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141.
R. Soria R. P. Fender D. C. Hannikainen A. M. Read I. R. Stevens 《Monthly notices of the Royal Astronomical Society》2006,368(4):1527-1539
We studied the radio source associated with the ultraluminous X-ray source in NGC 5408 ( L X ≈ 1040 erg s−1 ) . The radio spectrum is steep (index ≈−1 ), consistent with optically thin synchrotron emission, not with flat-spectrum core emission. Its flux density (≈0.28 mJy at 4.8 GHz, at a distance of 4.8 Mpc) was the same in the March 2000 and December 2004 observations, suggesting steady emission rather than a transient outburst. However, it is orders of magnitude higher than expected from steady jets in stellar-mass microquasar. Based on its radio flux and spectral index, we suggest that the radio source is either an unusually bright supernova remnant, or, more likely, a radio lobe powered by a jet from the black hole (BH). Moreover, there is speculative evidence that the source is marginally resolved with a radius ∼30 pc. A faint H ii region of similar size appears to coincide with the radio and X-ray sources, but its ionization mechanism remains unclear. Using a self-similar solution for the expansion of a jet-powered electron–positron plasma bubble, in the minimum-energy approximation, we show that the observed flux and (speculative) size are consistent with an average jet power ≈ 7 × 1038 erg s−1 ∼ 0.1 L X ∼ 0.1 L Edd , an age ≈105 yr, a current velocity of expansion ≈80 km s−1 . We briefly discuss the importance of this source as a key to understand the balance between luminosity and jet power in accreting BHs. 相似文献
142.
Steven R. Furlanetto 《Monthly notices of the Royal Astronomical Society》2006,370(4):1867-1875
We examine the prospects for studying the pre-reionization intergalactic medium (IGM) through the so-called 21-cm forest in spectra of bright high-redshift radio sources. We first compute the evolution of the mean optical depth τ for models that include X-ray heating of the IGM gas, Wouthuysen–Field coupling, and reionization. Under most circumstances, the spin temperature T S grows large well before reionization begins in earnest; this occurs so long as the X-ray luminosity of high-redshift starbursts (per unit star formation rate) is comparable to that in nearby galaxies. As a result, τ≲ 10−3 throughout most of reionization, and background sources must sit well beyond the reionization surface in order to experience absorption that is measurable by square-kilometre class telescopes. H ii regions produce relatively large 'transmission gaps' and may therefore still be observable during the early stages of reionization. Absorption from sheets and filaments in the cosmic web fades once T S becomes large and should be rare during reionization. Minihaloes can produce strong (albeit narrow) absorption features. Measuring their abundance would yield useful limits on the strength of feedback processes in the IGM as well as their effect on reionization. 相似文献
143.
A statistical analysis of the auroral kilometric radiation (AKR) measurements in the POLRAD experiment on the INTERBALL-2 satellite has revealed a dependences of the size and location of the AKR generation region on geomagnetic activity: the generation region rises upward and expands with increasing magnetic disturbances. Based on our two-year measurements, we found seasonal AKR intensity variations: the AKR maximum and minimum are observed in winter and summer, respectively. The seasonal variations and the dependence of the spectrum on geomagnetic activity are assumed to have a common physical nature—the background-plasma density variations in the region of the AKR source attributable to plasma flows from the ionosphere into the magnetosphere. 相似文献
144.
145.
John D. Bunton 《Experimental Astronomy》2004,17(1-3):251-259
When the SKA was proposed, a major technical obstacle to its feasibility was the cost of the correlator. Significant advances made in correlator design since then are described. These advances have made SKA correlator possible within reasonable cost constrains. At the same time performance issues with the proposed FX architecture have been addressed. 相似文献
146.
S. P. Ellingsen 《Monthly notices of the Royal Astronomical Society》2005,359(4):1498-1516
The Australia Telescope National Facility Mopra millimetre telescope has been used to search for 95.1-GHz class I methanol masers towards 62 6.6-GHz class II methanol masers. A total of 26 95.1-GHz masers were detected, 18 of these being new discoveries. Combining the results of this search with observations reported in the literature, a near complete sample of 66 6.6-GHz class II methanol masers has been searched in the 95.1-GHz transition, with detections towards 38 per cent (25 detections; not all of the sources studied in this paper qualify for the complete sample, and some of the sources in the sample were not observed in the present observations).
There is no evidence of an anticorrelation between either the velocity range, or peak flux density of the class I and II transitions, contrary to suggestions from previous studies. The majority of class I methanol maser sources have a velocity range that partly overlaps with the class II maser transitions. The presence of a class I methanol maser associated with a class II maser source is not correlated with the presence (or absence) of main-line OH or water masers. Investigations of the properties of the infrared emission associated with the maser sources shows no significant difference between those class II methanol masers with an associated class I maser and those without. This may be consistent with the hypothesis that the objects responsible for driving class I methanol masers are generally not those that produce main-line OH, water or class II methanol masers. 相似文献
There is no evidence of an anticorrelation between either the velocity range, or peak flux density of the class I and II transitions, contrary to suggestions from previous studies. The majority of class I methanol maser sources have a velocity range that partly overlaps with the class II maser transitions. The presence of a class I methanol maser associated with a class II maser source is not correlated with the presence (or absence) of main-line OH or water masers. Investigations of the properties of the infrared emission associated with the maser sources shows no significant difference between those class II methanol masers with an associated class I maser and those without. This may be consistent with the hypothesis that the objects responsible for driving class I methanol masers are generally not those that produce main-line OH, water or class II methanol masers. 相似文献
147.
148.
A molecular face-on view of the Galactic Centre region 总被引:1,自引:0,他引:1
149.
随着数字技术的发展,数字信号处理芯片的速度越来越快,这为高速数字滤波的实时实现提供了可能。简要阐述了数字滤波的原理,并对两种数字滤波的实现方法进行了分析;给出了数字滤波较模拟滤波的优势;介绍了数字滤波在射电天文测量中的各种应用。 相似文献
150.
N.I. Rovenskaya 《Astrophysics and Space Science》2004,291(2):113-122
The intensities of carbon radio recombination lines (RRL’s) are definedallowing for the effect of dielectronic-like recombination. The rate ofdielectronic-like recombination is calculated as functions of line number,electron density and temperature accurate to 0.05. Following from the balanceequation solutions for populations, the RRL intensities are analytically foundby the method of successive approximations to an accuracy of 0.15. Theobservations of carbon RRL’s are analyzed toward Cassiopeia A. The averageelectron temperature, density, expanded CII region lengths and inaccuraciesare found with the experimental values of RRL widths and intensities. 相似文献