828 research outputs found
Simultaneous two-photon resonant optical laser locking (STROLLing) in the hyperfine Paschen-Back regime
We demonstrate a technique to lock simultaneously two laser frequencies to each step of a two-photon transition in the presence of a magnetic field sufficiently large to gain access to the hyperfine Paschen–Back regime. A ladder configuration with the 5S1/2, 5P3/2, and 5D5/2 terms in a thermal vapor of Rb87 atoms is used. The two lasers remain locked for more than 24 h. For the sum of the laser frequencies, which represents the stability of the two-photon lock, we measure a frequency instability of less than the Rb D2 natural linewidth of 6 MHz for nearly all measured timescales
Single-photon interference due to motion in an atomic collective excitation
We experimentally demonstrate the generation of heralded bi-chromatic single photons from an atomic collective spin excitation (CSE). The photon arrival times display collective quantum beats, a novel interference effect resulting from the relative motion of atoms in the CSE. A combination of velocity-selective excitation with strong laser dressing and the addition of a magnetic field allows for exquisite control of this collective beat phenomenon. The present experiment uses a diamond scheme with near-IR photons that can be extended to include telecommunications-wavelengths or modified to allow storage and retrieval in an inverted-Y scheme
An annotated bibliography of the research on marine organisms and environments at Christmas Island and the Cocos (Keeling) Islands
Christmas Island and the Cocos (Keeling) Islands represent a unique marine biodiversity hotspot because of the overlap between two major biogeographic provinces (Indian and Pacific Ocean) and the high proportion of endemic species. In this paper, we compile existing scientific literature pertaining to marine organisms and environments at these islands to determine the current state of knowledge and identify major knowledge gaps. In total, 1066 studies have been published, including 582 peer-reviewed journal articles (55% of all publications), 332 reports, 141 books or book chapters, and 11 theses. These studies extend back to 1697, but most (83%) are post-1970. Seabirds have been the most studied group (43% of all publications), followed by land crabs (13%). There has been very little research on plankton (<0.3% of all studies), despite the diversity of marine species that have larval stages (including land crabs) and the importance of plankton to ecosystem function. Most invertebrate groups have received little attention or have not been studied. The taxonomic bias in marine research at these islands means that most of the invertebrates are yet to be documented. Some of these groups (e.g., Polychaeta, Copepoda, and anchialine fauna) are known for their high degree of endemism and are likely to contain new species, thereby increasing the biodiversity value of the islands. That whole families (even phyla) are yet to be studied highlights the infancy in some areas of marine research and adding to species lists for unstudied or understudied groups is one priority that would increase the conservation importance of these islands. Without this knowledge, the ability to monitor, detect or predict anthropogenic impacts on marine species is severely restricted, and therefore limits the development of management strategies aimed at conserving the unique marine biodiversity of these islands. Further studies on functional processes and research related directly to impacts are also needed. Increasing studies that directly relate to management questions will provide guidance to managers charged with protecting the environment. Improved decision making in conservation management will occur through increased directed research and monitoring
The Performance and Calibration of the CRAFT Fly's Eye Fast Radio Burst Survey
Since January 2017, the Commensal Real-time ASKAP Fast Transients survey
(CRAFT) has been utilising commissioning antennas of the Australian SKA
Pathfinder (ASKAP) to survey for fast radio bursts (FRBs) in fly's eye mode.
This is the first extensive astronomical survey using phased array feeds
(PAFs), and a total of 20 FRBs have been reported. Here we present a
calculation of the sensitivity and total exposure of this survey, using the
pulsars B1641-45 (J1644-4559) and B0833-45 (J0835-4510, i.e.\ Vela) as
calibrators. The design of the survey allows us to benchmark effects due to PAF
beamshape, antenna-dependent system noise, radio-frequency interference, and
fluctuations during commissioning on timescales from one hour to a year.
Observation time, solid-angle, and search efficiency are calculated as a
function of FRB fluence threshold. Using this metric, effective survey
exposures and sensitivities are calculated as a function of the source counts
distribution. The implied FRB rate is significantly lower than the
\,sky\,day calculated using nominal exposures and
sensitivities for this same sample by \citet{craft_nature}. At the Euclidean
power-law index of , the rate is \,sky\,day above a threshold of \,Jy\,ms, while for the best-fit index for this sample of , it is
\,sky\,day above a threshold of \,Jy\,ms. This strongly suggests that these calculations be performed
for other FRB-hunting experiments, allowing meaningful comparisons to be made
between them.Comment: 21 pages, 15 figures, 2 tables, accepted for publication in PAS
Cold gas outflows from the Small Magellanic Cloud traced with ASKAP
Feedback from massive stars plays a critical role in the evolution of the
Universe by driving powerful outflows from galaxies that enrich the
intergalactic medium and regulate star formation. An important source of
outflows may be the most numerous galaxies in the Universe: dwarf galaxies.
With small gravitational potential wells, these galaxies easily lose their
star-forming material in the presence of intense stellar feedback. Here, we
show that the nearby dwarf galaxy, the Small Magellanic Cloud (SMC), has atomic
hydrogen outflows extending at least 2 kiloparsecs (kpc) from the star-forming
bar of the galaxy. The outflows are cold, , and may have formed
during a period of active star formation million years (Myr) ago. The
total mass of atomic gas in the outflow is solar masses, , or % of the total atomic gas of the galaxy. The inferred
mass flux in atomic gas alone, , is up to an order of magnitude greater than the star
formation rate. We suggest that most of the observed outflow will be stripped
from the SMC through its interaction with its companion, the Large Magellanic
Cloud (LMC), and the Milky Way, feeding the Magellanic Stream of hydrogen
encircling the Milky Way.Comment: Published in Nature Astronomy, 29 October 2018,
http://dx.doi.org/10.1038/s41550-018-0608-
WALLABY Early Science - I. The NGC 7162 Galaxy Group
We present Widefield ASKAP L-band Legacy All-sky Blind Survey (WALLABY) early
science results from the Australian Square Kilometre Array Pathfinder (ASKAP)
observations of the NGC 7162 galaxy group. We use archival HIPASS and Australia
Telescope Compact Array (ATCA) observations of this group to validate the new
ASKAP data and the data reduction pipeline ASKAPsoft. We detect six galaxies in
the neutral hydrogen (HI) 21-cm line, expanding the NGC 7162 group membership
from four to seven galaxies. Two of the new detections are also the first HI
detections of the dwarf galaxies, AM 2159-434 and GALEXASC J220338.65-431128.7,
for which we have measured velocities of and km s,
respectively. We confirm that there is extended HI emission around NGC 7162
possibly due to past interactions in the group as indicated by the
offset between the kinematic and morphological major axes for NGC 7162A, and
its HI richness. Taking advantage of the increased resolution (factor of
) of the ASKAP data over archival ATCA observations, we fit a tilted
ring model and use envelope tracing to determine the galaxies' rotation curves.
Using these we estimate the dynamical masses and find, as expected, high dark
matter fractions of for all group members. The
ASKAP data are publicly available.Comment: 20 pages, 11 figures, accepted for publication in MNRA
A successful search for intervening 21 cm HI absorption in galaxies at 0.4 < z <1.0 with the Australian Square Kilometre Array Pathfinder (ASKAP)
We have used the Australian Square Kilometre Array Pathfinder (ASKAP) radio
telescope to search for intervening 21 cm neutral hydrogen (HI) absorption
along the line of sight to 53 bright radio continuum sources. Our observations
are sensitive to HI column densities typical of Damped Lyman Alpha absorbers
(DLAs) in cool gas with an HI spin temperature below about 300-500 K. The
six-dish Boolardy Engineering Test Array (BETA) and twelve-antenna Early
Science array (ASKAP-12) covered a frequency range corresponding to redshift
and respectively for the HI line. Fifty of the 53
radio sources observed have reliable optical redshifts, giving a total redshift
path = 21.37. This was a spectroscopically-untargeted survey, with
no prior assumptions about the location of the lines in redshift space. Four
intervening HI lines were detected, two of them new. In each case, the
estimated HI column density lies above the DLA limit for HI spin temperatures
above 50-80 K, and we estimate a DLA number density at redshift of
. This value lies somewhat above the
general trend of with redshift seen in optical DLA studies. Although the
current sample is small, it represents an important proof of concept for the
much larger 21cm First Large Absorption Survey in HI (FLASH) project to be
carried out with the full 36-antenna ASKAP telescope, probing a total redshift
path .Comment: 21 pages, 11 figures. Accepted for publication in MNRA
Energy of Isolated Systems at Retarded Times as the Null Limit of Quasilocal Energy
We define the energy of a perfectly isolated system at a given retarded time
as the suitable null limit of the quasilocal energy . The result coincides
with the Bondi-Sachs mass. Our is the lapse-unity shift-zero boundary value
of the gravitational Hamiltonian appropriate for the partial system
contained within a finite topologically spherical boundary . Moreover, we show that with an arbitrary lapse and zero shift the same
null limit of the Hamiltonian defines a physically meaningful element in the
space dual to supertranslations. This result is specialized to yield an
expression for the full Bondi-Sachs four-momentum in terms of Hamiltonian
values.Comment: REVTEX, 16 pages, 1 figur
Comparative study of in situ N2 rotational Raman spectroscopy methods for probing energy thermalisation processes during spin-exchange optical pumping
Spin-exchange optical pumping (SEOP) has been widely used to produce enhancements in nuclear spin polarisation for hyperpolarised noble gases. However, some key fundamental physical processes underlying SEOP remain poorly understood, particularly in regards to how pump laser energy absorbed during SEOP is thermalised, distributed and dissipated. This study uses in situ ultra-low frequency Raman spectroscopy to probe rotational temperatures of nitrogen buffer gas during optical pumping under conditions of high resonant laser flux and binary Xe/N2 gas mixtures. We compare two methods of collecting the Raman scattering signal from the SEOP cell: a conventional orthogonal arrangement combining intrinsic spatial filtering with the utilisation of the internal baffles of the Raman spectrometer, eliminating probe laser light and Rayleigh scattering, versus a new in-line modular design that uses ultra-narrowband notch filters to remove such unwanted contributions. We report a ~23-fold improvement in detection sensitivity using the in-line module, which leads to faster data acquisition and more accurate real-time monitoring of energy transport processes during optical pumping. The utility of this approach is demonstrated via measurements of the local internal gas temperature (which can greatly exceed the externally measured temperature) as a function of incident laser power and position within the cell
Does a radio jet drive the massive multi-phase outflow in the ultra-luminous infrared galaxy IRAS 10565+2448?
We present new upgraded Giant Metrewave Radio Telescope (uGMRT) HI 21-cm
observations of the ultra-luminous infrared galaxy IRAS 10565+2448, previously
reported to show blueshifted, broad, and shallow HI absorption indicating an
outflow. Our higher spatial resolution observations have localised this
blueshifted outflow, which is 1.36 kpc southwest of the radio centre and
has a blueshifted velocity of and a full width at
half maximum (FWHM) of . The spatial extent and
kinematic properties of the HI outflow are consistent with the previously
detected cold molecular outflows in IRAS 10565+2448, suggesting that they
likely have the same driving mechanism and are tracing the same outflow. By
combining the multi-phase gas observations, we estimate a total outflowing mass
rate of at least and a total energy loss rate of
at least , where the contribution from the
ionised outflow is negligible, emphasising the importance of including both
cold neutral and molecular gas when quantifying the impact of outflows. We
present evidence of the presence of a radio jet and argue that this may play a
role in driving the observed outflows. The modest radio luminosity
of the jet in IRAS
10565+2448 implies that the jet contribution to driving outflows should not be
ignored in low radio luminosity AGN.Comment: 12 pages, 9 figures, accepted for publication in MNRA
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