Xin-Ji Wu

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 mu(c) = 10(30-34) erg/cm(3) and critical stress sigma(c) = 10(18similar to24) erg/cm(3). The post-glitch behavior may represent a kind of damped oscillations

A Large Glitch in the Crab Pulsar

Using a new pulsar timing system at the 25-m radio telescope of Urumqi Astronomical Observatory, we have detected a large glitch in the Crab pulsar which occurred in 2000 July. The size of the glitch is Deltav/v similar to 2.4 x 10(-8), with a relative increment in frequency derivative Delta (v) overdot / (v) over dot similar to 5 x 10(-3). The observing system is introduced and the observed properties of the glitch axe discussed.

A study of the long-term flux density variation of PSRs B0329+54 and B1508+55

We have observed the flux densities of PSRs B0329+54 and B1508+55 for 460 days at 327 MHz using the 25 m telescope of NAOC Urumqi Observatory. The flux density time series and the corresponding structure functions are presented. Our observational results at 327 MHz and other observations at 74 MHz and 610 MHz are compared with the predictions of the refractive scintillation theory. The frequency dependence of the refractive scintillation parameters for PSR B0329+54 has been investigated. The results show that the long-term flux density modulations of PSR B0329+54 at the three frequencies are best described by a Kolmogorov spectrum for electron density inhomogeneities. For PSR B 1508+55, we suggest that the spectrum of the electron density fluctuation has a power exponent beta<4.

The Radial Structure of Pulsar Radio Emission Regions

An important parameter in the study of the radial structure of the pulsar radio emission region is the altitude of the emission, but this cannot be derived directly from the observations. The altitude can be expressed as a function of frequency, ; r ν p ν -ξ , and the method of K analysis can be used to calculate the power law index ξ for 18 pulsars observed at two frequencies, 610 MHz and 1408 MHz and for three pulsars observed at three or more frequencies. The average value of ξ is 0.27, which indicates that the emission altitude increases with decreasing frequency and that the radial structure is compact.

Two-component model for the interstellar scattering in direction to the Vela pulsar

This paper discusses a rigorous treatment of the refractive scintillation of pulsar PSR B0833-45 caused by a two-component interstellar scattering medium. It is assumed that the interstellar scattering medium is composed of a thin screen ISM and an extended interstellar medium. We consider that the scattering of the thin screen concentrates in a thin layer presented by a delta function distribution and that the scattering density of the extended irregular medium satisfies the Gaussian distribution. We investigate and develop equations for the flux density structure function corresponding to this two-component ISM geometry in the scattering density distribution and compare our result with that of the Vela pulsar observations. We conclude that the refractive scintillation caused by this two-component ISM scattering gives a more satisfactory explanation for the observed flux density variation of the Vela pulsar than does the single extended medium model. The level of refractive scintillation is strongly sensitive to the distribution of scattering material along the line of sight. The logarithmic slope of the structure function is sensitive to thin screen location and is relatively insensitive to the scattering strength of the thin screen medium. Therefore, the proposed model can be applied to interpret the structure function of flux density observed in pulsar PSR B0833-45. The result suggests that the medium consists of a discontinuous distribution of plasma turbulence embedded in the Vela supernova remnant. Thus our work provides some insight into the distribution of the scattering along the line of sight to the Vela pulsar.

An Analysis of Average Pulsar Profiles and A Study of the ρ-P relation of Pulsars

Using the method of Gaussian Fit Separation of Average Profile (GFSAP), we re-examine the average profiles of nine pulsars at several frequencies, ranging from 408–1642 MHz. This method enables us to obtain the number of components for each pulsar, and the parameters for each component, the width, position and amplitude. The ρ-P relation for the inner cone and outer cone are studied separately, and the results are, respectively, ρ = P−0.51±0.05 and ρ = P−0.42±0.06. The results can be interpreted as a confirmation of the double-cone structure of pulsar emission beams. The altitudes of emission region, and the radius-to-frequency-map (RFM) are also examined; for the outer cone, we obtained r(v) ∝ v−0.19±0.09.

An Estimation of the Initial Period of Pulsars

The initial period of a pulsar is an important factor in our understanding of the formation of neutron stars and of the nature of the equation of state of neutron star matter. Up to now this quantity can only be obtained for a few pulsars for which accurate age and braking index are known. Based on the theory of the off-center dipole emission, in which pulsars obtain their high velocities depending on the initial periods, we calculate the initial period using the proper motion data. Because the orbital velocity of the progenitor and asymmetric kick in the supernova explosion may also contribute to the observed velocity of the pulsar, the derived values of initial periods are lower limits. For normal pulsars, the initial periods are in the range of 0.6 similar to 2.6 ms. For the millisecond pulsars, the initial periods are comparable to their current periods, and the ratio between the initial period and the current period increases with the decrease of the current period. For PSR B1937+21 with the shortest period of 1.56 ms, the ratio is 0.77.

A Monte Carlo study of the evolution of the scale height of normal pulsars in the Galaxy

Based on the undisturbed, finite thickness disk gravitational potential, we carried out 3-D Monte Carlo simulations of normal pulsars. We find that their scale height evolves in a similar way for different velocity dispersions (sigma(nu)): it first increases linearly with time, reaches a peak, then gradually decreases, and finally approaches a stable asymptotic value. The initial velocity dispersion has a very large influence on the scale height. The time evolution of the scale height is studied. When the magnetic decay age is used as the time variable, the observed scale height has a similar trend as the simulated results in the linear stage, from which we derive velocity dispersions in the range 70 similar to 178 km s(-1), which are near the statistical result of 90 similar to 270 km s(-1) for 92 pulsars with known transverse velocities. If the characteristic age is used as the time variable, then the observed and theoretical curves roughly agree for t > 10(8) yr only if sigma(nu) < 25 km s(-1).

Radio Observations at 232 MHz and Multifrequency Spectral Studies of the SNR HB21

Radio observational results at 232 MHz and multifrequency studies of supernova remnant (SNR) HB21 are presented. Its integrated flux density at 232 MHz is about 390 ′ 30 Jy. Both the integrated spectral index and the spatial variations of spectral index of the remnant were calculated by combining the new map at 232 MHz with previously published maps made at 408, 1420, 2695, and 4750 MHz. The SNR has an integrated spectral index of about a = -0.43 (S ν ν α ) between 232 and 4750 MHz. In general the spectral index varies from -0.5 in southeast and west regions of the remnant to -0.3 in the central region and near the northwest edge. The new data of 232 MHz reveals that there is interaction between the remnant and the surrounding gas along the east edge of the remnant which causes the spectrum flattening at low frequency,while the very good agreement between the structure of X_ray emission and the central flat spectrum area suggests that the existence of thermal emission is the reason of spectrum flattening in the area.

Pulsar timing at Urumqi Observatory

We report on recent pulsar timing observations undertaken by the Urumqi Observatory. Observations for 74 radio pulsars have been made regularly at an observing frequency of 1540 MHz as part of the Urumqi Observatory timing program, which commenced in 1999 November at the 25-m Urumqi Nanshan telescope. Currently 284 pulsars are monitored with an average interval between observations of ~9 days. The dedispersion is provided by a 2×128×2.5 MHz filterbank/digitiser system. Accurate and updated periods, period derivatives and positions have been obtained for the majority of the 74 pulsars with timing observations spanning more than one year. For datasets spanning more than one year we have obtained improved positions, proper motions and velocities. Comparing our measurements of period and period derivative with earlier observations, we conclude that, long-term period and period-derivative fluctuations may be dominated by unseen glitches. We also present the observed results for ten glitches of four young pulsars detected in the last five years.