6,373 research outputs found
PT-symmetry broken by point-group symmetry
We discuss a PT-symmetric Hamiltonian with complex eigenvalues. It is based
on the dimensionless Schr\"{o}dinger equation for a particle in a square box
with the PT-symmetric potential . Perturbation theory clearly
shows that some of the eigenvalues are complex for sufficiently small values of
. Point-group symmetry proves useful to guess if some of the eigenvalues
may already be complex for all values of the coupling constant. We confirm
those conclusions by means of an accurate numerical calculation based on the
diagonalization method. On the other hand, the Schr\"odinger equation with the
potential exhibits real eigenvalues for sufficiently small
values of . Point group symmetry suggests that PT-symmetry may be broken
in the former case and unbroken in the latter one
Radio Continuum Observations of the Galactic Center: Photoevaporative Proplyd-like Objects near Sgr A*
We present radio images within 30 of Sgr A* based on recent VLA
observations at 34 GHz with 7.8 microJy sensitivity and resolution
milliarcseconds (mas). We report 44 partially resolved compact
sources clustered in two regions in the E arm of ionized gas that orbits Sgr
A*. These sources have size scales ranging between ~50 and 200 mas (400 to 1600
AUs), and a bow-shock appearance facing the direction of Sgr A*. Unlike the
bow-shock sources previously identified in the near-IR but associated with
massive stars, these 34 GHz sources do not appear to have near-IR counterparts
at 3.8 m. We interpret these sources as a candidate population of
photoevaporative protoplanetary disks (proplyds) that are associated with newly
formed low mass stars with mass loss rates ~10^{-7} - 10^{-6} solar mass per
year and are located at the edge of a molecular cloud outlined by ionized gas.
The disks are externally illuminated by strong Lyman continuum radiation from
the ~100 OB and WR massive stars distributed within 10'' of Sgr A*. The
presence of proplyds implies current in-situ star formation activity near Sgr
A* and opens a window for the first time to study low mass star, planetary and
brown dwarf formations near a supermassive black hole.Comment: 13 pages, 4 figures, ApJL (in press
The Variability of Polarized Radiation from Sgr A*
Sgr A* is variable at radio and submillimeter wavelengths on hourly time
scales showing time delays between the peaks of flare emission as well as
linearly polarized emission at millimeter and sub-mm wavelengths. To determine
the polarization characteristics of this variable source at radio frequencies,
we present VLA observations of Sgr A* and report the detection of polarized
emission at a level of 0.77\pm0.01% and 0.2\pm0.01% at 43 and 22 GHz,
respectively. The change in the time averaged polarization angle between 22 and
43 GHz corresponds to a RM of -2.5\pm0.6 x10^3 rad m{-2} with no phase wrapping
(or \sim 5x10^4 rad m^2 with 2\pi phase wrap). We also note a rise and fall
time scale of 1.5 -- 2 hours in the total polarized intensity. The light curves
of the degree of linearly polarized emission suggests a a correlation with the
variability of the total intensity at 43 GHz. The available polarization data
at radio and sub-mm wavelengths suggest that the rotation measure decreases
with decreasing frequency. This frequency dependence, and observed changes in
polarization angle during flare events, may be caused by the reduction in
rotation measure associated with the expansion of synchrotron-emitting blobs.Comment: 11 pages, 3 figures, ApJL (in press
Compact Radio Sources within 30" of Sgr A*: Proper Motions, Stellar Winds and the Accretion Rate onto Sgr A*
Recent broad-band 34 and 44 GHz radio continuum observations of the Galactic
center have revealed 41 massive stars identified with near-IR counterparts, as
well as 44 proplyd candidates within 30" of Sgr A*. Radio observations obtained
in 2011 and 2014 have been used to derive proper motions of eight young stars
near Sgr A*. The accuracy of proper motion estimates based on near-IR
observations by Lu et al. and Paumard et al. have been investigated by using
their proper motions to predict the 2014 epoch positions of near-IR stars and
comparing the predicted positions with those of radio counterparts in the 2014
radio observations. Predicted positions from Lu et al. show an rms scatter of 6
mas relative to the radio positions, while those from Paumard et al. show rms
residuals of 20 mas, which is mainly due to uncertainties in the IR-based
proper motions. Under the assumption of homogeneous ionized winds, we also
determine the mass-loss rates of 11 radio stars, finding rates that are on
average 2 times smaller than those determined from model atmosphere
calculations and near-IR data. Clumpiness of ionized winds would reduce the
mass loss rate of WR and O stars by additional factors of 3 and 10,
respectively. One important implication of this is a reduction in the expected
mass accretion rate onto Sgr A* from stellar winds by nearly an order of
magnitude to a value of few \msol\ yr. Finally, we
present the positions of 318 compact 34.5 GHz radio sources within 30\arcs\ of
Sgr A*. At least 45 of these have stellar counterparts in the near-IR
(2.18 m) and (3.8m) bands.Comment: 30 pages, 4 figures, ApJ (in press
Radio Continuum Emission from the Magnetar SGR J1745-2900: Interaction with Gas Orbiting Sgr A*
We present radio continuum light curves of the magnetar SGR J17452900 and
Sgr A* obtained with multi-frequency, multi-epoch Very Large Array observations
between 2012 and 2014. During this period, a powerful X-ray outburst from SGR
J17452900 occurred on 2013-04-24. Enhanced radio emission is delayed with
respect to the X-ray peak by about seven months. In addition, the flux density
of the emission from the magnetar fluctuates by a factor of 2 to 4 at
frequencies between 21 and 41 GHz and its spectral index varies erratically.
Here we argue that the excess fluctuating emission from the magnetar arises
from the interaction of a shock generated from the X-ray outburst with the
orbiting ionized gas at the Galactic center. In this picture, variable
synchrotron emission is produced by ram pressure variations due to
inhomogeneities in the dense ionized medium of the Sgr A West bar. The pulsar
with its high transverse velocity is moving through a highly blue-shifted
ionized medium. This implies that the magnetar is at a projected distance of
pc from Sgr A* and that the orbiting ionized gas is partially or
largely responsible for a large rotation measure detected toward the magnetar.
Despite the variability of Sgr A* expected to be induced by the passage of the
G2 cloud, monitoring data shows a constant flux density and spectral index
during this periodComment: 12 pages, 3 figures, ApJL (in press
Surveillance of healthcare-associated infection in hospitalised South African children: Which method performs best?
Background. In 2012, the South African (SA) National Department of Health mandated surveillance of healthcare-associated infection (HAI), but made no recommendations of appropriate surveillance methods.Methods. Prospective clinical HAI surveillance (the reference method) was conducted at Tygerberg Children’s Hospital, Cape Town, from 1 May to 31 October 2015. Performance of three surveillance methods (point prevalence surveys (PPSs), laboratory surveillance and tracking of antimicrobial prescriptions) was compared with the reference method using surveillance evaluation guidelines. Factors associated with failure to detect HAI were identified by logistic regression analysis.Results. The reference method detected 417 HAIs among 1 347 paediatric hospitalisations (HAI incidence of 31/1000 patient days; 95% confidence interval (CI) 28.2 - 34.2). Surveillance methods had variable sensitivity (S) and positive predictive value (PPV): PPS S = 24.9% (95% CI 21 - 29.3), PPV = 100%; laboratory surveillance S = 48.4% (95% CI 43.7 - 53.2), PPV = 55.2% (95% CI 50.1 - 60.2); and antimicrobial prescriptions S = 66.4% (95% CI 61.8 - 70.8%), PPV = 88.5% (95% CI 84.5 - 91.6). Combined laboratory-antimicrobial surveillance achieved superior HAI detection (S = 84.7% (95% CI 80.9 - 87.8%), PPV = 97% (95% CI 94.6 - 98.4%)). Factors associated with failure to detect HAI included patient transfer (odds ratio (OR) 2.0), single HAI event (OR 2.8), age category 1 - 5 years (OR 2.1) and hospitalisation in a general ward (OR 2.3).Conclusions. Repeated PPSs, laboratory surveillance and/or antimicrobial prescription tracking are feasible HAI surveillance methods for low-resource settings. Combined laboratory-antimicrobial surveillance achieved the best sensitivity and PPV. SA paediatric healthcare facilities should individualise HAI surveillance, selecting a method suited to available resources and practice context
ALMA and VLA Observations: Evidence for Ongoing Low-mass Star Formation near Sgr A*
Using the VLA, we recently detected a large number of protoplanetary disk
(proplyd) candidates lying within a couple of light years of the massive black
hole Sgr A*. The bow-shock appearance of proplyd candidates point toward the
young massive stars located near Sgr A*. Similar to Orion proplyds, the strong
UV radiation from the cluster of massive stars at the Galactic center is
expected to photoevaporate and photoionize the circumstellar disks around
young, low mass stars, thus allowing detection of the ionized outflows from the
photoionized layer surrounding cool and dense gaseous disks. To confirm this
picture, ALMA observations detect millimeter emission at 226 GHz from five
proplyd candidates that had been detected at 44 and 34 GHz with the VLA. We
present the derived disk masses for four sources as a function of the assumed
dust temperature. The mass of protoplanetary disks from cool dust emission
ranges between 0.03 -- 0.05 solar mass. These estimates are consistent with the
disk masses found in star forming sites in the Galaxy. These measurements show
the presence of on-going star formation with the implication that gas clouds
can survive near Sgr A* and the relative importance of high vs low-mass star
formation in the strong tidal and radiation fields of the Galactic center.Comment: 13 pages, 3 figures, MNRAS (in press
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