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1.
On the escape of particles from cosmic ray modified shocks   总被引：1，自引：0，他引：1  
Stationary solutions to the problem of particle acceleration at shock waves in the non-linear regime, when the dynamical reaction of the accelerated particles on the shock cannot be neglected, are known to show a prominent energy flux escaping from the shock towards upstream infinity. On physical grounds, the escape of particles from the upstream region of a shock has to be expected in all those situations in which the maximum momentum of accelerated particles,   p max  , decreases with time, as is the case for the Sedov–Taylor phase of expansion of a shell supernova remnant, when both the shock velocity and the cosmic ray induced magnetization decrease. In this situation, at each time t , particles with momenta larger than   p max( t )  leave the system from upstream, carrying away a large fraction of the energy if the shock is strongly modified by the presence of cosmic rays. This phenomenon is of crucial importance for explaining the cosmic ray spectrum detected at the Earth. In this paper, we discuss how this escape flux appears in the different approaches to non-linear diffusive shock acceleration, and especially in the quasi-stationary semi-analytical kinetic ones. We apply our calculations to the Sedov–Taylor phase of a typical supernova remnant, including in a self-consistent way particle acceleration, magnetic field amplification and the dynamical reaction on the shock structure of both particles and fields. Within this framework, we calculate the temporal evolution of the maximum energy reached by the accelerated particles and of the escape flux towards upstream infinity. The latter quantity is directly related to the cosmic ray spectrum detected at the Earth.  相似文献   

2.
I study the behaviour of the maximum rms fractional amplitude, r max, and the maximum coherence, Q max, of the kilohertz quasi-periodic oscillations (kHz QPOs) in a dozen low-mass X-ray binaries. I find that (i) the maximum rms amplitudes of the lower- and upper-kHz QPOs,   r ℓmax  and   r umax  , respectively, decrease more or less exponentially with increasing luminosity of the source; (ii) the maximum coherence of the lower-kHz QPO,   Q ℓmax  , first increases and then decreases exponentially with luminosity, at a faster rate than both   r ℓmax  and   r umax  ; (iii) the maximum coherence of the upper-kHz QPO,   Q umax  , is more or less independent of luminosity; and (iv) r max and Q max show the opposite behaviour with hardness of the source, consistent with the fact that there is a general anticorrelation between luminosity and spectral hardness in these sources. Both r max and Q max in the sample of sources, and the rms amplitude and coherence of the kHz QPOs in individual sources show a similar behaviour with hardness. This similarity argues against the interpretation that the drop of coherence and rms amplitude of the lower-kHz QPO at high QPO frequencies in individual sources is a signature of the innermost stable circular orbit around a neutron star. I discuss possible interpretations of these results in terms of the modulation mechanisms that may be responsible for the observed variability.  相似文献   

3.
The plethora of recent cosmologically relevant data has indicated that our Universe is very well fitted by a standard Friedmann–Lemaître–Robertson–Walker (FLRW) model, with     and  ΩΛ≈ 0.73  – or, more generally, by nearly flat FLRW models with parameters close to these values. Additional independent cosmological information, particularly the maximum of the angular-diameter (observer area) distance and the redshift at which it occurs, would improve and confirm these results, once sufficient precise Type Ia supernovae data in the range  1.5 < z < 1.8  become available. We obtain characteristic FLRW-closed functional forms for   C = C ( z )  and     , the angular-diameter distance and the density per source counted, respectively, when  Λ≠ 0  , analogous to those we have for  Λ= 0  . More importantly, we verify that for flat FLRW models z max– as is already known but rarely recognized – the redshift of C max, the maximum of the angular-diameter distance, uniquely gives  ΩΛ  , the amount of vacuum energy in the universe, independent of H 0, the Hubble parameter. For non-flat models, determination of both z max and C max gives both  ΩΛ  and ΩM, the amount of matter in the universe, as long as we know H 0 independently. Finally, determination of C max automatically gives a very simple observational criterion for whether or not the universe is flat – presuming that it is FLRW.  相似文献   

4.
We present the BVR broad-band polarimetric observations of 51 stars belonging to the young open cluster IC 1805. Along with the photometric data from the literature, we have modelled and subtracted the foreground dust contribution from the maximum polarization ( P max) and colour excess [ E ( B − V )]. The mean value of the P max for intracluster medium and the foreground are found to be 5.008 ± 0.005 and 4.865 ± 0.022 per cent, respectively. Moreover, the mean value of the wavelength of maximum polarization (λmax) for intracluster medium is 0.541 ± 0.003 μ m, which is quite similar as the general interstellar medium (ISM). The resulting intracluster dust component is found to have negligible polarization efficiency as compared to interstellar dust. Some of the observed stars in IC 1805 have shown the indication of intrinsic polarization in their measurements.  相似文献   

5.
Star formation happens in a clustered way which is why the star cluster population of a particular galaxy is closely related to the star formation history of this galaxy. From the probabilistic nature of a mass function follows that the mass of the most massive cluster of a complete population, M max, has a distribution with the total mass of the population as a parameter. The total mass of the population is connected to the star formation rate (SFR) by the length of a formation epoch.
Since due to evolutionary effects only massive star clusters are observable up to high ages, it is convenient to use this M max(SFR) relation for the reconstruction of a star formation history. The age distribution of the most massive clusters can therefore be used to constrain the star formation history of a galaxy. The method, including an assessment of the inherent uncertainties, is introduced with this contribution, while following papers will apply this method to a number of galaxies.  相似文献   

6.
We present a numerical simulation of the bulk Lorentz factor of a relativistic electron–positron jet driven by the Compton rocket effect from accretion disc radiation. The plasma is assumed to have a power-law distribution n e(γ) ∝ γ− s with 1 < γ < γmax and is continuously reheated to compensate for radiation losses. We include the full Klein–Nishina (hereafter KN) cross-section, and study the role of the energy upper cut-off γmax, spectral index s and source compactness. We determine the terminal bulk Lorentz factor in the cases of supermassive black holes, relevant to AGN, and stellar black holes, relevant to galactic microquasars. In the latter case, Klein–Nishina cross-section effects are more important and induce a terminal bulk Lorentz factor smaller than in the former case. Our result are in good agreement with bulk Lorentz factors observed in Galactic (GRS 1915+105, GRO J1655−40) and extragalactic sources. Differences in scattered radiation and acceleration mechanism efficiency in the AGN environment can be responsible for the variety of relativistic motion in those objects. We also take into account the influence of the size of the accretion disc; if the external radius is small enough, the bulk Lorentz factor can be as high as 60.  相似文献   

7.
We present optical and near-infrared photometry and spectroscopy of the Type Ia SN 2003cg, which exploded in the nearby galaxy NGC 3169. The observations cover a period between −8.5 and +414 d post-maximum. SN 2003cg is a normal but highly reddened Type Ia event. Its B magnitude at maximum   B max= 15.94 ± 0.04  and  Δ m 15( B )obs= 1.12 ± 0.04 [Δ m 15( B )intrinsic= 1.25 ± 0.05]  . Allowing   RV   to become a free parameter within the Cardelli et al. extinction law, simultaneous matches to a range of colour curves of normal SNe Ia yielded   E ( B − V ) = 1.33 ± 0.11  , and   RV = 1.80 ± 0.19  . While the value obtained for   RV   is small, such values have been invoked in the past, and may imply a grain size which is small compared with the average value for the local interstellar medium.  相似文献   

8.
We use non-linear scaling relations (NSRs) to investigate the effects arising from the existence of negative correlations on the evolution of gravitational clustering in an expanding universe. It turns out that such anticorrelated regions have important dynamical effects on all scales. In particular, the mere existence of negative values for the linear two-point correlation function ξ¯ L over some range of scales starting from l = L 0 implies that the non-linear correlation function is bounded from above at all scales x < L 0 . This also results in the relation ξ¯   ∝  x −3 , at these scales, at late times, independent of the original form of the correlation function. Current observations do not rule out the existence of negative ξ¯ for 200  h −1 Mpc≲ ξ¯ ≲1000  h −1 Mpc; the present work may thus have relevance for the real Universe. The only assumption made in the analysis is the existence of NSR; the results are independent of the form of the NSR as well as of the stable clustering hypothesis.  相似文献   

9.
One of the methods discussed in deflecting the orbit of an Earth-colliding asteroid is the use of nuclear explosives. In assessing its feasibility, apart from political considerations, it is important to quantify how effective it is in orbit deflection. The transfer of radiation incident at the surface is governed by a non-linear diffusion equation. For low-yield explosions with a slab geometry ( S 0≃108 kJ μs−1), the temperature at depth x and time t is well approximated by a similarity solution of the form T ( x , t )= T 0 f (ξ), ξ= x/ ( T n 0 t )1/2, with T 0 given by ( S 0/σ)1/4, where σ is the Stefan–Boltzmann constant, n is an index that specifies the radiation transfer and f (ξ) is the solution of a non-linear differential equation subject to the condition f (0)=1 and limξ→∞ f (ξ)=0. For high-yield explosions ( S 0≃1010 kJ μs−1), numerical solutions to the non-linear diffusion equation can be obtained. These solutions have properties similar to the case of low-yield explosions. If the duration of the explosion is d ×10−8 s, where d is close to 3, the fraction of energy absorbed by the surface is found to be 7, 12 and 23 per cent for S 0=108, 109 and 1010 kJ μs−1 respectively.  相似文献   

10.
We use the spherical evolution approximation to investigate non-linear evolution from the non-Gaussian initial conditions characteristic of the local f nl model. We provide an analytic formula for the non-linearly evolved probability distribution function (PDF) of the dark matter which shows that the underdense tail of the non-linear PDF in the f nl model should differ significantly from that for Gaussian initial conditions. Measurements of the underdense tail in numerical simulations may be affected by discreteness effects, and we use a Poisson counting model to describe this effect. Once this has been accounted, our model is in good quantitative agreement with the simulations. In principle, our calculation is an important first step in programs which seek to reconstruct the shape of the initial PDF from observations of large-scale structures in the Lyα forest and the galaxy distribution at later times.  相似文献   

11.
We have computed optical absorption-line profiles of CH+ and CH, as predicted by a model of a C-type shock propagating in a diffuse interstellar cloud. Both these species are produced in the shock wave in the reaction sequence that is initiated by C+(H2, H)CH+. Whilst CH+ flows at the ion speed, CH, which forms in the dissociative recombination reaction CH+3(e−, H2)CH, flows at a speed which is intermediate between those of the ions and the neutrals. The predicted velocity shift between the CH+ and CH line profiles is found to be no more than approximately 2 km s−1, which is smaller than has previously been assumed. We also investigate OH and HCO+, finding that the correlation between their column densities, recently observed in the diffuse interstellar medium, can be reproduced by the model.  相似文献   

12.
We look for a non-Gaussian signal in the Wilkinson Microwave Anisotropy Probe ( WMAP ) 5-year temperature anisotropy maps by performing a needlet-based data analysis. We use the foreground-reduced maps obtained by the WMAP team through the optimal combination of the W, V and Q channels, and perform realistic non-Gaussian simulations in order to constrain the non-linear coupling parameter f NL. We apply a third-order estimator of the needlet coefficients skewness and compute the  χ2  statistics of its distribution. We obtain  −80 < f NL < 120  at 95 per cent confidence level, which is consistent with a Gaussian distribution and comparable to previous constraints on the non-linear coupling. We then develop an estimator of f NL based on the same simulations and we find consistent constraints on primordial non-Gaussianity. We finally compute the three-point correlation function in needlet space: the constraints on f NL improve to  −50 < f NL < 110  at 95 per cent confidence level.  相似文献   

13.
We test the consistency of estimates of the non-linear coupling constant f NL using non-Gaussian cosmic microwave background (CMB) maps generated by the method described in the work of Liguori, Matarrese & Moscardini. This procedure to obtain non-Gaussian maps differs significantly from the method used in previous works on the estimation of f NL. Nevertheless, using spherical wavelets, we find results in very good agreement with Mukherjee & Wang, showing that the two ways of generating primordial non-Gaussian maps give equivalent results. Moreover, we introduce a new method for estimating the non-linear coupling constant from CMB observations by using the local curvature of the temperature fluctuation field. We present both Bayesian credible regions (assuming a flat prior) and proper (frequentist) confidence intervals on f NL, and discuss the relation between the two approaches. The Bayesian approach tends to yield lower error bars than the frequentist approach, suggesting that a careful analysis of the different interpretations is needed. Using this method, we estimate   f NL=−10+270−260  at the 2σ level (Bayesian) and   f NL=−10+310−270  (frequentist). Moreover, we find that the wavelet and the local curvature approaches, which provide similar error bars, yield approximately uncorrelated estimates of f NL and therefore, as advocated in the work of Cabella et al., the estimates may be combined to reduce the error bars. In this way, we obtain   f NL=−5 ± 85  and   f NL=−5 ± 175  at the 1σ and 2σ level respectively using the frequentist approach.  相似文献   

14.
We consider a situation where the density and peculiar velocities in real space are linear, and we calculate ξ s , the two-point correlation function in redshift space, incorporating all non-linear effects which arise as a consequence of the map from real to redshift space. Our result is non-perturbative and it includes the effects of possible multi-streaming in redshift space. We find that the deviations from the predictions of the linear redshift distortion analysis increase for the higher spherical harmonics of ξ s . While the deviations are insignificant for the monopole ξ 0, the hexadecapole ξ 4 exhibits large deviations from the linear predictions. For a COBE normalized     ,     cold dark matter (CDM) power spectrum, our results for ξ 4 deviate from the linear predictions by a factor of two on the scale of ∼10  h −1 Mpc. The deviations from the linear predictions depend separately on f (Ω) and b . This holds the possibility of removing the degeneracy that exists between these two parameters in the linear analysis of redshift surveys which yields only     .
We also show that the commonly used phenomenological model, where the non-linear redshift two-point correlation function is calculated by convolving the linear redshift correlation function with an isotropic pair velocity distribution function, is a limiting case of our result.  相似文献   

15.
We calculate the secondary anisotropies in the cosmic microwave background (CMB) produced by inhomogeneous reionization from simulations in which the effects of radiative and stellar feedback effects on galaxy formation have been included. This allows us to determine self-consistently the beginning ( z i≈30), the duration ( δz ≈20) and the (non-linear) evolution of the reionization process for a critical density cold dark matter (CDM) model. In addition, from the simulated spatial distribution of ionized regions, we are able to calculate the evolution of the two-point ionization correlation function, C χ , and obtain the power spectrum of the anisotropies, C ℓ, in the range 5000<ℓ<106. The power spectrum has a broad maximum around ℓ≈30 000, where it reaches the value 2×10−12. We also show that the ionization correlation function C χ is not Gaussian, but at separation angles θ ≲10−4 rad it can be approximated by a modified Lorentzian shape; at larger separations an anticorrelation signal is predicted for both C χ and C ( θ ). Detection of signals as above will be possible with future millimetre-wavelength interferometers like the Atacama Large Millimeter Array (ALMA) , which appears as an optimum instrument to search for signatures of inhomogeneous reionization.  相似文献   

16.
We examine the possibility of the decay of the vacuum energy into a homogeneous distribution of a thermalized cosmic microwave background (CMB), which is characteristic of an adiabatic vacuum energy decay into photons. It is shown that observations of the primordial density fluctuation spectrum, obtained from CMB and galaxy distribution data, restrict the possible decay rate. When photon creation due to an adiabatic vacuum energy decay takes place, the standard linear temperature dependence   T ( z ) = T 0(1 + z )  is modified, where T 0 is the present CMB temperature, and can be parametrized by a modified CMB temperature dependence     . From the observed CMB and galaxy distribution data, a strong limit on the maximum value of the decay rate is obtained by placing a maximum value  βmax≃ 3.4 × 10−3  on the β parameter.  相似文献   

17.
Compact regions of enhanced HCO+ and NH3 emission have been detected close to a number of Herbig–Haro objects. An interpretation of these detections is the following: a transient clump within the molecular cloud has been irradiated by the shock that generates the Herbig–Haro object. The irradiation releases icy mantles from the grains within the transient clump and initiates a photochemistry. On the basis of this picture, we have developed an extensive chemical model which predicts that a wide range of species, other than NH3 and HCO+, should also be detectable. These include CH3OH, H2S, C3H4, H2CO, SO, SO2, H2CS and NS. The chemical effects should last ∼  104 yr  .  相似文献   

18.
We present indications of rotation in the galaxy cluster A2107 by a method that searches for the maximum gradient in the velocity field in a flat disc-like model of a cluster. Galaxies from cumulative subsamples containing more and more distant members from the cluster centre are projected on to an axis passing through the centre and we apply a linear regression model on the projected distances x and the line-of-sight velocities V . The axis with the maximum linear correlation coefficient   r max= max[ r ( V , x )]  defines the direction of the maximum velocity gradient, and consequently it presents the major axis of the apparently elliptical cluster. Because the effects of rotation are subtle, we put strong emphasis on the estimation of the uncertainties of the results by implementing different bootstrap techniques. We have found that rotational effects are more strongly expressed at distances of 0.26–0.54 Mpc from the cluster centre. The total virial mass of the cluster is  (3.2 ± 0.6) × 1014 M⊙  , while the virial mass, corrected for rotation, is  (2.8 ± 0.5) × 1014 M⊙  .  相似文献   

19.
We have found a bar of shocked molecular hydrogen (H2) towards the OH(1720 MHz) maser located at the projected intersection of supernova remnant (SNR)  G359.1–0.5  and the non-thermal radio filament known as the Snake. The H2 bar is well aligned with the SNR shell and almost perpendicular to the Snake. The OH(1720 MHz) maser is located inside the sharp western edge of the H2 emission, which is consistent with the scenario in which the SNR drives a shock into a molecular cloud at that location. The spectral line profiles of 12CO, HCO+ and CS towards the maser show broad-line absorption, which is absent in the 13CO spectra and most probably originates from the pre-shock gas. A density gradient is present across the region and is consistent with the passage of the SNR shock, while the H2 filament is located at the boundary between the pre-shock and post-shock regions.  相似文献   

20.
High-redshift galaxies and quasi-stellar objects (QSOs) are most likely to be strongly lensed by intervening haloes between the source and the observer. In addition, a large fraction of lensed sources is expected to be seen in the submillimetre region, as a result of the enhanced magnification bias on the steep intrinsic number counts. We extend in three directions Blain's earlier study of this effect.
First, we use a modification of the Press–Schechter mass function and detailed lens models to compute the magnification probability distribution. We compare the magnification cross-sections of populations of singular isothermal spheres and Navarro, Frenk & White (NFW) haloes and find that they are very similar, in contrast to the image-splitting statistics which were recently investigated in other studies. The distinction between the two types of density profile is therefore irrelevant for our purposes.
Secondly, we discuss quantitatively the maximum magnification, μ max, that can be achieved for extended sources (galaxies) with realistic luminosity profiles, taking into account the possible ellipticity of the lensing potential. We find that μ max plausibly falls into the range     for sources of     effective radius at redshifts within     .
Thirdly, we apply our model for the lensing magnification to a class of sources following the luminosity evolution typical for a unified scheme of QSO formation. As a result of the peculiar steepness of their intrinsic number counts, we find that the lensed source counts at a fiducial wave length of 850 μm can exceed the unlensed counts by several orders of magnitude at flux densities ≳100 mJy, even with a conservative choice of the maximum magnification.  相似文献   

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