58 research outputs found
The Northern ROSAT All-Sky (NORAS) Galaxy Cluster Survey I: X-ray Properties of Clusters Detected as Extended X-ray Sources
In the construction of an X-ray selected sample of galaxy clusters for
cosmological studies, we have assembled a sample of 495 X-ray sources found to
show extended X-ray emission in the first processing of the ROSAT All-Sky
Survey. The sample covers the celestial region with declination and galactic latitude and comprises sources with
a count rate counts s and a source extent likelihood of 7. In
an optical follow-up identification program we find 378 (76%) of these sources
to be clusters of galaxies. ...Comment: 61 pages; ApJS in press; fixed bug in table file; also available at
(better image quality) http://www.xray.mpe.mpg.de/theorie/NORAS
Limits on the cosmological abundance of supermassive compact objects from a search for multiple imaging in compact radio sources
Using Very Long Baseline Interferometry we have searched a sample of 300
compact radio sources for examples of multiple imaging produced by
gravitational lensing; no multiple images were found with separations in the
angular range 1.5--50 milliarcsec. This null result allows us to place a limit
on the cosmological abundance of intergalactic supermassive compact objects in
the mass range to M; such objects cannot
make up more than of the closure density (95% confidence). A
uniformly distributed population of supermassive black holes forming soon after
the Big Bang do not, therefore, contribute significantly to the dark matter
content of the Universe.Comment: 2 figure
Astrometry and geodesy with radio interferometry: experiments, models, results
Summarizes current status of radio interferometry at radio frequencies
between Earth-based receivers, for astrometric and geodetic applications.
Emphasizes theoretical models of VLBI observables that are required to extract
results at the present accuracy levels of 1 cm and 1 nanoradian. Highlights the
achievements of VLBI during the past two decades in reference frames, Earth
orientation, atmospheric effects on microwave propagation, and relativity.Comment: 83 pages, 19 Postscript figures. To be published in Rev. Mod. Phys.,
Vol. 70, Oct. 199
Active Galactic Nuclei at the Crossroads of Astrophysics
Over the last five decades, AGN studies have produced a number of spectacular
examples of synergies and multifaceted approaches in astrophysics. The field of
AGN research now spans the entire spectral range and covers more than twelve
orders of magnitude in the spatial and temporal domains. The next generation of
astrophysical facilities will open up new possibilities for AGN studies,
especially in the areas of high-resolution and high-fidelity imaging and
spectroscopy of nuclear regions in the X-ray, optical, and radio bands. These
studies will address in detail a number of critical issues in AGN research such
as processes in the immediate vicinity of supermassive black holes, physical
conditions of broad-line and narrow-line regions, formation and evolution of
accretion disks and relativistic outflows, and the connection between nuclear
activity and galaxy evolution.Comment: 16 pages, 5 figures; review contribution; "Exploring the Cosmic
Frontier: Astrophysical Instruments for the 21st Century", ESO Astrophysical
Symposia Serie
The role of input materials in shallow seismogenic slip and forearc plateau development: International Ocean Discovery Program Expedition 362 Preliminary Report Sumatra Seismogenic Zone
Drilling the input materials of the north Sumatran subduction zone, part of the 5000 km long Sunda subduction zone system and the origin of the Mw âŒ9.2 earthquake and tsunami that devastated coastal communities around the Indian Ocean in 2004, was designed to groundtruth the material properties causing unexpectedly shallow seismogenic slip and a distinctive forearc prism structure. The intriguing seismogenic behavior and forearc structure are not well explained by existing models or by relationships observed at margins where seismogenic slip typically occurs farther landward. The input materials of the north Sumatran subduction zone are a distinctively thick (as thick as 4-5 km) succession of primarily Bengal-Nicobar Fan-related sediments. The correspondence between the 2004 rupture location and the overlying prism plateau, as well as evidence for a strengthened input section, suggest the input materials are key to driving the distinctive slip behavior and long-term forearc structure. During Expedition 362, two sites on the Indian oceanic plate âŒ250 km southwest of the subduction zone, Sites U1480 and U1481, were drilled, cored, and logged to a maximum depth of 1500 meters below seafloor. The succession of sediment/rocks that will develop into the plate boundary detachment and will drive growth of the forearc were sampled, and their progressive mechanical, frictional, and hydrogeological property evolution will be analyzed through postcruise experimental and modeling studies. Large penetration depths with good core recovery and successful wireline logging in the challenging submarine fan materials will enable evaluation of the role of thick sedimentar y subduction zone input sections in driving shallow slip and amplifying earthquake and tsunami magnitudes, at the Sunda subduction zone and globally at other subduction zones where submarine fan-influenced sections are being subducted
Remarks on the Cauchy functional equation and variations of it
This paper examines various aspects related to the Cauchy functional equation
, a fundamental equation in the theory of functional
equations. In particular, it considers its solvability and its stability
relative to subsets of multi-dimensional Euclidean spaces and tori. Several new
types of regularity conditions are introduced, such as a one in which a complex
exponent of the unknown function is locally measurable. An initial value
approach to analyzing this equation is considered too and it yields a few
by-products, such as the existence of a non-constant real function having an
uncountable set of periods which are linearly independent over the rationals.
The analysis is extended to related equations such as the Jensen equation, the
multiplicative Cauchy equation, and the Pexider equation. The paper also
includes a rather comprehensive survey of the history of the Cauchy equation.Comment: To appear in Aequationes Mathematicae (important remark: the
acknowledgments section in the official paper exists, but it appears before
the appendix and not before the references as in the arXiv version);
correction of a minor inaccuracy in Lemma 3.2 and the initial value proof of
Theorem 2.1; a few small improvements in various sections; added thank
Gamma-Ray and Parsec-Scale Jet Properties of a Complete Sample of Blazars From the MOJAVE Program
We investigate the Fermi LAT gamma-ray and 15 GHz VLBA radio properties of a
joint gamma-ray- and radio-selected sample of AGNs obtained during the first 11
months of the Fermi mission (2008 Aug 4 - 2009 Jul 5). Our sample contains the
brightest 173 AGNs in these bands above declination -30 deg. during this
period, and thus probes the full range of gamma-ray loudness (gamma-ray to
radio band luminosity ratio) in the bright blazar population. The latter
quantity spans at least four orders of magnitude, reflecting a wide range of
spectral energy distribution (SED) parameters in the bright blazar population.
The BL Lac objects, however, display a linear correlation of increasing
gamma-ray loudness with synchrotron SED peak frequency, suggesting a universal
SED shape for objects of this class. The synchrotron self-Compton model is
favored for the gamma-ray emission in these BL Lacs over external seed photon
models, since the latter predict a dependence of Compton dominance on Doppler
factor that would destroy any observed synchrotron SED peak - gamma-ray
loudness correlation. The high-synchrotron peaked (HSP) BL Lac objects are
distinguished by lower than average radio core brightness temperatures, and
none display large radio modulation indices or high linear core polarization
levels. No equivalent trends are seen for the flat-spectrum radio quasars
(FSRQ) in our sample. Given the association of such properties with
relativistic beaming, we suggest that the HSP BL Lacs have generally lower
Doppler factors than the lower-synchrotron peaked BL Lacs or FSRQs in our
sample.Comment: 22 pages, 11 figures, 4 tables; accepted for publication in the
Astrophysical Journa
The Fine Structure of the Subducted Investigator Fracture Zone in Western Sumatra as Seen by Local Seismicity
The Sumatran margin suffered three great earthquakes in recent years (Aceh-Andaman 26 December 2004Mw = 9.1, Nias 28 March 2005 Mw = 8.7, Bengkulu 12 September 2007 Mw = 8.5). Here we present local earthquake data from a dense, amphibious local seismic network covering a segment of the Sumatran margin that last ruptured in 1797. The occurrence of forearc islands along this part of the Sumatran margin allows the deployment of seismic land-stations above the shallow part of the thrust fault. In combination with ocean bottom seismometers this station geometry provides high quality hypocentre location for the updip end of the seismogenic zone in an area where geodetic data are also available. In this region, the Investigator Fracture Zone (IFZ), which consists of 4 sub-ridges, is subducted below the Sunda plate. This topography appears to influence seismicity at all depth intervals. A well-defined linear streak of seismicity extending from 80 to 200 km depth lies along the prolongation of closely spaced IFZ sub-ridges. More intermediate depth seismicity is located to the southeast of this string of seismicity and is related to subducted rough oceanic seafloor. The plate interface beneath Siberut Island which ruptured last in 1797 is characterised by almost complete absence of seismicity
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