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971.
Standard shot-noise models, which seek to explain the broadband noise variability that characterizes the X-ray light curves of X-ray binaries and active galaxies, predict that the power spectrum of the X-ray light curve is stationary (i.e. constant amplitude and shape) on short time-scales. We show that the broadband noise power spectra of the black hole candidate Cyg X-1 and the accreting millisecond pulsar SAX J1808.4−3658 are intrinsically non-stationary, in that rms variability scales linearly with flux. Flux-selected power spectra confirm that this effect is due to changes in power-spectral amplitude and not shape. The light curves of three Seyfert galaxies are also consistent with a linear relationship between rms variability and flux, suggesting that it is an intrinsic feature of the broadband noise variability in compact accreting systems over more than six decades of central object mass. The rms variability responds to flux variations on all measured time-scales, raising fundamental difficulties for shot-noise models which seek to explain this result by invoking variations in the shot parameters. We suggest that models should be explored where the longest time-scale variations are fundamental and precede the variations on shorter time-scales. Possible models which can explain the linear rms-flux relation include the fractal break-up of large coronal flares, or the propagation of fluctuations in mass accretion rate through the accretion disc. The linear relationship between rms variability and flux in Cyg X-1 and SAX J1808.4−3658 is offset on the flux axis, suggesting the presence of a second, constant-flux component to the light curve which contributes ∼25 per cent of the total flux. The spectrum of this constant component is similar to the total spectrum, suggesting that it may correspond to quiet, non-varying regions in the X-ray emitting corona. 相似文献
972.
A. Del Popolo 《Monthly notices of the Royal Astronomical Society》2001,325(3):1190-1196
I study the evolution of halo density profiles as a function of time in the SCDM and ΛCDM cosmologies. Following Del Popolo, I calculate the concentration parameter c = r v / a and study its time evolution. For a given halo mass, I find that c ( z ) ∝ 1/(1+ z ) in both the ΛCDM and SCDM cosmology, in agreement with the analytic model of Bullock et al. and N -body simulations. In both models, a ( z ) is roughly constant. The present model predicts a stronger evolution of c ( z ) with respect to the Navarro, Frenk & White model. Finally I show some consequences of the results on galaxy modelling. 相似文献
973.
The microlensing optical depth to Baade's Window constrains the minimum total mass in baryonic matter within the Solar circle to be greater than ∼ , assuming the inner Galaxy is barred with viewing angle ∼20°. From the kinematics of solar neighbourhood stars, the local surface density of dark matter is ∼ . We construct cuspy haloes normalized to the local dark matter density and calculate the circular-speed curve of the halo in the inner Galaxy. This is added in quadrature to the rotation curve provided by the stellar and ISM discs, together with a bar sufficiently massive so that the baryonic matter in the inner Galaxy reproduces the microlensing optical depth. Such models violate the observational constraint provided by the tangent-velocity data in the inner Galaxy (typically at radii . The high baryonic contribution required by the microlensing is consistent with implications from hydrodynamical modelling and the pattern speed of the Galactic bar. We conclude that the cuspy haloes favoured by the cold dark matter cosmology (and its variants) are inconsistent with the observational data on the Galaxy. 相似文献
974.
Shude Mao Hans J. Witt Leon V. E. Koopmans 《Monthly notices of the Royal Astronomical Society》2001,323(2):301-307
Recent observations indicate that many if not all galaxies host massive central black holes. In this paper we explore the influence of black holes on the lensing properties. We model the lens as an isothermal ellipsoid with a finite core radius plus a central black hole. We show that the presence of the black hole substantially changes the critical curves and caustics. If the black hole mass is above a critical value, then it will completely suppress the central images for all source positions. Realistic central black holes are likely to have masses below this critical value. Even in such subcritical cases, the black hole can suppress the central image when the source is inside a zone of influence, which depends on the core radius and black hole mass. In the subcritical cases, an additional image may be created by the black hole in some regions, which for some radio lenses may be detectable with high-resolution and large dynamic range VLBI maps. The presence of central black holes should also be taken into account when one constrains the core radius from the lack of central images in gravitational lenses. 相似文献
975.
A.J. Young † A.C. Fabian R.R. Ross Y. Tanaka 《Monthly notices of the Royal Astronomical Society》2001,325(3):1045-1052
The galactic black hole candidate Cygnus X-1 is observed to be in one of two X-ray spectral states: either the low/hard (low soft X-ray flux and a flat power-law tail) or high/soft (high blackbody-like soft X-ray flux and a steep power-law tail) state. The physical origin of these two states is unclear. We present here a model of an ionized accretion disc, the spectrum of which is blurred by relativistic effects, and fit it to the ASCA , Ginga and EXOSAT data of Cygnus X-1 in both spectral states. We confirm that relativistic blurring provides a much better fit to the low/hard state data and, contrary to some previous results, find the data of both states to be consistent with an ionized thin accretion disc with a reflected fraction of unity extending to the innermost stable circular orbit around the black hole. Our model is an alternative to those that, in the low/hard state, require the accretion disc to be truncated at a few tens of Schwarzschild radii, within which there is a Thomson-thin, hot accretion flow. We suggest a mechanism that may cause the changes in spectral state. 相似文献
976.
977.
通过编制区域性1:300万“青藏高原板块构造图”和1:50万“柴达木北缘板块构造地质图”的体会,并结合国内学者对这两份图件的审查意见及建议,重点讨论有关区域性板块构造图编制的理论与方法,供区域地质工作者参考。 相似文献
978.
979.
S. L. Bridle I. Zehavi A. Dekel O. Lahav M. P. Hobson A. N. Lasenby 《Monthly notices of the Royal Astronomical Society》2001,321(2):333-340
We compare and combine likelihood functions of the cosmological parameters Ωm , h and σ 8 , from peculiar velocities, cosmic microwave background (CMB) and type Ia supernovae. These three data sets directly probe the mass in the Universe, without the need to relate the galaxy distribution to the underlying mass via a 'biasing' relation. We include the recent results from the CMB experiments BOOMERANG and MAXIMA-1. Our analysis assumes a flat Λ cold dark matter (ΛCDM) cosmology with a scale-invariant adiabatic initial power spectrum and baryonic fraction as inferred from big-bang nucleosynthesis. We find that all three data sets agree well, overlapping significantly at the 2 σ level. This therefore justifies a joint analysis, in which we find a joint best-fitting point and 95 per cent confidence limits of (0.17,0.39), (0.64,0.86) and (0.98,1.37). In terms of the natural parameter combinations for these data (0.40,0.73), (0.16,0.27). Also for the best-fitting point, and the age of the Universe is 13.2 Gyr. 相似文献
980.
Alexander Knebe Julien E. G. Devriendt Asim Mahmood Joseph Silk 《Monthly notices of the Royal Astronomical Society》2002,329(4):813-828
Observations on galactic scales seem to be in contradiction with recent high-resolution N -body simulations. This so-called cold dark matter (CDM) crisis has been addressed in several ways, ranging from a change in fundamental physics by introducing self-interacting cold dark matter particles to a tuning of complex astrophysical processes such as global and/or local feedback. All these efforts attempt to soften density profiles and reduce the abundance of satellites in simulated galaxy haloes. In this paper, we explore a different approach that consists of filtering the dark matter power spectrum on small scales, thereby altering the formation history of low-mass objects. The physical motivation for damping these fluctuations lies in the possibility that the dark matter particles have a different nature, i.e. are warm (WDM) rather than cold. We show that this leads to some interesting new results in terms of the merger history and large-scale distribution of low-mass haloes, compared with the standard CDM scenario. However, WDM does not appear to be the ultimate solution, in the sense that it is not able to fully solve the CDM crisis, even though one of the main drawbacks, namely the abundance of satellites, can be remedied. Indeed, the cuspiness of the halo profiles still persists, at all redshifts, and for all haloes and sub-haloes that we investigated. Despite the persistence of the cuspiness problem of DM haloes, WDM seems to be still worth taking seriously, as it alleviates the problems of over-abundant sub-structures in galactic haloes and possibly the lack of angular momentum of simulated disc galaxies. WDM also lessens the need to invoke strong feedback to solve these problems, and may provide a natural explanation of the clustering properties and ages of dwarfs. 相似文献