arXiv subjects

R. X. Xu

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

At least 109 records · Page 6Linked to original sources

arXiv2005

To Understand the UV-optical Excess of RX J1856.5-3754

The enigma source, RX J1856.5-3754, is one of the so-called dim thermal neutron stars. Two puzzles of RXJ1856.5-3754 exist: (1) the observational X-ray spectrum is completely featureless; (2) the UV-optical intensity is about seven times larger than that given by the continuation of the blackbody model yielded by the X-ray data. Both the puzzles would not exist anymore if RX J1856.5-3754 is a low mass bare strange quark star, which is in a propeller phase with a low accretion rate. A boundary layer of RX J1856.5-3754 is suggested and modelled, from which the UV-optical emission is radiated. Free-free absorption dominates the opacity of the boundary layer, which results in the opacity to be high in UV-optical but low in X-ray bands. The star's magnetic field, spin period, as well as the accretion rate are constrained by observations.

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arXiv2004

Quarks and Quakes

A quake model of bare strange stars for normal pulsar glitches is summarized. Three mechanisms being responsible for developing elastic stress energy are presented. It is suggested that other kinds of glitches (e.g, the frequency glitch in KS 1947+300 and in AXP/SGRs) could represent the bulk-strain-induced quakes. The apparent field increase of normal pulsars into ``magnetars'' could be the result of stellar catastrophic shrinking when the elastic force raises to a critical point.

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arXiv2004

Low-mass bare strange stars

Strange stars with low masses are suggested to exist in reality, the origin of which could be via accretion-induced collapse (AIC) of white dwarfs. Such a strange star is likely bare, and would thus spin very fast, even to a period of < 0.1 ms. Strange stars with low masses may differ from those with solar-masses in various astrophysical appearances. Observations to test this ``low-mass'' idea are proposed.

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arXiv2004

A model for the challenging "bi-drifting" phenomenon in PSR J0815+09

A new drifting pulsar, PSR J0815+09, was discovered in the Arecibo drift-scan searches. An intriguing feature of this source is that within the four pulse components in the integrated pulse profile, the sub-pulse drifting direction in the two leading components is opposite from that in the two trailing components. In view that the leading theoretical model for pulsar sub-pulse drifting can only interpret one-direction sub-pulse drifting, the observed "bi-drifting" phenomenon from PSR J0815+09 poses a great challenge to the pulsar theory. The inner annular gap (IAG), a new type of inner particle accelerator, was recently proposed to explain both $γ$-ray and radio emission from pulsars. Here we show that the coexistence of the IAG and the conventional inner core gap (ICG) offers a natural interpretation to the bi-drifting phenomenon. In particular, the peculiar drifting behavior in PSR J0815+09 can be reproduced within the inverse Compton scattering (ICS) model for pulsar radio emission.

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arXiv2004

1E 1207.4-5209: a low-mass bare strange star?

Both rotation- and accretion-powered low-mass bare strange stars are studied, the astrophysical appearances of which are especially focused. It is suggested that low-mass bare strange stars, with weaker ferromagnetic fields than that of normal pulsars, could result from accretion-induced collapses (AIC) of white dwarfs. According to its peculiar timing behavior, we propose that the radio-quiet object, 1E 1207.4-5209, could be a low-mass bare strange star with polar surface magnetic field ~ 6 x 10^10 G and a few kilometers in radius. The low-mass bare strange star idea is helpful to distinguish neutron and strange stars, and is testable by imaging pulsar-like stars with the future Constellation-X telescope.

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arXiv2004

Astrophysical Quark Matter

The quark matter may have great implications in astrophysical studies, which could appear in the early Universe, in compact stars, and/or as cosmic rays. After a general review of astrophysical quark matter, the density-dominated quark matter is focused.

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arXiv2004

Quakes in Solid Quark Stars

A starquake mechanism for pulsar glitches is developed in the solid quark star model. It is found that the general glitch natures (i.e., the glitch amplitudes and the time intervals) could be reproduced if solid quark matter, with high baryon density but low temperature, has properties of shear modulus μ= 10^{30~34} erg/cm^3 and critical stress σ_c = 10^{18~24} erg/cm^3. The post-glitch behavior may represent a kind of damped oscillations.

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arXiv2004

The inner annular Gap for pulsar radiation: gamma-ray and radio emission

The inner annular gap (IAG), a new type of inner gap whose magnetic field lines intersect the null charge surface (NCS), is proposed to explain $γ$-ray and radio emission from pulsars. The IAG can be an important source for high-energy particles. The particles can radiate between the NCS and the IAG. Some observational characteristics in both $γ$-ray and radio bands, such as the $γ$-ray emission beams of Crab-like, Vela-like and Geminga-like, can be reproduced by numerical method. It is predicted that the view angle $ζ$ should be larger than the inclination angle ($ζ>α$), otherwise the $γ$-ray radiation will have little possibility to be observed. Whether the inner annular gap (or cap) is sparking (or free flow) depends on the surface binding energy of the pulsar. In stead of neutron star models, the scenario of the IAG is favorable for bare strange star models, because bare strange stars can easily satisfy the requisite condition to form an IAG for both pulsars ($\vecΩ\cdot \vec{B}<0$) and anti-pulsars ($\vecΩ\cdot \vec{B}>0$).

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arXiv2004

A note on solid pulsars

After a brief introduction to the necessary of quark stars in modelling pulsars, I present a qualitative analysis of the solidification of quark matter with low-temperature but high-density. The reason, that a solid neutron star could not be possible, is also given.

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arXiv2003

RX J1856.5-3754: A strange star with solid quark surface?

Within the realms of the possibility of solid quark matter, we fitted the 500ks Chandra LETG/HRC data for RX J1856.5-3754 with a phenomenological spectral model, and found that electric conductivity of quark matter on the stellar surface is about > 1.2 x 10^{18} s^{-1}.

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arXiv2003

Solid Bare Strange Quark Stars

The reason, we need three terms of `strange', `bare', and `solid' before quark stars, is presented concisely though some fundamental issues are not certain. Observations favoring these stars are introduced.

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arXiv2003

The braking indices in pulsar emission models

Using the method proposed in a previous paper, we calculate pulsar braking indices in the models with torque contributions from both inner and outer accelerating regions, assuming that the interaction between them is negligible. We suggest that it is likely that the inverse Compton scattering induced polar vacuum gap and the outer gap coexist in the pulsar magnetosphere. We include the new near threshold vacuum gap models with curvature-radiation and inverse Compton scattering induced cascades, respectively; and find that these models can well reproduce the measured values of the braking indices.

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arXiv2003

Can the age discrepancies of neutron stars be circumvented by an accretion-assisted torque?

It is found that 1E 1207.4-5209 could be a low-mass bare strange star if its small radius or low altitude cyclotron formation can be identified. The age problems of five sources could be solved by a fossil-disk-assisted torque. The magnetic dipole radiation dominates the evolution of PSR B1757-24 at present, and the others are in propeller (or tracking) phases.

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arXiv2003

Solid Quark Stars?

It is conjectured that cold quark matter with much high baryon density could be in a solid state, and strange stars with low temperatures should thus be solid stars. The speculation could be close to the truth if no peculiar polarization of thermal X-ray emission (in, e.g., RXJ1856), or no gravitational wave in post-glitch phases, is detected in future advanced facilities, or if spin frequencies beyond the critical ones limited by r-mode instability are discovered. The shear modulus of solid quark matter could be ~ 10^{32} erg/cm^3 if the kHz QPOs observed are relevant to the eigenvalues of the center star oscillations.

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arXiv2003

An accretion disk model for periodic timing variations of pulsars

The long-term, highly periodic and correlated variations in both the pulse shape and the rate of slow-down of two isolated pulsars (PSRs) PSR B1828-11 and PSR B1642-03 were discovered recently. This phenomenon may provide evidence for "free precession" as suggested in the literature. Some authors presented various kinds of models to explain this phenomenon within the framework of free precession. Here we present an accretion disk model for this precession phenomenon instead. Under reasonable parameters, the observed phenomenon can be explained by an isolated pulsar with a fossil disk. This may link radio pulsars and anomalous X-ray pulsars (AXPs) and present an indirect evidence for the existence of the fossil disk in nature.

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arXiv2003

Ultra High Energy Cosmic Rays: Strangelets? -- Extra dimensions, TeV-scale black holes and strange matter

The conjecture that ultra high energy cosmic rays (UHECRs) are actually strangelets is discussed. Besides the reason that strangelets can do as cosmic rays beyond the GZK-cutoff, another argument to support the conjecture is addressed in this letter via the study of formation of TeV-scale microscopic black holes when UHECRs bombarding bare strange stars. It is proposed that the exotic quark surface of a bare strange star could be an effective astro-laboratory in the investigations of the extra dimensions and of the detection of ultra-high energy neutrino fluxes. The flux of neutrinos (and other point-like particles) with energy >2.3 x 10^{20} eV could be expected to be smaller than 10^{-26} cm^{-2}$ s^{-1} if there are two extra spatial dimensions.

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arXiv2003

A Joint model for radio and gamma-ray emission from pulsars

Although pulsars can radiate electromagnetic wave from radio to gamma ray bands, we still have no a united model to understand the multi-band emission. In this paper the effort for a joint model is presented. The inverse Compton scattering (ICS) and a second acceleration process near the null charge surface are involved to account for the radio and the gama-ray emission, respectively. Various kind of pulse profiles and other observational properties can be reproduced.

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arXiv2003

Radio and gamma-ray emission from pulsars

The radiation of pulsars have been observed for many years. A few pulsars are discovered to have both radio and gamma-ray emission. Many models on pulsar radiation have been developed, but so far we are still lacking an elaborate model which can explain the emission from radio to gamma-rays in detail. In this paper we present a joint model for radio and gamma-ray emission, in which both the dominate emission mechanisms are inverse Compton scattering. The pulse profiles at radio and gamma-ray bands are reproduced for the Crab-like, Vela-like and Geminga-like pulsars, in good agreement with observations.

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