arXiv subjects

X. H. Sun

Publications and source records attributed to X. H. Sun.

42 records · Page 3Linked to original sources

arXiv2007

J1128+592: a highly variable IDV source

Short time-scale radio variations of compact extragalactic radio quasars and blazars known as IntraDay Variability (IDV) can be explained in at least some sources as a propagation effect; the variations are interpreted as scintillation of radio waves in the turbulent interstellar medium of the Milky Way. One of the most convincing observational arguments in favor of a propagation-induced variability scenario is the observed annual modulation in the characteristic time scale of the variation due to the Earth's orbital motion. So far there are only two sources known with a well-constrained seasonal cycle. Annual modulation has been proposed for a few other less well-documented objects. However, for some other IDV sources source-intrinsic structural variations which cause drastic changes in the variability time scale were also suggested. J1128+592 is a recently discovered, highly variable IDV source. Previous, densely time-sampled flux-density measurements with the Effelsberg 100-m radio telescope (Germany) and the Urumqi 25-m radio telescope (China), strongly indicate an annual modulation of the time scale. The most recent 4 observations in 2006/7, however, do not fit well to the annual modulation model proposed before. In this paper, we investigate a possible explanation of this discrepancy.

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arXiv2007

The IDV source J1128+5925, a new candidate for annual modulation?

Short time-scale radio variations of compact extragalactic radio sources, known as IntraDay Variability, can be explained in at least some sources by a source-extrinsic effect, in which the variations are interpreted as scintillation of radio waves caused by the turbulent ISM of the Milky Way. One of the most convincing observational arguments in favour of propagation-induced variability is the so called annual modulation of the characteristic variability time-scale, which is due to the orbital motion of the Earth. Data for the recently discovered and highly variable IDV source J1128+5925 are presented. We study the frequency and time dependence of the IDV in this compact quasar. We measure the characteristic variability time-scale of the IDV throughout the year, and analyze whether the observed changes in the variability time-scale are consistent with annual modulation. We monitored the flux density variability of J1128+5925 with dense time sampling between 2.7 and 10.45GHz with the 100m Effelsberg radio telescope of the MPIfR and with the 25m Urumqi radio telescope. From ten observing sessions, we determine the variability characteristics and time-scales. The observed pronounced changes of the variability time-scale of J1128+5925 are modelled with an anisotropic annual modulation model. The observed frequency dependence of the variation is in good agreement with the prediction from interstellar scintillation. Adopting a simple model for the annual modulation model and using also the frequency dependence of the IDV, we derive a lower limit to the distance of the scattering screen and an upper limit to the scintillating source size. The latter is found to be consistent with the measured core size from VLBI.

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arXiv2006

A Sino-German lambda6 cm polarization survey of the Galactic plane I. Survey strategy and results for the first survey region

(Abridged) Polarization measurements of the Galactic plane at lambda6 cm probe the interstellar medium (ISM) to larger distances compared to measurements at longer wavelengths, hence enable us to investigate properties of the Galactic magnetic fields and electron density. We are conducting a new lambda6 cm continuum and polarization survey of the Galactic plane covering 10degr < l < 230degr and |b|<5degr. Missing large-scale structures in the U and Q maps are restored based on extrapolated polarization K-band maps from the WMAP satellite. The lambda6 cm data are analyzed together with maps at other bands. We discuss some results for the first survey region, 7degr X 10degr in size, centered at (l,b)=(125.5degr, 0degr). Two new passive Faraday screens, G125.6-1.8 and G124.9+0.1, were detected. They cause significant rotation of background polarization angles but little depolarization. G124.9+0.1 was identified as a new faint HII region at a distance of 2.8 kpc. G125.6-1.8, with a size of about 46 pc, has neither correspondence in enhanced Halpha emission nor a counterpart in total intensity. A model combining foreground and background polarization modulated by the Faraday screen was developed. Using this model, we estimated the strength of the ordered magnetic field along the line of sight to be 3.9 microGauss for G124.9+0.1, and exceeding 6.4 microGauss for G125.6-1.8. We obtained an estimate of 2.5 and 6.3 mK/kpc for the average polarized and total synchrotron emissivity towards G124.9+0.1. The synchrotron emission beyond the Perseus arm is quite weak.

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arXiv2005

New lambda6cm observations of the Cygnus Loop

Radio continuum and polarization observations of the entire Cygnus Loop at 6cm wavelength were made with the Urumqi 25m telescope. The 6cm map is analysed together with recently published maps from the Effelsberg 100m telescope at 21cm and 11cm. The integrated flux density of the Cygnus Loop at 6cm is 90+/-9Jy, which implies a spectral index of -0.40+/-0.06 being consistent with that of Uyaniker et al. (2004) in the wavelength range up to 11cm. This rules out any global spectral steepening up to 6cm. However, small spectral index variations in some regions of the source are possible, but there are no indications for any spectral curvature. The linear polarization data at 6cm show percentage polarizations up to 35% in some areas of the Cygnus Loop, exceeding those observed at 11cm. The Rotation Measure is around -21rad/m^2 in the southern area, which agrees with previous observations. However, the distribution of Rotation Measures is rather complex in the northern part of the Cygnus Loop, where the 21cm emission is totally depolarized. Rotation Measures based on 11cm and 6cm data are significantly larger than in the southern part. The difference in the polarization characteristic between the northern and southern part supports previous ideas that the Cygnus Loop consists of two supernova remnants.

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arXiv2004

Pulsar motions in our Galaxy

Pulsar motions in our Galaxy from their birth until 2 Gyr are studied statistically via Monte-Carlo simulation of $2\times 10^5$ pulsars with the best available representation of the Galactic potential. We find that the distribution of height above the Galactic plane for pulsars with characteristic ages less than about 8 Myr could be well fitted by a Gaussian function. For older pulsars, an extra exponential function is necessary to fit the distribution. The scale-height of the Gaussian component increases linearly with time until about 40 Myr. The height distribution becomes stabilized after about 200 Myr. These results are not sensitive to initial height or radial distributions. Taking the relationship between the initial velocity and height distribution, we found from the latest pulsar catalog that the height distribution of pulsars younger than 1 Myr directly implies the mean initial velocity of $280\pm96$ km s$^{-1}$. Comparison of simulated sample of pulsars with the current available millisecond pulsars shows that their 1D initial velocity dispersion should be most probably $60\pm10$ km s$^{-1}$.

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arXiv2004

Structure Function Studies for Turbulent Interstellar Medium

We study structure functions of rotation measures in the Canadian Galactic Plane Survey (CGPS) region and the North Galactic Pole (NGP) to extract the interstellar medium (ISM) fluctuation information. The CGPS data are divided into three longitude intervals: $82^\circ 1^\circ$. The power law indices get smaller from CGPS1 to CGPS3 and the amplitudes decrease. The variations of the large-scale field and the electron density have only negligible effects on the structure function and thus cannot account for the changes, indicating that the turbulent properties of the Galactic ISM are intrinsically longitude-dependent. The Kolmogorov-like fluctuation spectrum of the electron density or the magnetic field should produce a power law structure function with an index of 5/3 or 2/3, neither of which is consistent with our results of small indices in the three sub-CGPS regions. For the NGP region, the structure function is flat, showing that the rotation measures are mostly intrinsic to the extragalactic sources, and the ISM is very random in that part of our Galaxy. It is obvious that the ISM fluctuation is latitude-dependent when comparing the results in the NGP region and the CGPS regions.

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