73 research outputs found
Context-Aware Computation Offloading for Mobile Cloud Computing: Requirements Analysis, Survey and Design Guideline
AbstractAlong with the rise of mobile handheld devices the resource demands of respective applications grow as well. However, mobile devices are still and will always be limited related to performance (e.g., computation, storage and battery life), context adaptation (e.g., intermittent connectivity, scalability and heterogeneity) and security aspects. A prominent solution to overcome these limita- tions is the so-called computation offloading, which is the focus of mobile cloud computing (MCC). However, current approaches fail to address the complexity that results from quickly and constantly changing context conditions in mobile user scenarios and hence developing effective and efficient MCC applications is still challenging. Therefore, this paper first presents a list of re- quirements for MCC applications together with a survey and classification of current solutions. Furthermore, it provides a design guideline for the selection of suitable concepts for different classes of common cloud-augmented mobile applications. Finally, it presents open issues that developers and researchers should be aware of when designing their MCC-approach
Interaction with Interconnected Data in Participatory Processes
This paper proposes a conceptual graphical user interface for the interaction with interconnected data in participatory processes that play an important role for future smart cities. The presented idea is based on identifying important tasks for data exploration and data editing. The data to consider is structured, semi-structured or unstructured and of different facets. For example, participatory processes like planning and decision processes involve text, time and spatial data. In other words, the handling of the data is a complex endeavor in terms of representation and interaction. In this respect, we utilize and describe a graph-based data model that properly reflects the connected data
Discovery of an Ultrasoft X-ray Transient Source in the 2XMM Catalog: a Tidal Disruption Event Candidate
We have discovered an ultrasoft X-ray transient source, 2XMMi
J184725.1-631724, which was detected serendipitously in two XMM-Newton
observations in the direction of the center of the galaxy IC 4765-f01-1504 at a
redshift of 0.0353. These two observations were separated by 211 days, with the
0.2-10 keV absorbed flux increasing by a factor of about 9. Their spectra are
best described by a model dominated by a thermal disk or a single-temperature
blackbody component (contributing >80% of the flux) plus a weak power-law
component. The thermal emission has a temperature of a few tens of eV, and the
weak power-law component has a photon index of ~3.5. Similar to the black hole
X-ray binaries in the thermal state, our source exhibits an accretion disk
whose luminosity appears to follow the relation. This would
indicate that the black hole mass is about 10^5-10^6 M_sun using the
best-fitting inner disk radius. Both XMM-Newton observations show variability
of about 21% on timescales of hours, which can be explained as due to fast
variations in the mass accretion rate. The source was not detected by ROSAT in
an observation in 1992, indicating a variability factor of >64 over longer
timescales. The source was not detected again in X-rays in a Swift observation
in 2011 February, implying a flux decrease by a factor of >12 since the last
XMM-Newton observation. The transient nature, in addition to the extreme
softness of the X-ray spectra and the inactivity of the galaxy implied by the
lack of strong optical emission lines, makes it a candidate tidal disruption
event. If this is the case, the first XMM-Newton observation would have been in
the rising phase, and the second one in the decay phase.Comment: 12 pages, 6 figures. Accepted for publication in Ap
Statistical Properties of Narrow-Line Seyfert 1 galaxies
The number of publications considering Narrow-Line Seyfert 1 galaxies has
increased dramatically in recent years. Especially after the launch of the
X-ray missions ROSAT and ASCA, Narrow-Line Seyfert 1s have become very popular.
In these proceedings I will give an overview of how they are distributed over
the electromagnetic spectrum. I will describe what we know about them at radio,
infrared, optical, and X-ray bands, and how they differ and how they are
similar to Broad-Line Seyfert 1s. Finally I will introduce a method to find
them with high probability.Comment: Invited talk presented at the Joint MPE,AIP,ESO workshop on NLS1s,
Bad Honnef, Dec. 1999, to appear in New Astronomy Reviews; also available at
http://wave.xray.mpe.mpg.de/conferences/nls1-worksho
Radio-loud Narrow-Line Type 1 Quasars
We present the first systematic study of (non-radio-selected) radio-loud
narrow-line Seyfert 1 (NLS1) galaxies. Cross-correlation of the `Catalogue of
Quasars and Active Nuclei' with several radio and optical catalogues led to the
identification of 11 radio-loud NLS1 candidates including 4 previously known
ones. Most of the radio-loud NLS1s are compact, steep spectrum sources
accreting close to, or above, the Eddington limit. The radio-loud NLS1s of our
sample are remarkable in that they occupy a previously rarely populated regime
in NLS1 multi-wavelength parameter space. While their [OIII]/H_beta and
FeII/H_beta intensity ratios almost cover the whole range observed in NLS1
galaxies, their radio properties extend the range of radio-loud objects to
those with small widths of the broad Balmer lines. Among the radio-detected
NLS1 galaxies, the radio index R distributes quite smoothly up to the critical
value of R ~ 10 and covers about 4 orders of magnitude in total. Statistics
show that ~7% of the NLS1 galaxies are formally radio-loud while only 2.5%
exceed a radio index R > 100. Several mechanisms are considered as explanations
for the radio loudness of the NLS1 galaxies and for the lower frequency of
radio-louds among NLS1s than quasars. While properties of most sources (with
2-3 exceptions) generally do not favor relativistic beaming, the combination of
accretion mode and spin may explain the observations. (abbreviated)Comment: Astronomical Journal (first submitted in Dec. 2005); 45 pages incl. 1
colour figur
A Quasar Catalog with Simultaneous UV, Optical and X-ray Observations by Swift
We have compiled a catalog of optically-selected quasars with simultaneous
observations in UV/optical and X-ray bands by the Swift Gamma Ray Burst
Explorer. Objects in this catalog are identified by matching the Swift
pointings with the Sloan Digital Sky Survey Data Release 5 quasar catalog. The
final catalog contains 843 objects, among which 637 have both UVOT and XRT
observations and 354 of which are detected by both instruments. The overall
X-ray detection rate is ~60% which rises to ~85% among sources with at least 10
ks of XRT exposure time. We construct the time-averaged spectral energy
distribution for each of the 354 quasars using UVOT photometric measurements
and XRT spectra. From model fits to these SEDs, we find that the big blue bump
contributes about 0.3 dex to the quasar luminosity. We re-visit the
alpha_ox-L_uv relation by selecting a clean sample with only type 1 radio-quiet
quasars; the dispersion of this relation is reduced by at least 15% compared to
studies that use non-simultaneous UV/optical and X-ray data. We only found a
weak correlation between L/L_Edd and alpha_uv. We do not find significant
correlations between alpha_x and alpha_ox, alpha_ox and alpha_uv, and alpha_x
and Log L(0.3-10 keV). The correlations between alpha_uv and alpha_x, alpha_ox
and alpha_x, alpha_ox and alpha_uv, L/L_Edd and alpha_x, and L/L_Edd and
alpha_ox are stronger amongst low-redshift quasars, indicating that these
correlations are likely driven by the changes of SED shape with accretion
state.Comment: 63 pages, 22 figures, accepted by ApJ
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