Total flux density radio observations as a tool for understanding AGN behaviour     

Anne Lähteenmäki  Merja Tornikoski  Talvikki Hovatta  Esko Valtaoja 《Astrophysics and Space Science》2007,311(1-3):347-351

A wealth of information on the properties of jets in Active Galactic Nuclei (AGNs) can be derived from total flux density observations at high radio frequencies. This includes, for example, the Doppler factor, the Lorentz factor, and the viewing angle of the jet. We have earlier calculated these parameters for a sample of ～80 sources of different AGN types using almost 20 years of 22 and 37 GHz data from Metsähovi Radio Observatory. We have now gathered data for an additional ten years, and studied the long term characteristic variability time scales of a large sample of AGNs using the first order structure function, the discrete autocorrelation function and the Lomb-Scargle periodogram. Some of the results will be presented in this paper. We also stress the importance of long term observations of AGNs, the main reason for this being misinterpretations of source properties due to insufficient time coverage. Only a few observing epochs will too easily lead to incorrect conclusions about variability, continuum spectra, and the general detectability of the source, not to mention the exclusion of interesting objects from further studies. This is particularly important when considering, for example, the Planck satellite for which the quality of the main mission product, the accurate cosmic microwave background anisotropy maps, depends heavily on the elimination of foreground sources such as AGNs.  相似文献   

Creep of saturated materials as a chemically enhanced rate‐dependent damage process     

Liang Bo Hu  Tomasz Hueckel 《国际地质力学数值与分析法杂志》2007,31(14):1537-1565

Material behaviour that exhibits characteristics of creep induced by a spontaneous mineral dissolution enhanced by material damage is studied. It is believed that the characteristic rates of the chemical processes involved determine the time‐rate dependence of the resulting strain. A basic model of a combined chemo‐plastic softening and chemically enhanced deviatoric strain hardening for saturated geomaterials is presented. Chemical softening is postulated to occur as a consequence of the net mass removal resulting from dissolution and precipitation of specific minerals occurring at the damage‐generated inter‐phase interfaces. Closed and open systems are discussed. In the former case, deformation at constant stress results entirely from a local compensation mechanism between the chemical softening and strain hardening. The classical three stages of creep are interpreted in terms of mechanisms of dissolution and precipitation, as well as the variation in the reaction surface areas involved in the mass exchange. In an open system, the above local mechanism is enhanced by the removal of mass via diffusion of species affecting the mass balance. Such a system is addressed via a boundary value problem as shown in an example. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

Compressible turbulence in galaxy clusters: physics and stochastic particle re-acceleration     

G. Brunetti  A. Lazarian 《Monthly notices of the Royal Astronomical Society》2007,378(1):245-275

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The search for hot spots associated with the Cygnus X-3 relativistic jet     

J. Martí  D. Pérez-Ramírez  P. Luque-Escamilla  J. L. Garrido  J. M. Paredes  A. Muñoz-Arjonilla  J. R. Sánchez-Sutil 《Astrophysics and Space Science》2007,309(1-4):309-313

We report and shortly discuss here the observational work carried out in order to test the possibility that two previously detected radio sources, in the vicinity of the well known microquasar Cygnus X-3, could be hot spot tracers of interaction between its relativistic jet and the interstellar medium (ISM). The motivation behind this search is in part justified considering recent theoretical models of high energy γ-ray emission which strongly rely on the interaction sites of galactic relativistic jets with nearby ISM clouds. The results presented in this paper include an improved radio exploration of the several arc-minute field around Cygnus X-3 using the Very Large Array (VLA), as well as deep near infrared (NIR) imaging with the Calar Alto 3.5 m telescope. We anticipate here that our observations do not appear to support the initial hot spot hypothesis. Instead, the resulting images suggest that the two radio sources, originally believed to be hot spot candidates, are most likely background or foreground objects.  相似文献   

Superdiscs in radio galaxies: jet–wind interactions     

Gopal-Krishna    Paul J. Wiita  Santosh Joshi † 《Monthly notices of the Royal Astronomical Society》2007,380(2):703-711

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Density gradients in Galactic ultra-compact H ii regions     

J. P. Phillips 《Monthly notices of the Royal Astronomical Society》2007,380(1):369-373

We have used recent radiative transfer solutions for cavity-centred shells to investigate the prevalence of density gradients in Galactic ultra-compact H  ii regions. We find that an analysis of 5 and 1.4 GHz data, taken from the recent compilation of Giveon et al., implies that ∼76 per cent of sources may have appreciable density gradients. It would also seem that the central cavities of these sources must be relatively small, with radii no greater than ∼20 per cent of those of the outer shells. The remainder of these sources are presumably homogenous, have much larger cavities, or possess reverse density gradients (densities which increase with increasing radius). A good fraction of the H  ii regions also appear to have high brightness temperatures, implying mean electron temperatures  〈 T _e〉  of the order of  ≈1.3 × 10⁴ K  . This value is higher than has been determined for other such sources.  相似文献   

Radio galaxies in the 2SLAQ Luminous Red Galaxy Survey – I. The evolution of low-power radio galaxies to  z∼ 0.7      

Elaine M. Sadler  Russell D. Cannon  Tom Mauch  Paul J. Hancock  David A. Wake  Nic Ross  Scott M. Croom  Michael J. Drinkwater  Alastair C. Edge  Daniel Eisenstein  rew M. Hopkins  Helen M. Johnston  Robert Nichol  Kevin A. Pimbblet  Roberto De Propris  Isaac G. Roseboom  Donald P. Schneider  Tom Shanks 《Monthly notices of the Royal Astronomical Society》2007,381(1):211-227

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High-redshift Fanaroff–Riley type II radio sources: large-scale X-ray environment     

E. Belsole  D. M. Worrall  M. J. Hardcastle  J. H. Croston 《Monthly notices of the Royal Astronomical Society》2007,381(3):1109-1126

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On the origins of part-time radio pulsars   总被引：1，自引：0，他引：1  

Bing Zhang  Janusz Gil  Jaroslaw Dyks 《Monthly notices of the Royal Astronomical Society》2007,374(3):1103-1107

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河南一次西南涡暴雨的大气排熵指数特征分析     

张霞  王新敏  高媛媛 《气象与环境科学》2015,38(1):26-33

应用耗散结构理论,基于广义相当位温构建大气排熵指数,利用常规观测资料、地面加密自动站雨量资料、NCEP/NCAR 1°×1°再分析资料等,对2008年7月21—23日一次西南低涡东移造成的河南省大范围暴雨过程的大气排熵指数进行诊断分析,结果表明:大气排熵指数的演变与此次西南涡暴雨落区和雨强关系密切,暴雨落在负排熵指数中心偏南一侧,大雨以上降水分布在排熵指数负值中心轴线附近及其偏南侧;强降水开始前,排熵指数明显减小,强降水持续时间与排熵指数低值维持时间联系紧密;雨强不仅与排熵指数低值有关,且与低值维持时间、6h变化量也有密切关系。排熵指数低值中心位置和中心值的强弱变化与该个例中西南低涡中心位置和其强弱变化具有较好一致性。  相似文献