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arXiv2007

Greybody Factors for d-Dimensional Black Holes

Gravitational greybody factors are analytically computed for static, spherically symmetric black holes in d-dimensions, including black holes with charge and in the presence of a cosmological constant (where a proper definition of greybody factors for both asymptotically dS and AdS spacetimes is provided). This calculation includes both the low-energy case --where the frequency of the scattered wave is small and real-- and the asymptotic case --where the frequency of the scattered wave is very large along the imaginary axis-- addressing gravitational perturbations as described by the Ishibashi-Kodama master equations, and yielding full transmission and reflection scattering coefficients for all considered spacetime geometries. At low frequencies a general method is developed, which can be employed for all three types of spacetime asymptotics, and which is independent of the details of the black hole. For asymptotically dS black holes the greybody factor is different for even or odd spacetime dimension, and proportional to the ratio of the areas of the event and cosmological horizons. For asymptotically AdS black holes the greybody factor has a rich structure in which there are several critical frequencies where it equals either one (pure transmission) or zero (pure reflection, with these frequencies corresponding to the normal modes of pure AdS spacetime). At asymptotic frequencies the computation of the greybody factor uses a technique inspired by monodromy matching, and some universality is hidden in the transmission and reflection coefficients. For either charged or asymptotically dS black holes the greybody factors are given by non-trivial functions, while for asymptotically AdS black holes the greybody factor precisely equals one (corresponding to pure blackbody emission).

hep-th↗

arXiv2007

The end of the Galactic spectrum

We use a diffusion galactic model to analyze the end of the Galactic cosmic ray spectrum and its mixing with the extragalactic cosmic ray flux. We analyze the transition between Galactic and extragalactic components using two different extragalactic models. We compare the sum of the diffusive galactic spectrum and extragalactic spectrum with the available experimental data.

astro-ph↗

arXiv2007

Indirect signals from braneworlds

It has been suggested that our universe could be a 3-dimensional brane where the SM fields live embedded in a D-dimensional space-time. In flexible raneworlds, in addition to the SM fields, new degrees of freedom appear on the brane associated to brane fluctuations. These new fields, known as branons, are standard WIMPs (Weakly Interacting Massive Particles) and therefore natural dark matter candidates, whose spontaneous annihilations can provide first evidences for this scenario.

astro-ph↗

arXiv2007

Dark Matter Decaying Now

The instability of dark matter may produce visible signals in the spectrum of cosmic gamma-rays. We consider this possibility in frameworks with additional spatial dimensions and supersymmetry. Examples of particles include superweakly-interacting massive particles such as gravitinos in supersymmetry models, the lightest Kaluza-Klein (KK) state in models with universal extra dimensions, and weakly-interacting massive particles such as branons in flexible brane-worlds.

astro-ph↗

arXiv2007

Observational Constraints on Agegraphic Dark Energy

In this paper, we use the Type Ia supernova data as well as the CMB and LSS data to constrain the agegraphic dark energy model recently proposed by Cai. Due to its peculiar nature, the parameter $n$ of this model cannot be well constrained by the SNIa data, while the other parameter $Ω_{m0}$ can be constrained to be $0.34\pm0.04$. When combined with CMB and LSS data, the range of $1σ$ confidence level for $n$ is greatly narrowed, albeit still very large. The best fit result is $Ω_{m0}=0.28\pm0.02$, which is consistent with most observations like WMAP and SDSS, and $n=3.4$, of which a meaningful range of confidence level can not be obtained due to the fact that the contours are not closed. Despite of this result, we conclude that for $n>1$ this model is consistent with SNIa, CMB and LSS observations. Furthermore, the fitting results indicate a generalized definition for the agegraphic dark energy.

astro-ph↗

arXiv2007

Seven-color Vilnius photometry of the open cluster NGC 752

New photoelectric seven-color observations in the Vilnius system are presented for 65 stars in the region of the open cluster NGC 752. Based on individual stars with accurate photometric classifications, we determine the apparent distance modulus (m-M)=8.38 and the mean reddening to the cluster E(Y-V)=0.027, or E(B-V)=0.034. The mean photometric metallicity for the main-sequence stars, [Fe/H]=-0.14, is found to be slightly lower than that derived for the red clump giants, [Fe/H]=-0.08. This difference suggests that red giants in later evolutionary phases may not have zero-age surface values of [Fe/H]. We made use of the least-squares minimization techniques to fit the Padova theoretical isochrones to the CMD, when the binary star population is taken into account. By varying the distance modulus, metallicity and age, the best match has been found between the seven magnitudes and colors of the observed stars and those of model binaries, which gives the distance modulus by 0.2 mag smaller than that derived from individual stars, i.e., (m-M)=8.18, a closely similar metallicity ([Fe/H]=-0.12), and age of 1.6 Gyr. With these results, the fraction of photometric binaries among the main-sequence stars is ~40%.

astro-ph↗

arXiv2008

The pattern speeds of NGC 6946

We study the kinematics of the barred spiral galaxy NGC 6946 by investigating the velocity field from H-alpha Fabry-Perot observations, determined the pattern speed of the bar by using the Tremaine-Weinberg method, and find a main pattern speed of 21.7 (+4.0,-0.8) km/s/kpc. Our data clearly suggest the presence of an additional pattern with a pattern speed more than twice that of the large pattern in this galaxy. We use the epicycle approximation to deduce the location of the resonance radii and subsequently determine the pattern speed between the radii, and find that inside the inner Inner Lindblad Resonance radius, a bar-like system has evolved.

astro-ph↗

arXiv2007

Jet Interactions with the Hot Halos of Clusters and Galaxies

X-ray observations of cavities and shock fronts produced by jets streaming through hot halos have significantly advanced our understanding of the energetics and dynamics of extragalactic radio sources. Radio sources at the centers of clusters have dynamical ages between ten and several hundred million years. They liberate between 1E58-1E62 erg per outburst, which is enough energy to regulate cooling of hot halos from galaxies to the richest clusters. Jet power scales approximately with the radio synchrotron luminosity to the one half power. However, the synchrotron efficiency varies widely from nearly unity to one part in 10,000, such that relatively feeble radio source can have quasar-like mechanical power. The synchrotron ages of cluster radio sources are decoupled from their dynamical ages, which tend to be factors of several to orders of magnitude older. Magnetic fields and particles in the lobes tend to be out of equipartition. The lobes may be maintained by heavy particles (e.g., protons), low energy electrons, a hot, diffuse thermal gas, or possibly magnetic (Poynting) stresses. Sensitive X-ray images of shock fronts and cavities can be used to study the dynamics of extragalactic radio sources.

astro-ph↗

arXiv2007

Analytical description of the Day-Night neutrino asymmetry

We present a new treatment of the Earth matter effects on the neutrino oscillations that is valid for an arbitrary density profile. When applied to the the study of the day-night effect on the solar neutrino flux it renders us a simple analytical expression, which is more accurate than those derived by using the perturbation theory and can be extended to higher energies.

astro-ph↗

arXiv2007

Spin-down of compact stars and energy release of a first-order phase transition

The deconfinement phase transition from hadronic matter to quark matter can continuously occur during spins down of neutron stars. It will lead to the release of latent heat if the transition is the first-order one. We have investigated the energy release of such deconfinement phase transition for rotating hybrid stars model which include mixed phase of hadronic matter and quark matter. The release of latent heat per baryon is calculated through studying a randomly process of infinitesimal compressing. Finally, we can self-consistently get the heating luminosity of deconfinement phase transition by imputing the EOS of mixed phase, and based on the equation of rotation structure of stars.

astro-ph↗

arXiv2007

The Evolution of Galaxies

The evolution of galaxies results from a combination of internal and external processes. The star formation is an internal process transforming cold and dense cores of molecular clouds to stars. It may be triggered internally by expanding shells, or externally, e. g. by galaxy collisions. The gas accretion and galaxy merging events are external contributors to galaxy evolution. They compete with another internal process of galaxy evolution, which is the secular evolution redistributing the mass and angular momentum inside of galaxies as a consequence of bar and spiral arms formation. As a nearby example of gas accretion we mention the interacting system Milky Way - Large and Small Magellanic Clouds. Stripping of ISM in galaxy clusters is reviewed as another example of interaction of galaxies with their environment.

astro-ph↗

arXiv2008

Testing modified gravity with motion of satellites around galaxies

A modified gravity (MOG) theory, that has been successfully fitted to galaxy rotational velocity data, cluster data, the Bullet Cluster 1E0657-56 and cosmological observations, is shown to be in good agreement with the motion of satellite galaxies around host galaxies at distances 50-400 kpc.

astro-ph↗

arXiv2007

H.E.S.S. observations of galaxy clusters

Clusters of galaxies, the largest gravitationally bound objects in the universe, are expected to contain a significant population of hadronic and leptonic cosmic rays. Potential sources for these particles are merger and accretion shocks, starburst driven galactic winds and radio galaxies. Furthermore, since galaxy clusters confine cosmic ray protons up to energies of at least 1 PeV for a time longer than the Hubble time they act as storehouses and accumulate all the hadronic particles which are accelerated within them. Consequently clusters of galaxies are potential sources of VHE (> 100 GeV) gamma rays. Motivated by these considerations, promising galaxy clusters are observed with the H.E.S.S. experiment as part of an ongoing campaign. Here, upper limits for the VHE gamma ray emission for the Abell 496 and Coma cluster systems are reported.

astro-ph↗

arXiv2008

Cosmic rays from thermal sources

Energy spectrum of cosmic rays (CR) exhibits power-like behavior with very characteristic "knee" structure. We consider a generalized statistical model for the production process of cosmic rays which accounts for such behavior in a natural way either by assuming existence of temperature fluctuations in the source of CR, or by assuming specific temperature distribution of CR sources. Both possibilities yield the so called Tsallis statistics and lead to the power-like distribution. We argue that the "knee" structure arises as result of abrupt change of fluctuations in the source of CR. Its possible origin is briefly discussed.

astro-ph↗

arXiv2007

Radio detection of high-energy cosmic rays at the Pierre Auger Observatory

The southern Auger Observatory provides an excellent test bed to study the radio detection of extensive air showers as an alternative, cost-effective, and accurate tool for cosmic-ray physics. The data from the radio setup can be correlated with those from the well-calibrated baseline detectors of the Pierre Auger Observatory. Furthermore, human-induced radio noise levels at the southern Auger site are relatively low. We have started an R&D program to test various radio-detection concepts. Our studies will reveal Radio Frequency Interferences (RFI) caused by natural effects such as day-night variations, thunderstorms, and by human-made disturbances. These RFI studies are conducted to optimise detection parameters such as antenna design, frequency interval, antenna spacing and signal processing. The data from our initial setups, which presently consist of typically 3 - 4 antennas, will be used to characterise the shower from radio signals and to optimise the initial concepts. Furthermore, the operation of a large detection array requires autonomous detector stations. The current design is aiming at stations with antennas for two polarisations, solar power, wireless communication, and local trigger logic. The results of this initial phase will provide an important stepping stone for the design of a few tens kilometers square engineering array

astro-ph↗

arXiv2011

Specific Angular Momentum of Extrasolar Planetary Systems

As the number of known planetary systems increases, the ability to follow-up and characterize the extent of any system becomes limited. This paper considers the use of specific angular momentum as a metric to prioritize future observations. We analyze 431 planets in 367 known extrasolar planetary systems from Butler et al. (2006) (including updates to their online catalog, current to April, 2011) and estimate each system's orbital angular momentum. The range of partition- ing of specific angular momentum in these systems is found to be large, spanning several orders of magnitude. The analysis shows that multi-planet systems tend to have the highest values of specific angular momentum normalized against the planetary masses. This suggests that in high angular momentum systems, the dominant contributors have already been discovered, and that single-planet sys- tems with low observed angular momentum may be the most likely candidates for additional undiscovered companions compared to their high angular momentum, single-planet counterparts. The multi-planet system, GJ 581, is considered as a historical case study to demonstrate the concept, examining how the specific angular momentum of the know planetary system evolved with each discovery.

astro-ph↗

arXiv2007

Simulation Results for U(1) Gauge Theory on Non-Commutative Spaces

We present numerical results for U(1) gauge theory in 2d and 4d spaces involving a non-commutative plane. Simulations are feasible thanks to a mapping of the non-commutative plane onto a twisted matrix model. In d=2 it was a long-standing issue if Wilson loops are (partially) invariant under area-preserving diffeomorphisms. We show that non-perturbatively this invariance breaks, including the subgroup SL(2,R). In both cases, d=2 and d=4, we extrapolate our results to the continuum and infinite volume by means of a Double Scaling Limit. In d=4 this limit leads to a phase with broken translation symmetry, which is not affected by the perturbatively known IR instability. Therefore the photon may survive in a non-commutative world.

hep-lat↗

arXiv2007

Mapping the dark trajectory of the Bullet Cluster

We model the electrons/positrons produced by dark matter annihilations in the colliding galaxy cluster system 1E0657-56 (the bullet cluster). These charged particles, confined by the Magnetic field, clearly trace the path of the bullet, which passes through the main cluster with a speed of 3000-5000 km/s. Adding the effect of dark matter substructure (subhalos) in each cluster we find the annihilation rate is enhancedgreatly and the density of positrons in the trail is similar to that within the bullet cluster. This opens the door to unambiguous detection of the annihilation signal through the SZ effect, at significant separation from the thermal electrons.

astro-ph↗