The hard X-ray spectrum of the Seyfert galaxy IRAS 18325–5926: reflection from an ionized disc and variable iron K emission     

K. Iwasawa  J. C. Lee †  A. J. Young  C. S. Reynolds  A. C. Fabian 《Monthly notices of the Royal Astronomical Society》2004,347(2):411-420

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How the X-ray spectrum of a narrow-line Seyfert 1 galaxy may be reflection-dominated     

A. C.Fabian  D. R.Ballantyne  A.Merloni  S.Vaughan  K.Iwasawa  Th.Boller 《Monthly notices of the Royal Astronomical Society》2002,331(3):L35-L39

A model for the inner regions of accretion flows is presented where, owing to disc instabilities, cold and dense material is clumped into deep sheets or rings. Surrounding these density enhancements is hot, tenuous gas where coronal dissipation processes occur. We expect this situation to be most relevant when the accretion rate is close to Eddington and the disc is radiation-pressure-dominated, and so may apply to narrow-line Seyfert 1 (NLS1) galaxies. In this scenario, the hard X-ray source is obscured for most observers, and so the detected X-ray emission would be dominated by reflection off the walls of the sheets. A simple Comptonization calculation shows that the large photon-indices characteristic of NLS1s would be a natural outcome of two reprocessors closely surrounding the hard X-ray source. We test this model by fitting the XMM-Newton spectrum of the NLS1 1H  0707–495  between 0.5 and 11 keV with reflection-dominated ionized disc models. A very good fit is found with three different reflectors each subject to the same  Γ=2.35  power law. An iron overabundance is still required to fit the sharp drop in the spectrum at around 7 keV. We note that even a small corrugation of the accretion disc may result in  Γ>2  and a strong reflection component in the observed spectrum. Therefore, this model may also explain the strength and the variability characteristics of the MCG–6-30-15 Fe K α line. The idea needs to be tested with further broad-band XMM-Newton observations of NLS1s.  相似文献   

Evidence for an intermediate-mass black hole and a multi-zone warm absorber in NGC 4395     

D. C. Shih  K. Iwasawa  A. C. Fabian 《Monthly notices of the Royal Astronomical Society》2003,341(3):973-980

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The lack of variability of the iron line in MCG–6-30-15: general relativistic effects     

G. Miniutti  A. C. Fabian  R. Goyder  A. N. Lasenby 《Monthly notices of the Royal Astronomical Society》2003,344(2):L22-L26

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A deep Chandra observation of the cluster environment of the  z= 1.786  radio galaxy 3C 294     

A. C. Fabian  J. S. Sanders  C. S. Crawford  S. Ettori 《Monthly notices of the Royal Astronomical Society》2003,341(3):729-738

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Ariel 1118-61 — A very close binary system or a slowly rotating neutron star?     

A. C. Fabian  J. E. Pringle  R. F. Webbink 《Astrophysics and Space Science》1976,42(1):161-164

The transient X-ray source Ariel 1118-61 has a period of 6.75 min. We review possible models for the X-ray source and in particular we consider orbital and rotational origins for the periodicity. Finally we discuss the possible identification of Ariel 1181-61 with the Mira-type variable RS Cen.Paper presented at the COSPAR Symposium on Fast Transients in X- and Gamma-Rays, held at Varna, Bulgaria, 29–31 May, 1975.  相似文献   

Non-Continuation of Integrals of the Rotating Two-body Problem in Hill's Lunar Problem     

Fabian Josef Winterberg  Efi Meletlidou 《Celestial Mechanics and Dynamical Astronomy》2004,88(1):37-49

We consider Hill's lunar problem as a perturbation of the integrable two-body problem. For this we avoid the usual normalization in which the angular velocity of the rotating frame of reference is put equal to unity and consider as the perturbation parameter. We first express the Hamiltonian H of Hill's lunar problem in the Delaunay variables. More precisely we deduce the expressions of H along the orbits of the two-body problem. Afterwards with the help of the conserved quantities of the planar two-body problem (energy, angular momentum and Laplace–Runge–Lenz vector) we prove that Hill's lunar problem does not possess a second integral of motion, independent of H, in the sense that there exist no analytic continuation of integrals, which are linear functions of in the rotating two-body problem. In connection with the proof of this main result we give a further restrictive statement to the nonintegrability of Hill's lunar problem.  相似文献   

Sediment cascades and the entangled relationship between human impact and natural dynamics at the pre-pottery Neolithic site of Göbekli Tepe,Anatolia     

Moritz Nykamp  Fabian Becker  Ricarda Braun  Nadja Pöllath  Daniel Knitter  Joris Peters  Brigitta Schütt 《地球表面变化过程与地形》2021,46(2):430-442

This study presents a meta-analysis of radiocarbon ages for the environs of Göbekli Tepe – one of the oldest monumental structures worldwide – using cumulative probability functions to diachronically assess phases of geomorphodynamic activity as controlled by natural or anthropogenic drivers. We employ sediment cascades as a heuristic framework to study the complex responses of the geomorphological system to various triggers at local to supra-regional scales. Possible triggers include climatic variability as documented by supra-regional hydroclimatic proxy data, regional demographic trends, and local to regional socioeconomic developments such as the emergence of sedentism or the introduction and dispersal of livestock herding. Our results show that phases of intensified geomorphodynamic activity occurred between ca. 7.4–7.0 and 5.8–3.3 ka BP. These phases roughly coincide with phases of population growth in southern Turkey and climatic variations in Turkey and the Levant. The phase between ca. 5.8–3.3 ka BP also corresponds to the time when organized agriculture and the seeder plough were introduced. Also, the identified phases are in agreement with the general trend of varying geomorphodynamic activity in the Eastern Mediterranean as driven by human impact and climatic change. However, neither the Younger Dryas–Holocene transition nor the development of herding during the Pre-Pottery Neolithic left a clear signature. We demonstrate how the different depositional environments in the studied landscape compartments vary with respect to their spatiotemporal coverage and discuss challenges when trying to understand processes that once shaped landscapes of past societies. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd  相似文献   

Improved constraints on dark energy from Chandra X-ray observations of the largest relaxed galaxy clusters     

S. W. Allen  D. A. Rapetti  R. W. Schmidt  H. Ebeling  R. G. Morris  A. C. Fabian 《Monthly notices of the Royal Astronomical Society》2008,383(3):879-896

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Tropospheric refractivity and zenith path delays from least-squares collocation of meteorological and GNSS data   总被引：1，自引：0，他引：1  

Karina Wilgan  Fabian Hurter  Alain Geiger  Witold Rohm  Jarosław Bosy 《Journal of Geodesy》2017,91(2):117-134

Precise positioning requires an accurate a priori troposphere model to enhance the solution quality. Several empirical models are available, but they may not properly characterize the state of troposphere, especially in severe weather conditions. Another possible solution is to use regional troposphere models based on real-time or near-real time measurements. In this study, we present the total refractivity and zenith total delay (ZTD) models based on a numerical weather prediction (NWP) model, Global Navigation Satellite System (GNSS) data and ground-based meteorological observations. We reconstruct the total refractivity profiles over the western part of Switzerland and the total refractivity profiles as well as ZTDs over Poland using the least-squares collocation software COMEDIE (Collocation of Meteorological Data for Interpretation and Estimation of Tropospheric Pathdelays) developed at ETH Zürich. In these two case studies, profiles of the total refractivity and ZTDs are calculated from different data sets. For Switzerland, the data set with the best agreement with the reference radiosonde (RS) measurements is the combination of ground-based meteorological observations and GNSS ZTDs. Introducing the horizontal gradients does not improve the vertical interpolation, and results in slightly larger biases and standard deviations. For Poland, the data set based on meteorological parameters from the NWP Weather Research and Forecasting (WRF) model and from a combination of the NWP model and GNSS ZTDs shows the best agreement with the reference RS data. In terms of ZTD, the combined NWP-GNSS observations and GNSS-only data set exhibit the best accuracy with an average bias (from all stations) of 3.7 mm and average standard deviations of 17.0 mm w.r.t. the reference GNSS stations.  相似文献