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At least 433 records · Page 24Linked to original sources
arXiv2008

The Zurich Extragalactic Bayesian Redshift Analyzer (ZEBRA), version 1.0 -- User manual

ZEBRA, the Zurich Extragalactic Bayesian Redshift Analyzer, is a tool for estimating redshifts and template types of galaxies using medium- and broad-band photometric data. ZEBRA employs novel techniques within the template-fitting approach to produce high-quality Maximum-Likelihood and Bayesian redshift estimates. This manuscript serves as a user guide to ZEBRA. It explains how to use ZEBRA, specifies input and output formats, and gives a short account of the available options. ZEBRA is a free and open-source software distributed under the GNU Public License 3 and available at http://www.exp-astro.phys.ethz.ch/ZEBRA . Several upgrades are currently being implemented in ZEBRA. An updated documentation will be provided at each new release. Any problems, comments and suggestions on the code and the manual should be sent via e-mail to zebra@phys.ethz.ch.

astro-ph↗
arXiv2008

V723 Cassiopeia still on in X-rays: A bright Super Soft Source 12 years after outburst

We find that the classical nova V723 Cas (1995) is still an active X-ray source more than 12 years after outburst and analyze seven X-ray observations carried out with Swift between 2006 January 31 and 2007 December 3. The average count rate is 0.022+/-0.01 cts s^-1 but the source is variable within a factor of two of the mean and does not show any signs of turning off. We present supporting optical observations which show that between 2001 and 2006 an underlying hot source was present with steadily increasing temperature. In order to confirm that the X-ray emission is from V723 Cas, we extract a ROSAT observation taken in 1990 and find that there was no X-ray source at the position of the nova. The Swift XRT spectra resemble those of the Super Soft X-ray binary Sources (SSS) which is confirmed by RXTE survey data which show no X-ray emission above 2 keV between 1996 and 2007. Using blackbody fits we constrain the effective temperature to between T_eff=(2.8-3.8)x10^5 K and a bolometric luminosity >5x10^36 erg s^-1 and caution that luminosities from blackbodies are generally overestimated and temperatures underestimated. We discuss a number of possible explanations for the continuing X-ray activity, including the intriguing possibility of steady hydrogen burning due to renewed accretion.

astro-ph↗
arXiv2009

Relativistic versus Newtonian orbit model: the Relativistic Motion Integrator (RMI) software. Illustration with the LISA mission

The Relativistic Motion Integrator (RMI) consists in integrating numerically the EXACT relativistic equations of motion, for a given metric (corresponding to a gravitational field at first post-Newtonian order or higher), instead of Newtonian equations plus relativistic corrections. The aim of the present paper is to validate the method, and to illustrate how RMI can be used for space missions to produce relativistic ephemerides of test-bodies (or satellites). Indeed, nowadays, relativistic effects have to be taken into account, and comparing a RMI model with a classical keplerian one helps to quantify such effects. LISA is a relevant example to use RMI. A precise orbit model for the LISA spacecraft is needed not only for the sake of satellite ephemerides but also to compute the photon flight time in laser links between spacecraft, required in LISA data pre-processing in order to reach the gravitational wave detection level. Relativistic effects in LISA orbit model needed to be considered and quantified. Using RMI, we show that the numerical classical model for LISA orbits in the gravitational field of a non-rotating spherical Sun without planets can be wrong, with respect to the numerical relativisitic version of the same model, by as much as about ten kilometers in radial distance during a year and up to about 60 kilometers in along track distance after a year... with consequences on estimated photon flight times. We validated RMI numerical results with a 1PN analytical developpement.

gr-qc↗
arXiv2008

The formation of disk galaxies in computer simulations

The formation of disk galaxies is one of the most outstanding problems in modern astrophysics and cosmology. We review the progress made by numerical simulations carried out on large parallel supercomputers. Recent progress stems from a combination of increased resolution and improved treatment of the astrophysical processes modeled in the simulations, such as the phenomenological description of the interstellar medium and of the process of star formation. High mass and spatial resolution is a necessary condition in order to obtain large disks comparable with observed spiral galaxies avoiding spurious dissipation of angular momentum. A realistic model of the star formation history. gas-to-stars ratio and the morphology of the stellar and gaseous component is instead controlled by the phenomenological description of the non-gravitational energy budget in the galaxy. We show that simulations of gas collapse within cold dark matter halos including a phenomenological description of supernovae blast-waves allow to obtain stellar disks with nearly exponential surface density profiles as those observed in real disk galaxies, counteracting the tendency of gas collapsing in such halos to form cuspy baryonic profiles. However, the ab-initio formation of a realistic rotationally supported disk galaxy with a pure exponential disk in a fully cosmological simulation is still an open problem. We argue that the suppression of bulge formation is related to the physics of galaxy formation during the merger of the most massive protogalactic lumps at high redshift, where the reionization of the Universe likely plays a key role. A sufficiently high resolution during this early phase of galaxy formation is also crucial to avoid artificial angular momentum loss (Abridged).

astro-ph↗
arXiv2008

Constraints on the lepton asymmetry and radiation energy density: Implications for PLANCK

By using most of the present CMB and LSS measurements and the BBN constraints on the primordial helium abundance, Y_p, we set bounds on the radiation content of the Universe and neutrino properties. We consider lepton asymmetric cosmological models parametrized by the neutrino degeneracy parameter ξ_ν and the variation of the relativistic degrees of freedom, ΔN_{oth}^{eff}, due to possible other physical processes that occurred between BBN and structure formation epoch. We found that present CMB and LSS data constraints the neutrino degeneracy parameter at ξ_ν \leq 0.722, implying a lepton asymmetry of the neutrino background {\cal L}_ν \leq 0.614 (2-σ). We also found ΔN^{eff}_{oth}=0.572^{+1.972}_{-1.780}, the contribution to the effective number of relativistic neutrino species N^{eff}=3.058^{+1.971}_{-1.178} and a primordial helium abundance Y_p=0.249^{+0.014}_{-0.016} (2-σerrors). These results bring an important improvement over the similar ones obtained by using WMAP~1-year and older LSS data or the WMAP~3-year data alone and the standard primordial helium abundance value Y_p=0.24, relaxing the stringent BBN constraint on the neutrino degeneracy parameter (ξ_ν \leq 0.07). We forecast that the CMB temperature and polarization maps observed with high angular resolutions and sensitivity by the future Planck Mission will constraint the primordial primordial helium abundance at Y_p=0.247 \pm 0.002 (2-σerrors) in agreement with the most stringent limits on Y_p given by the BBN and the neutrino degeneracy parameter at ξ_ν \leq 0.280 (2-σ), not excluding the possibility of larger lepton asymmetry. This work has been done on behalf of Planck-LFI activities.

astro-ph↗
arXiv2008

Evolution of Substructure in Galaxy Clusters as Observed in X-Rays

Clusters of galaxies are the largest gravitationally-bound objects in the Universe, having diameters on order of Mpc. Our work asked whether their shapes (morphologies) change over time as the Universe ages. We observed a sample of 165 galaxy clusters, at 0.1 < z < 1.3. A variety of measures were used to quantify the shapes of galaxy clusters. Archive observations from the Chandra X-Ray Observatory were used. Morphology evolution was probed at two different distances from clusters' centers -- 300 kpc and 500 kpc -- for comparison. In almost all cases, we were able to rule out that clusters retain their morphology over the history of the Universe, which is in agreement with our current picture of large-scale structure formation. In addition, we found that ellipticities, as a means of quantifying morphologies, are of limited use in studies such as this work.

astro-ph↗
arXiv2009

Modification from Noncommutative Inflation

We explore the noncommutative effect on single field inflation and compare with WMAP five-year data. First, we calculate the noncommutative effect from the potential and dynamical terms, and construct the general form of modified power spectrum. Second, we consider the leading order modification of slow-roll, DBI and K-inflation and unite the modification, which means the modification is nearly model independent at this level. Finally, comparing with the WMAP5 data, we find that the modified can be well realized as the origin of the relative large spectral index and the quite small running.

astro-ph↗
arXiv2008

Multiple stellar systems under photometric and astrometric analysis

The light-time effect method, its limitations and applications were studied. A powerful combined method of simultaneous analysis of the O-C diagrams and astrometric orbit in triple eclipsing-astrometric binaries was presented. Eleven eclipsing systems were studied in detail according to their O-C diagrams (RY Aqr, BF CMi, RW Cap, TY Cap, SS Cet, RR Dra, TY Del, TZ Eri, RV Per, UZ Sge, and BO Vul). The introduced method for studying the astrometric-eclipsing binaries was applied to QS Aql, VW Cep, Zeta Phe, V505 Sgr, HT Vir, and V2388 Oph. The algorithm for such an analysis was introduced and the its limitations were discussed. The catalogue of another systems, which contain eclipsing binaries in astrometric binaries, was presented. Such systems could be useful for prospective analysis. The method itself could be easily modified for estimation of the parallax of the individual systems.

astro-ph↗
arXiv2009

Huge entropy production inside black holes

We show that the entropy created by Ohmic dissipation inside an accreting charged black hole may exceed the Bekenstein-Hawking entropy by a large factor. If the black hole subsequently evaporates, radiating only the Bekenstein-Hawking entropy, then the black hole appears to destroy entropy, violating the second law of thermodynamics. A companion paper discusses the implications of this startling result. Bousso's covariant entropy bound is not violated.

gr-qc↗
arXiv2008

Multi-wavelength surveys towards the SKA pathfinders

In these proceedings I discuss various extragalactic surveys which will be undertaken over the next few years and which will be complementary to any HI and/or continuum surveys with the SKA-pathfinder telescopes. I concentrate on the near-infrared public surveys which will be undertaken with the Visible and Infrared Survey Telscope for Astronomy (VISTA), and in particular the VISTA Deep Extragalactic Observations (VIDEO) survey which will provide the ideal data set to combine with any deep SKA-pathfinder observations of the extragalactic sky. After highlighting the links that the SKA pathfinders have with the various VISTA surveys, I briefly describe an approved large area survey to be carried out with the Herschel Space Observatory which has a large scientific overlap with the SKA pathfinder telescopes.

astro-ph↗
arXiv2008

Passage of small black hole through the Earth. Is it detectable?

We examine the energy losses of a small black hole passing through the Earth, and in particular, the excitations created in the frequency range accessible to modern acoustic detectors. The dominating contributions to the effect are due to the coherent sound radiation of the Cherenkov type and to the conversion of black hole radiation into sound waves.

hep-ex↗
arXiv2008

Binary recycled pulsars, as a most precise physical laboratory

The following problems are discussed. 1. Pulsars and close binaries. 2. Hulse-Taylor pulsar. 3. Disrupted pulsar pairs. 4. RP statistics. 5. Enhanced evaporation: formation of single RP. 6. General relativity effects: NS+NS. 7. A Double pulsar system. 8. Checking general relativity. 9. Variability of the gravitational constant. 10. Space Watch.

astro-ph↗
arXiv2008

Cosmic Neutrinos

I recall the place of neutrinos in the electroweak theory and summarize what we know about neutrino mass and flavor change. I next review the essential characteristics expected for relic neutrinos and survey what we can say about the neutrino contribution to the dark matter of the Universe. Then I discuss the standard-model interactions of ultrahigh-energy neutrinos, paying attention to the consequences of neutrino oscillations, and illustrate a few topics of interest to neutrino observatories. I conclude with short comments on the remote possibility of detecting relic neutrinos through annihilations of ultrahigh-energy neutrinos at the $Z$ resonance.

hep-ph↗
arXiv2008

A deep survey of the low-surface-brightness radio sky

We have made a radio survey--the Australia Telescope Low Brightness Survey (ATLBS)--of 8.4 square degrees sky area, using the Australia Telescope Compact Array in the 20-cm band, in an observing mode designed to provide wide-field images with exceptional sensitivity in surface brightness, and thereby explore a new parameter space in radio source populations. The goals of this survey are to quantify the distribution in angular sizes, particularly at weak surface brightness levels: this has implications for the confusion in deep surveys with the SKA. The survey is expected to lead to a census of the radio emission associated with low-power radio galaxies at redshifts 1-3, without any missing extended emission, and hence a study of the cosmic evolution of low-power radio galaxies to higher redshift and a comprehensive study of the AGN feedback during the intense black hole growth phase during this redshift range.

astro-ph↗
arXiv2008

Uniformly Rotating Homogeneous Rings in Newtonian Gravity

In this paper, we describe an analytical method for treating uniformly rotating homogeneous rings without a central body in Newtonian gravity. We employ series expansions about the thin ring limit and use the fact that in this limit the cross-section of the ring tends to a circle. The coefficients can in principle be determined up to an arbitrary order. Results are presented here to the 20th order and compared with numerical results.

astro-ph↗
arXiv2008

Gravitational vacuum polarization as an alternative to dark matter

It is assumed that the quantum vacuum may be studied as consisting of two contributions, with positive and negative energy respectively, which interact but slightly and may be displaced from each other. Then it is proposed that dark matter may be just an increase of the quantum vacuum energy, with respect to the normal dark energy level, induced by the gravitational field of galaxies or clusters. A simple model is worked out able to reproduce astronomical observations.

gr-qc↗