9,954 research outputs found
On Derandomizing Local Distributed Algorithms
The gap between the known randomized and deterministic local distributed
algorithms underlies arguably the most fundamental and central open question in
distributed graph algorithms. In this paper, we develop a generic and clean
recipe for derandomizing LOCAL algorithms. We also exhibit how this simple
recipe leads to significant improvements on a number of problem. Two main
results are:
- An improved distributed hypergraph maximal matching algorithm, improving on
Fischer, Ghaffari, and Kuhn [FOCS'17], and giving improved algorithms for
edge-coloring, maximum matching approximation, and low out-degree edge
orientation. The first gives an improved algorithm for Open Problem 11.4 of the
book of Barenboim and Elkin, and the last gives the first positive resolution
of their Open Problem 11.10.
- An improved distributed algorithm for the Lov\'{a}sz Local Lemma, which
gets closer to a conjecture of Chang and Pettie [FOCS'17], and moreover leads
to improved distributed algorithms for problems such as defective coloring and
-SAT.Comment: 37 page
The iron K line complex in NGC1068: implications for X-ray reflection in the nucleus
We report a new analysis of ASCA data on the iron K line complex in NGC1068.
The line complex basically consists of three components, as previously
reported. A weak red wing of the 6.4 keV fluoresence iron K line is found. A
plausible explanation is Compton scattering in optically thick, cold matter
which can be identified with an obscuring torus or cold gas in the host galaxy.
We also show that this `Compton shoulder' should be observable with ASCA using
a simulated reflection spectrum. In order to explain the two higher energy
lines as well as the cold 6.4 keV line, we fit the ASCA data with a composite
model of cold and warm reflection. This shows that cold reflection dominates
the observed X-ray emission above 4 keV. The two higher energy lines have large
equivalent width with respect to the warm-scattered continuum, suggesting that
efficient resonant scattering operates. The line energies are systematically
lower than those expected from resonant lines for FeXXV and FeXXVI by 100 eV.
The redshifts may be due to either the ionized gas of the warm mirror receding
at a radial velocity of 4000-5000 km/s, or effects of Compton scattering in a
complicated geometry.Comment: 7 pages, 4 figures, to appear in MNRA
Bulk Comptonization spectra in blazars
We study the time dependent spectra produced via the bulk Compton process by
a cold, relativistic shell of plasma moving (and accelerating) along the jet of
a blazar, scattering on external photons emitted by the accretion disc and
reprocessed in the broad line region. Bulk Comptonization of disc photons is
shown to yield a spectral component contributing in the far UV band, and would
then be currently unobservable. On the contrary, the bulk Comptonization of
broad line photons may yield a significant feature in the soft X-ray band. Such
a feature is time-dependent and transient, and dominates over the non thermal
continuum only when: a) the dissipation occurs close to, but within, the broad
line region; b) other competing processes, like the synchrotron self-Compton
emission, yield a negligible flux in the X-ray band. The presence of a bulk
Compton component may account for the X-ray properties of high redshift blazars
that show a flattening (and possibly a hump) in the soft X-rays, previously
interpreted as due to intrinsic absorption. We discuss why the conditions
leading to a detectable bulk Compton feature might be met only occasionally in
high redshift blazars, concluding that the absence of such a feature in the
spectra of most blazars should not be taken as evidence against
matter--dominated relativistic jets. The detection of such a component carries
key information on the bulk Lorentz factor and kinetic energy associated to
(cold) leptons.Comment: 8 pages; 4 figures; MNRAS, accepte
Job Developer Types, Placement Practices and Outcomes Technical Report
Despite numerous employment initiatives, people with disabilities are significantly more likely to experience unemployment and consequently, reduced economic and social well-being and a dminished quality of life than their non disabled peers. In a recent national survey of employers, less than 14% of companies indicated that they actively recruit jobseekers with disabilities. Thus, the role of the job development professional is pivotal to helping job seekers with disabilities to find, secure and maintain employment. This research report examined the attitudes and beliefs of job development processionals articulated in a previous technical report by TransCen, Inc. and looked to further explore the relationship between the types, other personal characteristics and placement outcomes of the various job developer types
Radio and X-ray observations of an exceptional radio flare in the extreme z=4.72 blazar GB B1428+4217
We report on the extreme behaviour of the high redshift blazar GB B1428+4217
at z=4.72. A continued programme of radio measurements has revealed an
exceptional flare in the lightcurve, with the 15.2 GHz flux density rising by a
factor ~3 from ~140 mJy to ~430 mJy in a rest-frame timescale of only ~4 months
-- much larger than any previous flares observed in this source. In addition to
new measurements of the 1.4-43 GHz radio spectrum we also present the analysis
and results of a target-of-opportunity X-ray observation using XMM-Newton, made
close to the peak in radio flux. Although the X-ray data do not show a flare in
the high energy lightcurve, we are able to confirm the X-ray spectral
variability hinted at in previous observations. GB B1428+4217 is one of several
high-redshift radio-loud quasars that display a low energy break in the X-ray
spectrum, probably due to the presence of excess absorption in the source.
X-ray spectral analysis of the latest XMM-Newton data is shown to be consistent
with the warm absorption scenario which we have hypothesized previously. Warm
absorption is also consistent with the observed X-ray spectral variability of
the source, in which the spectral changes can be successfully accounted-for
with a fixed column density of material in which the ionization state is
correlated with hardness of the underlying power-law emission.Comment: 8 pages, 5 figures, MNRAS accepte
Two-temperature coronae in active galactic nuclei
We show that coronal magnetic dissipation in thin active sheets that sandwich
standard thin accretion disks in active galactic nuclei may account for
canonical electron temperatures of a few K if protons acquire most
of the dissipated energy. Coulomb collisions transfer energy from the ions to
the electrons, which subsequently cool rapidly by inverse-Compton scattering.
In equilibrium, the proton energy density likely exceeds that of the magnetic
field and both well exceed the electron and photon energy densities. The
Coulomb energy transfer from protons to electrons is slow enough to maintain a
high proton temperature, but fast enough to explain observed rapid X-ray
variabilities in Seyferts. The K electron temperature is insensitive
to the proton temperature when the latter is K.Comment: 5 pages LaTex, and 2 .ps figures, submitted to MNRAS, 4/9
Variability of the extreme z=4.72 blazar, GB 1428+4217
We report X-ray and radio variability of GB 1428+4217 which confirm its
blazar nature. IR observations reveal a powerful optical-UV component, not
obscured by dust, which is suggestive of the presence of a billion solar mass
black hole, already formed by z ~ 5. A detailed comparison of the broad band
spectral properties of GB 1428+4217 with those of nearby blazars shows it to be
extreme, but nevertheless consistent with the trend found for nearby sources.Comment: MNRAS, in press - 5 pages, 5 figure
- âŠ