Exact quasinormal modes for a special class of black holes

Analytic exact expressions for the quasinormal modes of scalar and electromagnetic perturbations around a special class of black holes are found in d\ge3 dimensions. It is shown that, the size of the black hole provides a bound for the angular momentum of the perturbation. Quasinormal modes appear when this bound is fulfilled, otherwise the excitations become purely damped.Comment: 8 pages, no figures. Slightly updated version of the plenary talk given at the General Relativity Conference: "50 Years of FaMAF and Workshop on Global Problems in Relativity", hosted during November 2006 at FaMAF, Universidad Nacional de Cordoba, Cordoba, Argentina

Dust in active nuclei. II. Powder or gravel?

In a companion paper, Maiolino et al. (2000) presented various observational evidences for "anomalous" dust properties in the circumnuclear region of AGNs and, in particular, the reduced E(B-V)/N_H and Av/N_H ratios, the absence of the silicate absorption feature in mid-IR spectra of Sy2s and the absence of the carbon dip in UV spectra of reddened Sy1s. In this paper we discuss various explanations for these facts. The observational constraints favor a scenario where coagulation, catalyzed by the high densities in the circumnuclear region, yields to the formation of large grains. The resulting extinction curve is featureless, flatter than Galactic and the E(B-V)/N_H and Av/N_H ratios are significantly reduced. These results should warn about an unappropriate use of the standard Galactic extinction curve and Av/N_H ratio when dealing with the extreme gas conditions typical of the circumnuclear clouds of AGNs. We also investigated alternative scenarios for the observed anomalous properties of dust in AGNs. Some of these scenarios might explain some of the observed properties for a few objects, but they generally fail to account for all of the observational constraints obtained for the large sample of AGNs studied in these works.Comment: 13 pages, 7 figures, accepted for publication in A&

Near-infrared K-band Spectroscopic Investigation of Seyfert 2 Nuclei in the CfA and 12 Micron Samples

We present near-infrared K-band slit spectra of the nuclei of 25 Seyfert 2 galaxies in the CfA and 12 micron samples. The strength of the CO absorption features at 2.3-2.4 micron produced by stars is measured in terms of a spectroscopic CO index. A clear anti-correlation between the observed CO index and the nuclear K-L color is present, suggesting that a featureless hot dust continuum heated by an AGN contributes significantly to the observed K-band fluxes in the nuclei of Seyfert 2 galaxies. After correction for this AGN contribution, we estimate nuclear stellar K-band luminosities for all sources, and CO indices for sources with modestly large observed CO indices. The corrected CO indices for 10 (=40%) Seyfert 2 nuclei are found to be as high as those observed in star-forming or elliptical (=spheroidal) galaxies. We combine the K-band data with measurements of the L-band 3.3 micron polycyclic aromatic hydrocarbon (PAH) emission feature, another powerful indicator for star-formation, and find that the 3.3 micron PAH to K-band stellar luminosity ratios are substantially smaller than those of starburst galaxies. Our results suggest that the 3.3 micron PAH emission originates in the putative nuclear starbursts in the dusty tori surrounding the AGNs, because of its high surface brightness, whereas the K-band CO absorption features detected at the nuclei are dominated by old bulge (=spheroid) stars, and thus may not be a powerful indicator for the nuclear starbursts. We see no clear difference in the strength of the CO absorption and PAH emission features between the CfA and 12 micron Seyfert 2s.Comment: 28 pages, 6 figures, accepted for publication in ApJ (10 October 2004, v614 issue

On the evolution of flow topology in turbulent Rayleigh-Bénard convection

Copyright 2016 AIP Publishing. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing.Small-scale dynamics is the spirit of turbulence physics. It implicates many attributes of flow topology evolution, coherent structures, hairpin vorticity dynamics, and mechanism of the kinetic energy cascade. In this work, several dynamical aspects of the small-scale motions have been numerically studied in a framework of Rayleigh-Benard convection (RBC). To do so, direct numerical simulations have been carried out at two Rayleigh numbers Ra = 10(8) and 10(10), inside an air-filled rectangular cell of aspect ratio unity and pi span-wise open-ended distance. As a main feature, the average rate of the invariants of the velocity gradient tensor (Q(G), R-G) has displayed the so-calledPeer ReviewedPostprint (author's final draft

The AMS-02 experiment status

The Alpha Magnetic Spectrometer (AMS) is a high-energy physics experiment built to operate in space. The prototype of the AMS detector was AMS-01, flown in 1998 on-board of the space shuttle Discovery (mission STS-91). Starting from the experience acquired in the high successful AMS-01 mission the detector AMS-02 has been designed improving the AMS-01 energetic range, geometric acceptance and particle identification capabilities. In 2010 the AMS-02 detector has been validated for the space/scientific operations by means of a wide test campaign (including beam tests, TVT test and EMI test). A major change in the design of AMS-02 has been decided after the thermo-vacuum test to extend as much as possible the endurance of the experiment, profiting also of the extended endurance of the International Space Station (ISS) program toward 2020. The final AMS-02 configuration has been integrated during summer 2010, then tested on the H8 beam-line at CERN, and finally delivered to the launch site (Kennedy Space Center, Florida) at the end of August. AMS-02 is planned to be installed on the International Space Station in 2011 by the space shuttle Endeavour (mission STS-134)

Performance of the AMS-02 silicon detector

The Alpha Magnetic Spectrometer (AMS) is a high-energy particle physics experiment in space. Eight layers of double-sided silicon detectors, embedded in a 0.8 T magnetic field created by a superconducting cryo-magnet, are the core of the AMS-02 detector. With a 10 μm spatial resolution, the silicon tracker is able to measure the magnetic rigidity and the sign of the passing particle up to few TV, with a resolution of ΔR/R ∼ 2.5%. In addition the energy loss of the particle in silicon enable the measurement of the absolute charge of nuclei until iron. In this work are presented, in terms of spatial and charge resolution, the performance of the silicon detector in a test beam performed in October 2003 at the CERN SPS