23 research outputs found
Blazar Optical Variability in the Palomar-QUEST Survey
We study the ensemble optical variability of 276 FSRQs and 86 BL Lacs in the
Palomar-QUEST Survey with the goal of searching for common fluctuation
properties, examining the range of behavior across the sample, and
characterizing the appearance of blazars in such a survey so that future work
can more easily identify such objects. The survey, which covers 15,000 square
degrees multiple times over 3.5 years, allows for the first ensemble blazar
study of this scale. Variability amplitude distributions are shown for the FSRQ
and BL Lac samples for numerous time lags, and also studied through structure
function analyses. Individual blazars show a wide range of variability
amplitudes, timescales, and duty cycles. Of the best sampled objects, 35% are
seen to vary by more than 0.4 magnitudes; for these, the fraction of
measurements contributing to the high amplitude variability ranges constantly
from about 5% to 80%. Blazar variability has some similarities to that of type
I quasars but includes larger amplitude fluctuations on all timescales. FSRQ
variability amplitudes are particularly similar to those of QSOs on timescales
of several months, suggesting significant contributions from the accretion disk
to the variable flux at these timescales. Optical variability amplitudes are
correlated with the maximum apparent velocities of the radio jet for the subset
of FSRQs with MOJAVE VLBA measurements, implying that the optically variable
flux's strength is typically related to that of the radio emission. We also
study CRATES radio-selected FSRQ candidates, which show similar variability
characteristics to known FSRQs; this suggests a high purity for the CRATES
sample.Comment: 29 pages, 12 figures. Accepted for publication in Ap
Stronger and More Vulnerable: A Balanced View of the Impacts of the NICU Experience on Parents
For parents, the experience of having an infant in the NICU is often psychologically traumatic. No parent can be fully prepared for the extreme stress and range of emotions of caring for a critically ill newborn. As health care providers familiar with the NICU, we thought that we understood the impact of the NICU on parents. But we were not prepared to see the children in our own families as NICU patients. Here are some of the lessons our NICU experience has taught us. We offer these lessons in the hope of helping health professionals consider a balanced view of the NICU's impact on families
The Lifetime and Powers of FR IIs in Galaxy Clusters
We have identified and studied a sample of 151 FR IIs found in brightest
cluster galaxies (BCGs) in the MaxBCG cluster catalog with data from FIRST and
NVSS. We have compared the radio luminosities and projected lengths of these FR
IIs to the projected length distribution of a range of mock catalogs generated
by an FR II model and estimate the FR II lifetime to be 1.9 x 10^8 yr. The
uncertainty in the lifetime calculation is a factor of two, due primarily to
uncertainties in the ICM density and the FR II axial ratio. We furthermore
measure the jet power distribution of FR IIs in BCGs and find that it is well
described by a log-normal distribution with a median power of 1.1 x 10^37 W and
a coefficient of variation of 2.2. These jet powers are nearly linearly related
to the observed luminosities, and this relation is steeper than many other
estimates, although it is dependent on the jet model. We investigate
correlations between FR II and cluster properties and find that galaxy
luminosity is correlated with jet power. This implies that jet power is also
correlated with black hole mass, as the stellar luminosity of a BCG should be a
good proxy for its spheroid mass and therefore the black hole mass. Jet power,
however, is not correlated with cluster richness, nor is FR II lifetime
strongly correlated with any cluster properties. We calculate the enthalpy of
the lobes to examine the impact of the FR IIs on the ICM and find that heating
due to adiabatic expansion is too small to offset radiative cooling by a factor
of at least six. In contrast, the jet power is approximately an order of
magnitude larger than required to counteract cooling. We conclude that if
feedback from FR IIs offsets cooling of the ICM, then heating must be primarily
due to another mechanism associated with FR II expansion.Comment: 22 pages, 20 figures. Accepted to ApJ. Added minor clarifications
throughout the paper and restructured section 6.2 in response to the referee.
A brief video explaining the paper can be found at
http://youtu.be/DOq85qUSU-
Magnetohydrodynamic relaxation of AGN ejecta: radio bubbles in the intracluster medium
X-ray images of galaxy clusters often display underdense bubbles which are
apparently inflated by AGN outflow. I consider the evolution of the magnetic
field inside such a bubble, using a mixture of analytic and numerical methods.
It is found that the field relaxes into an equilibrium filling the entire
volume of the bubble. The timescale on which this happens depends critically on
the magnetisation and helicity of the outflow as well as on properties of the
surrounding ICM. If the outflow is strongly magnetised, the magnetic field
undergoes reconnection on a short timescale, magnetic energy being converted
into heat whilst the characteristic length scale of the field rises; this
process stops when a global equilibrium is reached. The strength of the
equilibrium field is determined by the magnetic helicity injected into the
bubble by the AGN: if the outflow has a consistent net flux and consequently a
large helicity then a global equilibrium will be reached on a short timescale,
whereas a low-helicity outflow results in no global equilibrium being reached
and at the time of observation reconnection will be ongoing. However, localised
flux-tube equilibria will form. If, on the other hand, the outflow is very
weakly magnetised, no reconnection occurs and the magnetic field inside the
bubble remains small-scale and passive. These results have implications for the
internal composition of the bubbles, their interaction with ICM -- in
particular to explain how bubbles could move a large distance through the ICM
without breaking up -- as well as for the cooling flow problem in general. In
addition, reconnection sites in a bubble could be a convenient source of
energetic particles, circumventing the problem of synchrotron emitters having a
shorter lifetime than the age of the bubble they inhabit.Comment: MNRAS accepted. 15 pages, 10 figures
The Lifetime of FRIIs in Groups and Clusters: Implications for Radio-Mode Feedback
We determine the maximum lifetime t_max of 52 FRII radio sources found in 26
central group galaxies from cross correlation of the Berlind SDSS group catalog
with the VLA FIRST survey. Mock catalogs of FRII sources were produced to match
the selection criteria of FIRST and the redshift distribution of our parent
sample, while an analytical model was used to calculate source sizes and
luminosities. The maximum lifetime of FRII sources was then determined via a
comparison of the observed and model projected length distributions. We
estimate the average FRII lifetime is 1.5x10^7 years and the duty cycle is
~8x10^8 years. Degeneracies between t_max and the model parameters: jet power
distribution, axial ratio, energy injection index, and ambient density
introduce at most a factor of two uncertainty in our lifetime estimate. In
addition, we calculate the radio active galactic nuclei (AGN) fraction in
central group galaxies as a function of several group and host galaxy
properties. The lifetime of radio sources recorded here is consistent with the
quasar lifetime, even though these FRIIs have substantially sub-Eddington
accretion. These results suggest a fiducial time frame for energy injection
from AGN in feedback models. If the morphology of a given extended radio source
is set by large-scale environment, while the lifetime is determined by the
details of the accretion physics, this FRII lifetime is relevant for all
extended radio sources.Comment: 18 pages, 7 figures. Accepted for publication in ApJ. High resolution
paper available at http://www.astronomy.ohio-state.edu/~bird/BMK07.pd
G64.5+0.9, a new shell supernova remnant with unusual central emission
We present observations between 1.4 and 18 GHz confirming that G64.5+0.9 is
new Galactic shell supernova remnant, using the Very Large Array and the
Arcminute Microkelvin Imager. The remnant is a shell ~8 arcmin in diameter with
a spectral index of alpha = 0.47 +/- 0.03 (with alpha defined such that flux
density S varies with frequency nu as S proportional to nu to the power of
-alpha). There is also emission near the centre of the shell, ~1 arcmin in
extent, with a spectral index of alpha = 0.81 +/- 0.02. We do not find any
evidence for spectral breaks for either source within our frequency range. The
nature of the central object is unclear and requires further investigation, but
we argue that is most unlikely to be extragalactic. It is difficult to avoid
the conclusion that it is associated with the shell, although its spectrum is
very unlike that of known pulsar wind nebulae.Comment: 5 pages, 6 figures, submitted to MNRA
Discovery of a Sub-Parsec Radio Counterjet in the Nucleus of Centaurus A
A sub-parsec scale radio counterjet has been detected in the nucleus of the
closest radio galaxy, Centaurus A (NGC 5128), with VLBI imaging at 2.3 and 8.4
GHz. This is one of the first detections of a VLBI counterjet and provides new
constraints on the kinematics of the radio jets emerging from the nucleus of
Cen A. A bright, compact core is seen at 8.4 GHz, along with a jet extending
along P.A. 51 degrees. The core is completely absorbed at 2.3 GHz. Our images
show a much wider gap between the base of the main jet and the counterjet at
2.3 GHz than at 8.4 GHz and also that the core has an extraordinarily inverted
spectrum. These observations provide evidence that the innermost 0.4-0.8 pc of
the source is seen through a disk or torus of ionized gas which is opaque at
low frequencies due to free-free absorption.Comment: 3 pages, 2 postscript figures, scheduled for publication in August 1,
1996 issue of Ap.J. Letter
The Pandemic Allocation of Ventilators Model Penalizes Infants with Bronchopulmonary Dysplasia
During the COVID-19 pandemic, institutions developed ventilator allocation models. In one proposed model, neonates compete with adults for ventilators using a scoring system. Points are given for conditions that increase one- and five-year (y) mortality. For example, comparable points were added for adult conditions with mortality of 71.3% and for neonates with moderate or severe bronchopulmonary dysplasia (mod/sBPD). We hypothesized that this model overestimates mortality in neonates with BPD and would penalize these infants unfairly. There was little information available on 1 y and 5 y mortality risk for mod/sBPD. To evaluate this allocation protocol, a retrospective chart review was performed on infants born ≥22 weeks and weighing <1500 g admitted to Rainbow Babies and Children’s Hospital in 2015 to identify babies with BPD. The main outcomes were 1 and 5 y mortality. In 2015, 28 infants were diagnosed with mod/s BPD based on NIH 2001 definition; 4 infants had modBPD and 24 had sBPD. All infants (100%) with modBPD survived to 5 y; 2 infants with sBPD died by 1 y (8%) and 22 survived (92%) to 1 y; 3 died (12.5%) by 5 y; and at least 13 survived (54%) to 5 y. Infants with mod/s BPD had lower-than-predicted 1 and 5 y mortality, suggesting the points assigned in the model are too high for these conditions. We believe this model would unfairly penalize these babies