493 research outputs found
Chromatin proteins and RNA are associated with DNA during all phases of mitosis.
Mitosis brings about major changes to chromosome and nuclear structure. We used recently developed proximity ligation assay-based techniques to investigate the association with DNA of chromatin-associated proteins and RNAs in Drosophila embryos during mitosis. All groups of tested proteins, histone-modifying and chromatin-remodeling proteins and methylated histones remained in close proximity to DNA during all phases of mitosis. We also found that RNA transcripts are associated with DNA during all stages of mitosis. Reduction of H3K27me3 levels or elimination of RNAs had no effect on the association of the components of PcG and TrxG complexes to DNA. Using a combination of proximity ligation assay-based techniques and super-resolution microscopy, we found that the number of protein-DNA and RNA-DNA foci undergoes significant reduction during mitosis, suggesting that mitosis may be accompanied by structural re-arrangement or compaction of specific chromatin domains
Ultra-high resolution observations of selected blazars
Active Galactic Nuclei are the luminous centres of active galaxies that produce
powerful relativistic jets from central super massive black holes (SMBH). When
these jets are oriented towards the observer’s line-of-sight, they become very
bright, very variable and very energetic. These sources are known as blazars
and Very Long Baseline Interferometry (VLBI) provides a direct means of observing
into the heart of these objects. VLBI performed at 3 mm with the Global
mm-VLBI Array (GMVA) and 7 mm VLBI performed with the Very Long Baseline
Array (VLBA), allows some of the highest angular resolution images of blazars to
be produced. In this thesis, we present the first results of an ongoing monitoring
program of blazars known to emit at γ-ray energies.
The physical processes that produce these jets and the γ-ray emission are still not
well known. The jets are thought to be produced by converting gravitational energy
around the black hole into relativistic particles that are accelerated away
at near the speed of light. However, the exact mechanisms for this and the role
that magnetic fields play is not fully clear. Similarly, γ-rays have been long known
to have been emitted from blazars and that their production is often related to
the up-scattering of synchrotron radiation from the jet. However, the origin of
seed photons for the up-scattering (either from within the jet itself or from an external
photon field) and the location of the γ-ray emission regions has remained
inconclusive. In this thesis, we aim to describe the likely location of γ-ray emission
in jets, the physical structure of blazar jets, the location of the VLBI features
relative to the origin of the jet and the nature of the magnetic field, both of the
VLBI scale jet and in the region where the jet is produced.
We present five sources that have been monitored at 3 mm using the GMVA
from 2008 until 2012. These sources have been analysed with near-in-time 7 mm
maps from the Very Long Baseline Array (VLBA), γ-ray light curves from the Fermi/LAT
space telescope and cm to mm-wave total-intensity light curves. In one
source, OJ 287, the source has additionally been analysed with monthly imaging
at 7 mm with the VLBA and near-in-time 2 cm VLBI maps. We use these resources
to analyse high angular resolution structural and spectral changes and
see if they correlate with flaring (both radio and γ-ray) activity and with VLBI
component ejections. By spectrally decomposing sources, we can determine
the spatially resolved magnetic field structure in the jets at the highest yet performed
resolutions and at frequencies that are near or above the turnover frequency
for synchrotron self-absorption (SSA). We compute the magnetic field
estimates from SSA theory and by assuming equipartition between magnetic
fields and relativistic particle energies.
v
All sources analysed exhibit downstream quasi-stationary features which sometimes
exhibit higher brightness temperatures and flux density variability than the
VLBI “core”, which we interpret as being recollimation or oblique shocks. We
find that γ-ray flaring, mm-wave radio flaring and changes in opacity from optically
thick to optically thin, is in many cases consistent with component ejections
past both the VLBI “core” and these quasi-stationary downstream features.
We find decreasing apparent brightness temperatures and Doppler factors as
a function of increased “core” separation, which is interpreted as consistent
with a slowly accelerating jet over the de-projected inner ∼10-20 pc. Assuming
equipartition between magnetic energy and relativistic particle energy, the
magnetic field strengths within the jets at these scales are, on average, between
B ∼ 0.3 − 0.9 G, with the highest strengths found within the VLBI “core”.
From the observed gradient in magnetic field strengths, we can place the mmwave
“core” ∼1-3 pc downstream of the base of the jet. Additionally, we estimate
the the magnetic field is Bapex ∼ 3000 − 18000 G at the base of the jet. We
computed theoretical estimates based on jet production under magnetically
arrested disks (MAD) and find our estimates to be consistent.
In the BL Lac source OJ 287, we included monthly 7 mm and near-in-time 2 cm
VLBA maps to provide full kinematics and increased spectral coverage. Following a previously reported radical change in inner-jet PA of ∼100◦we find unusually discrepant PAs compared with the previous jet direction, that follow very different trajectories. The source exhibits a downstream quasi-stationary feature that at times has higher brightness temperatures than the “core”. The source also exhibited a large change in apparent component speeds as compared with previous epochs, which we propose could be due to changes in jet pressure causing changes in the location of downstream recollimation or oblique shocks and hence their line-of-sight viewing angle. The addition of 2 cm VLBA data allows for a comparison of magnetic fields derived from SSA and equipartition.
The magnetic field estimates are consistent within 20%, with BSSA ≥ 1.6 G and Bequi ≥ 1.2 G in the “core” and BSSA ≤ 0.4 G and Bequi ≤ 0.3 G in the stationary feature. Gamma-ray emission appears to originate in the “core” and the stationary feature. The decrease in magnetic field strengths places the mmwave “core” downstream of the jet base by ≤6 pc and likely outside of the broad line region (BLR).
This, combined with the results in other sources are consistent with γ-rays being produced in the vicinity of the VLBI “core” of in further downstream stationary features, which are likely over a parsec downstream of the central black hole, favouring the scenario of photons being up-scattered within the relativistic jet
Recommended from our members
Graduate Automotive Technology Education (GATE) Center
The Graduate Automotive Technology Education (GATE) Center at the University of Tennessee, Knoxville has completed its sixth year of operation. During this period the Center has involved thirteen GATE Fellows and ten GATE Research Assistants in preparing them to contribute to advanced automotive technologies in the center's focus area: hybrid drive trains and control systems. Eighteen GATE students have graduated, and three have completed their course work requirements. Nine faculty members from three departments in the College of Engineering have been involved in the GATE Center. In addition to the impact that the Center has had on the students and faculty involved, the presence of the center has led to the acquisition of resources that probably would not have been obtained if the GATE Center had not existed. Significant industry interaction such as internships, equipment donations, and support for GATE students has been realized. The value of the total resources brought to the university (including related research contracts) exceeds $4,000,000. Problem areas are discussed in the hope that future activities may benefit from the operation of the current program
Capturing flow transformation processes across an uneven seabed in coarse-grained sediment gravity flow deposits
The upper part of the Jurassic stratigraphy of the Los Molles Formation corresponds to deepwater succession deposited with a regional marine transgression during the early postrift stage of the Neuquén Basin. The present study conducted in the location of Chacay Melehue (Argentina) is used to document the interactions between coarse-grained sediment gravity flows and the depositional relief of a seismic-scale mass transport complex (MTC), with metre-scale mounds and decametric protruding clasts, as an analogue for similar configurations in subsurface systems. The 60 m thick succession is exposed along a 6.5 km long oblique downdip longitudinal profile. Exposure quality permits walking out of individual beds. A total of 16 stratigraphic sections (1560 m thick) spaced between 500 m and 100 m, were logged at 1:50 and 1:25 scale. The basal datum of the studied interval is the top of a MTC and the top marker bed is an extensive sandstone bed. Two units studied correspond to very coarse to fine-grained medium-bedded sandstones with abundant pebble-sized clasts (Unit1) and three thick plurimetric distinct beds of poorly-sorted, granular to medium-grained mud-rich sandstone matrix supporting polygenic gravels (Unit2). Distinct thick extensive beds in both units are intercalated with heterolithic successions of thin to medium-bedded very coarse- to coarse-grained or fine-grained sandstones, siltstones and mudstones. The sandstone beds in Unit 1 abruptly pinchout in the proximal part of study area. They are associated with evidence of erosion, sediment bypass and transformation of subaqueous sand-bearing flows. Stratigraphically, sandstone packages in the Unit 2 are increasingly more laterally extensive upwards in the succession. Thickness variations in these beds are related to compensational stacking. The research will inform studies in the architecture of deep-water successions above an uneven seabed inherited from the top of a MTC. The detailed analysis of exhumed examples that record depositional changes in the structure of the flow can lead to the development of predictive stratigraphic models which incorporate details and complexities observed in outcrop and can be applied for the evaluation of the quality of subsurface reservoirs.Fil: Privat, Aurélia. University of Leeds; Reino UnidoFil: Hodgson, David. University of Leeds; Reino UnidoFil: Peakall, Jeffrey. University of Leeds; Reino UnidoFil: Jackson, Christopher A. L.. University of Leeds; Reino UnidoFil: Schwarz, Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; Argentina2016 AAPG/SEG International Convention and ExhibitionBarcelonaEspañaAmerican Association of Petroleum GeologistsSociety of Exploration Geophysicist
- …