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At least 595 records · Page 33Linked to original sources
arXiv2008

On the Very-High-Energy Gamma Ray Spectra from Typical Supernovae Remnants

Recently measured VHE gamma ray spectra from supernovae remnants (SNRs) are best fit by power laws with an exponential cutoff. But this feature does not occur at the `knee-equivalent' energy at which VHE gamma ray spectra are expected to reflect the `knee' in the otherwise featureless cosmic ray spectrum. In this article, the VHE gamma ray feature is explained as a consequence of a recently deduced quantum effect. The effect distinguishes `trajectory energy' from `particle state energy' and requires that the particle state energy depends strongly on gravitational potential at very high energies. Based on this effect and the observed CR spectrum, a tight two-parameter fit is obtained to the combined VHE gamma ray spectra of SNRs RX J1713.7-3946 and RX J0852.0-4622 and the Galactic Center ridge.

astro-ph↗
arXiv2008

Relativistic Flows in TeV Blazars

Rapid variability of the TeV emission in several blazars implies a central black hole mass $M_{BH}<10^8M_\odot$, appreciably smaller than the values estimated from the $M_{BH}-L_{bulg}$ relation, and Doppler factors for the $γ$-ray emitting fluid much larger than those associated with radio patterns. We discuss the conditions in the central engine required to account for the short timescales and large luminosities observed, and propose some explanations for the inferred kinematics of the source on various scales.

astro-ph↗
arXiv2008

Can the cosmological "constant" run? - It may run

Using standard quantum field theory, we discuss several theoretical aspects of the possible running of the cosmological constant (CC) term in Einstein's equations. The basic motivation for the present work is to emphasize that this possibility should also be taken into account when considering dynamical models for the dark energy (DE), which are nowadays mainly focused on identifying the DE with the energy density associated to one or more ad hoc scalar fields. At the same time, we address some recent criticisms that have been published (or privately communicated to us) attempting to cast doubts on the fundamental possibility of such running. In this work, we argue that while there is no comprehensive proof of the CC running, there is no rigorous proof of the non-running either. In particular, some purported "non-running theorem" recently adduced in the literature is, in our opinion, completely insubstantial and formally incorrect. The way to the CC running is, therefore, still open and we take here the opportunity to present a pedagogical review of the present state of the art in this field, including a a brief historical account.

hep-th↗
arXiv2008

Dynamical effects of the long bar in the Milky Way

We examine the dynamical effects on disk stars of a "long bar" in the Milky Way by inserting a triaxial rotating bar into an axisymmetric disk+bulge+dark halo potential and integrating 3-D orbits of 104 tracer stars over a period of 2 Gyr. The long bar has been detected via "clump giants" in the IR by Lopez-Corredoira et al. (2007), and is estimated to have semi-major axes of (3.9 : 0.6 : 0.1) kpc and a mass of 6 10^9 Msun. We find such a structure has a slight impact on the inner disk-system, moving tracers near to the bar into the bar-region, as well as into the bulge. These effects are under continuing study.

astro-ph↗
arXiv2008

Flares as fingerprints of inner solar darkness

X-ray flares and other solar brightenings have been discarded as potential axion signatures. An axion X-ray signal must appear exclusively near the disk centre, and its spectrum must peak at ~4.2keV, contrary to observation. We argue here that due to Compton scattering off the (plasma) electrons the outward propagation of X-rays from axions converted near the Sun's surface can explain energy distribution and non direcivity. Simulation points at the photosphere as the birth place of the presumed axion conversion, implying an axion rest mass of ~0.01eV. At present, even optimistic parameter values can not reproduce the measured intensities. The simulated photon spectrum peaks at low energies. Quiet Sun hard X-rays are in favour of massive and/or light axion involvement.

astro-ph↗
arXiv2008

From inflation to late acceleration: A new cosmological paradigm

A new idea of deriving a cosmological term from an underlying theory has been proposed in order to explain the expansion history of the universe. We obtain the scale factor with this derived cosmological term and demonstrate that it reflects all the characteristics of the expanding universe in different era so as to result in a transition from inflation to late acceleration through intermediate decelerating phases by this single entity. We further discuss certain observational aspects of this paradigm.

gr-qc↗
arXiv2010

Is "Spike" a Reliable Feature in Porb Distribution of AM HER Stars?

Orbital periods in AM Her stars (polars) are synchronized with spin periods of white dwarf by its high magnetic field. Since the last study of Porb distribution of these systems, the number of known objects of such type has more than doubled. This challenged us to compile a new updated catalogue of cataclysmic variables with highly magnetic white dwarfs (polars) and to study their Porb distribution. In this paper we also discus if "spike" is reliable feature in the distribution. ("Spike" is a concentration of polars in the distribution of their orbital periods near Porb = 114 min and was previously discussed by Ritter & Kolb (1992) and Shahbaz & Wood (1996).)

astro-ph↗
arXiv2009

Evolution of scalar perturbations in cosmology with quintessential dark energy

The dynamics of expansion of the Universe and evolution of scalar perturbations are studied for the quintessential scalar fields $Q$ with the classical Lagrangian satisfying the additional condition $w=const$ or $c^2_a=0$. Both quintessential fields are studied for the same cosmological model. The systems of evolutionary equations for gauge-invariant perturbations of metrics, matter and quintessence have been analysed analyticaly for the early stage of the Universe life and numerically up to the present epoch. It is shown that amplitudes of the adiabatic matter density perturbations grow like in $Λ$CDM-model but time dependences of amplitudes of the quintessence perturbations are more varied: gauge-invariant variables $D_g^{(Q)}$ and $D_s^{(Q)}$ decay from initial constant value after entering the particle horizon while $D^{(Q)}$ and $V^{(Q)}$ grow at the early stage before entering the horizon and decay after that -- in the quintessence-dominated epoch, when gravitational potential starts to decay -- so, that at the current epoch they are approximately two orders lower than matter ones at the supercluster scale. Therefore, at the subhorizon scales the quintessential scalar fields are smoothed out while the matter clusters. It is also shown that both quintessential scalar fields suppress the growth of matter density perturbations and the amplitude of gravitational potential. In these QCDM-models -- unlike $Λ$CDM ones -- such suppression is scale dependent and more visible for the quintessence with $c^2_a=0$.

astro-ph↗
arXiv2009

Is there a redshift cutoff for submillimetre galaxies?

We present new optical and infrared photometry for a statistically complete sample of seven 1.1 mm selected sources with accurate Submillimetre Array coordinates. We determine photometric redshifts for four of the seven sources of 4.47, 4.50, 1.49 and 0.64. Of the other three sources two are undetected at optical wavelengths down to the limits of very deep Subaru and Canada-France-Hawaii Telescope images ($\sim$27 mag AB, i band) and the photometry of the remaining source is corrupted by a bright nearby galaxy. The sources with the highest redshifts are at higher redshifts than all but one of the $\sim$200 sources taken from the largest recent 850 $μ$m surveys, which may indicate that 1.1 mm surveys are more efficient at finding sources at very high redshifts than 850 $μ$m surveys. We investigate the evolution of the number density with redshift of our sample using a banded $V_{e}/V_{a}$ analysis and find no evidence for a redshift cutoff, although the number of sources is very small. We also perform the same analysis on a statistically complete sample of 38 galaxies selected at 850$μ$m from the GOODS-N field and find evidence for a drop-off in the number density beyond $z\sim1$ and 2 for hot and cold dust dominated SMGs respectively, confirming the earlier conclusion of Wall, Pope & Scott. We also find strong evidence for the existence of two differently evolving sub-populations separated in luminosity, with a higher relative density of the high luminosity galaxies at higher redshifts.

astro-ph↗
arXiv2008

The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. I. Classical Cepheids in the Large Magellanic Cloud

We present the first part of a new catalog of variable stars (OIII-CVS) compiled from the data collected in the course of the third phase of the Optical Gravitational Lensing Experiment (OGLE-III). In this paper we describe the catalog of 3361 classical Cepheids detected in the ~40 square degrees area in the Large Magellanic Cloud. The sample consists of 1848 fundamental-mode (F), 1228 first-overtone (1O), 14 second-overtone (2O), 61 double-mode F/1O, 203 double-mode 1O/2O, 2 double-mode 1O/3O, and 5 triple-mode classical Cepheids. This sample is supplemented by the list of 23 ultra-low amplitude variable stars which may be Cepheids entering or exiting instability strip. The catalog data include VI high-quality photometry collected since 2001, and for some stars supplemented by the OGLE-II photometry obtained between 1997 and 2000. We provide basic parameters of the stars: coordinates, periods, mean magnitudes, amplitudes and parameters of the Fourier light curve decompositions. Individual objects of particular interest are discussed, including single-mode second-overtone Cepheids, multiperiodic pulsators with unusual period ratios or Cepheids in eclipsing binary systems. We discuss the variations of the Fourier coefficients with periods and point out on the sharp feature for periods around 0.35 days of first-overtone Cepheids, which can be explained by the occurrence of 2:1 resonance between the first and fifth overtones. Similar behavior at P=3 days for 1O Cepheids and P=10 days for F Cepheids are also interpreted as an effect of resonances between two radial modes. We fit the period-luminosity relations to our sample of Cepheids and compare these functions with previous determinations.

astro-ph↗
arXiv2008

Mirror dark matter discovered?

Recent astrophysical data indicates that dark matter shows a controversial behaviour in galaxy cluster collisions. In case of the notorious Bullet cluster, dark matter component of the cluster behaves like a collisionless system. However, its behaviour in the Abell 520 cluster indicates a significant self-interaction cross-section. It is hard for the WIMP based dark matter models to reconcile such a diverse behaviour. Mirror dark matter models, on the contrary, are more flexible and for them diverse behaviour of the dark matter is a natural expectation.

astro-ph↗
arXiv2010

Worlds Beyond: A Strategy for the Detection and Characterization of Exoplanets

This is a scientific strategy for the detection and characterization of extrasolar planets; that is, planets orbiting other stars. As such, it maps out over a 15-year horizon the techniques and capabilities required to detect and measure the properties of planets as small as Earth around stars as large as our own Sun. It shows how the technology pieces and their development fit together to achieve the primary goal of the strategy: if planets like Earth exist around stars within some tens of light years of our own Solar System, those planets will be found and their basic properties characterized. Essential to this strategy is not only the search for and examination of individual planets, but also a knowledge of the arrangement, or architecture, of planetary systems around as large a number of stars as possible; this is the second goal of the strategy. The final goal of the strategy is the study of disks around stars, important both to understand the implications of the variety of exoplanet systems for planet formation, and to determine how many nearby stars have environments around them clean enough of debris that planets may be sought and, if found, characterized.

astro-ph↗
arXiv2008

Asteroseismology and the Solar-Stellar Connection

In March 2009, NASA will launch the Kepler satellite -- a mission designed to discover habitable Earth-like planets around distant Sun-like stars. The method that Kepler will use to detect distant worlds will only reveal the size of the planet relative to the size of the host star, so part of the mission is devoted to characterizing other suns using asteroseismology. In this proceedings, I give a broad overview of the Kepler mission and the data that it will produce, with a special emphasis on how it could improve our understanding of solar and stellar dynamos. I conclude with an update on the development of a stellar modeling pipeline for interpreting asteroseismic observations.

astro-ph↗
arXiv2008

An oscillating, homogeneous and isotropic Universe from Scalar-Tensor gravity

An oscillating, homogeneous and isotropic Universe which arises from Scalar-Tensor gravity is discussed in the linearized approach, showing that some observative evidences like the Hubble Law and the Cosmological Redshift are in agreement with the model. In this context Dark Energy appears like a pure curvature effect arising by the scalar field.

astro-ph↗
arXiv2008

Temporal behaviour of global perturbations in compressible axisymmetric flows with free boundaries

The dynamics of small global perturbations in the form of linear combination of a finite number of non-axisymmetric eigenmodes is studied in two-dimensional approximation. The background flow is assumed to be an axisymmetric perfect fluid with the adiabatic index $γ=5/3$ rotating with power law angular velocity distribution $Ω\propto r^{-q}$, $1.5<q<2.0$, confined by free boundaries in the radial direction. The substantial transient growth of acoustic energy of optimized perturbations is discovered. An optimal energy growth $G$ is calculated numerically for a variety of parameters. Its value depends essentially on the perturbation azimuthal wavenumber $m$ and increases for higher values of $m$. The closer the rotation profile to the Keplerian law, the larger growth factors can be obtained but over a longer time. The highest acoustic energy increase found numerically is of order $\sim 10^2$ over $\sim 6$ typical Keplerian periods. Slow neutral eigenmodes with corotation radius beyond the outer boundary mostly contribute to the transient growth. The revealed linear temporal behaviour of perturbations may play an important role in angular momentum transfer in toroidal flows near compact relativistic objects.

astro-ph↗
arXiv2008

AGILE detection of variable gamma-ray activity from the blazar S5 0716+714 during September-October 2007

We report the gamma-ray activity from the Intermediate BL Lac S5 0716+714 during 2007 September-October observations by the AGILE satellite, coincident with a period of intense optical activity of the source monitored by GASP-WEBT. AGILE observed the source with its two co-aligned imagers, the Gamma-Ray Imaging Detector (GRID) and the hard X-ray imager (Super-AGILE) sensitive in the energy range 30 MeV-50 GeV and 18-60 keV respectively, in two different periods: the first between 4 and 23 September 2007, the second between 24 October and 1 November 2007. Over the period 7-12 September, AGILE detected gamma-ray emission from the source at a significance level of 9.6-sigma with an average flux (E>100 MeV) of (97+/-15) x 10^{-8} photons cm^{-2} s^{-1}, increasing by a factor of at least four within three days. No emission was detected by Super-AGILE in the energy range 18-60 keV, with a 3-sigma upper limit of 10 mCrab in 335 ksec. The gamma-ray flux of S5 0716+714 detected by AGILE is the highest ever detected for this blazar and one of the most intense gamma-ray fluxes detected from a BL Lac object. The Spectral Energy Distribution (SED) of mid-September seems to be consistent with the synchrotron self-Compton (SSC) emission model, but only by including two SSC components with different variability. In October 2007 AGILE repointed toward S5 0716+714 following an intense optical flare, measuring an average flux of (47+/-11) x 10^{-8} photons cm^{-2} s^{-1} at a significance level of 6.0-sigma. The gamma-ray flux during both AGILE pointings appears to be highly variable on timescales of 1 day.

astro-ph↗
arXiv2008

Minimal Dark Matter predictions and the PAMELA positron excess

We present Minimal Dark Matter and its univocal predictions for Dark Matter observables. During the idm2008 conference, PAMELA presented preliminary results showing an excess in the positron fraction: we find a good agreement, with a modest astrophysical boost factor.

astro-ph↗