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1.
The fluctuating-accretion model of Lyubarskii and its extension by Kotov, Churazov & Gilfanov seek to explain the spectral-timing properties of the X-ray variability of accreting black holes in terms of inward-propagating mass accretion fluctuations produced at a broad range of radii. The fluctuations modulate the X-ray emitting region as they move inwards and can produce temporal-frequency-dependent lags between energy bands, and energy-dependent power spectral densities (PSDs) as a result of the different emissivity profiles, which may be expected at different X-ray energies. Here, we use a simple numerical implementation to investigate in detail the X-ray spectral-timing properties of the model and their relation to several physically interesting parameters, namely the emissivity profile in different energy bands, the geometrical thickness and viscosity parameter of the accretion flow, the strength of damping on the fluctuations and the temporal coherence (measured by the 'quality factor', Q ) of the fluctuations introduced at each radius. We find that a geometrically thick flow with large viscosity parameter is favoured, and we confirm that the predicted lags are quite robust to changes in the emissivity profile and physical parameters of the accretion flow, which may help to explain the similarity of the lag spectra in the low/hard and high/soft states of Cyg X-1. We also demonstrate the model regime where the light curves in different energy bands are highly spectrally coherent. We compare model predictions directly to X-ray data from the narrow line Seyfert 1 galaxy NGC 4051 and the black hole X-ray binary (BHXRB) Cyg X-1 in its high/soft state, and we show that this general scheme can reproduce simultaneously the time lags and energy-dependence of the PSD.  相似文献   

2.
We present ISOPHOT observations of eight interstellar regions in the 60–200 μm wavelength range. The regions belong to mostly quiescent high-latitude clouds and have optical extinction peaks from   AV ∼1–6 mag  . From the 150- and 200-μm emission, we derived colour temperatures for the classical big grain component which show a clear trend of decreasing temperature with increasing 200-μm emission. The 200-μm emission per unit   AV   , however, does not drop at lower temperatures. This fact can be interpreted in terms of an increased far-infrared (FIR) emissivity of the big grains. We developed a two-component model including warm dust with the temperature of the diffuse interstellar medium (ISM) of   T = 17.5 K  , and cold dust with   T = 13.5 K  and FIR emissivity increased by a factor of >4. A mixture of the two components can reproduce the observed colour variations and the ratios   I 200/ AV   and  τ200/ AV   . The relative abundance of small grains with respect to the big grains shows significant variations from region to region at low column densities. However, in lines of sight of higher column density, our data indicate the disappearance of small grains, perhaps a signature of adsorption/coagulation of dust. The larger size and porous structure could also explain the increased FIR emissivity. Our results from eight independent regions suggest that these grains might be ubiquitous in the galactic ISM.  相似文献   

3.
We are carrying out a project to calculate power spectra of variability, assuming a model of a ‘spotted’ accretion disc near a black hole. We consider relativistic effects that change photon energy and produce light‐bending and time‐delays acting on the X‐ray signal received by an observer. We assume that the life‐time and the intrinsic emissivity of individual.aring events are described in terms of a simple stochastic process. This allows us to give approximate analytical formulae and compare them with numerical computations. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

4.
Previous calculations of Fe K line profiles are based on axisymmetric emissivity laws. In this paper, we show line profiles driven by non-axial symmetric illumination which results from an off-axis X-ray point source. We find that source location and motion have significant effects on the red wing and blue horn of the line profiles. The disc region under the source will receive more flux, which is the most important factor to affect the line profiles. We suggest that at least part of the variation in Fe K line profiles is caused by the motion of X-ray sources. Future observations of Fe K line profiles will provide more information about the distribution and motion of the X-ray sources around black holes, and hence the underlying physics.  相似文献   

5.
In Paper I we presented a methodology to recover the spatial variations of properties of the intracluster gas from ASCA X-ray satellite observations of galaxy clusters. We verified the correctness of this procedure by applying it to simulated cluster data sets that we had subjected to the various contaminants common in ASCA data. In this paper we present the results that we obtain when we apply this method to real galaxy cluster observations. We determine broad-band temperature and cooling-flow mass-deposition rates for the 106 clusters in our sample, and obtain temperature, abundance and emissivity profiles (i.e., at least two annular bins) for 98 of these clusters. We find that 90 per cent of these temperature profiles are consistent with isothermality at the 3 σ confidence level. This conflicts with the prevalence of steeply declining cluster temperature profiles found by Markevitch et al. from a sample of 30 clusters.  相似文献   

6.
研究了磁场对奇异星模型中夸克直接Urca过程的中微子能量损失率的影响,首先改进了弱场条件下的近似计算方法,这一方法可以推广到其他弱作用过程．在甚强磁场下,严格地计算Urca过程的中微子能量损失率,结果显示辐射率强烈地依赖于磁场,与磁场的二次方成正比,更重要的是对温度的依赖关系不同于弱场及没有磁场时的情形．  相似文献   

7.
We present a comprehensive treatment of the spectrum of electric dipole emission from spinning dust grains, updating the commonly used model of Draine & Lazarian. Grain angular velocity distributions are computed using the Fokker–Planck equation; we revisit the drift and diffusion coefficients for the major torques on the grain, including collisions, grain-plasma interactions and infrared emission. We use updated grain optical properties and size distributions. The theoretical formalism is implemented in the companion code, spdust , which is publicly available. The effect of some environmental and grain parameters on the emissivity is shown and analysed.  相似文献   

8.
Recent analyses of Newton-XMM and Chandra data of the cores of X-ray bright clusters of galaxies show that modelling with a multi-phase gas in which several temperatures and densities are in equilibrium might not be appropriate. Instead, a single-phase model seems able to reproduce properly the spectra collected in annuli from the central region. The measured single-phase temperature profiles indicate a steep positive gradient in the central  100–200 kpc  and the gas density shows a flat profile in the central few 10s of kpc. Given this observational evidence, we estimate the contribution to the projected-on-the-sky rings from the cluster emissivity as function of the shell volume fraction sampled. We show that the observed projected X-ray emission mimics the multi-phase status of the plasma even though the input distribution is single-phase. This geometrical projection affects (i) analyses of data where insufficient spatial resolution is accessible, (ii) the central bin when its dimension is comparable to the extension of any flatness in the central gas density profile.  相似文献   

9.
We studied the evolution of isolated strange stars (SSs) synthetically, considering the influence of r -mode instability. Our results show that the cooling of SSs with non-ultrastrong magnetic fields is delayed by heating due to r -mode damping for millions of years, while the spin-down of the stars is dominated by gravitational radiation (GR). Especially for the SSs in a possible existing colour–flavour locked (CFL) phase, the effect of r -mode instability on the evolution of stars becomes extremely important because the viscosity, neutrino emissivity and specific heat involving pairing quarks are blocked. It leads to the cooling of these colour superconducting stars being very slow and the stars can remain at high temperature for millions of years, which differs completely from previous understanding. In this case, an SS in CFL phase can be located at the bottom of its r -mode instability window for a long time, but does not spin-down to a very low frequency for hours.  相似文献   

10.
Synchro-curvature radiation describes the emission from a relativistic charged par- ticle which is moving and spiralling in a curved magnetic field. We investigate the maser emission for synchro-curvature radiation including drift of the guiding center of the radiating electron. It is shown that under some conditions the absorption coefficient can be negative, so maser can happen. These conditions are different from those needed for maser emission of curvature radiation including drift of the charged particles. We point out that our results, in- cluding the emissivity, can reduce to these of curvature radiation. Previously it was found that synchro-curvature radiation can not generate maser in vacuum, but we argue that synchro- curvature radiation including drift can generate maser even in vacuum. We discuss the possi- bilities of the potential applications of the synchro-curvature maser in modeling gamma ray bursts and pulsars.  相似文献   

11.
We estimate the evolution of the contribution of galaxies to the cosmic background flux at 912 Å by means of a semi-analytic model of galaxy formation and evolution. Such modelling has been quite successful in reproducing the optical properties of galaxies. We assume that high-redshift damped Lyman α systems are the progenitors of present-day galaxies, and we design a series of models that are consistent with the evolution of cosmic comoving emissivities in the available near-infrared, optical, ultraviolet and far-infrared bands along with the evolution of the neutral hydrogen content and average metallicity of damped Lyman α systems. We use these models to compute the galactic contribution to the Lyman-limit emissivity and background flux for 0 ≃  z  ≤ 4. We take into account the absorption of Lyman-limit photons by H  I and dust in the interstellar medium of the galaxies. We find that the background Lyman-limit flux due to galaxies might dominate (or be comparable to) the contribution from quasars at almost all redshifts if the absorption by H  I in the interstellar medium is neglected. Such H  I absorption would result in a severe diminishing of this flux — by almost three orders of magnitude at high redshifts and by one to two orders at z  ≃ 0. Though the resulting galaxy flux is completely negligible at high redshifts, it is comparable to the quasar flux at z  ≃ 0.  相似文献   

12.
The inner parts of black‐hole accretion discs shine in X‐rays which can be monitored and the observed spectra can be used to trace strong gravitational fields in the place of emission and along paths of light rays. This paper summarizes several aspects of how the spectral features are influenced by relativistic effects. We focus our attention onto variable and broad emission lines, origin of which can be attributed to the presence of orbiting patterns – spots and spiral waves in the disc. We point out that the observed spectrum can determine parameters of the central black hole provided the intrinsic local emissivity is constrained by theoretical models. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

13.
In this work, we present a comprehensive X-ray picture of the interaction between a super star cluster and the interstellar medium. In order to do that, we compare and combine the X-ray emission from the superwind driven by the cluster with the emission from the wind-blown bubble. Detailed analytical models for the hydrodynamics and X-ray luminosity of fast polytropic superwinds are presented. The superwind X-ray luminosity models are an extension of the results obtained in Paper I. Here, the superwind polytropic character allows us to parametrize a wide variety of effects, for instance, radiative cooling. Additionally, X-ray properties that are valid for all bubble models taking thermal evaporation into account are derived. The final X-ray picture is obtained by calculating analytically the expected surface brightness and weighted temperature of each component. All of our X-ray models have an explicit dependence on metallicity and admit general emissivities as functions of the hydrodynamical variables. We consider a realistic X-ray emissivity that separates the contributions from hydrogen and metals. The paper ends with a comparison of the models with observational data.  相似文献   

14.
We report the results of the Cornell Mid-IR Asteroid Spectroscopy (MIDAS) survey, a program of ground-based observations designed to characterize the 8-13 μm spectral properties of a statistically significant sample of asteroids from a wide variety of visible to near-IR spectral classes. MIDAS is conducted at Palomar Observatory using the Spectrocam-10 (SC-10) spectrograph on the 200-in Hale telescope. We have measured the mid-infrared spectra of twenty-nine asteroids and have derived temperature estimates from our data that are largely consistent with the predictions of the standard thermal model. We have also generated relative emissivity spectra for the target asteroids. On only one asteroid, 1 Ceres, have we found emissivity features with spectral contrast greater than 5%. Our spectrum of 4 Vesta suggests emissivity variation at the 2-3% level. Published spectra of several of the small number of asteroids observed with ISO (six of which are also included in our survey), which appeared to exhibit much stronger emissivity features, are difficult to reconcile with our measurements. Laboratory work on mineral and meteorite samples has shown that the contrast of mid-IR spectral features is greatly reduced at fine grain sizes. Moreover, the NEAR mission found that 433 Eros is covered by a relatively thick fine-grained regolith. If small bodies in general possess such regoliths, their mid-IR spectral features may be quite subtle. This may explain the evident absence of strong emissivity variation in the majority of the MIDAS spectra.  相似文献   

15.
We have investigated the ionization structure of the post-shock regions of magnetic cataclysmic variables, using an analytic density and temperature structure model in which effects caused by bremsstrahlung and cyclotron cooling are considered. We find that in the majority of the shock-heated region where H- and He-like lines of the heavy elements are emitted, the collisional-ionization and corona-condition approximations are justified. We have calculated the line emissivity and ionization profiles for iron as a function of height within the post-shock flow. For low-mass white dwarfs, line emission takes place near the shock. For high-mass white dwarfs, most of the line emission takes place in regions well below the shock and hence it is less sensitive to the shock temperature. Thus, the line ratios are useful to determine the white dwarf masses for the low-mass white dwarfs, but the method is less reliable when the white dwarfs are massive. Line spectra can, however, be used to map the hydrodynamic structure of the post-shock accretion flow.  相似文献   

16.
We exploit observations at 1.25 mm with the ESO–SEST telescope of a southern galaxy sample, selected from the IRAS PSC and complete to S 60=2 Jy, to derive the FIR and mm luminosity functions and the conditional probability distributions of FIR and mm luminosity of galaxies. The reliability of these estimates is ensured by the good observed correlation of the far-infrared and mm emissions. This detailed knowledge of the millimetric properties of galaxies is used to simulate the extragalactic sub-mm sky (background intensity, small-scale anisotropy signals and discrete source statistics), which is the target of a variety of ground-based and space observatories. We find, in particular, that a recent tentative detection of a sub-mm background would require, if confirmed, strong evolution with cosmic time of the galaxy long-wavelength emissivity. We finally discuss ways to test such evolution with present and forthcoming facilities: while emphasizing the difficulty of achieving this with large mm telescopes on the ground (because of the poor atmospheric conditions of current sub-mm sites), we mention an interesting opportunity with the long-wavelength camera on ISO . Preliminary results of deep surveys, both from space and from the ground, seem indeed to require excess emission in the past by dusty galaxies with respect to no-evolution predictions.  相似文献   

17.
Deep SCUBA observations of NGC 1275 at 450 and 850 μm along with the application of deconvolution algorithms have permitted us to separate the strong core emission in this galaxy from the fainter extended emission around it. The core has a steep spectral index and is likely caused primarily by the active galactic nucleus. The faint emission has a positive spectral index and is clearly caused by extended dust in a patchy distribution out to a radius of ∼20 kpc from the nucleus. These observations have now revealed that a large quantity of dust, ∼     (two orders of magnitude larger than that inferred from previous optical absorption measurements), exists in this galaxy. We estimate the temperature of this dust to be ∼20 K (using an emissivity index of     and the gas/dust ratio to be 360. These values are typical of spiral galaxies. The dust emission correlates spatially with the hot X-ray emitting gas, which may be a result of collisional heating of broadly distributed dust by electrons. As the destruction time-scale is short, the dust cannot be replenished by stellar mass loss and must be externally supplied, via either the infalling galaxy or the cooling flow itself.  相似文献   

18.
We have conducted a search for emissivity features in the thermal infrared spectrum of the icy satellites of Saturn, Phoebe, Iapetus, Enceladus, Tethys, and Hyperion, observed by the Composite Infrared Spectrometer (CIRS) on board the Cassini spacecraft. Despite the heterogeneity of the composition of these bodies depicted by Earth-based and Cassini/VIMS observations, the CIRS spectra of all satellites are undistinguishable from black-body spectra, with no detectable emissivity feature. However, several materials, which have been detected on the surface of the same bodies, present emissivity features in the analyzed spectral range. In particular, water ice presents features with sufficient contrast to be detected by CIRS. Here we study the physical causes of the absence of features by simulating the effects of intimate mixtures using models of directional emissivity for optically thick surfaces for different particle sizes and abundances, and porosities. The simulations include a set of materials detected on the Phoebe's surface, like water ice, hydrated silicates, and organics. We find that featureless spectra can be produced in three scenarios: (1) ice particles with large sizes, (2) mixtures of ices dominated by dark contaminants, and (3) small particles with large porosity. Constraints imposed by the NIR spectra of the satellites favors the latter scenario as the more likely explanation to the absence of emissivity features on the icy satellites of Saturn.  相似文献   

19.
Spectroscopy at 8–13 μm with T-ReCS on Gemini-S is presented for three galaxies with substantial silicate absorption features, NGC 3094, NGC 7172 and NGC 5506. In the galaxies with the deepest absorption bands, the silicate profile towards the nuclei is well represented by the emissivity function derived from the circumstellar emission from the red supergiant, μ Cephei which is also representative of the mid-infrared absorption in the diffuse interstellar medium in the Galaxy. There is spectral structure near 11.2 μm in NGC 3094 which may be due to a component of crystalline silicates. In NGC 5506, the depth of the silicate absorption increases from north to south across the nucleus, suggestive of a dusty structure on scales of tens of parsecs. We discuss the profile of the silicate absorption band towards galaxy nuclei and the relationship between the 9.7-μm silicate and 3.4-μm hydrocarbon absorption bands.  相似文献   

20.
We present results from a multiwavelength study of 29 sources (false detection probabilities <5 per cent) from a survey of the Great Observatories Origins Deep Survey-North (GOODS-N) field at 1.1 mm using the Astronomical Thermal Emission Camera (AzTEC). Comparing with existing 850 μm Submillimetre Common-User Bolometer Array (SCUBA) studies in the field, we examine differences in the source populations selected at the two wavelengths. The AzTEC observations uniformly cover the entire survey field to a 1σ depth of ∼1 mJy. Searching deep 1.4 GHz Very Large Array (VLA) and Spitzer 3–24 μm catalogues, we identify robust counterparts for 21 1.1 mm sources, and tentative associations for the remaining objects. The redshift distribution of AzTEC sources is inferred from available spectroscopic and photometric redshifts. We find a median redshift of   z = 2.7  , somewhat higher than   z = 2.0  for  850 μm  selected sources in the same field, and our lowest redshift identification lies at a spectroscopic redshift   z = 1.1460  . We measure the 850 μm to 1.1 mm colour of our sources and do not find evidence for '850 μm dropouts', which can be explained by the low signal-to-noise ratio of the observations. We also combine these observed colours with spectroscopic redshifts to derive the range of dust temperatures T , and dust emissivity indices β for the sample, concluding that existing estimates   T ∼ 30 K  and  β∼ 1.75  are consistent with these new data.  相似文献   

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