60 research outputs found
Shocks and dust survival in nearby active galaxies: implications for the alignment effect
One of the most popular explanations for the so-called alignment effect in
high redshift (z>0.7) radio galaxies is the scattering by dust of the hidden
quasar light. As shown by De Young (1998) a problem with the dust scattering
model is that the short destruction time-scale for dust grains means that they
will not survive the passage of the radio jet.
We investigate the survival of dust in the extended ionised gas of nearby
active galaxies with jet/gas interactions. We discuss the implications on the
alignment effect of high redshift (>0.7) radio galaxies. We conclude that
although shocks are likely to destroy dust grains in regions of interaction,
dust might survive in enough quantities to scatter light from the active
nucleus and produce alignment between scattered light and the radio structures.
We propose an observational test to investigate the existence of dust in
shocked regions based on the sensitivity of calcium to depletion onto dust
grains.Comment: 8 pages, 1 Figure, 3 tables, accepted for publication in MNRA
Following the TraCS of exoplanets with Pan-Planets: Wendelstein-1b and Wendelstein-2b
Hot Jupiters seem to get rarer with decreasing stellar mass. The goal of the
Pan-Planets transit survey was the detection of such planets and a statistical
characterization of their frequency. Here, we announce the discovery and
validation of two planets found in that survey, Wendelstein-1b and
Wendelstein-2b, which are two short-period hot Jupiters that orbit late K host
stars. We validated them both by the traditional method of radial velocity
measurements with the HIgh Resolution Echelle Spectrometer (HIRES) and the
Habitable-zone Planet Finder (HPF) instruments and then by their Transit Color
Signature (TraCS). We observed the targets in the wavelength range of Angstr\"om and performed a simultaneous multiband transit fit and
additionally determined their thermal emission via secondary eclipse
observations. Wendelstein-1b is a hot Jupiter with a radius of
and mass of ,
orbiting a K7V dwarf star at a period of d, and has an estimated surface
temperature of about K. Wendelstein-2b is a hot Jupiter with
a radius of and a mass of
, orbiting a K6V dwarf star at a period of
d, and has an estimated surface temperature of about
K. With this, we demonstrate that multiband photometry is
an effective way of validating transiting exoplanets, in particular for fainter
targets since radial velocity (RV) follow-up becomes more and more costly for
those targets.Comment: 14 pages, 12 figures. Accepted for publication in A&
A Brief History of AGN
Astronomers knew early in the twentieth century that some galaxies have
emission-line nuclei. However, even the systematic study by Seyfert (1943) was
not enough to launch active galactic nuclei (AGN) as a major topic of
astronomy. The advances in radio astronomy in the 1950s revealed a new universe
of energetic phenomena, and inevitably led to the discovery of quasars. These
discoveries demanded the attention of observers and theorists, and AGN have
been a subject of intense effort ever since. Only a year after the recognition
of the redshifts of 3C 273 and 3C 48 in 1963, the idea of energy production by
accretion onto a black hole was advanced. However, acceptance of this idea came
slowly, encouraged by the discovery of black hole X-ray sources in our Galaxy
and, more recently, supermassive black holes in the center of the Milky Way and
other galaxies. Many questions remain as to the formation and fueling of the
hole, the geometry of the central regions, the detailed emission mechanisms,
the production of jets, and other aspects. The study of AGN will remain a
vigorous part of astronomy for the foreseeable future.Comment: 37 pages, no figures. Uses aaspp4.sty. To be published in
Publications of the Astronomical Society of the Pacific, 1999 Jun
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
Прецеденты снижения распространенности немецкого языка в Бразилии во второй половине XX вв. (на примере опроса учащихся школ штата Санта-Катарина)
Although functional magnetic resonance imaging (fMRI) has gained increasing importance in investigating neural substrates of anxiety disorders, less is known about the stress eliciting properties of the scanner environment itself. The aim of the study was to investigate feasibility, self-reported distress and anxiety management strategies during an fMRI experiment in a comprehensive sample of patients with panic disorder and agoraphobia (PD/AG). Within the national research network PANIC-NET, n = 89 patients and n = 90 controls participated in a multicenter fMRI study. Subjects completed a retrospective questionnaire on self-reported distress, including a habituation profile and exploratory questions about helpful strategies. Drop-out rates and fMRI quality parameters were employed as markers of study feasibility. Different anxiety measures were used to identify patients particularly vulnerable to increased scanner anxiety and impaired data quality. Three (3.5%) patients terminated the session prematurely. While drop-out rates were comparable for patients and controls, data quality was moderately impaired in patients. Distress was significantly elevated in patients compared to controls; claustrophobic anxiety was furthermore associated with pronounced distress and lower fMRI data quality in patients. Patients reported helpful strategies, including motivational factors and cognitive coping strategies. The feasibility of large-scale fMRI studies on PD/AG patients could be proved. Study designs should nevertheless acknowledge that the MRI setting may enhance stress reactions. Future studies are needed to investigate the relationship between self-reported distress and fMRI data in patient groups that are subject to neuroimaging research
Intranight polarization variability in radio-loud and radio-quiet AGN
(Abriged) Intranight polarization variability in AGN has not been studied
extensively so far. Studying the variability in polarization makes it possibly
to distinguish between different emission mechanisms. Thus it can help
answering the question if intranight variability in radio-loud and radio-quiet
AGN is of the same or of fundamentally different origin. In this paper we
investigate intranight polarization variability in AGN. Our sample consists of
28 AGN at low to moderate redshifts (0.048 < z < 1.036), 12 of which are
radio-quiet quasars (RQQs) and 16 are radio-loud blazars. The subsample of
blazars consists of eight flat-spectrum radio-quasars (FSRQs) and eight BL Lac
objects. We find clear differences between the two samples. A majority of the
radio-loud AGN show moderate to high degrees of polarization, more than half of
them also show variability in polarization. There seems to be a dividing line
for polarization intranight variability at P~5 per cent over which all objects
vary in polarization. Only two out of 12 radio-quiet quasars show polarized
emission, both at levels of P<1 per cent. The lack of polarization intranight
variability in radio-quiet AGN points towards accretion instabilities being the
cause for intranight flux variability whereas the high duty cycle of
polarization variability in radio-loud objects is more likely caused by
instabilities in the jet or changes of physical conditions in the jet plasma.Comment: Accepted for Publication in MNRAS (17 pages, 14 figures, 4 tables
The XMM-Newton view of PG quasars I. X-ray continuum and absorption
We present results of a systematic analysis of the XMM-Newton spectra of 40
quasars (QSOs) (z<1.72) from the Palomar-Green (PG) Bright Quasar Survey sample
(M_B<-23). The sample includes 35 radio-quiet quasars (RQQs) and 5 radio-loud
quasars (RLQs). The analysis of the spectra above 2 keV reveals that the hard
X-ray continuum emission can be modeled with a power law component with
= 1.89+/-0.11 and = 1.63^(+0.02)_(-0.01) for the
RQQs and RLQs, respectively. Below 2 keV, a strong, broad excess is present in
most QSO spectra. This feature has been fitted with four different models
assuming several physical scenarios. All tested models (blackbody, multicolor
blackbody, bremsstrahlung and power law) satisfactorily fitted the majority of
the spectra. However, none of them is able to provide an adequate
parameterization for the soft excess emission in all QSOs, indicating the
absence of an universal shape for this spectral feature. An additional cold
absorption component was required only in three sources. On the other hand, as
recently pointed out by Porquet et al. (2004) for a smaller sample of PG QSOs,
warm absorber features are present in ~50% of the QSO spectra in contrast with
their rare occurrence (~5-10%) found in previous studies. The XMM-Newton view
of optically-selected bright QSOs therefore suggests that there are no
significant difference in the X-ray spectral properties once compared with the
low-luminosity Seyfert 1 galaxies. Properties of the Fe Kalpha emission lines
are presented in a companion paper.Comment: To be published in A&
Time dependent simulations of multiwavelength variability of the blazar Mrk 421 with a Monte Carlo multi-zone code
(abridged) We present a new time-dependent multi-zone radiative transfer code
and its application to study the SSC emission of Mrk 421. The code couples
Fokker-Planck and Monte Carlo methods, in a 2D geometry. For the first time all
the light travel time effects (LCTE) are fully considered, along with a proper
treatment of Compton cooling, which depends on them. We study a set of simple
scenarios where the variability is produced by injection of relativistic
electrons as a `shock front' crosses the emission region. We consider emission
from two components, with the second one either being pre-existing and
co-spatial and participating in the evolution of the active region, or
spatially separated and independent, only diluting the observed variability.
Temporal and spectral results of the simulation are compared to the
multiwavelength observations of Mrk 421 in March 2001. We find parameters that
can adequately fit the observed SEDs and multiwavelength light curves and
correlations. There remain however a few open issues, most notably: i)
systematic soft intra-band X-ray lags. ii) The quadratic correlation between
the TeV and X-ray flux during the flare decay has not been reproduced. These
features are among those affected by the spatial extent and geometry of the
source. The difficulty of producing hard X-ray lags is exacerbated by a bias
towards soft lags caused by the combination of energy dependent radiative
cooling time-scales and LCTE. About the second emission component, our results
strongly favor the scenario where it is co-spatial and it participates in the
flare evolution, suggesting that different phases of activity may occur in the
same region. The cases presented in this paper represent only an initial study,
and despite their limited scope they make a strong case for the need of true
time-dependent and multi-zone modeling.Comment: Accepted for publication in MNRAS, 20 pages, 9 figure
MOJAVE. X. Parsec-Scale Jet Orientation Variations and Superluminal Motion in AGN
We describe the parsec-scale kinematics of 200 AGN jets based on 15 GHz VLBA data obtained between 1994 Aug 31 and 2011 May 1. We present new VLBA 15 GHz images of these and 59 additional AGN from the MOJAVE and 2 cm Survey programs. Nearly all of the 60 most heavily observed jets show significant changes in their innermost position angle over a 12 to 16 year interval, ranging from 10º to 150º on the sky, corresponding to intrinsic variations of ∼0.5º to ∼2º. The BL Lac jets show smaller variations than quasars. Roughly half of the heavily observed jets show systematic position angle trends with time, and 20 show indications of oscillatory behavior. The time spans of the data sets are too short compared to the fitted periods (5 to 12 y), however, to reliably establish periodicity. The rapid changes and large jumps in position angle seen in many cases suggest that the superluminal AGN jet features occupy only a portion of the entire jet cross section, and may be energized portions of thin instability structures within the jet. We have derived vector proper motions for 887 moving features in 200 jets having at least five VLBA epochs. For 557 well-sampled features, there are sufficient data to additionally study possible accelerations. We find that the moving features are generally non-ballistic, with 70% of the well-sampled features showing either significant accelerations or non-radial motions. Inward motions are rare (2% of all features), are slow (<0.1 mas per y), are more prevalent in BL Lac jets, and are typically found within 1 mas of the unresolved core feature. There is a general trend of increasing apparent speed with distance down the jet for both radio galaxies and BL Lac objects. In most jets, the speeds of the features cluster around a characteristic value, yet there is a considerable dispersion in the distribution. Orientation variations within the jet cannot fully account for the dispersion, implying that the features have a range of Lorentz factor and/or pattern speed. Very slow pattern speed features are rare, comprising only 4% of the sample, and are more prevalent in radio galaxy and BL Lac jets. We confirm a previously reported upper envelope to the distribution of speed versus beamed luminosity for moving jet features. Below 10^26 W Hz−1 there is a fall-off in maximum speed with decreasing 15 GHz radio luminosity. The general shape of the envelope implies that the most intrinsically powerful AGN jets have a wide range of Lorentz factors up to ∼40, while intrinsically weak jets are only mildly relativistic
Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector
A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements
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