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   

Some effects of small-scale metallicity variations in cooling flows     

R. Glenn Morris  A. C. Fabian 《Monthly notices of the Royal Astronomical Society》2003,338(4):824-836

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Exciting molecular hydrogen in the central galaxies of cooling flows     

R. J. Wilman  A. C. Edge  R. M. Johnstone  A. C. Fabian  S. W. Allen  C. S. Crawford 《Monthly notices of the Royal Astronomical Society》2002,337(1):63-70

The origin of rovibrational H₂ emission in the central galaxies of cooling flow clusters is poorly understood. Here we address this issue using data from our near-infrared spectroscopic survey of 32 of the most line-luminous such systems, presented in the companion paper by Edge et al.
We consider excitation by X-rays from the surrounding intracluster medium (ICM), ultra-violet (UV) radiation from young stars, and shocks. The   v = 1–0  K -band lines with upper levels within  10⁴ K  of the ground state appear to be mostly thermalized (implying gas densities  ≳10⁵ cm⁻³  ), with the excitation temperature typically exceeding 2000 K, as found earlier by Jaffe, Bremer & van der Werf. Together with the lack of strong   v = 2–0  lines in the H -band, this rules out UV radiative fluorescence.
Using the cloudy photoionization code, we deduce that the H₂ lines can originate in a population of dense clouds, exposed to the same hot  ( T ∼ 50 000 K)  stellar continuum as the lower density gas which produces the bulk of the forbidden optical line emission in the Hα-luminous systems. This dense gas may be in the form of self-gravitating clouds deposited directly by the cooling flow, or may instead be produced in the high-pressure zones behind strong shocks. Furthermore, the shocked gas is likely to be gravitationally unstable, so collisions between the larger clouds may lead to the formation of globular clusters.  相似文献   

An X-ray absorption analysis of the high-velocity system in NGC 1275     

K. Gillmon  J. S. Sanders  A. C. Fabian 《Monthly notices of the Royal Astronomical Society》2004,348(1):159-164

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Chandra X-ray observations of the 3C 295 cluster core     

S.W. Allen  G.B. Taylor  P.E.J. Nulsen  R.M. Johnstone  L.P. David  S. Ettori  A.C. Fabian  W. Forman  C. Jones  B. McNamara 《Monthly notices of the Royal Astronomical Society》2001,324(4):842-858

We examine the properties of the X-ray gas in the central regions of the distant ( z =0.46) , X-ray luminous cluster of galaxies surrounding the powerful radio source 3C 295, using observations made with the Chandra Observatory . Between radii of 50 and 500 kpc, the cluster gas is approximately isothermal with an emission-weighted temperature, kT ∼5 keV . Within the central 50-kpc radius this value drops to kT ∼3.7 keV . The spectral and imaging Chandra data indicate the presence of a cooling flow within the central 50-kpc radius of the cluster, with a mass deposition rate of approximately 280 M_⊙ yr⁻¹. We estimate an age for the cooling flow of 1–2 Gyr , which is approximately 1000 times older than the central radio source. We find no evidence in the X-ray spectra or images for significant heating of the X-ray gas by the radio source. We report the detection of an edge-like absorption feature in the spectrum for the central 50-kpc region, which may be caused by oxygen-enriched dust grains. The implied mass in metals seen in absorption could have been accumulated by the cooling flow over its lifetime. Combining the results on the X-ray gas density profile with radio measurements of the Faraday rotation measure in 3C 295, we estimate the magnetic field strength in the region of the cluster core to be B ∼12 μG .  相似文献   

Stratospheric trace gas analysis from ions: H₂O and HNO₃     

F. Arnold  R. Fabian  G. Henschen  W. Joos 《Planetary and Space Science》1980,28(7):681-685

A new method for stratospheric trace gas analysis based upon in situ ion composition measurements is presented. The strength of the method lies in its extremely high sensitivity which at present allows for detection limits of the order of 100–1000 molecules cm^?3 corresponding to volume mixing ratios of the order of 10^?15–10^?14 around 35 km altitude. The typical uncertainty of derived trace gas abundances is plus or minus a factor of about two. A disadvantage lies in the selectivity of the method which is restricted to trace gases having either large proton affinities, dipole moments or gas phase acidities. Analyses of water vapor and nitric acid vapor in the upper stratosphere are presented.  相似文献   

The effect of SST on the stratospheric distribution of odd nitrogen     

Peter Fabian 《Pure and Applied Geophysics》1974,112(6):901-913

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On the determination of magnetic grain-size distributions of superparamagnetic particle ensembles using the frequency dependence of susceptibility at different temperatures   总被引：1，自引：0，他引：1  

Valera P. Shcherbakov  Karl Fabian 《Geophysical Journal International》2005,162(3):736-746

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Fuelling quasars with hot gas     

P. E. J. Nulsen  A. C. Fabian 《Monthly notices of the Royal Astronomical Society》2000,311(2):346-356

We consider a model for quasar formation in which massive black holes are formed and fuelled largely by the accretion of hot gas during the process of galaxy formation. In standard hierarchical collapse models, objects about the size of normal galaxies and larger form a dense hot atmosphere when they collapse. We show that if such an atmosphere forms a nearly 'maximal' cooling flow, then a central black hole can accrete at close to its Eddington limit. This leads to exponential growth of a seed black hole, resulting in a quasar in some cases. In this model, the first quasars form soon after the first collapses to produce hot gas. The hot gas is depleted as time progresses, mostly by cooling, so that the accretion rate eventually falls below the threshold for advection-dominated accretion, at which stage radiative efficiency plummets and any quasar turns off. A simple implementation of this model, incorporated into a semi-analytical model for galaxy formation, overproduces quasars when compared with observed luminosity functions, but is consistent with models of the X-ray background, which indicate that most accretion is obscured. It produces few quasars at high redshift owing to the lack of time needed to grow massive black holes. Quasar fuelling by hot gas provides a minimum level, sufficient to power most quasars at redshifts between one and two, to which other sources of fuel can be added. The results are sensitive to feedback effects, such as might result from radio jets and other outflows.  相似文献   

A physical model for the hard X-ray background     

R. J. Wilman  A. C. Fabian  P. E. J. Nulsen 《Monthly notices of the Royal Astronomical Society》2000,319(2):583-590

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