377 research outputs found
From radio-quiet to radio-silent: low luminosity Seyfert radio cores
A strong effort has been devoted to understand the physical origin of radio
emission from low-luminosity AGN (LLAGN), but a comprehensive picture is still
missing. We used high-resolution (1 arcsec), multi-frequency (1.5, 5.5, 9
and 14 GHz) NSF's Karl G. Jansky Very Large Array (VLA) observations to
characterise the state of the nuclear region of ten Seyfert nuclei, which are
the faintest members of a complete, distance-limited sample of 28 sources. With
the sensitivity and resolution guaranteed by the VLA-A configuration, we
measured radio emission for six sources (NGC3185, NGC3941, NGC4477, NGC4639,
NGC4698 and NGC4725), while for the remaining four (NGC0676, NGC1058, NGC2685
and NGC3486) we put upper limits at tens uJy/beam level, below the previous
0.12 mJy/beam level of Ho&Ulvestad (2001), corresponding to luminosities down
to L W/Hz at 1.5 GHz for the highest RMS observation. Two sources,
NGC4639 and NGC4698, exhibit spectral slopes compatible with inverted spectra
(0, ), hint for radio emission
from an optically-thick core, while NGC4477 exhibits a steep (+0.520.09)
slope. The detected sources are mainly compact on scales arcseconds,
predominantly unresolved, except NGC3185 and NGC3941, in which the resolved
radio emission could be associated to star-formation processes. A significant
X-ray - radio luminosities correlation is extended down to very low
luminosities, with slope consistent with inefficient accretion, expected at
such low Eddington ratios. Such sources will be one of the dominant Square
Kilometre Array (SKA) population, allowing a deeper understanding of the
physics underlying such faint AGN.Comment: accepted for publication on MNRAS (19 pages, 26 figures
Hard - X-rays selected Active Galactic Nuclei. I. A radio view at high-frequencies
A thorough study of radio emission in Active Galactic Nuclei (AGN) is of
fundamental importance to understand the physical mechanisms responsible for
the emission and the interplay between accretion and ejection processes. High
frequency radio observations can target the nuclear contribution of smaller
emitting regions and are less affected by absorption. We present JVLA 22 and 45
GHz observations of 16 nearby (0.003z0.3) hard - X-rays selected AGN
at the (sub)-kpc scale with tens uJy beam sensitivity. We detected 15/16
sources, with flux densities ranging from hundreds uJy beam to tens Jy
(specific luminosities from 10 to 10 at
22 GHz). All detected sources host a compact core, with 8 being core-dominated
at either frequencies, the others exhibiting also extended structures. Spectral
indices range from steep to flat/inverted. We interpret this evidence as either
due to a core+jet system (6/15), a core accompanied by surrounding star
formation (1/15), to a jet oriented close to the line of sight (3/15), to
emission from a corona or the base of a jet (1/15), although there might be
degeneracies between different processes. Four sources require more data to
shed light on their nature. We conclude that, at these frequencies, extended,
optically-thin components are present together with the flat-spectrum core. The
relation is roughly followed, indicating a possible
contribution to radio emission from a hot corona. A weakly significant
correlation between radio core (22 and 45 GHz) and X-rays luminosities is
discussed in the light of an accretion-ejection framework.Comment: Accepted for publication on MNRA
Radiography of a normal fault system by 64,000 high-precision earthquake locations: The 2009 L’Aquila (central Italy) case study
We studied the anatomy of the fault system where the 2009 L’Aquila earthquake
(MW 6.1) nucleated by means of ~64 k high-precision earthquake locations spanning
1 year. Data were analyzed by combining an automatic picking procedure for P and S
waves, together with cross-correlation and double-difference location methods reaching a
completeness magnitude for the catalogue equal to 0.7 including 425 clusters of similar
earthquakes. The fault system is composed by two major faults: the high-angle L’Aquila
fault and the listric Campotosto fault, both located in the first 10 km of the upper crust. We
detect an extraordinary degree of detail in the anatomy of the single fault segments
resembling the degree of complexity observed by field geologists on fault outcrops. We
observe multiple antithetic and synthetic fault segments tens of meters long in both the
hanging wall and footwall along with bends and cross fault intersections along the main
fault and fault splays. The width of the L’Aquila fault zone varies along strike from 0.3 km
where the fault exhibits the simplest geometry and experienced peaks in the slip
distribution, up to 1.5 km at the fault tips with an increase in the geometrical complexity.
These characteristics, similar to damage zone properties of natural faults, underline the key
role of aftershocks in fault growth and co-seismic rupture propagation processes.
Additionally, we interpret the persistent nucleation of similar events at the seismicity cutoff depth as the presence of a rheological (i.e., creeping) discontinuity explaining how normal faults detach at depth
Fault structure and slip localization in carbonate-bearing normal faults: An example from the Northern Apennines of Italy
Carbonate-bearing normal faults are important structures for controlling fluid flow and seismogenesis
within the brittle upper crust. Numerous studies have tried to characterize fault zone structure and
earthquake slip processes along carbonate-bearing faults. However, due to the different scales of
investigation, these studies are not often integrated to provide a comprehensive fault image. Here we
present a multi-scale investigation of a normal fault exhumed from seismogenic depths. The fault extends
for a length of 10 km with a maximum width of about 1.5 km and consists of 5 sub-parallel and
interacting segments. The maximum displacement (370e650 m) of each fault segment is partitioned
along sub-parallel slipping zones extending for a total width of about 50 m. Each slipping zone is
characterized by slipping surfaces exhibiting different slip plane phenomena. Fault rock development is
controlled by the protolith lithology. In massive limestone, moving away from the slip surface, we
observe a thin layer (<2 cm) of ultracataclasite, cataclasite (2e10 cm) and fault breccia. In marly limestone,
the fault rock consists of a cataclasite with hydrofractures and smectite-rich pressure solution
seams. At the micro-nanoscale, the slip surface consists of a continuous and thin (<300 mm) layer
composed of coarse calcite grains (~5e20 mm in size) associated with sub-micrometer grains showing
fading grain boundaries, voids and/or vesicles, and suggesting thermal decomposition processes.
Micrometer-sized calcite crystals show nanoscale polysynthetic twinning affected by the occurrence of
subgrain boundaries and polygonalized nanostructures. Investigations at the kilometres-tens of meter
scale provide fault images that can be directly compared with high-resolution seismological data and
when combined can be used to develop a comprehensive characterization of seismically active fault
structures in carbonate lithologies. Micro and nanoscale investigations along the principal slipping zone
suggest that different deformation processes, including plastic deformation and thermal decomposition,
were active during seismic slip
Zn-induced interactions between SARS-CoV-2 orf7a and BST2/Tetherin
We present in this work a first X-ray Absorption Spectroscopy study of the interactions of Zn with human BST2/tetherin and SARS-CoV-2 orf7a proteins as well as with some of their complexes. The analysis of the XANES region of the measured spectra shows that Zn binds to BST2, as well as to orf7a, thus resulting in the formation of BST2-orf7a complexes. This structural information confirms the the conjecture, recently put forward by some of the present Authors, according to which the accessory orf7a (and possibly also orf8) viral protein are capable of interfering with the BST2 antiviral activity. Our explanation for this behavior is that, when BST2 gets in contact with Zn bound to the orf7a Cys(15) ligand, it has the ability of displacing the metal owing to the creation of a new disulfide bridge across the two proteins. The formation of this BST2-orf7a complex destabilizes BST2 dimerization, thus impairing the antiviral activity of the latter
A discovery of young radio sources in the cores of giant radio galaxies selected at hard X-rays
Giant Radio Galaxies (GRG) are the largest single entities in the Universe,
having a projected linear size exceeding 0.7 Mpc, which implies that they are
also quite old objects. They are not common, representing a fraction of only
about 6% in samples of bright radio galaxies. While a census of about 300 of
these objects has been built in the past years, still no light has been shed on
the conditions necessary to allow such an exceptional growth, whether of
environmental nature or linked to the inner accretion properties. Recent
studies found that samples of radio galaxies selected from hard X-ray AGN
catalogs selected from INTEGRAL/IBIS and Swift/BAT (thus at energies >20 keV)
present a fraction of GRG four times larger than what found in radio-selected
samples. We present radio observations of 15 nuclei of hard X-ray selected GRG,
finding for the first time a large fraction (61%) of young radio sources at the
center of Mpc-scale structures. Being at the center of GRG, these young nuclei
may be undergoing a restarting activity episode, suggesting a link between the
detected hard X-ray emission - due to the ongoing accretion - and the
reactivation of the jets.Comment: Accepted for publication on Ap
The role of Zn ions in the interaction between SARS-CoV-2 orf7a protein and BST2/tetherin
In this paper, we provide evidence that Zn2+ ions play a role in the SARS-CoV-2 virus strategy to escape the immune response mediated by the BST2-tetherin host protein. This conclusion is based on sequence analysis and molecular dynamics simulations as well as X-ray absorption experiments [1]
The role of Zn ions in the interaction between SARS-CoV-2 orf7a protein and BST2/tetherin
In this paper, we provide evidence that Zn2+ ions play a role in the SARS-CoV-2 virus strategy to escape the immune response mediated by the BST2-tetherin host protein. This conclusion is based on sequence analysis and molecular dynamics simulations as well as X-ray absorption experiments
Metal ion binding in wild-type and mutated frataxin: a stability study
This work studies the stability of wild-type frataxin and some of its variants found in cancer tissues upon Co2+ binding. Although the physiologically involved metal ion in the frataxin enzymatic activity is Fe2+, as it is customarily done, Co2+ is most often used in experiments because Fe2+ is extremely unstable owing to the fast oxidation reaction Fe2+ → Fe3+. Protein stability is monitored following the conformational changes induced by Co2+ binding as measured by circular dichroism, fluorescence spectroscopy, and melting temperature measurements. The stability ranking among the wild-type frataxin and its variants obtained in this way is confirmed by a detailed comparative analysis of the XAS spectra of the metal-protein complex at the Co K-edge. In particular, a fit to the EXAFS region of the spectrum allows positively identifying the frataxin acidic ridge as the most likely location of the metal-binding sites. Furthermore, we can explain the surprising feature emerging from a detailed analysis of the XANES region of the spectrum, showing that the longer 81-210 frataxin fragment has a smaller propensity for Co2+ binding than the shorter 90-210 one. This fact is explained by the peculiar role of the N-terminal disordered tail in modulating the protein ability to interact with the metal
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