arXiv subjects

N. Wang

Publications and source records attributed to N. Wang.

At least 109 records · Page 6Linked to original sources

arXiv2012

Population Synthesis for Symbiotic X-ray Binaries

Symbiotic X-ray binaries (SyXBs) comprise a rare class of low-mass X-ray binaries. We study the Galactic SyXBs, which we consider as detached binaries composed of low-mass giants and wind-fed neutron star companions, by simulation of the interaction of a magnetized neutron star (NS) with its environment and utilizing a population synthesis code. We focus mainly on the parameters that influence observational appearance of the SyXB: the donor wind velocity (vw) and the angular momentum distribution in the shell of matter settling onto NS. We estimate the birthrate of SyXB as $\sim 4.1\times 10^{-5}$ yr$^{-1}$ to $ \sim 6.6\times 10^{-6}$ yr$^{-1}$ and their number in the Galaxy as $\sim$(100 -- 1000). Assumed stellar wind velocity from cool giants is the input parameter that influences the model SyXBs population most. Among known SyXBs or candidate systems, 4U 1954+31 and IGR J16358-4724 in which NS have very long spin periods may host quasi-spherically accreting NSs. GX 1+4 has a peculiar long-term spin behaviour and it may also be a quasi-spherical wind-accreting source. We cannot identify whether there are wind-fed accretion disks in 4U 1700+24, Sct X-1, IRXS J180431.1-273932 and 2XMM J174016.0-290337.

astro-ph.SR↗

arXiv2012

Dynamical nucleus-nucleus potential and incompressibility of nuclear matter

The dynamical nucleus-nucleus potentials for some fusion reactions are investigated by using the improved quantum molecular dynamics (ImQMD) model with different sets of parameters in which the corresponding incompressibility coefficient of nuclear matter is different. Two new sets of parameters SKP* and IQ3 for the ImQMD model are proposed with the incompressibility coefficient of 195 and 225 MeV, respectively. The measured fusion excitation function for 16O+208Pb and the charge distribution of fragments for Ca+Ca and Au+Au in multi-fragmentation process can be reasonably well reproduced. Simultaneously, the influence of the nuclear matter incompressibility and the range of nucleon-nucleon interaction on the nucleus-nucleus dynamic potential is investigated.

nucl-th↗

arXiv2011

Long term monitoring of mode switching for PSR B0329+54

The mode switching phenomenon of PSR B0329+54 is investigated based on the long-term monitoring from September 2003 to April 2009 made with the Urumqi 25m radio telescope at 1540 MHz. At that frequency, the change of relative intensity between the leading and trailing components is the predominant feature of mode switching. The intensity ratios between the leading and trailing components are measured for the individual profiles averaged over a few minutes. It is found that the ratios follow normal distributions, where the abnormal mode has a wider typical width than the normal mode, indicating that the abnormal mode is less stable than the normal mode. Our data show that 84.9% of the time for PSR B0329+54 was in the normal mode and 15.1% was in the abnormal mode. From the two passages of eight-day quasi-continuous observations in 2004, and supplemented by the daily data observed with 15 m telescope at 610 MHz at Jodrell Bank Observatory, the intrinsic distributions of mode timescales are constrained with the Bayesian inference method. It is found that the gamma distribution with the shape parameter slightly smaller than 1 is favored over the normal, lognormal and Pareto distributions. The optimal scale parameters of the gamma distribution is 31.5 minutes for the abnormal mode and 154 minutes for the normal mode. The shape parameters have very similar values, i.e. 0.75^{+0.22}_{-0.17} for the normal mode and 0.84^{+0.28}_{-0.22} for the abnormal mode, indicating the physical mechanisms in both modes may be the same. No long-term modulation of the relative intensity ratios was found for both the modes, suggesting that the mode switching was stable. The intrinsic timescale distributions, for the first time constrained for this pulsar, provide valuable information to understand the physics of mode switching.

astro-ph.SR↗

arXiv2011

Superlattices of Bi2Se3/In2Se3: Growth Characteristics and Structural Properties

Superlattices (SLs) consisted of alternating Bi2Se3 and In2Se3 layers are grown on Si(111) by molecular-beam epitaxy. Bi2Se3, a three-dimensional topological insulator (TI), showed good chemical and structural compatibility with In2Se3, a normal band insulator with large energy bandgap. The individual layers in the SLs are very uniform and the hetero-interfaces are sharp. Therefore, such SL structures are potential candidates for explorations of the quantum size effects of TIs.

cond-mat.mes-hall↗

arXiv2011

On the nature of the first transient Z-source XTE J1701-462: its accretion disk structure, neutron star magnetic field strength, and hard tail

Using the data from the Rossi X-Ray Timing Explorer satellite, we investigate the spectral evolution along a "Z" track and a "v" track on the hardness-intensity diagrams of the first transient Z source XTE J1701-462. The spectral analyses suggest that the inner disk radius depends on the mass accretion rate, in agreement with the model prediction, R_in \propto ((dM/dt)_disk)^{2/7}, for a radiation pressure dominated accretion disk interacting with the magnetosphere of a neutron star (NS). The changes in the disk mass accretion rate (dM/dt)_disk are responsible for the evolution of the "Z" or "v" track. The radiation pressure thickens the disk considerably, and also produces significant outflows. The NS surface magnetic field strength, derived from the interaction between the magnetosphere and the radiation pressure dominated accretion disk, is ~(1--3)X10^9 G, which is possibly between normal atoll and Z sources. A significant hard tail is detected in the horizontal branches and we discuss several possible origins of the hard tail.

astro-ph.HE↗

arXiv2011

Strong pulses detected from a rotating radio transient J1819$-$1458

We analyze individual pulses detected from RRAT J1819$-$1458. From April 2007 to April 2010, we carried out observations using the Nanshan 25-m radio telescope of Urumqi Observatory at a central frequency of 1541.25 MHz. We obtain a dispersion measure $DM=195.7\pm0.3$ pc cm^{-3} by analyzing all the 423 detected bursts. The tri-band pattern of arrival time residuals is confirmed by a single pulse timing analysis. Twenty-seven bimodal bursts located in the middle residual band are detected, and, profiles of two typical bimodal bursts and two individual single-peak pulses are presented. We determine the statistical properties of SNR and W$_{50}$ of bursts in different residual bands. The W$_{50}$ variation with SNR shows that the shapes of bursts are quite different from each other. The cumulative probability distribution of intensity for a possible power law with index $α=1.6\pm0.2$ is inferred from the number of those bursts with $SNR\ge6$ and high intensities.

astro-ph.SR↗

arXiv2010

Multiwavelength properties of a new Geminga-like pulsar: PSR J2021+4026

In this paper, we report a detailed investigation of the multiwavelength properties of a newly detected gamma-ray pulsar, PSR J2021+4026, in both observational and theoretical aspects. We firstly identify an X-ray source in the XMM-Newton serendipitous source catalogue, 2XMM J202131.0+402645, located within the 95% confidence circle of PSR J2021+4026. With an archival Chandra observation, this identification provides an X-ray position with arcsecond accuracy which is helpful in facilitating further investigations. Searching for the pulsed radio emission at the position of 2XMM J202131.0+402645 with a 25-m telescope at Urumqi Astronomical Observatory resulted in null detection and places an upper-limit of 0.1~mJy for any pulsed signal at 18~cm. Together with the emission properties in X-ray and gamma-ray, the radio quietness suggests PSR J2021+4026 to be another member of Geminga-like pulsars. In the radio sky survey data, extended emission features have been identified in the gamma-ray error circle of PSR J2021+4026. We have also re-analyzed the gamma-ray data collected by FERMI's Large Area Telescope. We found that the X-ray position of 2XMM J202131.0+402645 is consistent with that of the optimal gamma-ray timing solution. We have further modeled the results in the context of outer gap model which provides us with constraints for the pulsar emission geometry such as magnetic inclination angle and the viewing angle. We have also discussed the possibility of whether PSR J2021+4026 has any physical association with the supernova remnant G78.2+2.1 (gamma-Cygni).

astro-ph.HE↗

arXiv2010

A very large glitch in PSR B2334+61

Seven years of pulse time-of-arrival measurements have been collected from observations of the young pulsar PSR B2334+61 using the Nanshan radio telescope of Urumqi Observatory. A phase-connected solution has been obtained over the whole data span, 2002 August to 2009 August. This includes a very large glitch that occurred between 2005 August 26 and September 8 (MJDs 53608 and 53621). The relative increase in rotational frequency for this glitch, $Δν_{g}/ν~\sim~20.5\times10^{-6}$, is the largest ever seen. Although accounting for less than 1\% of the glitch, there were two well-defined exponential decay terms with time constants of 21 and 147 days respectively. There was also a large long-term increase in the spindown rate with $Δ\dotν_p/\dotν\sim 0.011$ at the time of the glitch. A highly significant oscillation with a period of close to one year is seen in the post-glitch residuals. It is very unlikely that this can be accounted for by a pulsar position error or proper motion -- it appears to result from effects interior to the neutron star. Implications of these results for pulsar glitch models are discussed.

astro-ph.HE↗

arXiv2010

Van der Waals epitaxy of Bi2Se3 on Si(111) vicinal surface: An approach to prepare high-quality thin films of topological insulator

Epitaxial growth of topological insulator Bi2Se3 thin films on nominally flat and vicinal Si(111) substrates is studied. In order to achieve planner growth front and better quality epifilms, a two-step growth method is adopted for the van der Waal epitaxy of Bi2Se3 to proceed. By employing vicinal Si(111) substrate surfaces, the in-pane growth rate anisotropy of Bi2Se3 is explored to achieve single crystalline Bi2Se3 epifilms, in which threading defects and twins are effectively suppressed. Optimization of the growth parameters has resulted in vicinal Bi2Se3 films showing a carrier mobility of ~ 2000 cm2V-1s-1 and the background doping of ~ 3 x 1018 cm-3 of the as-grown layers. Such samples not only show relatively high magnetoresistance but also a linear dependence on magnetic field.

cond-mat.mtrl-sci↗

arXiv2010

29 Glitches Detected at Urumqi Observatory

Glitches detected in pulsar timing observations at the Nanshan radio telescope of Urumqi Observatory between 2002 July and 2008 December are presented. In total, 29 glitches were detected in 19 young pulsars, with this being the first detection of a glitch in 12 of these pulsars. Fractional glitch amplitudes range from a few parts in 10^{-11} to 3.9 x 10^{-6}. Three "slow" glitches are identified in PSRs J0631+1036, B1822-09 and B1907+10. Post-glitch recoveries differ greatly from pulsar to pulsar and for different glitches in the same pulsar. Most large glitches show some evidence for exponential post-glitch recovery on timescales of 100 -- 1000 days, but in some cases, e.g., PSR B1758-23, there is little or no recovery. Beside exponential recoveries, permanent increases in slowdown rate are seen for the two large glitches in PSRs B1800-21 and B1823-13. These and several other pulsars also show a linear increase in nudot following the partial exponential recovery, which is similar to the Vela pulsar post-glitch behaviour. Analysis of the whole sample of known glitches show that fractional glitch amplitudes are correlated with characteristic age with a peak at about 10^5 years, but there is a spread of two or three orders of magnitude at all ages. Glitch activity is positively correlated with spin-down rate, again with a wide spread of values.

astro-ph.GA↗

arXiv2009

Multi-phase-field analysis of short-range forces between diffuse interfaces

We characterize both analytically and numerically short-range forces between spatially diffuse interfaces in multi-phase-field models of polycrystalline materials. During late-stage solidification, crystal-melt interfaces may attract or repel each other depending on the degree of misorientation between impinging grains, temperature, composition, and stress. To characterize this interaction, we map the multi-phase-field equations for stationary interfaces to a multi-dimensional classical mechanical scattering problem. From the solution of this problem, we derive asymptotic forms for short-range forces between interfaces for distances larger than the interface thickness. The results show that forces are always attractive for traditional models where each phase-field represents the phase fraction of a given grain. Those predictions are validated by numerical computations of forces for all distances. Based on insights from the scattering problem, we propose a new multi-phase-field formulation that can describe both attractive and repulsive forces in real systems. This model is then used to investigate the influence of solute addition and a uniaxial stress perpendicular to the interface. Solute addition leads to bistability of different interfacial equilibrium states, with the temperature range of bistability increasing with strength of partitioning. Stress in turn, is shown to be equivalent to a temperature change through a standard Clausius-Clapeyron relation. The implications of those results for understanding grain boundary premelting are discussed.

cond-mat.mtrl-sci↗

arXiv2009

Hurst parameter analysis of radio pulsar timing noise

We present an analysis of timing residual (noise) of 54 pulsars obtained from 25-m radio telescope at Urumqi Observatory with a time span of 5~8 years, dealing with statistics of the Hurst parameter. The majority of these pulsars were selected to have timing noise that look like white noise rather than smooth curves. The results are compared with artificial series of different constant pairwise covariances. Despite the noise like appearance, many timing residual series showed Hurst parameters significantly deviated from that of independent series. We concluded that Hurst parameter may be capable of detecting dependence in timing residual and of distinguishing chaotic behavior from random processes.

astro-ph.IM↗

arXiv2008

Pulsar Timing for the Fermi Gamma-ray Space Telescope

We describe a comprehensive pulsar monitoring campaign for the Large Area Telescope (LAT) on the {\em Fermi Gamma-ray Space Telescope} (formerly GLAST). The detection and study of pulsars in gamma rays give insights into the populations of neutron stars and supernova rates in the Galaxy, into particle acceleration mechanisms in neutron star magnetospheres, and into the "engines" driving pulsar wind nebulae. LAT's unprecedented sensitivity between 20 MeV and 300 GeV together with its 2.4 sr field-of-view makes detection of many gamma-ray pulsars likely, justifying the monitoring of over two hundred pulsars with large spin-down powers. To search for gamma-ray pulsations from most of these pulsars requires a set of phase-connected timing solutions spanning a year or more to properly align the sparse photon arrival times. We describe the choice of pulsars and the instruments involved in the campaign. Attention is paid to verifications of the LAT pulsar software, using for example giant radio pulses from the Crab and from PSR B1937+21 recorded at Nançay, and using X-ray data on PSR J0218+4232 from XMM-Newton. We demonstrate accuracy of the pulsar phase calculations at the microsecond level. Data Table 1 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/ .

astro-ph↗

arXiv2008

Timing observations of RRAT J1819-1458 at Urumqi Observatory

We report the timing-analysis results obtained for RRAT J1819-1458 from regular timing observations at 1.54 GHz using the Urumqi 25 m radio telescope between 2007 April to 2008 March. RRAT J1819-1458 is a relatively young and highly magnetized neutron star discovered by its sporadic short bursts in the Parkes Multibeam Pulsar Survey data. In 94 hrs of observation data, we detected a total of 162 dispersed bursts of RRAT J1819-1458 with the signal-to-noise ratios (S/Ns) above 5-$σ$ threshold. Among them, 5 bursts clearly show two-component structure. The S/N of the strongest burst is 13.3. The source's DM measured through our data is 196.0 $\pm$0.4 pc cm$^{-3}$. The timing position, frequency and its first derivative were determined using standard pulsar timing techniques on the arrival times of these individual bursts. The accuracy of the solved rotating parameters are improved comparing with that in previous publication. Our timing position with 2-$σ$ error is consistent with the position of its X-ray counterpart CXOU J181934.1-145804. The effect of timing noise and the phase fluctuation of the individual short bursts on the timing residuals is briefly discussed. The distribution of the timing residuals is bimodal, which cannot be explained readily by timing irregularity.

astro-ph↗

arXiv2008

Step-up converter for electromagnetic vibrational energy scavenger

This paper introduces a voltage multiplier (VM) circuit which can step up a minimum voltage of 150 mV (peak). The operation and characteristics of this converter circuit are described. The voltage multiplier circuit is also tested with micro and macro scale electromagnetic vibrational generators and the effect of the VM on the optimum load conditions of the electromagnetic generator is presented. The measured results show that 85% efficiency can be achieved from this VM circuit at a power level of 18 ?W.

cs.OH↗

arXiv2008

Daily Observations of Interstellar Scintillation in PSR B0329+54

Quasi-continuous observations of PSR B03239+54 over 20 days using the Nanshan 25-m telescope at 1540 MHz have been used to study the effects of refractive scintillation on the pulsar flux density and diffractive scintillation properties. Dynamic spectra were obtained from datasets of 90 min duration and diffractive parameters derived from a two-dimensional auto-correlation analysis. Secondary spectra were also computed but these showed no significant evidence for arc structure. Cross correlations between variations in the derived parameters were much lower than predicted by thin screen models and in one case was of opposite sign to the prediction. Observed modulation indices were larger than predicted by thin screen models with a Kolmogorov fluctuation spectrum. Structure functions were computed for the flux density, diffractive timescale and decorrelation bandwidth. These indicated a refractive timescale of $8\pm 2$ h, much shorter than predicted by the thin screen model. The measured structure-function slope of $0.4\pm 0.2$ is also inconsistent with scattering by a single thin screen for which a slope of 2.0 is expected. All observations are consistent with scattering by an extended medium having a Kolmogorov fluctuation spectrum which is concentrated towards the pulsar. This interpretation is also consistent with recent observations of multiple diffuse scintillation arcs for this pulsar.

astro-ph↗

arXiv2007

Observations of Six Glitches in PSR B1737-30

Six glitches have been recently observed in the rotational frequency of the young pulsar PSR B1737-30 (J1740-3015) using the 25-m Nanshan telescope of Urumqi Observatory. With a total of 20 glitches in 20 years, it is one of the most frequently glitching pulsars of the about 1750 known pulsars. Glitch amplitudes are very variable with fractional increases in rotation rate ranging from 10^{-9} to 10^{-6}. Inter-glitch intervals are also very variable, but no relationship is observed between interval and the size of the preceding glitch. There is a persistent increase in |\dotν|, opposite in sign to that expected from slowdown with a positive braking index, which may result from changes in the effective magnetic dipole moment of the star during the glitch.

astro-ph↗

arXiv2007

Pulsar Nulling and Mode Changing

With its relatively long observation time per pointing, the Parkes multibeam survey was effective in detecting nulling pulsars. We have made 2-hour observations of 23 pulsars which showed evidence for pulse nulling or mode changing in the survey data. Because of the low flux density of these pulsars, in most cases averaging times of between 10 s and 60 s were necessary and so this analysis is insensitive to very short nulls. Seven of the pulsars had null fractions of more than 40% with the largest having a lower limit of 95%. Mode changes were observed in six pulsars with clear relationships between nulling and mode changing in some cases. Combined with earlier results, the data suggest that large null fractions are more related to large characteristic age than to long pulse period. The observations suggest that nulling and mode changing are different manifestations of the same phenomenon.

astro-ph↗