8187171
The IDV source J 1128+5925, a new candidate for annual modulation?
Context. Short time-scale radio variations of compact extragalactic radio sources, known as IntraDay Variability (IDV), 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 interstellar medium 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. So far there are only two sources known which show such a well-defined seasonal cycle, a few more sources with fewer data can be regarded as possible candidates for this effect. However, source-intrinsic effects, such as structural variations, can also cause the observed changes of the variability time-scale. Data for the new, recently discovered, and highly variable IDV source J 1128+5925 are presented. Aims. 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. Assuming a radio wave propagation effect as origin, we are able to constrain some physical properties (such as distance, scattering-strength, and possible anisotropy) of the “plasma” screen, which may cause the scintillation. Methods. We monitored the flux density variability of J 1128+5925 with dense time sampling between 2.7 and 10.45 GHz. We observed with the 100 m Effelsberg radio telescope of the Max-Planck-Institut fur Radioastronomie (MPIfR) at 2.70 GHz, 4.85 GHz, and 10.45 GHz, as well as with the 25 m Urumqi radio telescope (China) at 4.85 GHz. From ten observing sessions, each of which lasted several days during the period between 2004–2006, we determine the variability characteristics and time-scales which we investigate in view of possible scintillation and annual modulation. Results. The observed pronounced changes of the variability time-scale of J 1128+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 also using the frequency dependence of the IDV, we derive a lower limit to the distance the scattering screen and an upper limit for the scintillating source size. The latter is found to be consistent with the measured core size from Very Long Baseline Interferometry (VLBI).
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)
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
73710703
A seasonal cycle and an abrupt change in the variability characteristics of the intraday variable source S4 0954+65
The BLLac object S4 0954+65 is one of the main targets of the Urumqi monitoring program targeting intraday variable (IDV) sources. Between August 2005 and December 2009, the source was included in 41 observing sessions, carried out at a frequency of 4.8 GHz. The time analysis of the collected light curves, performed by applying both a structure function analysis and a specifically developed wavelet-based algorithm, discovered an annual cycle in the variability timescales, suggesting that there is a fundamental contribution by interstellar scintillation to the IDV pattern of the source. The combined use of the two analysis methods also revealed that there was a dramatic change in the variability characteristics of the source between February and March 2008, at the starting time of a strong outburst phase. The analysis' results suggest that the flaring state of the source coincides with the appearance of multiple timescales in its light curves, indicating that changes in the structure of the relativistically moving emitting region may strongly influence the variability observed on IDV timescales.
118516843
Intra-day variability observations of S5 0716+714 over 4.5 years at 4.8 GHz
Aims. We aim to search for evidence of annual modulation in the time scales of the BL Lac object S5 0716+714.
119257437
Very Long Baseline Array observations of the intraday variable source J1128+592
Context. Short timescale flux density variations in flat spectrum radio sources are often explained by the scattering of radio waves in the turbulent ionized interstellar medium of the Milky Way. One of the most convincing observational arguments in favor of this is the annual modulation of the variability timescale caused by the orbital motion of Earth around the Sun. J1128+5925 is a recently discovered IDV source with a possible annual modulation in its variability timescale. New observations suggest a change in its variability characteristics.
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.
73657451
Systematic parameter study of hadron spectra and elliptic flow from viscous hydrodynamic simulations of Au+Au collisions at √s_NN=200 GeV
Using the (2+1)-dimensional viscous hydrodynamic code vish2+1[H. Song and U. Heinz, Phys. Lett. B 658, 279 (2008); H. Song and U. Heinz, Phys. Rev. C 77, 064901 (2008); H. Song, Ph. D. thesis, The Ohio State University, 2009], we present systematic studies of the dependence of pion and proton transverse-momentum spectra and their elliptic flow in 200A GeV Au+Au collisions on the parameters of the hydrodynamic model (thermalization time, initial entropy density distribution, decoupling temperature, equation of state, and specific shear viscosity {eta}/s). We identify a tension between the slope of the proton spectra, which (within hydrodynamic simulations that assume a constant shear viscosity to entropy density ratio) prefer larger {eta}/s values, and the slope of the p{sub T} dependence of charged hadron elliptic flow, which prefers smaller values of {eta}/s. Changing other model parameters does not appear to permit dissolution of this tension.
118880227
A Broken Solar Type?II Radio Burst Induced by a Coronal Shock Propagating across the Streamer Boundary
We discuss an intriguing type II radio burst that occurred on 2011 March 27. The dynamic spectrum was featured by a sudden break at about 43 MHz on the well-observed harmonic branch. Before the break, the spectrum drifted gradually with a mean rate of about -0.05 MHz s(-1). Following the break, the spectrum jumped to lower frequencies. The post-break emission lasted for about 3 minutes. It consisted of an overall slow drift which appeared to have a few fast-drift sub-bands. Simultaneous observations from the Solar TErrestrial RElations Observatory and the Solar Dynamics Observatory were also available and are examined for this event. We suggest that the slow-drift period before the break was generated inside a streamer by a coronal eruption driven shock, and the spectral break as well as the relatively wide spectrum after the break is a consequence of the shock crossing the streamer boundary where density drops abruptly. It is suggested that this type of radio bursts can be taken as a unique diagnostic tool for inferring the coronal density structure, as well as the radio-emitting source region.
28563606
200 A GeV Au + Au collisions serve a nearly perfect quark-gluon liquid.
A new robust method to extract the specific shear viscosity (η/s)(QGP) of a quark-gluon plasma (QGP) at temperatures T(c) < T ≲ 2T(c) from the centrality dependence of the eccentricity-scaled elliptic flow v2/ε measured in ultrarelativistic heavy-ion collisions is presented. Coupling viscous fluid dynamics for the QGP with a microscopic transport model for hadronic freeze-out we find for 200 A GeV Au + Au collisions that v2/ε is a universal function of multiplicity density (1/S)(dN(ch)/dy) that depends only on the viscosity but not on the model used for computing the initial fireball eccentricity ε. Comparing with measurements we find 1<4π(η/s)(QGP) < 2.5 where the uncertainty range is dominated by model uncertainties for the values of ε used to normalize the measured v2.
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].