全文获取类型
收费全文 | 1341篇 |
免费 | 131篇 |
国内免费 | 199篇 |
专业分类
测绘学 | 14篇 |
大气科学 | 87篇 |
地球物理 | 65篇 |
地质学 | 563篇 |
海洋学 | 68篇 |
天文学 | 736篇 |
综合类 | 49篇 |
自然地理 | 89篇 |
出版年
2024年 | 11篇 |
2023年 | 11篇 |
2022年 | 32篇 |
2021年 | 42篇 |
2020年 | 34篇 |
2019年 | 35篇 |
2018年 | 19篇 |
2017年 | 29篇 |
2016年 | 22篇 |
2015年 | 28篇 |
2014年 | 51篇 |
2013年 | 40篇 |
2012年 | 42篇 |
2011年 | 55篇 |
2010年 | 46篇 |
2009年 | 124篇 |
2008年 | 99篇 |
2007年 | 141篇 |
2006年 | 134篇 |
2005年 | 114篇 |
2004年 | 111篇 |
2003年 | 98篇 |
2002年 | 64篇 |
2001年 | 53篇 |
2000年 | 49篇 |
1999年 | 63篇 |
1998年 | 40篇 |
1997年 | 14篇 |
1996年 | 10篇 |
1995年 | 11篇 |
1994年 | 12篇 |
1993年 | 8篇 |
1992年 | 6篇 |
1991年 | 2篇 |
1990年 | 6篇 |
1989年 | 4篇 |
1988年 | 5篇 |
1986年 | 4篇 |
1985年 | 1篇 |
1981年 | 1篇 |
排序方式: 共有1671条查询结果,搜索用时 31 毫秒
111.
Sandip K. Chakrabarti † Santabrata Das 《Monthly notices of the Royal Astronomical Society》2001,327(3):808-812
We analytically study how the behaviour of accretion flows changes when the flow model is varied. We study the transonic properties of the conical flow, a flow of constant height and a flow in vertical equilibrium, and show that all these models are basically identical, provided that the polytropic constant is suitably changed from one model to another. We show that this behaviour is extendible even when standing shocks are produced in the flow. The parameter space where shocks are produced remains roughly identical in all these models when the same transformation among the polytropic indices is used. We present applications of these findings. 相似文献
112.
Soumen Mondal Sandip K. Chakrabarti 《Monthly notices of the Royal Astronomical Society》2006,371(3):1418-1426
Standing, propagating or oscillating shock waves are common in accretion and winds around compact objects. We study the topology of all possible solutions using the pseudo-Kerr geometry. We present the parameter space spanned by the specific energy and angular momentum and compare it with that obtained from the full general relativity to show that the potential can work satisfactorily in fluid dynamics also, provided the polytropic index is suitably modified. We then divide the parameter space depending on the nature of the solution topology. We specifically study the nature of the standing Rankine–Hugoniot shocks. We also show that as the Kerr parameter is increased, the shock location generally moves closer to the black hole. In future, these solutions can be used as guidelines to test numerical simulations around compact objects. 相似文献
113.
114.
M. Freeland Z. Kuncic R. Soria G. V. Bicknell 《Monthly notices of the Royal Astronomical Society》2006,372(2):630-638
We calculate the broad-band radio–X-ray spectra predicted by microblazar and microquasar models for ultraluminous X-ray sources (ULXs), exploring the possibility that their dominant power-law component is produced by a relativistic jet, even at near-Eddington mass accretion rates. We do this by first constructing a generalized disc–jet theoretical framework in which some fraction of the total accretion power, P a , is efficiently removed from the accretion disc by a magnetic torque responsible for jet formation. Thus, for different black hole masses, mass accretion rates and magnetic coupling strength, we self-consistently calculate the relative importance of the modified disc spectrum, as well as the overall jet emission due to synchrotron and Compton processes. In general, transferring accretion power to a jet makes the disc fainter and cooler than a standard disc at the same mass accretion rate; this may explain why the soft spectral component appears less prominent than the dominant power-law component in most bright ULXs. We show that the apparent X-ray luminosity and spectrum predicted by the microquasar model are consistent with the observed properties of most ULXs. We predict that the radio synchrotron jet emission is too faint to be detected at the typical threshold of radio surveys to date. This is consistent with the high rate of non-detections over detections in radio counterpart searches. Conversely, we conclude that the observed radio emission found associated with a few ULXs cannot be due to beamed synchrotron emission from a relativistic jet. 相似文献
115.
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
116.
117.
118.
Recent Chandra and XMM-Newton surveys have uncovered a large fraction of the obscured AGN responsible of the hard X-ray background. One of the most intriguing
results of extensive programs of follow-up observations concerns the optical and near-infrared properties of the hard X-ray
sources counterparts. More specifically, for a significant fraction of hard X-ray obscured sources the AGN responsible of
the high X-ray luminosity remains elusive over a wide range of wavelengths from soft X-rays to near-infrared. This very observational
result opens the possibility to investigate the host of bright obscured quasars in some detail. Here we briefly report on
some preliminar results obtained for a small sample of elusive AGN in the HELLAS2XMM survey. 相似文献
119.
We present and discuss a short and simple derivation of orbital epicyclic frequencies for circular geodesic orbits in stationary
and axially symmetric spacetimes. Such spacetimes include as special cases analytically known black hole Kerr and Schwarzschild
spacetimes, as well as the analytic Hartle-Thorne spacetime and all numerically constructed spacetimes relevant for rotating
neutron stars. Our derivation follows directly from energy and angular momentum conservation and it uses the concept of the
effective potential. It has never been published, except for a few special cases, but it has already become a part of the
common knowledge in the field. 相似文献
120.