One more explanation of superluminal motion

The occurrence of superluminal motion in extragalactic radio sources is believed to be quite common. Among others, the geometrical scattering of radio radiation can also cause superluminal expansion and or motion and halo formation, In this paper, the effectiveness of the stimulated Raman scattering in producing these features is investigated. The scattering medium is a plasma whose position, density and temperature decide the rate and angle of scattering. When the radiation from a stationary and constant source gets scattered from a stationary plasma, a halo is formed around the source. However, the scattering of a rotating radiation beam does produce superluminal motion of the virtual source. It is found that the plasma should have the characteristics of the emission-line regions and the intercloud medium in order to Raman scatter the radiation. Since the scattering is polarization dependent, it is possible to estimate the rotation of the electric vector along the direction of the apparent motion of a radio source.     

X-ray emission from accretion disks in active galactic nuclei

We constructed a grid of relativistic models for standard high-relative-luminosity accretion α-disks around supermassive Kerr black holes (BHs) and computed X-ray spectra for their hot, effectively optically thin inner parts by taking into account general-relativity effects. They are known to be heated to high (～10⁶–10⁹ K) temperatures and to cool down through the Comptonization of intrinsic thermal radiation. Their spectra are power laws with an exponential cutoff at high energies; i.e., they have the same shape as those observed in active galactic nuclei (AGNs). Fitting the observed X-ray spectra of AGNs with computed spectra allowed us to estimate the fundamental parameters of BHs (their mass and Kerr parameter) and accretion disks (luminosity and inclination to the line of sight) in 28 AGNs. We show that the Kerr parameter for BHs in AGNs is close to unity and that the disk inclination correlates with the Seyfert type of AGN, in accordance with the unification model of activity. The estimated BH masses M_x are compared with the masses M_rev determined by the reverberation mapping technique. For AGNs with luminosities close to the Eddington limit, these masses agree and the model under consideration may be valid for them. For low-relative-luminosity AGNs, the differences in masses increase with decreasing relative luminosity and their X-ray emission cannot be explained by this model.     

Chinese Surveying and Control Network for Earth-Orbit Satellites and Deep Space Detection

Introduction Astorealizetheprojectofspatialgeodesyby geodeticsatellites,itisnecessarytoestablisha satellitecontrollingandsurveyingnetwork(SC SN)totrack,tosurvey,toremotelycontroland tocommunicatewithearth orbitsatellite,forre alizingtheprojectofdeepspaced…     

The complex gravitational lens system B1933+503

We report the discovery of the most complex arcsec-scale radio gravitational lens system yet known. B1933+503 was found during the course of the CLASS survey and MERLIN and VLA radio maps reveal up to 10 components. Four of these are compact and have flat spectra; the rest are more extended and have steep spectra. The background lensed object appears to consist of a flat-spectrum core (quadruply imaged) and two compact 'lobes' symmetrically disposed relative to the core. One of the lobes is quadruply imaged while the other is doubly imaged. An HST observation of the system with the WFPC2 shows a galaxy with an axial ratio of 0.5, but none of the images of the background object is detected. A redshift of 0.755 has been measured for the lens galaxy.     

Studies of ultracompact H ii regions — II. High-resolution radio continuum and methanol maser survey

High spatial resolution radio continuum and 6.67-GHz methanol spectral line data are presented for methanol masers previously detected by Walsh et al. (1997). Methanol maser and/or radio continuum emission is found in 364 cases towards IRAS -selected regions. For those sources with methanol maser emission, relative positions have been obtained to an accuracy of typically 0.05 arcsec, with absolute positions accurate to around 1 arcsec. Maps of selected sources are provided. The intensity of the maser emission does not seem to depend on the presence of a continuum source. The coincidence of water and methanol maser positions in some regions suggests there is overlap in the requirements for methanol and water maser emission to be observable. However, there is a striking difference between the general proximity of methanol and water masers to both cometary and irregularly shaped ultracompact (UC) H  ii regions, indicating that, in other cases, there must be differing environments conducive to stimulating their emission. We show that the methanol maser is most likely present before an observable UC H  ii region is formed around a massive star and is quickly destroyed as the UC H  ii region evolves. There are 36 out of 97 maser sites that are linearly extended. The hypothesis that the maser emission is found in a circumstellar disc is not inconsistent with these 36 maser sites, but is unlikely. It cannot, however, account for all other maser sites. An alternative model which uses shocks to create the masing spots can more readily reproduce the maser spot distributions.     

Optical and near-infrared imaging of faint Gigahertz Peaked Spectrum sources

A sample of 47 faint Gigahertz Peaked Spectrum (GPS) radio sources selected from the Westerbork Northern Sky Survey (WENSS) has been imaged in the optical and near-infrared, resulting in an identification fraction of 87 per cent. The R  −  I R  −  K colours of the faint optical counterparts are as expected for passively evolving elliptical galaxies, assuming that they follow the R -band Hubble diagram as determined for radio-bright GPS galaxies. We find evidence that the radio spectral properties of the GPS quasars are different from those of GPS galaxies. The observed distribution of radio spectral peak frequencies for GPS sources optically identified with bright stellar objects (presumably quasars) is shifted compared with GPS sources identified with faint or extended optical objects (presumably galaxies), in the sense that a GPS quasar is likely to have a higher peak frequency than a GPS galaxy. This means that the true peak frequency distribution is different for the GPS galaxies and quasars, because the sample selection effects are independent of optical identification. The correlation between peak frequency and redshift that has been suggested for bright sources has not been found in this sample; no correlation exists between R magnitude (and therefore redshift) and peak frequency for the GPS galaxies. We therefore believe that the claimed correlation is actually caused by the dependence of the peak frequency on optical host, because the GPS galaxies are generally at lower redshifts than the quasars. The difference in the peak frequency distributions of the GPS galaxies and quasars is further evidence against the hypothesis that they form a single class of object.     

A 408-MHz variability study of two fields in the region 6h30m, +45°

Variability in 408-MHz flux density, over a 1-yr interval between 1995 and 1996, has been investigated using the DRAO Synthesis Telescope for a complete sample of 322 sources with S _408 MHz > 80 mJy at galactic latitude b  = 15°; four sources are found to show significant flux density variations. These results, taken with those from a similar study at b  = 58°, indicate that 408-MHz variability over a time-scale of 1 yr is more uncommon than has previously been thought; variations at a level >10 per cent could have been detected in a statistically complete sample of about 150 of these sources and none was found. Variability over a period of 18 yr has also been investigated by comparing the DRAO data with those from the B3 catalogue for 182 sources with S _408 MHz > 200 mJy; five variables are found. Large flux density variations over a period of 18 yr are much more common than over 1 yr, with roughly one source in 15 showing variations at the 15 per cent level or greater. The statistics are not good enough to show any conclusive evidence for a dependence of variability on galactic latitude. Roughly half of the variable sources have spectra that are steep at high frequencies and flatten at low frequencies; the other half have flat radio spectra over the entire frequency range. Simultaneous observations at 1420 MHz, also with the DRAO Synthesis Telescope, have been used to investigate variability at this frequency as well, and indicate that over a period of 1 yr roughly half of the flat-spectrum sources vary at the 5 per cent level or greater.     

A disturbed molecular disc in the Circinus galaxy

We have used the Swedish ESO Submillimeter Telescope to observe the molecular gas in the Circinus galaxy using the CO(1 → 0) transition as a tracer. The central region and major axis have been mapped and several other points were also observed. The gas in the galaxy is concentrated towards the nucleus, the peak being coincident with the radio/optical core. The inclination of the molecular galactic disc is more comparable to that of the radio continuum than to that of the large-scale H  i emission. Evidence for an anomalous spur structure pointing radially away from the galactic centre is presented, and may indicate a causal link between it and similar features seen in optical lines and radio continuum. Our data suggest the presence of a central molecular ring or disc with radius 300 ± 50 pc and a rotation velocity of about 200 km s⁻¹ (assuming i  = 73°). The dynamical mass of the nucleus is estimated to be no greater than 3.9 × 10⁹ M_⊙. Assuming that the distribution of gas varies smoothly in the outer regions, we calculate the mass of molecular gas in the galaxy to be at least M _mol = 1.1 × 10⁹ M_⊙, and the star-forming efficiency to be 11 ± 2 L_⊙ M_⊙⁻¹. These results imply that Circinus is undergoing a massive central starburst which may be, at least partially, responsible for its extended minor axis emission seen in several wavebands.     

计算机跟踪伺服系统的保护电路

本文介绍了为紫台13.7米射电望远镜跟踪系统设计制造的一套的简单实用的保护电路。实验证明该电路性能可靠、效果显著。