Where are the X-ray quasi-periodic oscillations in active galaxies?     

S. Vaughan  P. Uttley 《Monthly notices of the Royal Astronomical Society》2005,362(1):235-244

In this paper, we address the question of whether existing X-ray observations of Seyfert galaxies are sufficiently sensitive to detect quasi-periodic oscillations (QPOs) similar to those observed in the X-ray variations of Galactic black holes (GBHs). We use data from XMM–Newton and simulated data based on the best Rossi X-ray Timing Explorer ( RXTE ) long-term monitoring light curves to show that if X-ray QPOs are present in Seyfert X-ray light curves – with similar shapes and strengths to those observed in GBHs, but at lower frequencies commensurate with their larger black hole masses – they would be exceedingly difficult to detect. Our results offer a simple explanation for the present lack of QPO detections in Seyferts. We discuss the improvements in telescope size and monitoring patterns needed to make QPO detections feasible. The most efficient type of future observatory for searching for X-ray QPOs in active Galactic nuclei (AGN) is an X-ray All-Sky Monitor (ASM). A sufficiently sensitive ASM would be ideally suited to detect low-frequency QPOs in nearby AGN. The detection of AGN QPOs would strengthen the AGN–GBH connection, and could serve as powerful diagnostics of the black hole mass and the structure of the X-ray emitting region in AGN.  相似文献   

The exceptional X-ray variability of the dwarf Seyfert nucleus NGC 4395     

S. Vaughan  K. Iwasawa  A. C. Fabian  K. Hayashida 《Monthly notices of the Royal Astronomical Society》2005,356(2):524-530

An analysis of the X-ray variability of the low-luminosity Seyfert nucleus NGC 4395, based on a long XMM–Newton observation, is presented. The power spectrum shows a clear break from a flat spectrum  (α≈ 1)  to a steeper spectrum  (α≈ 2)  at a frequency   f _br= 0.5–3.0 × 10⁻³ Hz  , comparable to the highest characteristic frequency found previously in a Seyfert galaxy. This extends the measured   M _BH− f _br  values to lower M _BH than previous studies of Seyfert galaxies, and is consistent with an inverse scaling of variability frequency with black hole mass. The variations observed are among the most violent seen in an active galactic nuclei to date, with the fractional rms amplitude  ( F _var)  exceeding 100 per cent in the softest band. The amplitude of the variations seems intrinsically higher in NGC 4395 than most other Seyfert galaxies, even after accounting for the differences in characteristic frequencies. The origin of this difference is not clear, but it is unlikely to be a high accretion rate (   L / L _Edd≲ 20  per cent for NGC 4395). The variations clearly follow the linear rms–flux relation, further supporting the idea that this is a ubiquitous characteristics of accreting black holes. The variations are highly coherent between different energy bands with any frequency-dependent time delay limited to ≲1 per cent.  相似文献   

Antithetic fault linkages in a deep water fold and thrust belt   总被引：4，自引：0，他引：4  

Simon Higgins  Richard J. Davies  Benjamin Clarke   《Journal of Structural Geology》2007,29(12):1900-1914

Deep water fold and thrust belts consist of both forethrusts and backthrusts that can link along strike to form continuous folds in the overburden. The interaction of faults of opposing dip are termed ‘antithetic thrust fault linkages’ and share the common feature of a switch in vergence of overlying hangingwall anticlines. Using three-dimensional seismic data, on the toe-of-slope of the Niger Delta, linkages are classified into three distinct structural styles. This preliminary classification is based on the vertical extent of faulting within a transfer zones relative to the branch line of the antithetic faults. The stratigraphic level of the lateral tip of the fault, the shape of lateral tip region of a fault plane and the stratal deformation within the transfer zones is also distinctive in each type of fault linkage. A Type 1 linkage comprises faults that overlap exclusively above the level of the branch line. A ‘pop-up’ structure forms within the transfer zone with sediments below remaining planar. The lower tip lines of faults climb stratigraphically towards the linkage zone creating asymmetric, upward-tapering lateral tip regions. In Type 2 linkages fault overlap occurs lower than the level of the branch line such that lateral fault tips are located within the footwall of the counterpart fault. Faulting is thus limited to the deeper section within the transfer zone and creates unfaulted, symmetric, bell-shaped folds in the overburden. Upper tip lines of faults lose elevation within the transfer zone creating asymmetric, downwards-tapering lateral tip regions. In Type 3 linkages both faults continue above and below the branch line within the transfer zone resulting in cross-cutting fault relationships. Horizon continuity across the folds, through the transfer zones, varies significantly with depth and with the type of fault intersection.  相似文献   

A lunar core and the moon's magnetic field     

G. C. Kennedy  G. H. Higgins 《Earth, Moon, and Planets》1975,12(4):401-406

The suggestion that the Moon's magnetic field is due to adiabatic magnetohydrodynamic convection of a molten core has been made by a number of recent authors. Considerations based on petrology, mass and rotational inertia limit the size of this hypothetical core to a few 100 km at the most. A proposal has been made that this core is either molten iron or iron sulfide. Fortunately, we know the properties of both molten iron and iron-sulfide at lunar core pressures. We can find no way of maintaining circulation in a hypothetical lunar core, as circulation is contingent upon a temperature gradient being greater than the adiabatic gradient, or an internal heat source.  相似文献   

Temperature gradients at the core-mantle interface     

Kennedy  G. C.  Higgins  G. H. 《Earth, Moon, and Planets》1973,7(1-2):14-21

Heat flowing out of the core must flow into the mantle. If the Earth's magnetic field is owing to adiabatic magnetohydrodynamic circulation of the outer core, whole mantle convection or melting at the core mantle boundary is required to keep the inner core from becoming isothermal, thereby preventing adiabatic circulation.Alternatively, the outer core fluid must have some unexpected and exotic property such as an extremely low coefficient of thermal expansion and resultant low Gruneisen's parameter.Paper dedicated to Professor Harold C. Urey on the occasion of his 80th birthday on 29 April 1973. Publication No. 1046 Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Calif., U.S.A.  相似文献   

Publications     

Clifford S. Russell  William J. Vaughan 《Marine Policy》1977,1(2):174-176

  相似文献   

Introduction: Professor Lance Thomas’s 70th Birthday     

G. Vaughan 《Annales Geophysicae》2000,18(7):729-729

  相似文献   

Letter to the editor A strange cloud in the Arctic summer stratosphere 1998 above Esrange (68°N), Sweden     

J. Siebert  C. Timmis  G. Vaughan  K. H. Fricke 《Annales Geophysicae》2000,18(4):505-509

When the University of Bonn lidar on the Esrange (68°N, 21°E), Sweden, was switched on in the evening of July 18, 1998, a geometrically and optically thin cloud layer was present near 14 km altitude or 400 K potential temperature, where it persisted for two hours. The tropopause altitude was 4 km below the cloud altitude. The cloud particles depolarized the lidar returns, thus must they have been aspherical and hence solid. Atmospheric temperatures near 230 K were approximately 40 K too high to support ice particles at stratospheric water vapour pressures of a few ppmv. The isentropic back trajectory on 400 K showed the air parcels to have stayed clear of active major rocket launch sites. The air parcels at 400 K had travelled from the Aleutians across Canada and the Atlantic Ocean arriving above central Europe and then turned northward to pass over above the lidar station. Parcels at levels at ±25 K from 400 K had come from the pole and joined the 400 K trajectory path above eastern Canada. Apparently the cloud existed in a filament of air with an origin different from those filaments both above and below. Possibly the 400 K level air parcels had carried soot particles from forest wild fires in northern Canada or volcanic ash from the eruption of the Korovin Volcano in the Aleutian Islands.  相似文献   

Earth,Wind, and Fire: Wildfire Risk Perceptions in a Hurricane-Prone Environment     

Soren M. Newman  Matthew S. Carroll  Pamela J. Jakes  Daniel R. Williams  Lorie L. Higgins 《社会与自然资源》2014,27(11):1161-1176

Wildfire is one of several potential disturbances that could have extraordinary impacts on individuals and communities in fire-prone areas. In this article we describe disturbance risk perceptions from interviews with residents in three Florida communities that face significant wildfire and hurricane risk. Although they live in areas characterized by emergency managers as having high wildfire risk and many participants have direct experience with wildfire, residents tended to share high hurricane and low wildfire risk perceptions. The different perceptions of risk seem linked to several factors: direct hurricane experience, different scales of impact, the local “hurricane culture,” effectiveness of local ordinances and development patterns, perceived predictability of the event, and perceived ability to control the event. This study shows that residents may perceive and act to reduce risk for one disturbance in relation to their perceptions, concern, and actions for another.  相似文献   

Exploring high-end scenarios for local sea level rise to develop flood protection strategies for a low-lying delta—the Netherlands as an example   总被引：4，自引：1，他引：3  

Caroline A. Katsman  A. Sterl  J. J. Beersma  H. W. van den Brink  J. A. Church  W. Hazeleger  R. E. Kopp  D. Kroon  J. Kwadijk  R. Lammersen  J. Lowe  M. Oppenheimer  H. -P. Plag  J. Ridley  H. von Storch  D. G. Vaughan  P. Vellinga  L. L. A. Vermeersen  R. S. W. van de Wal  R. Weisse 《Climatic change》2011,109(3-4):617-645

Sea level rise, especially combined with possible changes in storm surges and increased river discharge resulting from climate change, poses a major threat in low-lying river deltas. In this study we focus on a specific example of such a delta: the Netherlands. To evaluate whether the country’s flood protection strategy is capable of coping with future climate conditions, an assessment of low-probability/high-impact scenarios is conducted, focusing mainly on sea level rise. We develop a plausible high-end scenario of 0.55 to 1.15 m global mean sea level rise, and 0.40 to 1.05 m rise on the coast of the Netherlands by 2100 (excluding land subsidence), and more than three times these local values by 2200. Together with projections for changes in storm surge height and peak river discharge, these scenarios depict a complex, enhanced flood risk for the Dutch delta.  相似文献