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1.
We consider the possibility that the excess of cosmic rays near ∼1018 eV, reported by the AGASA and SUGAR groups from the direction of the Galactic Centre, is caused by a young, very fast pulsar in the high-density medium. The pulsar accelerates iron nuclei to energies ∼1020 eV, as postulated by the Galactic models for the origin of the highest-energy cosmic rays. The iron nuclei, about 1 yr after pulsar formation, leave the supernova envelope without energy losses and diffuse through the dense central region of the Galaxy. Some of them collide with the background matter creating neutrons (from disintegration of Fe), neutrinos and gamma-rays (in inelastic collisions). We suggest that neutrons produced at a specific time after the pulsar formation are responsible for the observed excess of cosmic rays at ∼1018 eV. From normalization of the calculated neutron flux to the one observed in the cosmic ray excess, we predict the neutrino and gamma-ray fluxes. It has been found that the 1 km2 neutrino detector of the IceCube type should detect from a few up to several events per year from the Galactic Centre, depending on the parameters of the considered model. Moreover, future systems of Cherenkov telescopes (CANGAROO III, HESS, VERITAS) should be able to observe  1–10 TeV  gamma-rays from the Galactic Centre if the pulsar was created inside a huge molecular cloud about  3–10×103 yr  ago.  相似文献   

2.
The CTA will mean a significant increase of the potential for dark matter detection, compared to present-day detectors like MAGIC, HESS and VERITAS. In particular, if – as it might be indicated from early LHC results – the dark matter sector is heavy, perhaps in the TeV mass range, imaging air Cherenkov arrays have a good opportunity to detect γ-rays from dark matter annihilation in the galactic halo, the galactic center, dwarf galaxies, or galaxy clusters. A review of the present situation is given and a few of the “miracles” that may enhance chances for detection in CTA are discussed, such as Sommerfeld enhancement and internal bremsstrahlung radiation. A few templates for dark matter are studied, and the importance of the acceptance of the detector at low energies is pointed out. Finally, the idea of a complement to CTA in the form of a high-altitude, low energy threshold dedicated dark matter array, DMA, is discussed.  相似文献   

3.
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4.
Astrophysical limits on massive dark matter   总被引：1，自引：0，他引：1  
Annihilations of weakly interacting dark matter particles provide an important signature for the possibility of indirect detection of dark matter in galaxy haloes. These self-annihilations can be greatly enhanced in the vicinity of a massive black hole. We show that the massive black hole present at the centre of our galaxy accretes dark matter particles, creating a region of very high particle density. Consequently the annihilation rate is considerably increased, with a large number of e+e− pairs being produced either directly or by successive decays of mesons. We evaluate the synchrotron emission (and self-absorption) associated with the propagation of these particles through the galactic magnetic field, and are able to constrain the allowed values of masses and cross sections of dark matter particles.  相似文献   

5.
We present stellar radial velocity data for the Draco dwarf spheroidal (dSph) galaxy obtained using the AF2/WYFFOS instrument combination on the William Herschel Telescope. Our data set consists of 186 member stars, 159 of which have good quality velocities, extending to a magnitude   V ≈19.5  with a mean velocity precision of ≈2 km s−1. As this survey is based on a high-precision photometric target list, it contains many more Draco members at large radii. For the first time, this allows a robust determination of the radial behaviour of the velocity dispersion in a dSph.
We find statistically strong evidence of a rising velocity dispersion consistent with a dark matter halo that has a gently rising rotation curve. There is a <2 σ signature of rotation about the long axis, inconsistent with tidal disruption as the source of the rising dispersion. By comparing our data set with earlier velocities, we find that Draco probably has a binary distribution and fraction comparable to those in the solar neighbourhood.
We apply a novel maximum likelihood algorithm and fit the velocity data to a two parameter spherical model with an adjustable dark matter content and velocity anisotropy. Draco is best fit by a weakly tangentially anisotropic distribution of stellar orbits in a dark matter halo with a very slowly rising rotation law  ( v circ∝ r 0.17)  . We are able to rule out both a mass-follows-light distribution and an extended halo with a harmonic core at the 2.5 to 3 σ significance level, depending on the details of our assumptions about Draco's stellar binary population. Our modelling lends support to the idea that the dark matter in dwarf spheroidals is distributed in the form of massive, nearly isothermal haloes.  相似文献   

6.
Possible orbital histories of the Sgr dwarf galaxy are explored. A special-purpose N -body code is used to construct the first models of the Milky Way–Sgr dwarf system in which both the Milky Way and the Sgr dwarf are represented by full N -body systems and followed for a Hubble time. These models are used to calibrate a semi-analytic model of the Sgr dwarf's orbit that enables us to explore a wider parameter space than is accessible to the N -body models. We conclude that the extant data on the Sgr dwarf are compatible with a wide range of orbital histories. At one extreme the Sgr dwarf initially possesses ∼1011 M⊙ and starts from a Galactocentric distance R D(0)≳200 kpc. At the other extreme the Sgr dwarf starts with ∼109 M⊙ and R D(0)∼60 kpc, similar to its present apocentric distance. In all cases the Sgr dwarf is initially dark matter dominated and the current velocity dispersion of the Sgr dwarf's dark matter is tightly constrained to be 21±2 km s−1. This number is probably compatible with the smaller measured dispersion of the Sgr dwarf's stars because of (i) the dynamical difference between dark and luminous matter, and (ii) velocity anisotropy.  相似文献   

7.
Using eight dark matter haloes extracted from fully self-consistent cosmological N -body simulations, we perform microlensing experiments. A hypothetical observer is placed at a distance of 8.5 kpc from the centre of the halo measuring optical depths, event durations and event rates towards the direction of the Large Magellanic Cloud. We simulate 1600 microlensing experiments for each halo. Assuming that the whole halo consists of massive astronomical compact halo objects (MACHOs),   f = 1.0  , and a single MACHO mass is   m M= 1.0 M⊙  , the simulations yield mean values of  τ= 4.7+5.0−2.2× 10−7  and  Γ= 1.6+1.3−0.6× 10−6  events star−1 yr−1. We find that triaxiality and substructure can have major effects on the measured values so that τ and Γ values of up to three times the mean can be found. If we fit our values of τ and Γ to the MACHO collaboration observations, we find   f = 0.23+0.15−0.13  and   m M= 0.44+0.24−0.16  . Five out of the eight haloes under investigation produce f and m M values mainly concentrated within these bounds.  相似文献   

8.
The 'Carina Flare' supershell, GSH 287+04−17, is a molecular supershell originally discovered in  12CO( J = 1–0)  with the NANTEN 4 m telescope. We present the first study of the shell's atomic ISM, using H  i 21-cm line data from the Parkes 64-m telescope Southern Galactic Plane Survey. The data reveal a gently expanding,  ∼230 × 360  pc H  i supershell that shows strong evidence of Galactic Plane blowout, with a break in its main body at   z ∼ 280  pc and a capped high-latitude extension reaching   z ∼ 450  pc. The molecular clouds form comoving parts of the atomic shell, and the morphology of the two phases reflects the supershell's influence on the structure of the ISM. We also report the first discovery of an ionized component of the supershell, in the form of delicate, streamer-like filaments aligned with the proposed direction of blowout. The distance estimate to the shell is re-examined, and we find strong evidence to support the original suggestion that it is located in the Carina Arm at a distance of  2.6 ± 0.4 kpc  . Associated H  i and H2 masses are estimated as   M H I≈ 7 ± 3 × 105 M⊙  and     , and the kinetic energy of the expanding shell as   E K ∼ 1 × 1051  erg. We examine the results of analytical and numerical models to estimate a required formation energy of several 1051 to  ∼1052  erg, and an age of  ∼107 yr  . This age is compatible with molecular cloud formation time-scales, and we briefly consider the viability of a supershell-triggered origin for the molecular component.  相似文献   

9.
We present a new and completely general technique for calculating the fine-grained phase-space structure of dark matter (DM) throughout the Galactic halo. Our goal is to understand this structure on the scales relevant for direct and indirect detection experiments. Our method is based on evaluating the geodesic deviation equation along the trajectories of individual DM particles. It requires no assumptions about the symmetry or stationarity of the halo formation process. In this paper we study general static potentials which exhibit more complex behaviour than the separable potentials studied previously. For ellipsoidal logarithmic potentials with a core, phase mixing is sensitive to the resonance structure, as indicated by the number of independent orbital frequencies. Regions of chaotic mixing can be identified by the very rapid decrease in the real-space density of the associated DM streams. We also study the evolution of stream-density in ellipsoidal NFW haloes with radially varying isopotential shape, showing that if such a model is applied to the Galactic halo, at least 105 streams are expected near the Sun. The most novel aspect of our approach is that general non-static systems can be studied through implementation in a cosmological N -body code. Such an implementation allows a robust and accurate evaluation of the enhancements in annihilation radiation due to fine-scale structure such as caustics. We embed the scheme in the current state-of-the-art code gadget -3 and present tests which demonstrate that N -body discreteness effects can be kept under control in realistic configurations.  相似文献   

10.
In this paper, we study the angular momentum properties of simulated dark matter haloes at high redshifts that likely host the first stars in the Universe. Calculating the spin distributions of these  106– 107 M⊙  haloes in redshift slices from   z = 15  to 6, we find that they are well fit by a lognormal distribution as is found for lower redshift and more massive haloes in earlier work. We find that both the mean value of the spin and dispersion are largely unchanged with redshift for all haloes. Our key result is that subsamples of low- and high-spin, 106 and  107 M⊙  , haloes show difference in clustering strength. In both mass bins, higher spin haloes are more strongly clustered in concordance with a tidal torquing picture for the growth of angular momentum in dark matter haloes in the cold dark matter paradigm.  相似文献   

11.
Analytic solutions for the evolution of cosmological linear density perturbations in the baryonic gas and collisionless dark matter are derived. The solutions are expressed in a closed form in terms of elementary functions, for arbitrary baryonic mass fraction. They are obtained assuming =1 and a time-independent comoving Jeans wavenumber, k J. By working with a time variable ln( t 2/3), the evolution of the perturbations is described by linear differential equations with constant coefficients. The new equations are then solved by means of Laplace transformation, assuming that the gas and dark matter trace the same density field before a sudden heating epoch. In a dark matter-dominated Universe, the ratio of baryonic to dark matter density perturbation decays with time roughly as exp(5 /4) t 5/6 to the limiting value 1/[1+( k k J)2]. For wavenumbers the decay is accompanied by oscillations with a period in . In comparison, as increases in a baryonic matter-dominated Universe, the ratio approaches 1( k k J)2 for k k J, and zero otherwise.  相似文献   

12.
We determine the velocity distribution and space density of a volume-complete sample of A and F stars, using parallaxes and proper motions from the Hipparcos satellite. We use these data to solve for the gravitational potential vertically in the local Galactic disc, by comparing the Hipparcos measured space density with predictions from various disc models. We derive an estimate of the local dynamical mass density of 0.102±0.010 pc−3, which may be compared with an estimate of 0.095 M⊙ pc−3 in visible disc matter. Our estimate is found to be in reasonable agreement with other estimates by Crézé et al. and Pham, also based on Hipparcos data. We conclude that there is no compelling evidence for significant amounts of dark matter in the disc.  相似文献   

13.
We discuss the possibility of observing the products of the dark matter annihilation that was going on in the early Universe. Of all the particles that could be generated by this process, we consider only photons, as they are both uncharged and easily detectable. The younger the Universe was, the higher the dark matter concentration n and the annihilation rate (proportional to n 2) were. However, the emission from the very early Universe cannot reach us because of the opacity. The main part of the signal was generated at the moment the Universe had just become transparent for the photons produced by the annihilation. Thus, the dark matter annihilation in the early Universe should have created a sort of relic emission. We obtain its flux and the spectrum.
If weakly interacting massive particles (WIMPs) constitute dark matter, it is shown that we may expect an extragalactic gamma-ray signal in the energy range 0.5–20 MeV with a maximum near 8 MeV. We show that an experimentally observed excess in the gamma-ray background at 0.5–20 MeV could be created by the relic signal from the annihilation of WIMPs only if the dark matter structures in the Universe had appeared before the Universe became transparent for the annihilation products  ( z ≃ 300)  . We discuss in more detail physical conditions whereby this interpretation could be possible.  相似文献   

14.
We describe a new implementation of a parallel TreeSPH code with the aim of simulating galaxy formation and evolution. The code has been parallelized using shmem , a Cray proprietary library to handle communications between the 256 processors of the Silicon Graphics T3E massively parallel supercomputer hosted by the Cineca Super-computing Center (Bologna, Italy). 1
The code combines the smoothed particle hydrodynamics (SPH) method for solving hydrodynamical equations with the popular Barnes & Hut tree-code to perform gravity calculation with an N ×log  N scaling, and it is based on the scalar TreeSPH code developed by Carraro et al. Parallelization is achieved by distributing particles along processors according to a workload criterion.
Benchmarks, in terms of load balance and scalability, of the code are analysed and critically discussed against the adiabatic collapse of an isothermal gas sphere test using 2×104 particles on 8 processors. The code results balance at more than the 95 per cent level. Increasing the number of processors, the load balance slightly worsens. The deviation from perfect scalability for increasing number of processors is almost negligible up to 32 processors. Finally, we present a simulation of the formation of an X-ray galaxy cluster in a flat cold dark matter cosmology, using 2×105 particles and 32 processors, and compare our results with Evrard's P3M–SPH simulations.
Additionally we have incorporated radiative cooling, star formation, feedback from SNe of types II and Ia, stellar winds and UV flux from massive stars, and an algorithm to follow the chemical enrichment of the interstellar medium. Simulations with some of these ingredients are also presented.  相似文献   

15.
We study the structure of a stationary and axisymmetric charge-deficient region (or a potential gap) in the outer magnetosphere of a spinning neutron star. A large electric field along the magnetic field lines is created in this potential gap and accelerates migratory electrons (e−) and/or positrons (e+) to ultrarelativistic energies. Assuming that the gap is immersed in a dense soft photon field, these relativistic e± radiate γ -ray photons via inverse Compton (IC) scattering. These γ -rays, in turn, produce yet more radiating particles by colliding with ambient soft photons, leading to a pair-production cascade in the gap. The replenished charges partially screen the longitudinal electric field, which is self-consistently solved together with the distribution of e± and γ -ray photons. It is demonstrated that the voltage drop in the gap is not more than 1010 V when the background X-ray radiation is as luminous as 1037 erg s−1. However, this value increases with decreasing X-ray luminosity and attains 1012 V when the X-ray radiation is 1036 erg s−1. In addition, we find useful expressions of the spatial distribution of the particle fluxes and longitudinal electric field, together with the relationship between the voltage drop and the current density. Amazingly, these expressions are valid not only when IC scattering dominates but also when curvature radiation dominates.  相似文献   

16.
In regions of very high dark matter density such as the Galactic Centre, the capture and annihilation of WIMP dark matter by stars has the potential to significantly alter their evolution. We describe the dark stellar evolution code D ark S tars , and present a series of detailed grids of WIMP-influenced stellar models for main-sequence stars. We describe the changes in stellar structure and main-sequence evolution which occur as a function of the rate of energy injection by WIMPs, for masses of  0.3–2.0 M⊙  and metallicities   Z = 0.0003–0.02  . We show what rates of energy injection can be obtained using realistic orbital parameters for stars at the Galactic Centre, including detailed consideration of the velocity and density profiles of dark matter. Capture and annihilation rates are strongly boosted when stars follow elliptical rather than circular orbits. If there is a spike of dark matter induced by the supermassive black hole at the Galactic Centre, single solar mass stars following orbits with periods as long as 50 yr and eccentricities as low as 0.9 could be significantly affected. Binary systems with similar periods about the Galactic Centre could be affected on even less eccentric orbits. The most striking observational effect of this scenario would be the existence of a binary consisting of a low-mass protostar and a higher mass evolved star. The observation of low-mass stars and/or binaries on such orbits would either provide a detection of WIMP dark matter, or place stringent limits on the combination of the WIMP mass, spin-dependent nuclear-scattering cross-section, halo density and velocity distribution near the Galactic Centre. In some cases, the derived limits on the WIMP mass and spin-dependent nuclear-scattering cross-section would be of comparable sensitivity to current direct-detection experiments.  相似文献   

17.
Some massive binaries should contain energetic pulsars which inject relativistic leptons from their inner magnetospheres and/or pulsar wind regions. If the binary system is compact enough, then these leptons can initiate inverse Compton (IC) e± pair cascades in the anisotropic radiation field of a massive star. γ-rays can be produced in the IC cascade during its development in a pulsar wind region and above a shock in a massive star wind region where the propagation of leptons is determined by the structure of a magnetic field around the massive star. For a binary system with specific parameters, we calculate phase-dependent spectra and fluxes of γ-rays escaping as a function of the inclination angle of the system and for different assumptions on injection conditions of the primary leptons (their initial spectra and location of the shock inside the binary). We conclude that the features of γ-ray emission from such massive binaries containing energetic pulsars should allow us to obtain important information on the acceleration of particles by the pulsars, and on interactions of a compact object with the massive star wind. Predicted γ-ray light curves and spectra in the GeV and TeV energy ranges from such binary systems within our Galaxy and Magellanic Clouds should be observed by future AGILE and GLAST satellites and low-threshold Cherenkov telescopes, such as MAGIC, HESS, VERITAS or CANGAROO III.  相似文献   

18.
We present a comprehensive multiwavelength temporal and spectral analysis of the 'fast rise exponential decay' GRB 070419A. The early-time emission in the γ-ray and X-ray bands can be explained by a central engine active for at least 250 s, while at late times the X-ray light curve displays a simple power-law decay. In contrast, the observed behaviour in the optical band is complex (from 102 up to 106 s). We investigate the light-curve behaviour in the context of the standard forward/reverse shock model; associating the peak in the optical light curve at ∼450 s with the fireball deceleration time results in a Lorenz factor  Γ≈ 350  at this time. In contrast, the shallow optical decay between 450 and 1500 s remains problematic, requiring a reverse shock component whose typical frequency is above the optical band at the optical peak time for it to be explained within the standard model. This predicts an increasing flux density for the forward shock component until   t ∼ 4 × 106 s  , inconsistent with the observed decay of the optical emission from   t ∼ 104 s  . A highly magnetized fireball is also ruled out due to unrealistic microphysic parameters and predicted light-curve behaviour that is not observed. We conclude that a long-lived central engine with a finely tuned energy injection rate and a sudden cessation of the injection is required to create the observed light curves, consistent with the same conditions that are invoked to explain the plateau phase of canonical X-ray light curves of γ-ray bursts.  相似文献   

19.
We constrain the distance of the gamma-ray burst (GRB) prompt emission site from the explosion centre R , by determining the location of the electron's self-absorption frequency in the GRB prompt optical-to-X/γ-ray spectral energy distribution, assuming that the optical and the γ-ray emissions are among the same synchrotron radiation continuum of a group of hot electrons. All possible spectral regimes are considered in our analysis. The method has only two assumed parameters, namely the bulk Lorentz factor of the emitting source Γ and the magnetic field strength B in the emission region (with a weak dependence). We identify a small sample of four bursts that satisfy the following three criteria: (1) they all have simultaneous optical and γ-ray detections in multiple observational time intervals, (2) they all show temporal correlations between the optical and γ-ray light curves and (3) the optical emission is consistent with belonging to the same spectral component as the γ-ray emission. For all the time intervals of these four bursts, it is inferred that   R ≥ 1014  (Γ/300)3/4 ( B /105 G)1/4  cm. For a small fraction of the sample, the constraint can be pinned down to   R ≈ 1014–1015 cm  for  Γ∼ 300  . For a second sample of bursts with prompt optical non-detections, only upper limits on R can be obtained. We find no inconsistency between the R -constraints for this non-detection sample and those for the detection sample.  相似文献   

20.
Assuming that the dark matter is entirely made up of neutralinos, we re-visit the role of their annihilation on the temperature of diffuse gas in the high-redshift universe  ( z > 10)  , before the formation of luminous structures. We consider neutralinos of particle mass 36 and 100 GeV. The former is able to produce  ∼7  e − e +  particles per annihilation through the fremionic channel, and the latter ∼53 particles assuming a purely bosonic channel. High-energy   e − e +  particles up-scatter the cosmic microwave background (CMB) photons into higher energies via the inverse-Compton scattering. The process produces a power-law   e − e +  energy spectrum of index −1 in the energy range of interest, independent of the initial energy distribution. The corresponding energy spectrum of the up-scattered photons is a power law of index −1/2, if absorption by the gas is not included. The scattered photons photoheat the gas by releasing electrons which deposit a fraction (14 per cent) of their energy as heat into the ambient medium. For uniformly distributed neutralinos, the heating is insignificant. The effect is greatly enhanced by the clumping of neutralinos into dense haloes. We use a time-dependent clumping model which takes into account the damping of density fluctuations on mass-scales smaller than  ∼10−6 M⊙  . With this clumping model, the heating mechanism boosts the gas temperature above that of the CMB after a redshift of   z ∼ 30  . By   z ≈ 10  , the gas temperature is nearly 100 times its temperature when no heating is invoked. Similar increase is obtained for the two neutralino masses considered.  相似文献   

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