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461.
Wolter Ruscica & Caccianiga 《Monthly notices of the Royal Astronomical Society》1998,299(4):1047-1058
We present optical identifications for a sample of 20 previously unknown X-ray/radio sources that are present both in the source catalogue of ROSAT PSPC pointed observations ( ROSAT SRC) and in the NRAO VLA Sky Survey (NVSS). The optical spectroscopy was carried out with the 2.1-m telescope at San Pedro Martir (Mexico) during 1995 April and September. We have identified 15 active galactic nuclei [including 12 broad-emission-line (FWHM >1000 km s−1 ) objects, one bona fide BL Lac, one BL Lac candidate and one narrow-line (FWHM < 1000 km s−1 ) radio galaxy] and five radio galaxies. We derive the X-ray fluxes and luminosities by analysing the PSPC exposures, and show the radio morphology from the NVSS maps. We find that the correlation between the monochromatic X-ray luminosity at 2 keV and the core radio luminosity at 5 GHz for the radio galaxies in our sample follows that found for the 3CR radio galaxies, suggesting a possible nuclear origin for the X-ray emission in these sources. This correlation is weaker in the case of broad-line objects, indicating the presence of another (unbeamed) mechanism for the X-ray emission only weakly related to the radio emission. 相似文献
462.
463.
C. J. Phillips R. P. Norris S. P. Ellingsen & P. M. McCulloch 《Monthly notices of the Royal Astronomical Society》1998,300(4):1131-1157
We have used the Australia Telescope Compact Array (ATCA) to make high-resolution images of the 6.7-GHz 51 → 60 A+ maser transition of methanol towards 33 sources in the Galactic plane. Including the results from 12 methanol sources in the literature, we find that 17 out of 45 sources have curved or linear morphology. Most of the 17 have a velocity gradient along the line, which is consistent with masers lying in an edge-on circumstellar disc surrounding a massive star. We also made simultaneous continuum observations of the sources at 8.6 GHz, in order to image any associated H ii region. 25 of the sources are associated with an ultracompact H ii region, with a detection limit of ∼0.5 mJy beam−1 . We argue that the methanol sources without an associated H ii region represent less massive embedded stars, not an earlier stage in the lifetime of the star, as previously suggested. 相似文献
464.
A. Pedlar B. Fernandez N. G. Hamilton M. P. Redman & P. E. Dewdney 《Monthly notices of the Royal Astronomical Society》1998,300(4):1071-1076
We present subarcsecond MERLIN 0.4-GHz (73 cm) and 1.6-GHz (18 cm) radio measurements of the nuclear region of the Seyfert galaxy NGC 4151. By comparison with higher frequency observations, we deduce that one component (C4) shows a low-frequency turnover which we interpret as evidence for free–free absorption by ionized gas with an emission measure between 3 × 105 and 106 pc cm−6 . The free–free absorption appears to be localized to a region ∼50 pc in extent, and we consider models in which the ionized gas may be closely associated with a neutral molecular torus. 相似文献
465.
G. H. J. Cole C. G. Mundell & A. Pedlar 《Monthly notices of the Royal Astronomical Society》1998,300(3):656-664
Using MERLIN with 0.2-arcsec resolution we have observed neutral hydrogen absorption against the central region of the starburst galaxy NGC 3628. The central region resolves into ∼16 continuum components at 1.4 GHz. From comparison with published 15-GHz data, we infer that these components are supernova remnants, although three components may be consistent with a weak active galactic nucleus. Neutral hydrogen absorption is seen against the continuum emission with column densities ∼1022 cm−2 . The absorption appears to be from two separate absorbing structures. Assuming a simple morphology, the main velocity structure can be attributed to a ring of neutral gas with a radius 130 pc rotating around a central starburst with a velocity gradient of 1270 km s−1 kpc−1 . From simple assumptions, the mass interior to this ring is 0.9 × 109 M⊙ . The second absorption structure may represent outflow from the starburst region or a large-scale galactic structure. Alternatively the absorption structure may be non-axisymmetric, such as a bar. 相似文献
466.
Shan-jie Qian Xi-zhen Zhang S. Britzen A. Witzel T. Krichbaum A. Kraus E. Valtaoja H.D. Aller 《Chinese Astronomy and Astrophysics》1998,22(4):409-418
During the period of 1991–1993 two strong high energy γ-ray flares were observed by the Compton Gamma Ray Observatory in the flat spectrum radio source PKS 0528+134. They were associated with strong mm-radio outbursts with a few months time-delays. In this paper the spectral energy distributions (SED) of the radiations in the γ-hand X-ray and the IR-optical bands are analysed. It is shown that the high energy γ-ray radiation may be due to the inverse Compton scattering of the ambient UV and soft X-ray photons by the relativistic electrons in the jet. Basing on the comparison between the properties of the synchrotron radiation of the γ-ray source and the spectral evolution of the mm-radio outbursts, the evolutional relationship between the γ-ray emitting blobs and the mm-radio emitting blobs is discussed. 相似文献
467.
We present and discuss in this paper the rest frame radio spectra (1–25 GHz) of a sample of fourteen radio galaxies atz >2 from the newly defined MRC/1Jy complete sample of 558 radio sources. These galaxies are among the most powerful radio sources
known and range in luminosity from 1028-1028·8 watt Hz-1 at 1 GHz. We find that the median rest frame spectral index of this
sample of galaxies atz >2 is significantly steeper than that of a matched luminosity sample of 3CRR galaxies which are at a much lower redshift (0.85
<z < 1.7). This indicates that spectral index correlates primarily with redshift, at least in the luminosity range considered
here. The difference between the distributions of rest frame spectral curvatures for the two samples does not appear to be
statistically significant.
We suggest a new explanation for the steeper spectra of radio galaxies at high redshift involving steeper electron energy
spectra at injection. Electron energy spectra are expected to steepen in a first-order Fermi acceleration process, at both
non-relativistic and relativistic shock fronts, as the upstream fluid velocity decreases. This may well be the case at high
redshifts: the hotter and denser circum-galactic medium at high redshifts could result in slower speeds for the hotspot and
the jet material behind it. The smaller sizes of radio sources at higher redshifts provide support to this scenario.
Since deceased. 相似文献
468.
本文介绍了1997年3月9日,紫金山天文台在3.2cm波段上日偏食观测概况及所得到的结果。 相似文献
469.
全球定位系统(GPS)技术的最新进展 第六讲 GPS气象学 总被引:8,自引:0,他引:8
介绍了地基GPS气象学及空基GPS气象学的概念、基本原理、方法以及它们在数值天气预报和气候变化研究中的应用。 相似文献
470.