g. some pulsars show that mean pulse profiles are wider and core emission is higher at higher frequencies than that at lower frequencies, but some other pulsars show completely the contrary results. We show that all these "beam evolution" figures can be understood by the Inverse Compton Scattering(ICS) model (see Qiao at al.2001 also). An important observing test is that, for a certain observing frequency different emission components are radiated from the different heights. For the $γ$-ray pulsars, the geometrical method (Wang et al. 2006) can be used to diagnose the radiation location for the $γ$-ray radiation. As an example, Wang et al. (2006) constrain the $γ$-ray radiation location of PSR B1055-52 to be the place near the null charge surface. Here we show that Wang's result matches the proposed radiation locations by the annular gap model as well as the outer gap models.

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arXiv2007

Pulsar's kicks and Gamma-ray bursts

The consistence of the distributions of pulsar's kick velocities from the model of GRB and from the pulsar observations is tested based on the supernova-GRB ($γ$-ray burst) association and under the assumption that the GRB asymmetric explosions produce pulsars. The deduced distribution of kick velocity from the model of GRB and the observed kick distribution of radio pulsars are checked by K-S test. These two distributions are found to come from a same parent population. This result may indicate that GRBs could be really related to supernova, and that the asymmetry of GRB associated with supernova would cause the kick of pulsars.

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arXiv2007

PSR B1828-11: a precession pulsar torqued by a quark planet?

The pulsar PSR B1828-11 has long-term, highly periodic and correlated variations in both pulse shape and the rate of slow-down. This phenomenon may provide evidence for precession of the pulsar as suggested previously within the framework of free precession as well as forced one. On a presumption of forced precession, we propose a quark planet model to this precession henomenon instead, in which the pulsar is torqued by a quark planet. We construct this model by constraining mass of the pulsar ($M_{\rm psr}$), mass of the planet ($M_{\rm pl}$) and orbital radius of the planet ($r_{\rm pl}$). Five aspects are considered: derived relation between $M_{\rm psr}$ and $r_{\rm pl}$, movement of the pulsar around the center of mass, ratio of $M_{\rm psr}$ and $M_{\rm pl}$, gravitational wave radiation timescale of the planetary system, and death-line criterion. We also calculate the range of precession period derivative and gravitational wave strength (at earth) permitted by the model. Under reasonable parameters, the observed phenomenon can be understood by a pulsar ($10^{-4}\sim10^{-1}M_{\odot}$) with a quark planet ($10^{-8}\sim10^{-3}M_{\odot}$) orbiting it. According to the calculations presented, the pulsar would be a quark star because of its low mass, which might eject a lump of quark matter (to become a planet around) during its birth.

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arXiv2007

An annular gap acceleration model for $γ$-ray emission of pulsars

If the binding energy of the pulsar's surface is not so high (the case of a neutron star), both the negative and positive charges will flow out freely from the surface of the star. The annular free flow model for $γ$-ray emission of pulsars is suggested in this paper. It is emphasized that: (1). Two kinds of acceleration regions (annular and core) need to be taken into account. The annular acceleration region is defined by the magnetic field lines that cross the null charge surface within the light cylinder. (2). If the potential drop in the annular region of a pulsar is high enough (normally the cases of young pulsars), charges in both the annular and the core regions could be accelerated and produce primary gamma-rays. Secondary pairs are generated in both regions and stream outwards to power the broadband radiations. (3). The potential drop in the annular region grows more rapidly than that in the core region. The annular acceleration process is a key point to produce wide emission beams as observed. (4). The advantages of both the polar cap and outer gap models are retained in this model. The geometric properties of the $γ$-ray emission from the annular flow is analogous to that presented in a previous work by Qiao et al., which match the observations well. (5). Since charges with different signs leave the pulsar through the annular and the core regions, respectively, the current closure problem can be partially solved.

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arXiv2007

On a synthetical method to constrain 3-D structure of emission regions of pulsars

The main idea and procedure are presented for a new synthetical method on constraining 3-D structure of emission regions of pulsars. With this method the emission regions can be synthetically constrained by fitting multi-wavelength observations features, e.g. pulse widths, phase offsets between radio pulse profiles and high energy light curves, and radio polarization properties. The main technique is based on numerically calculating the emission directions along any open field line via including aberration and retardation effects under a certain magnetic field configuration, e.g. vacuum dipole field. It can be easily extended by involving some further effects or magnetic field line configuration. The role of observed linear polarization properties in the method is discussed. The future application to known X/gamma-ray and radio pulsars will be helpful to test and improve current theoretical models on pulsar emission.

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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

"High-field" pulsars torqued by accretion disk?

The nature of AXPs/SGRs (anomalous X-ray pulsars/soft $γ$-ray repeaters) and high field radio pulsars is still unclear even in the magnetar and/or accretion models. The detection of radio emission from AXP XTE J1810$-$197 and the discovery of a debris disk around AXP 4U 0142+61 might shed light on the problem. We propose that AXPs/SGRs could be pulsars that have magnetic field $B\lesssim 10^{13} \mathrm{G}$ as normal pulsars, but in accretion environments. We investigate these issues under the accretion model and find that two of the AXPs/SGRs might be low mass quark stars if all AXPs and SGRs are likely grouped together.

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arXiv2006

To Constrain the Parameter Space of the ICS Model

The pulse profile of pulsar gives geometric information about pulsar's radiation model. After investigating the pulse profiles of PSR B1642-03 and PSR B0950+08, we calculate the ratios of beam width and the emission height between different frequencies. We find that the ratios are almost constants as inclination angle $α$ changes from $0^{\circ}$ to $90^{\circ}$. The ratios can be used to test pulsar's radiation model. In particular it can well constrain the parameter space of the inverse Compton scattering (ICS) model. These constrained parameters indicate some physical implication for the ICS model.

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arXiv2006

What can the braking indices tell us about pulsars' nature?

As a result of observational difficulties, braking indices of only six rotation-powered pulsars are obtained with certainty, all of which are remarkably smaller than the value ($n=3$) expected for pure magnetodipole radiation model. This is still a real fundamental question not being well answered after nearly forty years of the discovery of pulsar. The main problem is that we are shamefully not sure about the dominant mechanisms that result in pulsars' spin-down. Based on the previous works, the braking index is re-examined, with a conclusion of suggesting a constant gap potential drop for pulsars with magnetospheric activities. New constrains on model parameters from observed braking indices are presented.

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arXiv2006

Is PSR B0943+10 a low-mass quark star?

A recent X-ray observation has shown that the radio pulsar PSR B0943+10, with clear drifting subpulses, has a much smaller polar cap area than that of conventional pulsars with mass of $\sim\msun$ and radius of $\sim10$ km. Zhang et al. (2005) addressed then that this new result conflicts with the standard vacuum gap model. Nonetheless, the discrepancy could be explained if PSR B0943+10 is actually a low-mass quark star. It is found that the potential drop in the open-field-line region of oblique pulsars (i.e., inclination angle $α\neq 0$) might be $\sim 10^2$ times that of aligned pulsars, and that PSR B0943+10 with $α= 12.4^{\rm o}$ could be well above the deathline. We thus conclude that the Ruderman-Sutherland-type vacuum gap model still works well for this pulsar if it is a bare quark star with a mass of $\sim 0.02M_\odot$ and a radius of $\sim 2.6$ km.

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arXiv2006

The superflares of soft Gamma-ray repeatres: giant quakes in solid quark stars?

Three times of supergiant flares from soft $γ$-ray repeatres are observed, with typical released energy of $\sim 10^{44-47}$ erg. A conventional model (i.e., the magnetar model) for such events is catastrophic magnetism-powered instability through magnetohydrodynamic process, in which a significant part of short-hard $γ$-ray bursts could also be the results of magnetars. Based on various observational features (e.g., precession, glitch, thermal photon emission) and the underlying theory of strong interaction (quantum chromodynamics, QCD), it could not be ruled out yet that pulsar-like stars might be actually solid quark stars. Strain energy develops during a solid star's life, and starquakes could occur when stellar stresses reach a critical value, with huge energy released. An alternative model for supergiant flares of soft $γ$-ray repeatres is presented, in which energy release during a star quake of solid quark stars is calculated. Numerical results for spherically asymmetric solid stars show that the released gravitational energy during a giant quake could be as high as $10^{48}$ erg if the tangential pressure is slightly higher than the radial one. Difficulties in magnetar models may be overcome if AXPs/SGRs are accreting solid quark stars with mass $\sim (1-2)M_\odot$.

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arXiv2006

Toward an understanding of the periastron puzzle of PSR B1259-63

Efforts are made to understand the timing behaviors (e.g., the jumps of the projected pulsar semimajor axis in the periastron passages) observed via 13-year timing for PSR B1259-63. In the first, planet-like objects are suggested to orbit around the Be star, which may gravitationally perturb the (probably low mass) pulsar when it passes through periastron. Second, an accretion disk should exist outside the pulsar's light cylinder, which creates a spindown torque on the pulsar due to the propeller effect. The observed negative braking index and the discrepant timing residuals close to periastron could be relevant to the existence of the disk with varying accretion rates. Third, a speculation is presented that the accretion rate may increase on a long timescale in order to explain the negative braking index.

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arXiv2006

To probe into pulsar's interior through gravitational waves

The gravitational radiation from compact pulsar-like stars depends on the state of dense matter at supranuclear densities, i.e., the nature of pulsar (e.g., either normal neutron stars or quark stars). The solid quark star model is focused for the nature of pulsar-like compact objects. Possible gravitational emission of quark stars (either fluid or solid) during the birth and later lifetime is discussed. Several observational features to distinguish various models for pulsar-like stars are proposed. It is suggested that the gravitational wave behaviors should be mass-dependent. Based on the data from the second LIGO science run, the upper limits of R.theta^{1/5} and thus M.theta^{3/5} (M: mass, R: radius, and theta: wobble angle) are provided for those targets of millisecond pulsars.

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arXiv2005

GCRT J1745-3009: A precessing radio pulsar?

A unique transient bursting radio source, GCRT J1745-3009, has been discovered (Hyman et al. 2005a) near the direction of the Galactic center. It is still an open question to explain this phenomenon, although some efforts to understand its nature have been made. This paper shows that most of the observed features can be reproduced by our proposed precessing pulsar model. It is found that the precession angle of the pulsar should be larger ($\ga 15^{\rm o}$) than that of previously known precessing pulsars, which have a precession angle $\la 10^{\rm o}$, if the beam width of the pulsar is larger than 10 degree. The pulsar could be a nulling (or even extremely) radio pulsars to account for the transient nature of the source. This model can be confirmed if a pulsar is detected at the position of the source. The pulsar could hardly be a normal neutron star (but probably a solid quark star) if the spin period of the pulsar is detected to be $\ga 10$ ms in the future.

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