1,492 research outputs found
Fabrication and test of lightweight honeycomb sandwich structures Final report
Fabrication and testing of lightweight honeycomb sandwich structure
On the relation of optical obscuration and X-ray absorption in Seyfert galaxies
The optical classification of a Seyfert galaxy and whether it is considered
X-ray absorbed are often used interchangeably. But there are many borderline
cases and also numerous examples where the optical and X-ray classifications
appear to be in conflict. In this article we re-visit the relation between
optical obscuration and X-ray absorption in AGNs. We make use of our "dust
color" method (Burtscher et al. 2015) to derive the optical obscuration A_V and
consistently estimated X-ray absorbing columns using 0.3--150 keV spectral
energy distributions. We also take into account the variable nature of the
neutral gas column N_H and derive the Seyfert sub-classes of all our objects in
a consistent way.
We show in a sample of 25 local, hard-X-ray detected Seyfert galaxies (log
L_X / (erg/s) ~ 41.5 - 43.5) that there can actually be a good agreement
between optical and X-ray classification. If Seyfert types 1.8 and 1.9 are
considered unobscured, the threshold between X-ray unabsorbed and absorbed
should be chosen at a column N_H = 10^22.3 / cm^2 to be consistent with the
optical classification.
We find that N_H is related to A_V and that the N_H/A_V ratio is
approximately Galactic or higher in all sources, as indicated previously. But
in several objects we also see that deviations from the Galactic ratio are only
due to a variable X-ray column, showing that (1) deviations from the Galactic
N_H/A_V can simply be explained by dust-free neutral gas within the broad line
region in some sources, that (2) the dust properties in AGNs can be similar to
Galactic dust and that (3) the dust color method is a robust way to estimate
the optical extinction towards the sublimation radius in all but the most
obscured AGNs.Comment: 7 pages, 3 figures, accepted for publication by A&A; updated PDF to
include abstrac
A Hard Look at NGC 5347: Revealing a Nearby Compton-thick AGN
Current measurements show that the observed fraction of Compton-thick (CT) active galactic nuclei (AGN) is smaller than the expected values needed to explain the cosmic X-ray background. Prior fits to the X-ray spectrum of the nearby Seyfert-2 galaxy NGC 5347 (z = 0.00792, D = 35.5 Mpc ) have alternately suggested a CT and Compton-thin source. Combining archival data from Suzaku, Chandra, and—most importantly—new data from NuSTAR, ... See full text for complete abstrac
A population of luminous accreting black holes with hidden mergers
Major galaxy mergers are thought to play an important part in fuelling the
growth of supermassive black holes. However, observational support for this
hypothesis is mixed, with some studies showing a correlation between merging
galaxies and luminous quasars and others showing no such association. Recent
observations have shown that a black hole is likely to become heavily obscured
behind merger-driven gas and dust, even in the early stages of the merger, when
the galaxies are well separated (5 to 40 kiloparsecs). Merger simulations
further suggest that such obscuration and black-hole accretion peaks in the
final merger stage, when the two galactic nuclei are closely separated (less
than 3 kiloparsecs). Resolving this final stage requires a combination of
high-spatial-resolution infrared imaging and high-sensitivity hard-X-ray
observations to detect highly obscured sources. However, large numbers of
obscured luminous accreting supermassive black holes have been recently
detected nearby (distances below 250 megaparsecs) in X-ray observations. Here
we report high-resolution infrared observations of hard-X-ray-selected black
holes and the discovery of obscured nuclear mergers, the parent populations of
supermassive-black-hole mergers. We find that obscured luminous black holes
(bolometric luminosity higher than 2x10^44 ergs per second) show a significant
(P<0.001) excess of late-stage nuclear mergers (17.6 per cent) compared to a
sample of inactive galaxies with matching stellar masses and star formation
rates (1.1 per cent), in agreement with theoretical predictions. Using
hydrodynamic simulations, we confirm that the excess of nuclear mergers is
indeed strongest for gas-rich major-merger hosts of obscured luminous black
holes in this final stage.Comment: To appear in the 8 November 2018 issue of Nature. This is the
authors' version of the wor
Design and fabrication of adjustable red-green-blue LED light arrays for plant research
BACKGROUND: Although specific light attributes, such as color and fluence rate, influence plant growth and development, researchers generally cannot control the fine spectral conditions of artificial plant-growth environments. Plant growth chambers are typically outfitted with fluorescent and/or incandescent fixtures that provide a general spectrum that is accommodating to the human eye and not necessarily supportive to plant development. Many studies over the last several decades, primarily in Arabidopsis thaliana, have clearly shown that variation in light quantity, quality and photoperiod can be manipulated to affect growth and control developmental transitions. Light emitting diodes (LEDs) has been used for decades to test plant responses to narrow-bandwidth light. LEDs are particularly well suited for plant growth chambers, as they have an extraordinary life (about 100,000 hours), require little maintenance, and use negligible energy. These factors render LED-based light strategies particularly appropriate for space-biology as well as terrestrial applications. However, there is a need for a versatile and inexpensive LED array platform where individual wavebands can be specifically tuned to produce a series of light combinations consisting of various quantities and qualities of individual wavelengths. Two plans are presented in this report. RESULTS: In this technical report we describe the practical construction of tunable red-green-blue LED arrays to support research in plant growth and development. Two light fixture designs and corresponding circuitry are presented. The first is well suited for a laboratory environment for use in a finite area with small plants, such as Arabidopsis. The second is expandable and appropriate for growth chambers. The application of these arrays to early plant developmental studies has been validated with assays of hypocotyl growth inhibition/promotion and phototropic curvature in Arabidopsis seedlings. CONCLUSION: The presentation of these proven plans for LED array construction allows the teacher, researcher or electronics aficionado a means to inexpensively build efficient, adjustable lighting modules for plant research. These simple and effective designs permit the construction of useful tools by programs short on electronics expertise. These arrays represent a means to modulate precise quality and quantity in experimental settings to test the effect of specific light combinations in regulating plant growth, development and plant-product yield
X-ray bolometric corrections for Compton-thick active galactic nuclei
We present X-ray bolometric correction factors, (), for Compton-thick (CT) active galactic nuclei (AGN) with the aim
of testing AGN torus models, probing orientation effects, and estimating the
bolometric output of the most obscured AGN. We adopt bolometric luminosities,
, from literature infrared (IR) torus modeling and compile published
intrinsic 2--10 keV X-ray luminosities, , from X-ray torus modeling of
NuSTAR data. Our sample consists of 10 local CT AGN where both of these
estimates are available. We test for systematic differences in
values produced when using two widely used IR torus models and two widely used
X-ray torus models, finding consistency within the uncertainties. We find that
the mean of our sample in the range
erg/s is log
with an intrinsic scatter of dex, and that our derived
values are consistent with previously established relationships between
and and and Eddington ratio. We
investigate if is dependent on by comparing our results on
CT AGN to published results on less-obscured AGN, finding no significant
dependence. Since many of our sample are megamaser AGN, known to be viewed
edge-on, and furthermore under the assumptions of AGN unification whereby
unobscured AGN are viewed face-on, our result implies that the X-ray emitting
corona is not strongly anisotropic. Finally, we present values
for CT AGN identified in X-ray surveys as a function of their observed ,
where an estimate of their intrinsic is not available, and redshift,
useful for estimating the bolometric output of the most obscured AGN across
cosmic time.Comment: Accepted for publication in Ap
Crossover Scaling in Dendritic Evolution at Low Undercooling
We examine scaling in two-dimensional simulations of dendritic growth at low
undercooling, as well as in three-dimensional pivalic acid dendrites grown on
NASA's USMP-4 Isothermal Dendritic Growth Experiment. We report new results on
self-similar evolution in both the experiments and simulations. We find that
the time dependent scaling of our low undercooling simulations displays a
cross-over scaling from a regime different than that characterizing Laplacian
growth to steady-state growth
Optical, near-IR and sub-mm IFU Observations of the nearby dual AGN Mrk 463
We present optical and near-IR Integral Field Unit (IFU) and ALMA band 6
observations of the nearby dual Active Galactic Nuclei (AGN) Mrk 463. At a
distance of 210 Mpc, and a nuclear separation of 4 kpc, Mrk 463 is an
excellent laboratory to study the gas dynamics, star formation processes and
supermassive black hole (SMBH) accretion in a late-stage gas-rich major galaxy
merger. The IFU observations reveal a complex morphology, including tidal
tails, star-forming clumps, and emission line regions. The optical data, which
map the full extent of the merger, show evidence for a biconical outflow and
material outflowing at 600 km s, both associated with the Mrk 463E
nucleus, together with large scale gradients likely related to the ongoing
galaxy merger. We further find an emission line region 11 kpc south of
Mrk 463E that is consistent with being photoionized by an AGN. Compared to the
current AGN luminosity, the energy budget of the cloud implies a luminosity
drop in Mrk 463E by a factor 3-20 over the last 40,000 years. The ALMA
observations of CO(2-1) and adjacent 1mm continuum reveal the presence
of 10M in molecular gas in the system. The molecular gas
shows velocity gradients of 800 km/s and 400 km/s around the Mrk
463E and 463W nuclei, respectively. We conclude that in this system the infall
of 100s /yr of molecular gas is in rough balance with the
removal of ionized gas by a biconical outflow being fueled by a relatively
small, 0.01% of accretion onto each SMBH.Comment: Accepted by The Astrophysical Journal, 23 pages, 19 figure
Broadband Observations of the Compton-thick Nucleus of NGC 3393
We present new NuSTAR and Chandra observations of NGC 3393, a galaxy reported
to host the smallest separation dual AGN resolved in the X-rays. While past
results suggested a 150 pc separation dual AGN, three times deeper Chandra
imaging, combined with adaptive optics and radio imaging suggest a single,
heavily obscured, radio-bright AGN. Using VLA and VLBA data, we find an AGN
with a two-sided jet rather than a dual AGN and that the hard X-ray, UV,
optical, NIR, and radio emission are all from a single point source with a
radius <0.2". We find that the previously reported dual AGN is most likely a
spurious detection resulting from the low number of X-ray counts (<160) at 6-7
keV and Gaussian smoothing of the data on scales much smaller than the PSF
(0.25" vs. 0.80" FWHM). We show that statistical noise in a single Chandra PSF
generates spurious dual peaks of the same separation (0.550.07" vs. 0.6")
and flux ratio (399% vs. 32% of counts) as the purported dual AGN. With
NuSTAR, we measure a Compton-thick source (NH=
cm) with a large torus half-opening angle, {\theta}=79 which we
postulate results from feedback from strong radio jets. This AGN shows a 2-10
keV intrinsic to observed flux ratio of 150. Using simulations, we find that
even the deepest Chandra observations would severely underestimate the
intrinsic luminosity of NGC 3393 above z>0.2, but would detect an unobscured
AGN of this luminosity out to high redshift (z=5).Comment: Accepted for publication in ApJ. 15 Figures and 4 table
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