arXiv subjects

Xiang-Dong Li

Publications and source records attributed to Xiang-Dong Li.

187 records · Page 11Linked to original sources

arXiv2006

Exploration of the $\ps-\pb$ Relation for Wind-Fed X-Ray Pulsars

We have investigated the relation between the orbital periods ($\pb$) and the spin periods ($\ps$) of wind-fed X-ray pulsars in high-mass X-ray binaries (HMXBs), based on population synthesis calculations of the spin evolution of neutron stars during the pre-HMXB stage. We show that most of the neutron stars either have steady accretion or still reside in the radio pulsar phase when the donor star starts evolving off the main sequence. In the former case the values of $\ps$ can be decelerated to be $\sim 10^2-10^3$ s depending on $\pb$. We briefly discuss the possible origin of the $\ps-\pb$ correlation in Be/X-ray binaries, and the existence of HMXBs with main sequence donors. We have also investigated the evolution of the inclination angle between the magnetic and spin axes of neutron stars in a massive binary, suggesting secular alignment of the magnetic and spin axes during their evolution.

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arXiv2006

On the Nature of Part Time Radio Pulsars

The recent discovery of rotating radio transients and the quasi-periodicity of pulsar activity in the radio pulsar PSR B1931$+$24 has challenged the conventional theory of radio pulsar emission. Here we suggest that these phenomena could be due to the interaction between the neutron star magnetosphere and the surrounding debris disk. The pattern of pulsar emission depends on whether the disk can penetrate the light cylinder and efficiently quench the processes of particle production and acceleration inside the magnetospheric gap. A precessing disk may naturally account for the switch-on/off behavior in PSR B1931$+$24.

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arXiv2006

Accretion torque on magnetized neutron stars

The conventional picture of disk accretion onto magnetized neutron stars has been challenged by the spin changes observed in a few X-ray pulsars, and by theoretical results from numerical simulations of disk-magnetized star interactions. These indicate possible accretion during the propeller regime and the spin-down torque increasing with the accretion rate. Here we present a model for the accretion torque exerted by the disk on a magnetized neutron star, assuming accretion continues even for rapid rotators. The accretion torque is shown to have some different characteristics from that in the conventional model, but in accord with observations and numerical calculations of accretion-powered magnetized neutron stars. We also discuss its possible applications to the spin evolution in X-ray pulsars.

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arXiv2004

Evolution of Intermediate-Mass Black Hole X-Ray Binaries

The majority of the ultraluminous X-ray sources (ULXs) in external galaxies are believed to be accreting black holes in binary systems; some of the black holes could be as massive as $\sim 100-1000 \ms$. We have performed evolution calculations for intermediate-mass black hole X-ray binaries, assuming they are formed in dense star clusters via tidal capture. The results are compared with those for stellar-mass black holes X-ray binaries. We find that these two types of black holes may have similar companion stars and binary orbits if observed as ULXs. However, intermediate-mass black holes seem to be favored in explaining the most luminous ULXs. We also discuss the possibilities of transient behavior and beamed emission in the evolution of these binary systems.

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arXiv2003

A Fallback Disk Model for Ultraluminous X-ray Sources

Current stellar evolution models predict that during the core collapse of massive stars, a considerable amount of the stellar material will fall back onto the compact, collapsed remnants (neutron stars or black holes), usually in the form of an accretion disk. This triggers rapid mass accretion onto, e.g., the black hole, and produces energetic explosions known as Gamma-ray bursts. However it is very difficult to prove the existence of an accretion disk around newborn black holes observationally. Here we demonstrate that some of the ultraluminous X-ray sources in nearby galaxies, which are associated with supernova remnants, may be black holes accreting from their fallback disks, i.e., they have evolved from collapsars. Since it is almost certain that there is an accretion disk around these black holes, this would for the first time lend the observational support to the collapsar model.

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arXiv2000

Glimpses of a strange star

There are about 2000 gamma ray burst (GRB) events known to us with data pouring in at the rate of one per day. While the afterglows of GRBs in radio, optical and X-ray bands are successfully explained by the fireball model, a significant difficulty with the proposed mechanisms for GRBs is that a small amount ($\le 10^{-6} M_{\odot}$) of baryons in the ejecta can be involved. There are very few models that fulfill this criteria together with other observational features, among which are the differentially rotating collapsed object model and the "supernova" model. These models generally invoke rapidly rotating neutron stars, and may be subject to uncertainties in the formation mechanisms and the equations of state of neutron stars. According to Spruit, the problem of making a GRB from an X-ray binary is reduced to finding a plausible way to make the star rotate differentially. We suggest that a model of strange star (SS) can naturally explain many of these bursts with not only their low baryon content, but the differential rotation which leads to an enhanced magnetic field that surfaces up and is responsible for GRBs.

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arXiv1999

On the nature of the compact star in 4U 1728-34

The discovery of kilohertz quasi-periodic oscillations (kHz QPOs) in low-mass X-ray binaries (LMXBs) with the Rossi X-ray Timing Explorer has stimulated extensive studies of these sources. Recently, Osherovich & Titarchuk suggest a new model for kHz QPOs and the related correlations between kHz QPOs and low frequency features in LMXBs. Here we use their results to study the mass - radius relation for the atoll source 4U 1728-34. We find that, if this model is correct, 4U 1728-34 is possibly a strange star rather a neutron star.

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