474 research outputs found
Gigahertz repetition rate thermionic electron gun concept
We present a novel concept for the generation of gigahertz repetition rate
high brightness electron bunches. A custom design 100 kV thermionic gun
provides a continuous electron beam, with the current determined by the
filament size and temperature. A 1 GHz rectangular RF cavity deflects the beam
across a knife-edge, creating a pulsed beam. Adding a higher harmonic mode to
this cavity results in a flattened magnetic field profile which increases the
duty cycle to 30%. Finally, a compression cavity induces a negative
longitudinal velocity-time chirp in a bunch, initiating ballistic compression.
Adding a higher harmonic mode to this cavity increases the linearity of this
chirp and thus decreases the final bunch length. Charged particle simulations
show that with a 0.15 mm radius LaB6 filament held at 1760 K, this method can
create 279 fs, 3.0 pC electron bunches with a radial rms core emittance of
0.089 mm mrad at a repetition rate of 1 GHz.Comment: 12 pages, 12 figure
The demographics of neutron star - white dwarf mergers: rates, delay-time distributions and progenitors
The mergers of neutron stars (NSs) and white dwarfs (WDs) could give rise to
explosive transients, potentially observable with current and future transient
surveys. However, the expected properties and distribution of such events is
not well understood. Here we characterize the rates of such events, their delay
time distribution, their progenitors and the distribution of their properties.
We use binary populations synthesis models and consider a wide range of initial
conditions and physical processes. In particular we consider different
common-envelope evolution models and different NS natal kick distributions. We
provide detailed predictions arising from each of the models considered. We
find that the majority of NS-WD mergers are born in systems in which
mass-transfer played an important role, and the WD formed before the NS. For
the majority of the mergers the WDs have a carbon-oxygen composition (60-80%)
and most of the rest are with oxygen-neon WDs. The rates of NS-WD mergers are
in the range of 3-15% of the type Ia supernovae (SNe) rate. Their delay time
distribution is very similar to that of type Ia SNe, but slightly biased
towards earlier times. They typically explode in young 0.1-1Gyr environments,
but have a tail distribution extending to long, Gyrs-timescales. Models
including significant kicks give rise to relatively wide offset distribution
extending to hundreds of kpcs. The demographic and physical properties of NS-WD
mergers suggest they are likely to be peculiar type Ic-like SNe, mostly
exploding in late type galaxies. Their overall properties could be related to a
class of rapidly evolving SNe recently observed, while they are less likely to
be related to the class of Ca-rich SNe.Comment: updated version: accepted for publication in A&
Quantifying Resonant Structure in NGC 6946 from Two-dimensional Kinematics
We study the two-dimensional kinematics of the H-alpha-emitting gas in the
nearby barred Scd galaxy, NGC 6946, in order to determine the pattern speed of
the primary m=2 perturbation mode. The pattern speed is a crucial parameter for
constraining the internal dynamics, estimating the impact velocities of the
gravitational perturbation at the resonance radii, and to set up an
evolutionary scenario for NGC 6946. Our data allows us to derive the best
fitting kinematic position angle and the geometry of the underlying gaseous
disk, which we use to derive the pattern speed using the Tremaine-Weinberg
method. We find a main pattern speed Omega_p=22 km/s/kpc, but our data clearly
reveal the presence of an additional pattern speed Omega_p=47 km/s/kpc in a
zone within 1.25 kpc of the nucleus. Using the epicyclic approximation, we
deduce the location of the resonance radii and confirm that inside the outer
Inner Lindblad Resonance radius of the main oval, a primary bar has formed
rotating at more than twice the outer pattern speed. We further confirm that a
nuclear bar has formed inside the Inner Lindblad Resonance radius of the
primary bar, coinciding with the inner Inner Lindblad Resonance radius of the
large-scale m=2 mode oval.Comment: Accepted for publication in ApJ Letter
The Scope of Published Population Genetic Data for Indo-Pacific Marine Fauna and Future Research Opportunities in the Region
Marine biodiversity reaches its pinnacle in the tropical Indo-Pacific region, with high levels of both species richness and endemism, especially in coral reef habitats. While this pattern of biodiversity has been known to biogeographers for centuries, causal mechanisms remain enigmatic. Over the past 20 yrs, genetic markers have been employed by many researchers as a tool to elucidate patterns of biodiversity above and below the species level, as well as to make inferences about the underlying processes of diversification, demographic history, and dispersal. In a quantitative, comparative framework, these data can be synthesized to address questions about this bewildering diversity by treating species as âreplicates.â However, the sheer size of the Indo-Pacific region means that the geographic and genetic scope of many speciesâ data sets are not complementary. Here, we describe data sets from 116 Indo-Pacific species (108 studies). With a mind to future synthetic investigations, we consider the strengths and omissions of currently published population genetic data for marine fauna of the Indo-Pacific region, as well as the geographic and taxonomic scope of the data, and suggest some ways forward for data collection and collation
Neutron Star - White Dwarf Binaries: Probing Formation Pathways and Natal Kicks with LISA
Neutron star-white dwarf (NS+WD) binaries offer a unique opportunity for
studying NS-specific phenomena with gravitational waves. In this paper, we
employ the binary population synthesis technique to study the Galactic
population of NS+WDs with the future Laser Interferometer Space Antenna (LISA).
We anticipate approximately detectable NS+WDs by LISA,
encompassing both circular and eccentric binaries formed via different
pathways. Despite the challenge of distinguishing NS+WDs from more prevalent
double white dwarfs in the LISA data (especially at frequencies below 2 mHz),
we show that their eccentricity and chirp mass distributions may provide
avenues to explore the NS natal kicks and common envelope evolution.
Additionally, we investigate the spatial distribution of detectable NS+WDs
relative to the Galactic plane and discuss prospects for identifying
electromagnetic counterparts at radio wavelengths. Our results emphasise LISA's
capability to detect and characterise NS+WDs and to offer insights into the
properties of the underlying population. Our conclusions carry significant
implications for shaping LISA data analysis strategies and future data
interpretation.Comment: Submitted to MNRAS. Comments are welcom
Performance-based financing as a health system reform : mapping the key dimensions for monitoring and evaluation
Peer reviewedPublisher PD
Spatially Resolved X-Ray Study of Supernova Remnant G306.3-0.9 with Unusually High Calcium Abundance
The RAB3-RIM Pathway Is Essential for the Release of Neuromodulators
Neurons secrete neuromodulators/neuropeptides from dense-core vesicles (DCVs) by a largely unknown mechanism. Persoon et al. identify RAB3 and RIM1/2 as essential factors. RAB3âs indispensable role is the first distinct feature of DCV secretion as compared to synaptic vesicle secretion
- âŠ