High resolution radio refractive index studies in India using airborne microwave refractometers

We discuss high resolution measurements on radio refractive index, in the boundary layer and above the atmosphere in the Indian sub-continent spread over a period of more than a solar cycle by making use of airborne microwave refractometers, operating around 9 GHz. Some atmospheric turbulence parameters comparable with earlier results have also been derived. The results demonstrate sharp gradients of radio refractive index, layer structures and the variability of the boundary layer in time and space. An erratum to this article is available at .     

Doppler boosting, superluminal motion, and the kinematics of AGN jets

We discuss results from a decade long program to study the fine-scale structure and the kinematics of relativistic AGN jets with the aim of better understanding the acceleration and collimation of the relativistic plasma forming AGN jets. From the observed distribution of brightness temperature, apparent velocity, flux density, time variability, and apparent luminosity, the intrinsic properties of the jets including Lorentz factor, luminosity, orientation, and brightness temperature are discussed. Special attention is given to the jet in M87, which has been studied over a wide range of wavelengths and which, due to its proximity, is observed with excellent spatial resolution. Most radio jets appear quite linear, but we also observe curved non-linear jets and non-radial motions. Sometimes, different features in a given jet appear to follow the same curved path but there is evidence for ballistic trajectories as well. The data are best fit with a distribution of Lorentz factors extending up to γ∼30 and intrinsic luminosity up to ∼10²⁶ W Hz⁻¹. In general, gamma-ray quasars may have somewhat larger Lorentz factors than non gamma-ray quasars. Initially the observed brightness temperature near the base of the jet extend up to ∼5×10¹³ K which is well in excess of the inverse Compton limit and corresponds to a large excess of particle energy over magnetic energy. However, more typically, the observed brightness temperatures are ∼2×10¹¹ K, i.e., closer to equipartition.     

Secondary leptons synchrotron emission from microquasar jets

We present a model to estimate the synchrotron radio emission generated in microquasar (MQ) jets due to secondary pairs created via decay of charged pions produced in proton-proton collisions between stellar wind ions and jet relativistic protons. The synchrotron radiation produced by secondary electrons/positrons is computed using consistently derived particle energy distributions. Energy losses due to synchrotron and inverse Compton (IC) processes, and adiabatic expansion, are taken into account. The space parameter for the model is explored and the corresponding spectral energy distributions (SEDs) are presented. We conclude that secondary leptonic emission represents a significant though hardly dominant contribution to the total radio emission in MQs, with observational consequences that can be used to test some still unknown processes occurring in these objects as well as the nature of the matter outflowing in their jets.      

Large kinetic power in FRII radio jets

We investigate the total kinetic powers (L _j) and ages (t _age) of powerful jets of four FR II radio sources (Cygnus A, 3C 223, 3C 284, and 3C 219) by the detail comparison of the dynamical model of expanding cocoons with observed ones. It is found that these sources have quite large kinetic powers with the ratio of L _j to the Eddington luminosity (L _Edd) resides in 0.02<L _j/L _Edd<10. Reflecting the large kinetic powers, we also find that the total energy stored in the cocoon (E _c) exceed the energy derived from the minimum energy condition (E _min): 2<E _c/E _min<160. This implies that a large amount of kinetic power is carried by invisible components such as thermal leptons (electron and positron) and/or protons.     

利用反射率因子垂直廓线填补雷达波束遮挡区的方法研究

新一代天气雷达很多位于地形复杂的山区,地形遮挡形成观测盲区,严重影响了新一代天气雷达数据的应用效果。利用基于高分辨率地形数据计算的波束遮挡信息,依据反射率因子垂直廓线,由高仰角无遮挡的反射率因子观测数据得到低仰角完全遮挡区的数据。以杭州雷达为例,通过直接对比反射率因子值和对比填补前后雷达估算降水效果两种途径检验了填补效果,结果表明：填补与观测“真值”有很好的一致性,填补后降水估算效果优于填补前。本文提出一种填补低仰角完全遮挡区的方法,适用于均匀性降水系统。     

Radio OH observations of 9P/Tempel 1 before and after Deep Impact

We observed 18-cm OH emission in Comet 9P/Tempel 1 before and after Deep Impact. Observations using the Arecibo Observatory 305 m telescope took place between 8 April and 9 June, 2005, followed by post-impact observations using the National Radio Astronomy Observatory 100 m Green Bank Telescope 4-12 July, 2005. The resulting spectra were analyzed with a kinematic Monte Carlo model which allows estimation of the OH production rate, neutral gas outflow velocity, and distribution of the out-gassing from the nucleus. We detected typically 24% variability from the overall OH production rate trend in the two months leading up to the impact, and no dramatic increase in OH production in the days post-impact. Generally, the coma is well-described, within uncertainties, by a symmetric model with OH production rates from 1.6 to , and mean water outflow velocity of . At these low production rates, collisional quenching is expected to occur only within 20,000 km of the nucleus. However, our best-fit average quenching radius is 64,200 ± 22,000 km in April and May.     

Radio observations of Comet 9P/Tempel 1 before and after Deep Impact

Comet 9P/Tempel 1 was the target of a multi-wavelength worldwide investigation in 2005. The NASA Deep Impact mission reached the comet on 4.24 July 2005, delivering a 370-kg impactor which hit the comet at 10.3 km s⁻¹. Following this impact, a cloud of gas and dust was excavated from the comet nucleus. The comet was observed in 2005 prior to and after the impact, at 18-cm wavelength with the Nançay radio telescope, in the millimeter range with the IRAM and CSO radio telescopes, and at 557 GHz with the Odin satellite. OH observations at Nançay provided a 4-month monitoring of the outgassing of the comet from March to June, followed by the observation of H₂O with Odin from June to August 2005. The peak of outgassing was found to be around between May and July. Observations conducted with the IRAM 30-m radio telescope in May and July 2005 resulted in detections of HCN, CH₃OH and H₂S with classical abundances relative to water (0.12, 2.7 and 0.5%, respectively). In addition, a variation of the HCN production rate with a period of 1.73±0.10 days was observed in May 2005, consistent with the 1.7-day rotation period of the nucleus. The phase of these variations, as well as those of CN seen in July by Jehin et al. [Jehin, E., Manfroid, J., Hutsemékers, D., Cochran, A.L., Arpigny, C., Jackson, W.M., Rauer, H., Schulz, R., Zucconi, J.-M., 2006. Astrophys. J. 641, L145-L148], is consistent with a rotation period of the nucleus of 1.715 days and a strong variation of the outgassing activity by a factor 3 from minimum to maximum. This also implies that the impact took place on the rising phase of the “natural” outgassing which reached its maximum ≈4 h after the impact. Post-impact observations at IRAM and CSO did not reveal a significant change of the outgassing rates and relative abundances, with the exception of CH₃OH which may have been more abundant by up to one order of magnitude in the ejecta. Most other variations are linked to the intrinsic variability of the comet. The Odin satellite monitored nearly continuously the H₂O line at 557 GHz during the 38 h following the impact on the 4th of July, in addition to weekly monitoring. Once the periodic variations related to the nucleus rotation are removed, a small increase of outgassing related to the impact is present, which corresponds to the release of ≈5000±2000 tons of water. Two other bursts of activity, also observed at other wavelengths, were seen on 23 June and 7 July; they correspond to even larger releases of gas.     

Frequency Distributions of Microwave Pulses for the 18 March 2003 Solar Flare

We analyze the pulses in high-frequency drift radio structures observed by the spectrometer at Purple Mountain Observatory (PMO) over the frequency range of 4.5 – 7.5 GHz during the 18 March 2003 solar flare. A number of individual pulses are determined from the drifting radio structures after the detected gradual component subtraction. The frequency distributions of microwave pulse occurrence as functions of peak flux, duration, bandwidth, and time interval between two adjacent pulses exhibit a power-law behavior, i.e. . From regression fitting in log-log space, we obtain the power-law indexes, α _P=7.38±0.40 for the peak flux, α _D=5.39±0.86 for the duration, and α _B=6.35±0.56 for the bandwidth. We find that the frequency distribution for the time interval displays a broken power law. The break occurs at about 500 ms, and their indexes are α _W1=1.56±0.08 and α _W2=3.19±0.12, respectively. Our results are consistent with the previous findings of hard X-ray pulses, type III bursts, and decimetric millisecond spikes.