2,199 research outputs found
X-ray and optical counterparts of hard X-ray selected sources from the SHEEP survey: first results
We present followup observations of five hard X-ray sources from the ASCA
5-10 keV SHEEP survey, which has a limiting flux of erg
cm s. Chandra data have been obtained to improve the X-ray
positions from a few arcmin to , which allows unambiguous optical
identification. While the objects almost certainly house AGN based on their
X-ray luminosity, optical spectroscopy reveals a variety of properties. The
identifications indicate that the SHEEP survey samples the same populations as
deeper surveys which probe the origin of the X-ray background, but because the
SHEEP sources are far brighter, they are more amenable to detailed followup
work. We find a variety of classifications and properties, including a type II
QSO, a galaxy undergoing star formation, and a broad-line AGN which has a very
hard X-ray spectrum, indicating substantial absorption in the X-ray but none in
the optical. Two objects have X-ray/optical flux ratios which, were they at an
X-ray flux level typical of objects in Chandra deep surveys, would place them
in the ``optically faint'' category. They are both identified with broad line
QSOs at z. Clearly this survey - which is relatively unbiased against
obscured objects - is revealing a set of remarkable objects quite different to
the familiar classes of AGN found in previous optical and soft X-ray surveys.Comment: 5 pages, 3 figures. MNRAS, in pres
Probing BH mass and accretion through X-ray variability in the CDFS
Recent work on nearby AGNs has shown that X-ray variability is correlated
with the mass and accretion rate onto the central SMBH. Here we present the
application of the variability-luminosity relation to high redshift AGNs in the
CDFS, making use of XMM-Newton observations. We use Monte Carlo simulations in
order to properly account for bias and uncertainties introduced by the sparse
sampling and the very low statistics. Our preliminary results indicate that BH
masses span over the range from 10^5 to 10^9 solar mass while accretion rates
range from 10^-3 up to values greater than 1, in unit of Eddington accretion
rate.Comment: 2 pages, 2 figures,in press in the X-ray 2009 Conference Proceedings
(Bologna, 7-11 September 2009
Asymmetric periodic orbits in the photogravitational Copenhagen problem
AbstractIn this paper we study the Ox-asymmetric solutions of the planar photogravitational restricted three-body problem in the case of primaries with equal masses and equal values of the radiation pressure parameters. In particular, we are concerned with the families of asymmetric orbits which bifurcate from the well known families a, b, and c. Their evolution is examined via the numerical construction of series of the critical bifurcation points of a, b, and c with respect to the variation of the common radiation parameter q. We also present some illustrative cases of these families for several values of this parameter. In order to avoid the singularity due to binary collisions between the third body and one of the primaries, the equations of motion of the problem are regularized by using the Levi-Civita transformations
Reconstruction of large scale flow structures in a stirred tank from limited sensor data
We combine reduced order modelling and system identification to reconstruct the temporal evolution of large scale vortical structures behind the blades of a Rushton impeller. We performed Direct Numerical Simulations at Reynolds number 600 and employed proper orthogonal decomposition (POD) to extract the dominant modes and their temporal coefficients. We then applied the identification algorithm, N4SID, to construct an estimator that captures the relation between the velocity signals at sensor points (input) and the POD coefficients (output). We show that the first pair of modes can be very well reconstructed using the velocity time signal from even a single sensor point. A larger number of points improves accuracy and robustness, and also leads to better reconstruction for the second pair of POD modes. Application of the estimator derived at Re=600 to the flows at Re=500 and 700, shows that it is robust with respect to changes in operating conditions
Design and characterization of programmable DNA nanotubes
DNA self-assembly provides a programmable bottom-up approach for the synthesis of complex structures from nanoscale components. Although nanotubes are a fundamental form encountered in tile-based DNA self-assembly, the factors governing tube structure remain poorly understood. Here we report and characterize a new type of nanotube made from DNA double-crossover molecules (DAE-E tiles). Unmodified tubes range from 7 to 20 nm in diameter (4 to 10 tiles in circumference), grow as long as 50 μm with a persistence length of ~4 μm, and can be programmed to display a variety of patterns. A survey of modifications (1) confirms the importance of sticky-end stacking, (2) confirms the identity of the inside and outside faces of the tubes, and (3) identifies features of the tiles that profoundly affect the size and morphology of the tubes. Supported by these results, nanotube structure is explained by a simple model based on the geometry and energetics of B-form DNA
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