17574064
Observations of IntraDay Variable sources with the Effelsberg and Urumqi Radio Telescopes
National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR ChinaE-mail: hjl@bao.ac.cnA sample of classical IntraDay Variable (IDV) and IDV candidate sources has been monitoredwith the Urumqi 25m telescope and the Effelsberg 100m telescope. Aim of the project is to inves-tigate the origin of IntraDay Variability, a phenomenon which has been observed in about 30%of ﬂat spectrum radio quasars. Simultaneous Effelsberg-Urumqi observations demonstrated thatthe Urumqi antenna, although relatively small in diameter, is well suitable for IDV experiments.A few Urumqi datasets, however, turned out to be affected by a spurious ˘24 hours modulation,an effect which has been removed by means of a new procedure for data reduction. In about 14months, 12 epochs of observation have been collected, for a total observing time of more than 45days. The epochs are regularly distributed over the whole year, in order to check for the presenceof systematic annual changes in the variability time scales - a crucial test for verifying the con-sistency of source-extrinsic models of the variability. Preliminary time-analysis of the monitoredsources revealed some hint for a slowing down of the characteristic time scales of S5 0716+714,a result that, if conﬁrmed, would be compatible with a source-extrinsic origin of the variability, incontrast to previous IDV studies. No signiﬁcant modulation of the time scales has been detectedfor S4 0954+658.Bursts, Pulses and Flickering:Wide-ﬁeld monitoring of the dynamic radio skyJune 12-15 2007Kerastari, Tripolis, Greece
119209019
On the influence of the Sun on the rapid variability of compact extragalactic sources
Since December 2004, we have performed a program to monitor intraday variable sources at a frequency of 5 GHz at the Urumqi Observatory. We present our analysis of the variability characteristics of the flat-spectrum radio source AO 0235 + 164, which detects an annual cycle in the variability amplitude. This appears to correlate with the solar elongation of the source. A thorough analysis of the results of the MASIV IDV survey - which provides the variability characteristics of a large sample of compact radio sources confirms that there is a small but detectable component of the observed fractional modulation that increases with decreasing solar elongation. We discuss the hypothesis that the phenomenon is related to interplanetary scintillation.
16003577
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. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
117018554
Bursty Variations of Jovian 6cm Radio Emissions and Quasi-Periodic Jupiter's Polar Activities
In reference to Jupiter south polar quasi-periodic 40-50 min (QP-40) activities and the model scenario for global QP-40 oscillations of the Jovian inner radiation belt (IRB), we validate relevant predictions and confirmations by amassing empirical evidence from Ulysses, Cassini, Chandra, Galileo, XMM-Newton, and Advanced Composition Explorer for Jupiter north polar QP-40 activities. We report ground 6cm radio observations of Jupiter by Urumqi 25m telescope for synchrotron intensity bursty variations of the Jovian IRB and show their likely correlations with the recurrent arrival of high-speed solar winds at Jupiter.
119119786
Radio observations of the first three-months of Fermi AGN at 4.8 GHz
Using the Urumqi 25 m radio telescope, sources from the first t hree months of the Fermi-large area telescope detected active galactic nuclei (AGN) catalog with a declination of > 0 ◦ were observed in 2009 at 4.8 GHz. The radio flux density ap- peared to correlate with the -ray intensity. Intra-day variability (IDV) observations were performed in March, April and May 2009 for 42 selected -ray bright blazars, and ∼60% of them showed evidence of flux variability at 4.8 GHz duri ng the IDV observations. The IDV detection rate was higher than that in previous flat-spectrum AGN samples. IDV appeared more often in the very long baseline interferometry-core dominant blazars, and the non-IDV blazars showed relatively "steeper" spectral in- dices than the IDV blazars. Pronounced inter-month variability was also found in two BL Lac objects: J0112+2244 and J0238+1636.
119312762
Structure, spectra and variability of some GPS radio sources
We report the results of multifrequency-VLBI observations of GHz-Peaked-Spectrum (GPS) radio sources. The VLBI structure and component spectra of some GPS sources are presented. Our VLBI results show that about 80% of the GPS galaxies exhibit a compact double or CSO-like structure, white the GPS quasars tend to show a core-jet. The component spectra of the GPS galaxies are often steep/convex, and the core has a flat spectrum but it is usually hidden or weak. In addition, we studied the variability of GPS sources by comparing new flux density measures, acquired with the Urumqi 25-m telescope at 4.85 GHz, with previous 87GB data. The results show that 44% of the GPS quasars varied higher than 10% in passed 20 years, while the fraction is only 12% for the GPS galaxies meaning that the GPS quasars are much more variable than GPS galaxies. In total, 25% of GPS sources show > 10% variability at 4.85 GHz in our sample.
118788264
Hydrodynamic modeling and the QGP shear viscosity
In this article, we will briefly review the recent progress on hydrodynamic modeling and the extraction of the quark-gluon plasma (QGP) specific shear viscosity with an emphasis on the results obtained from the hybrid model VISHNU that couples viscous hydrodynamics for the macroscopic expansion of the QGP to the hadron cascade model for the microscopic evolution of the late hadronic stage.
118545612
A Statistical Study on the Morphology of Rays and Dynamics of Blobs in the Wake of Coronal Mass Ejections
In this paper, with a survey through the Large Angle and Spectrometric Coronagraph (LASCO) data from 1996 to 2009, we present 11 events with plasma blobs flowing outwards sequentially along a bright coronal ray in the wake of a coronal mass ejection. The ray is believed to be associated with the current-sheet structure that formed as a result of solar eruption, and the blobs are products of magnetic reconnection occurring along the current sheet. The ray morphology and blob dynamics are investigated statistically. It is found that the apparent angular widths of the rays at a fixed time vary in a range of 2.1 – 6.6○ (2.0 – 4.4○) with an average of 3.5○ (2.9○) at 3R⊙ (4R⊙), respectively, and the observed durations of the events vary from 12 h to a few days with an average of 27 h. It is also found, based on the analysis of blob motions, that 58% (26) of the blobs were accelerated, 20% (9) were decelerated, and 22% (10) moved with a nearly constant speed. Comparing the dynamics of our blobs and those that are observed above the tip of a helmet streamer, we find that the speeds and accelerations of the blobs in these two cases differ significantly. It is suggested that these differences of the blob dynamics stem from the associated magnetic reconnection involving different magnetic field configurations and triggering processes.
118341161
Suppression of elliptic flow in a minimally viscous quark-gluon plasma
The success of the hydrodynamic model in describing the bulk of hadron production in Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC) [1] has led to a paradigmatic shift in our view of the quark-gluon plasma (QGP): Instead of behaving like a gas of weakly interacting quarks and gluons [2], as naively expected on the basis of asymptotic freedom in QCD, its collective properties rather reflect those of a “perfect liquid” with (almost) vanishing viscosity. However, due to quantum mechanical uncertainty no fluid can have exactly zero viscosity [3], and recent work [4] on strongly coupled gauge field theories, based on techniques exploiting the AdS/CFT correspondence, suggests an absolute lower limit for the ratio of shear viscosity η to entropy density s: η/s ≥ ~/4π. This raises the interesting question how close to this limit the actual value of the shear viscosity of the QGP created at RHIC is. Answering this question requires hydrodynamic simulations for relativistic viscous fluids in which the ratio η/s enters as a parameter. To study the anisotropic (“elliptic”) collective flow in non-central heavy-ion collisions, from which limits on η/s can be extracted [5], requires a code that evolves the hydrodynamic fields at least in the two dimensions transverse to the heavy-ion beam. In this Letter we present our first results from such simulations [6]; a longer paper with a discussion of all technical details of our approach is in preparation [7].
118634430
The QGP shear viscosity–elusive goal or just around the corner?
With the new viscous hydrodynamic + hadron cascade hybrid code VISHNU, a rather precise () extraction of the quark gluon plasma (QGP) shear viscosity (?/s)QGP from heavy-ion elliptic flow data is possible if the initial eccentricity of the collision fireball is known with <5% accuracy. At this point, eccentricities from initial state models differ by up to 20%, leading to an uncertainty for (?/s)QGP. It is shown that a simultaneous comparison of elliptic and triangular flow, v2 and v3, puts strong constraints on initial state models and can largely eliminate the present uncertainty in (?/s)QGP. The variation of the differential elliptic flow v2(pT) for identified hadrons between RHIC and LHC energies provides additional tests of the evolution model.
119173058
Hadron spectra and elliptic flow for 200 A GeV Au + Au collisions from viscous hydrodynamics coupled to a Boltzmann cascade
It is shown that the recently developed hybrid code VISHNU, which couples a relativistic viscous fluid dynamical description of the quark-gluon plasma (QGP) with a microscopic Boltzmann cascade for the late hadronic rescattering stage, yields an excellent description of charged and identified hadron spectra and elliptic flow measured in 200 A GeV Au+Au collisions at the Relativistic Heavy-Ion Collider (RHIC). Using initial conditions that incorporate event-by-event fluctuations in the initial shape and orientation of the collision fireball and values {eta}/s for the specific shear viscosity of the quark-gluon plasma that were recently extracted from the measured centrality dependence of the eccentricity-scaled, p{sub T}-integrated charged hadron elliptic flow v{sub 2,ch}/{epsilon}, we obtain universally good agreement between theory and experiment for the p{sub T} spectra and differential elliptic flow v{sub 2}(p{sub T}) for both pions and protons at all collision centralities.