6,397 research outputs found
Lyman-alpha radiative transfer during the Epoch of Reionization: contribution to 21-cm signal fluctuations
During the epoch of reionization, Ly-alpha photons emitted by the first stars
can couple the neutral hydrogen spin temperature to the kinetic gas
temperature, providing the opportunity to observe the gas in emission or
absorption in the 21-cm line. Given the bright foregrounds, it is of prime
importance to determine precisely the fluctuations signature of the signal, to
be able to extract it by its correlation power.
LICORICE is a Monte-Carlo radiative transfer code, coupled to the dynamics
via an adaptative Tree-SPH code. We present here the Ly-alpha part of the
implementation, and validate it through three classical tests. Contrary to
previous works, we do not assume that P_alpha, the number of scatterings of
Ly-alpha photons per atom per second, is proportional to the Ly-alpha
background flux, but take into account the scatterings in the Ly-alpha line
wings. The latter have the effect to steepen the radial profile of P_alpha
around each source, and re-inforce the contrast of the fluctuations. In the
particular geometry of cosmic filaments of baryonic matter, Ly-alpha photons
are scattered out of the filament, and the large scale structure of P_alpha is
significantly anisotropic. This could have strong implications for the possible
detection of the 21-cm signal.Comment: 13 pages, 9 figures. To be published in A&
Persistence of singlet fluctuations in the coupled spin tetrahedra system Cu2Te2O5Br2 revealed by high-field magnetization and 79Br NQR - 125Te NMR
We present high-field magnetization and Br nuclear quadrupole
resonance (NQR) and Te nuclear magnetic resonance (NMR) studies in the
weakly coupled Cu () tetrahedral system CuTeOBr.
The field-induced level crossing effects were observed by the magnetization
measurements in a long-ranged magnetically ordered state which was confirmed by
a strong divergence of the spin-lattice relaxation rate 1/T1 at T0=13.5 K. In
the paramagnetic state, 1/T1 reveals an effective singlet-triplet spin gap much
larger than that observed by static bulk measurements. Our results imply that
the inter- and the intra-tetrahedral interactions compete, but at the same time
they cooperate strengthening effectively the local intratetrahedral exchange
couplings. We discuss that the unusual feature originates from the frustrated
intertetrahedral interactions.Comment: 5 pages, 4 figures, accepted in Phys. Rev. B as a Rapid
Communication
Distinctive rings in the 21 cm signal of the epoch of reionization
It is predicted that sources emitting UV radiation in the Lyman band during
the epoch of reionization (EoR) showed a series of discontinuities in their
Ly-alpha flux radial profile as a consequence of the thickness of the Lyman
line series in the primeval intergalactic medium. Through unsaturated
Wouthuysen-Field coupling, these spherical discontinuities are also present in
the 21 cm emission of the neutral IGM. In this article, we study the effects
these discontinuities have on the differential brightness temperature of the 21
cm signal of neutral hydrogen in a realistic setting including all other
sources of fluctuations. We focus on the early phases of the EoR, and we
address the question of the detectability by the planned Square Kilometre
Array. Such a detection would be of great interest, because these structures
could provide an unambiguous diagnostic for the cosmological origin of the
signal remaining after the foreground cleaning procedure. Also, they could be
used as a new type of standard rulers. We determine the differential brightness
temperature of the 21 cm signal in the presence of inhomogeneous
Wouthuysen-Field effect using simulations which include (hydro)dynamics and
both ionizing and Lyman lines 3D radiative transfer with the code LICORICE. We
find that the Lyman horizons are clearly visible on the maps and radial
profiles around the first sources of our simulations, but for a limited time
interval, typically \Delta z \approx 2 at z \sim 13. Stacking the profiles of
the different sources of the simulation at a given redshift results in
extending this interval to \Delta z \approx 4. When we take into account the
implementation and design planned for the SKA (collecting area, sensitivity,
resolution), we find that detection will be challenging. It may be possible
with a 10 km diameter for the core, but will be difficult with the currently
favored design of a 5 km core.Comment: 10 pages, 10 figures; v2: Section 5.5 rewritten; some new references
added; accepted for publication in Astronomy and Astrophysic
NEDD8 and ubiquitin ligation by cullin-RING E3 ligases
RING E3s comprise the largest family of ubiquitin (UB) and ubiquitin-like protein (UBL) ligases. RING E3s typically promote UB or UBL transfer from the active site of an associated E2 enzyme to a distally-recruited substrate. Many RING E3s – including the cullin-RING ligase family – are multifunctional, interacting with various E2s (or other E3s) to target distinct proteins, transfer different UBLs, or to initially modify substrates with UB or subsequently elongate UB chains. Here we consider recent structures of cullin-RING ligases, and their partner E2 enzymes, representing ligation reactions. The studies collectively reveal multimodal mechanisms – interactions between ancillary E2 or E3 domains, post-translational modifications, or auxiliary binding partners – directing cullin-RING E3-E2 enzyme active sites to modify their specific targets
Hysteresis Switching Loops in Ag-manganite memristive interfaces
Multilevel resistance states in silver-manganite interfaces are studied both
experimentally and through a realistic model that includes as a main ingredient
the oxygen vacancies diffusion under applied electric fields. The switching
threshold and amplitude studied through Hysteresis Switching Loops are found to
depend critically on the initial state. The associated vacancy profiles further
unveil the prominent role of the effective electric field acting at the
interfaces. While experimental results validate main assumptions of the model,
the simulations allow to disentangle the microscopic mechanisms behind the
resistive switching in metal-transition metal oxide interfaces.Comment: 14 pages, 3 figures, to be published in Jour. of Appl. Phy
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