3,386 research outputs found
On the evolutionary behaviour of BL Lac objects
We present a new well defined sample of BL Lac objects selected from the
ROSAT All-Sky Survey (RASS). The sample consists of 39 objects with 35 forming
a flux limited sample down to f_X = 8 x 10^{-13} cgs, redshifts are known for
33 objects (and 31 of the complete sample). X-ray spectral properties were
determined for each object individually with the RASS data. The luminosity
function of RASS selected BL Lac objects is compatible with results provided by
objects selected with the Einstein observatory, but the RASS selected sample
contains objects with luminosities at least tenfold higher. Our analysis
confirms the negative evolution for X-ray selected BL Lac objects found in a
sample by the Einstein observatory, the parameterization provides similar
results. A subdivision of the sample into halves according to the X-ray to
optical flux ratio yielded unexpected results. The extremely X-ray dominated
objects have higher redshifts and X-ray luminosities and only this subgroup
shows clear signs of strong negative evolution. The evolutionary behaviour of
objects with an intermediate spectral energy distribution between X-ray and
radio dominated is compatible with no evolution at all. Consequences for
unified schemes of X-ray and radio selected BL Lac objects are discussed.We
suggest that the intermediate BL Lac objects are the basic BL Lac population.
The distinction between the two subgroups can be explained if extreme X-ray
dominated BL Lac objects are observed in a state of enhanced X-ray activity.Comment: 14 pages incl. 8 figures, accepted by A&
Online instructional videos as a complimentary method of teaching practical rehabilitation skills for groups and individuals
Online instructional videos are becoming increasingly common within education. This study adopts a quasi-experimental 2 × 2 crossover design (control and experimental groups) to evaluate the efficacy of instructional videos to teach practical rehabilitation skills. The students performed practical sessions in class and were formatively assessed by their lecturers. The results demonstrate that the group effect was moderate with an effect size of 0.68, CI [0.04, 1.31]; the magnitude-based inferences indicate the probabilities the video was beneficial or trivial or harmful were 93.9% or 5.4% or 0.6%. The results of the study demonstrate that the instructional videos had a small to moderate beneficial effect on all assessed criteria
Dimensionality effects on non-equilibrium electronic transport in Cu nanobridges
We report on non-equilibrium electronic transport through normal-metal (Cu)
nanobridges coupled to large reservoirs at low temperatures. We observe a
logarithmic temperature dependence of the zero-bias conductance, as well as a
universal scaling behavior of the differential conductance. Our results are
explained by electron-electron interactions in diffusive metals in the
zero-dimensional limit.Comment: RevTex, 4 page
Whirlpool: Improving Dynamic Cache Management with Static Data Classification
Cache hierarchies are increasingly non-uniform and difficult to manage. Several techniques, such as scratchpads or reuse hints, use static information about how programs access data to manage the memory hierarchy. Static techniques are effective on regular programs, but because they set fixed policies, they are vulnerable to changes in program behavior or available cache space. Instead, most systems rely on dynamic caching policies that adapt to observed program behavior. Unfortunately, dynamic policies spend significant resources trying to learn how programs use memory, and yet they often perform worse than a static policy. We present Whirlpool, a novel approach that combines static information with dynamic policies to reap the benefits of each. Whirlpool statically classifies data into pools based on how the program uses memory. Whirlpool then uses dynamic policies to tune the cache to each pool. Hence, rather than setting policies statically, Whirlpool uses static analysis to guide dynamic policies. We present both an API that lets programmers specify pools manually and a profiling tool that discovers pools automatically in unmodified binaries.
We evaluate Whirlpool on a state-of-the-art NUCA cache. Whirlpool significantly outperforms prior approaches: on sequential programs, Whirlpool improves performance by up to 38% and reduces data movement energy by up to 53%; on parallel programs, Whirlpool improves performance by up to 67% and reduces data movement energy by up to 2.6x.National Science Foundation (U.S.) (grant CCF-1318384)National Science Foundation (U.S.) (CAREER-1452994)Samsung (Firm) (GRO award
The 1 keV to 200 keV X-ray Spectrum of NGC 2992 and NGC 3081
The Seyfert 2 galaxies NGC 2992 and NGC 3081 have been observed by INTEGRAL
and Swift. We report about the results and the comparison of the spectrum above
10 keV based on INTEGRAL IBIS/ISGRI, Swift/BAT, and BeppoSAX/PDS. A spectrum
can be extracted in the X-ray energy band ranging from 1 keV up to 200 keV.
Although NGC 2992 shows a complex spectrum below 10 keV, the hard tail observed
by various missions exhibits a slope with photon index = 2, independent on the
flux level during the observation. No cut-off is detectable up to the detection
limit around 200 keV. In addition, NGC 3081 is detected in the INTEGRAL and
Swift observation and also shows an unbroken Gamma = 1.8 spectrum up to 150
keV. These two Seyfert galaxies give further evidence that a high-energy
cut-off in the hard X-ray spectra is often located at energies E_C >> 100 keV.
In NGC 2992 a constant spectral shape is observed over a hard X-ray luminosity
variation by a factor of 11. This might indicate that the physical conditions
of the emitting hot plasma are constant, while the amount of plasma varies, due
to long-term flaring activity.Comment: 8 pages, 4 figures, accepted for publication in Ap
The HRX-BL Lac sample - evolution of BL Lac objects
The unification of X-ray and radio selected BL Lacs has been an outstanding
problem in the blazar research in the past years. Recent investigations have
shown that the gap between the two classes can be filled with intermediate
objects and that apparently all differences can be explained by mutual shifts
of the peak frequencies of the synchrotron and inverse Compton component of the
emission. We study the consequences of this scheme using a new sample of X-ray
selected BL Lac objects comprising 104 objects with z<0.9 and a mean redshift
z=0.34. 77 BL Lacs, of which the redshift could be determined for 64 (83%)
objects, form a complete sample. The new data could not confirm our earlier
result, drawn from a subsample, that the negative evolution vanishes below a
synchrotron peak frequency log (peak-frequency) = 16.5. The complete sample
shows negative evolution at the 2 sigma level ( = 0.42 +- 0.04). We
conclude that the observed properties of the HRX BL Lac sample show typical
behaviour for X-ray selected BL Lacs. They support an evolutionary model, in
which flat-spectrum radio quasars (FSRQ) with high energetic jets evolve
towards low frequency peaked (mostly radio-selected) BL Lac objects and later
on to high frequency peaked (mostly X-ray selected) BL Lacs.Comment: 24 pages, 35 figures, accepted by A&
Hard X-ray Variability of AGN
Aims: Active Galactic Nuclei are known to be variable throughout the
electromagnetic spectrum. An energy domain poorly studied in this respect is
the hard X-ray range above 20 keV.
Methods: The first 9 months of the Swift/BAT all-sky survey are used to study
the 14 - 195 keV variability of the 44 brightest AGN. The sources have been
selected due to their detection significance of >10 sigma. We tested the
variability using a maximum likelihood estimator and by analysing the structure
function.
Results: Probing different time scales, it appears that the absorbed AGN are
more variable than the unabsorbed ones. The same applies for the comparison of
Seyfert 2 and Seyfert 1 objects. As expected the blazars show stronger
variability. 15% of the non-blazar AGN show variability of >20% compared to the
average flux on time scales of 20 days, and 30% show at least 10% flux
variation. All the non-blazar AGN which show strong variability are
low-luminosity objects with L(14-195 keV) < 1E44 erg/sec.
Conclusions: Concerning the variability pattern, there is a tendency of
unabsorbed or type 1 galaxies being less variable than the absorbed or type 2
objects at hardest X-rays. A more solid anti-correlation is found between
variability and luminosity, which has been previously observed in soft X-rays,
in the UV, and in the optical domain.Comment: 9 pages, 7 figures, accepted for publication in A&
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