1,211 research outputs found
Variability of Active Galactic Nuclei from the Optical to X-ray Regions
Some progress in understanding AGN variability is reviewed. Reprocessing of
X-ray radiation to produce significant amounts of longer-wavelength continua
seems to be ruled out. In some objects where there has been correlated X-ray
and optical variability, the amplitude of the optical variability has exceeded
the amplitude of X-ray variability. We suggest that accelerated particles
striking material could be linking X-ray and optical variability (as in
activity in the solar chromosphere). Beaming effects could be significant in
all types of AGN. The diversity in optical/X-ray relationships at different
times in the same object, and between different objects, might be explained by
changes in geometry and directions of motion relative to our line of sight.
Linear shot-noise models of the variability are ruled out; instead there must
be large-scale organization of variability. Variability occurs on
light-crossing timescales rather than viscous timescales and this probably
rules out the standard Shakura-Sunyaev accretion disk. Radio-loud and
radio-quiet AGNs have similar continuum shapes and similar variability
properties. This suggests similar continuum origins and variability mechanisms.
Despite their extreme X-ray variability, narrow-line Seyfert 1s (NLS1s) do not
show extreme optical variability.Comment: Invited talk given at Euro Asian Astronomical Society meeting in
Moscow, June 2002. 20 pages, 4 figures. References update
A robust enhancement to the Clarke-Wright savings algorithm
We address the Clarke and Wright (CW) savings algorithm proposed for the Capacitated Vehicle Routing Problem (CVRP). We first consider a recent enhancement which uses the put first larger items idea originally proposed for the bin packing problem and show that the conflicting idea of putting smaller items first has a comparable performance. Next, we propose a robust enhancement to the CW savings formulation. The proposed formulation is normalized to efficiently solve different problems, independent from the measurement units and parameter intervals. To test the performance of the proposed savings function, we conduct an extensive computational study on a large set of well-known instances from the literature. Our results show that the proposed savings function provides shorter distances in the majority of the instances and the average performance is significantly better than previously presented enhancements
Modelling the flow of droplets of bio-pesticide on foliage
The flow of droplets of bio-pesticide, liquid laden with entomapathogenic nematodes (EPNs), over foliage approximated as a planar substrate is investigated theoretically via a simple analytical model and computationally by solving a subset of the Navier-Stokes equations arising from application of the long-wave approximation. That the droplets of interest can be represented as a homogeneous liquid is established via complementary experiments revealing the presence of EPNs to have negligible influence on bio-pesticide droplet spray distribution predeposition. Both approaches are used to study key issues affecting the migration of droplets over substrates relevant to pesticide deposition processes, including the effect (i) of droplet size and flow inertia on droplet morphology and coverage and (ii) of adaxial (above the leaf) or abaxial (under the leaf) flow orientations. The computational results obtained when inertia is accounted for are generally found to compare well with those given by the simple analytical model − a droplet's velocity relaxes to its terminal value very quickly, at which point gravitational, viscous, and hysteresis forces are in balance; substrate orientation is found to have only a minor influence on the extent of droplet migration
The End of the Lines for OX 169: No Binary Broad-Line Region
We show that unusual Balmer emission line profiles of the quasar OX 169,
frequently described as either self-absorbed or double peaked, are actually
neither. The effect is an illusion resulting from two coincidences. First, the
forbidden lines are quite strong and broad. Consequently, the [N II]6583 line
and the associated narrow-line component of H-alpha present the appearance of
twin H-alpha peaks. Second, the redshift of 0.2110 brings H-beta into
coincidence with Na I D at zero redshift, and ISM absorption in Na I D divides
the H-beta emission line. In spectra obtained over the past decade, we see no
substantial change in the character of the line profiles, and no indication of
intrinsic double-peaked structure. The H-gamma, Mg II, and Ly-alpha emission
lines are single peaked, and all of the emission-line redshifts are consistent
once they are correctly attributed to their permitted and forbidden-line
identifications. A systematic shift of up to 700 km/s between broad and narrow
lines is seen, but such differences are common, and could be due to
gravitational and transverse redshift in a low-inclination disk. Stockton &
Farnham (1991) had called attention to an apparent tidal tail in the host
galaxy of OX 169, and speculated that a recent merger had supplied the nucleus
with a coalescing pair of black holes which was now revealing its existence in
the form of two physically distinct broad-line regions. Although there is no
longer any evidence for two broad emission-line regions in OX 169, binary black
holes should form frequently in galaxy mergers, and it is still worthwhile to
monitor the radial velocities of emission lines which could supply evidence of
their existence in certain objects.Comment: 19 pages, 5 figures, accepted for publication in Ap.
Long-term variability of the optical spectra of NGC 4151: I. Light curves and flux correlations
Results of a long-term spectral monitoring of the active galactic nucleus of
NGC 4151 are presented (11 years, from 1996 to 2006). High quality spectra
(S/N>50 in the continuum near Halpha and Hbeta) were obtained in the spectral
range ~4000 to 7500 \AA, with a resolution between 5 and 15 A, using the 6-m
and the 1-m SAO's telescopes (Russia), the GHAO's 2.1-m telescope (Cananea,
Mexico), and the OAN-SPM's 2.1-m telescope (San-Pedro, Mexico). The observed
fluxes of the Halpha, Hbeta, Hgamma and HeII emission lines and of the
continuum at the observed wavelength 5117 A, were corrected for the position
angle, the seeing and the aperture effects.
We found that the continuum and line fluxes varied strongly (up to a factor
6) during the monitoring period. The emission was maximum in 1996-1998, and
there were two minima, in 2001 and in 2005. The Halpha, Hgamma and He II fluxes
were well correlated with the Hbeta flux. We considered three characteristic
periods during which the Hbeta and Halpha profiles were similar: 1996-1999,
2000-2001 and 2002-2006. The line to continuum flux ratios were different; in
particular during the first period, the lines were not correlated with the
continuum and saturated at high fluxes. In the second and third period, where
the continuum flux was small, the Halpha and Hbeta fluxes were well correlated
to the continuum flux, meaning that the ionizing continuum was a good
extrapolation of the optical continuum. The CCFs are often asymmetrical and the
time lags between the lines and the continuum are badly defined indicating the
presence of a complex BLR, with dimensions from 1 to 50 light-days.Comment: A&A, accepte
Multi-Wavelength Monitoring of the Changing-Look AGN NGC 2617 during State Changes
Optical and near-infrared photometry, optical spectroscopy, and soft X-ray and UV monitoring of the changing-look active galactic nucleus NGC 2617 show that it continues to have the appearance of a type-1 Seyfert galaxy. An optical light curve for 2010-2017 indicates that the change of type probably occurred between 2010 October and 2012 February and was not related to the brightening in 2013. In 2016 and 2017 NGC 2617 brightened again to a level of activity close to that in 2013 April. However, in 2017 from the end of the March to end of July 2017 it was in very low level and starting to change back to a Seyfert 1.8. We find variations in all passbands and in both the intensities and profiles of the broad Balmer lines. A new displaced emission peak has appeared in Hβ. X-ray variations are well correlated with UV-optical variability and possibly lead by ̃2-3 d. The K band lags the J band by about 21.5 ± 2.5 d and lags the combined B + J bands by ̃25 d. J lags B by about 3 d. This could be because J-band variability arises predominantly from the outer part of the accretion disc, while K-band variability is dominated by thermal re-emission by dust. We propose that spectral-type changes are a result of increasing central luminosity causing sublimation of the innermost dust in the hollow bi-conical outflow. We briefly discuss various other possible reasons that might explain the dramatic changes in NGC 2617.Fil: Oknyansky, V. L.. Sternberg Astronomical Institute; RusiaFil: Gaskell, C. M.. Department of Astronomy and Astrophysics. University of California. Santa Cruz; Estados UnidosFil: Mikailov, K. M.. Shamakhy Astrophysical Observatory, National Academy of Sciences.
Pirkuli; AzerbaiyánFil: Lipunov, V. M.. Sternberg Astronomical Institute. M.V.Lomonosov Moscow State University ; RusiaFil: Shatsky, N. I.. Sternberg Astronomical Institute. M.V.Lomonosov Moscow State University; RusiaFil: Tsygankov, S. S.. Tuorla Observatory, Department of Physics and Astronomy. University of Turku.; FinlandiaFil: Gorbovskoy, E. S.. Sternberg Astronomical Institute. M.V.Lomonosov Moscow State University; RusiaFil: Tatarnikov, A. M.. Sternberg Astronomical Institute. M.V.Lomonosov Moscow State University; RusiaFil: Metlov, V. G.. Sternberg Astronomical Institute. M.V.Lomonosov Moscow State University; RusiaFil: Malanchev, K. L.. Sternberg Astronomical Institute. M.V.Lomonosov Moscow State University; RusiaFil: Brotherton, M.B.. University of Wyoming; Estados UnidosFil: Kasper, D.. University of Wyoming; Estados UnidosFil: Du, P.. Institute of High Energy Physics. Chinese Academy of Sciences; ChinaFil: Chen, X.. School of Space Science and Physics. Shandong University; ChinaFil: Burlak, M. A.. Sternberg Astronomical Institute. M.V.Lomonosov Moscow State University; RusiaFil: Buckley, D. A. H.. The South African Astronomical Observatory; SudáfricaFil: Rebolo, R.. Instituto de Astrofisica de Canarias; EspañaFil: Serra-Ricart, M.. Instituto de Astrofisica de Canarias; EspañaFil: Podestá, R.. Universidad Nacional de San Juan; ArgentinaFil: Levato, O. H.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Instituto de Ciencias Astronómicas, de la Tierra y del Espacio. Universidad Nacional de San Juan. Instituto de Ciencias Astronómicas, de la Tierra y del Espacio; Argentin
Supermassive Binaries and Extragalactic Jets
Some quasars show Doppler shifted broad emission line peaks. I give new
statistics of the occurrence of these peaks and show that, while the most
spectacular cases are in quasars with strong radio jets inclined to the line of
sight, they are also almost as common in radio-quiet quasars. Theories of the
origin of the peaks are reviewed and it is argued that the displaced peaks are
most likely produced by the supermassive binary model. The separations of the
peaks in the 3C 390.3-type objects are consistent with orientation-dependent
"unified models" of quasar activity. If the supermassive binary model is
correct, all members of "the jet set" (astrophysical objects showing jets)
could be binaries.Comment: 31 pages, PostScript, missing figure is in ApJ 464, L105 (see
http://www.aas.org/ApJ/v464n2/5736/5736.html
Pressure induced structural and dynamical changes in liquid Si. An ab-initio study
The static and dynamic properties of liquid Si at high-pressure have been
studied using the orbital free ab-initio molecular dynamics method. Four
thermodynamic states at pressures 4, 8, 14 and 23 GPa are considered. The
calculated static structure shows qualitative agreement with the available
experimental data. We analize the remarkable structural changes occurring
between 8 and 14 GPa along with its effect on several dynamic properties.Comment: 10 pages, 11 figures. Accepted for publication in Journal of Physics:
Condensed Matte
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Do closed survey questions over-estimate public perceptions of food risks?
In this paper we show that the widely accepted methodology for the assessment of risk perception – Likert type survey questions featuring a set of risks with fixed response alternatives measuring the extent of worry or concern – may over-estimate food risk perception. Using a European representative sample survey (n=26,961) that included an open-ended question asking about problems and risks with food and eating, followed by a battery of closed questions assessing food risk perception we find a similar ranking of perceived food risks across the two methods. Across Europe the five priority concerns are chronic food related illness; food origins and quality; acute food related illness; chemical contamination, and adulteration of food. However, the discrepancies between mentioning a risk in the open ended question and the expression of worry about risks in the closed question are substantial. Of those who did not mention a specific risk category in the open question, between 60% and 83% (depending on risk category) expressed worry in the closed question. This parallels previous research on the fear of crime, showing that survey responses lead to greatly inflated estimates of the public’s fear of crime than is evidenced by qualitative questioning. It is also consistent with evidence from research on cognitive aspects of survey methodology suggesting that survey questions may frame the respondent’s thinking about an issue. We conclude with recommendations for the use of branched questions in the quantitative elicitation of public perceptions of risk
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