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Qu J.L.,CAS Institute of High Energy Physics | Lu F.J.,CAS Institute of High Energy Physics | Lu Y.,CAS Institute of High Energy Physics | Song L.M.,CAS Institute of High Energy Physics | And 3 more authors.
Astrophysical Journal | Year: 2010

We present a study of the centroid frequencies and phase lags of quasi-periodic oscillations (QPOs) as functions of photon energy for GRS 1915+105. It is found that the centroid frequencies of the 0.5-10 Hz QPOs and their phase lags are both energy dependent, and there exists an anticorrelation between the QPO frequency and phase lag. These new results challenge the popular QPO models, because none of them can fully explain the observed properties. We suggest that the observed QPO phase lags are partially due to the variation of the QPO frequency with energy, especially for those with frequency higher than 3.5 Hz. © 2010. The American Astronomical Society. All rights reserved. Source


Yan W.M.,Urumqi Observatory | Yan W.M.,University of Chinese Academy of Sciences | Manchester R.N.,CSIRO | van Straten W.,Swinburne University of Technology | And 20 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2011

Polarization profiles are presented for 20 millisecond pulsars that are being observed as part of the Parkes Pulsar Timing Array project. The observations used the Parkes multibeam receiver with a central frequency of 1369MHz and the Parkes digital filter bank pulsar signal-processing system PDFB2. Because of the large total observing time, the summed polarization profiles have very high signal-to-noise ratios and show many previously undetected profile features. 13 of the 20 pulsars show emission over more than half of the pulse period. Polarization variations across the profiles are complex, and the observed position angle variations are generally not in accord with the rotating vector model for pulsar polarization. Nevertheless, the polarization properties are broadly similar to those of normal (non-millisecond) pulsars, suggesting that the basic radio emission mechanism is the same in both classes of pulsar. The results support the idea that radio emission from millisecond pulsars originates high in the pulsar magnetosphere, probably close to the emission regions for high-energy X-ray and gamma-ray emission. Rotation measures were obtained for all 20 pulsars, eight of which had no previously published measurements. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS. Source


Li Y.,Northwest Agriculture and Forestry University | Li Y.,Iowa State University | Horton R.,Iowa State University | Ren T.,China Agricultural University | Chen C.,Urumqi Observatory
Agricultural Water Management | Year: 2010

It is important to determine how well ETo can be estimated from easily observed Epan (free water evaporation measured by a pan) measurements and the other climatic data. Our objectives are to predict annual ETo with Epan data (with a calibrated kp (=ETo/Epan)) and with a 4-variable regression function method. The significance of the trends of Epan, ETo and kp series were detected. The whole data series (ETo, Epan, mean temperature, sunlight hours, relative humidity and wind speed) were divided into the early (L-5) years for calibrating kp and coefficients of a 4-variable function and the last 5 years for predicting ETo. From the results, significance of series trends decreased when using the modified Mann-Kendall (MMK) test compared to the Mann-Kendall (MK) method. For ETo, five out of six sites showed significant trends according to the MK statistic Z, and two sites were significant in trend combining with the MMK statistic Z*(j). For Epan, two sites were significant in trends according to Z, and zero sites were significant in trends combining with Z*(j). For kp, two sites were significant in trends according to Z, and no sites were significant in trends combining with Z*(j). Thus the calibrated kp can be treated as a constant when using the Epan method. The predicted annual ETo using the Epan and the multi-variable methods showed generally good agreements with the estimated annual ETo (based on monthly PM equation) with low relative errors (RE). Mean ETo values were well predicted by both methods. When using Epan method, RE ranged from -14.7 to -3.3% for Urumqi, from 17.6 to 21.7% for Xning, from 1.8 to 10.7% for Lanzhou, from 4.7 to 17.0% for Huhehaote, from -7.4 to 9.1% for Beijing, and from -8.6 to 2.3% for Changchun. RE of predicting annual ETo with 4-variable regression function were even lower compared to Epan method. The main error source of the predictions came from the deviation between calibrated kp and the actual kp of the predicted years when using Epan method and from random fluctuations of climatic data when using the 4-varible regression function. In conclusion, the MMK test was a robust method for trend detection because it considered serial time dependence. Insignificant trend of the kp series supports the choice of a mean value as the calibrated kp and for ETo predictions. The Epan method is recommended for prediction of annual ETo. © 2009 Elsevier B.V. All rights reserved. Source


Yuan J.P.,Urumqi Observatory | Yuan J.P.,University of Chinese Academy of Sciences | Wang N.,Urumqi Observatory | Manchester R.N.,CSIRO | Liu Z.Y.,Urumqi Observatory
Monthly Notices of the Royal Astronomical Society | Year: 2010

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 × 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. Three small glitches in PSRs B0402+61, B0525+21 and J1853+0545 show evidence for normal post-glitch recovery, but for PSRs B0144+59 and B2224+65 the spin frequency ν continually increases relative to the pre-glitch solution for hundreds of days after their small glitches. Most large glitches show some evidence for exponential post-glitch recovery on timescales of 100-1000 d, but in some cases, e.g. PSR B1758-23, there is little or no recovery, i.e. no detectable increase in |ν| at the time of the glitch. 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 ν following the partial exponential recovery, which is similar to the Vela pulsar post-glitch behaviour. However, the rate of increase, ν̈, is an order of magnitude less than for the Vela pulsar. We present improved positions for 14 of the glitching pulsars. Analysis of the whole sample of known glitches shows that fractional glitch amplitudes are correlated with characteristic age with a peak at about 105 yr, 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. For some (but not all) pulsars, there is a correlation between glitch amplitude and the duration of the following interglitch interval. In no case is there a correlation of glitch amplitude with the duration of the preceding interglitch interval. © 2010 The Authors. Journal compilation. © 2010 RAS. Source


Gao Z.F.,Urumqi Observatory | Gao Z.F.,University of Chinese Academy of Sciences | Gao Z.F.,Nanjing University | Wang N.,Urumqi Observatory | And 4 more authors.
Astrophysics and Space Science | Year: 2011

In this paper, we consider the effect of Landau levels on the decay of superhigh magnetic fields of magnetars. Applying 3P2 anisotropic neutron superfluid theory yield a second-order differential equation for a superhigh magnetic field B and its evolutionary timescale t. The superhigh magnetic fields may evolve on timescales ~(106-107) yrs for common magnetars. According to our model, the activity of a magnetar may originate from instability caused by the high electron Fermi energy. © 2011 Springer Science+Business Media B.V. Source

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