5,011 research outputs found
Quasar outflow energetics from broad absorption line variability
Quasar outflows have long been recognized as potential contributors to the
co-evolution between supermassive black holes (SMBHs) and their host galaxies.
The role of outflows in AGN feedback processes can be better understood by
placing observational constraints on wind locations and kinetic energies. We
utilize broad absorption line (BAL) variability to investigate the properties
of a sample of 71 BAL quasars with PV broad absorption. The
presence of PV BALs indicates that other BALs like CIV
are saturated, such that variability in those lines favours clouds crossing the
line of sight. We use these constraints with measurements of BAL variability to
estimate outflow locations and energetics. Our data set consists of
multiple-epoch spectra from the Sloan Digital Sky Survey and MDM Observatory.
We detect significant (4) BAL variations from 10 quasars in our sample
over rest frame time-scales between < 0.2-3.8 yr. Our derived distances for the
10 variable outflows are nominally < 1-10 pc from the SMBH using the
transverse-motion scenario, and < 100-1000 pc from the central source using
ionization-change considerations. These distances, in combination with the
estimated high outflow column densities (i.e. > 10
cm), yield outflow kinetic luminosities between ~ 0.001-1 times the
bolometric luminosity of the quasar, indicating that many absorber energies
within our sample are viable for AGN feedback.Comment: 19 pages, 3 figures, 4 tables, 1 supplementary figure, accepted to
MNRA
Atmospheric absorption of high frequency noise and application to fractional-octave bands
Pure tone sound absorption coefficients were measured at 1/12 octave intervals from 4 to 100 KHz at 5.5K temperature intervals between 255.4 and 310.9 K and at 10 percent relative humidity increments between 0 percent and saturation in a large cylindrical tube (i.d., 25.4 cm; length, 4.8 m). Special solid-dielectric capacitance transducers, one to generate bursts of sound waves and one to terminate the sound path and detect the tone bursts, were constructed to fit inside the tube. The absorption was measured by varying the transmitter receiver separation from 1 to 4 m and observing the decay of multiple reflections or change in amplitude of the first received burst. The resulting absorption was compared with that from a proposed procedure for computing sound absorption in still air. Absorption of bands of noise was numerically computed by using the pure tone results. The results depended on spectrum shape, on filter type, and nonlinearly on propagation distance. For some of the cases considered, comparison with the extrapolation of ARP-866A showed a difference as large as a factor of 2. However, for many cases, the absorption for a finite band was nearly equal to the pure tone absorption at the center frequency of the band. A recommended prediction procedure is described for 1/3 octave band absorption coefficients
Constraining FeLoBAL outflows from absorption line variability
FeLoBALs are a rare class of quasar outflows with low-ionization broad
absorption lines (BALs), large column densities, and potentially large kinetic
energies that might be important for `feedback' to galaxy evolution. In order
to probe the physical properties of these outflows, we conducted a
multiple-epoch, absorption line variability study of 12 FeLoBAL quasars
spanning a redshift range between 0.7 and 1.9 over rest frame time-scales of
approximately 10 d to 7.6 yr. We detect absorption line variability with
greater than 8 sigma confidence in 3 out of the 12 sources in our sample over
time-scales of 0.6 to 7.6 yr. Variable wavelength intervals are associated with
ground and excited state Fe II multiplets, the Mg II 2796, 2803 doublet, Mg I
2852, and excited state Ni II multiplets. The observed variability along with
evidence of saturation in the absorption lines favors transverse motions of gas
across the line of sight (LOS) as the preferred scenario, and allows us to
constrain the outflow distance from the supermassive black hole (SMBH) to be
less than 69, 7, and 60 pc for our three variable sources. In combination with
other studies, these results suggest that the outflowing gas in FeLoBAL quasars
resides on a range of scales and includes matter within tens of parsecs of the
central source.Comment: 21 pages, 6 figures, 2 supplementary figures (attached at the end of
the manuscript), accepted to Monthly Notices of the Royal Astronomical
Societ
Return times, recurrence densities and entropy for actions of some discrete amenable groups
Results of Wyner and Ziv and of Ornstein and Weiss show that if one observes
the first k outputs of a finite-valued ergodic process, then the waiting time
until this block appears again is almost surely asymptotic to , where
is the entropy of the process. We examine this phenomenon when the allowed
return times are restricted to some subset of times, and generalize the results
to processes parameterized by other discrete amenable groups.
We also obtain a uniform density version of the waiting time results: For a
process on symbols, within a given realization, the density of the initial
-block within larger -blocks approaches , uniformly in ,
as tends to infinity. Again, similar results hold for processes with other
indexing groups.Comment: To appear in Journal d'Analyse Mathematiqu
Gigahertz quantum key distribution with InGaAs avalanche photodiodes
We report a demonstration of quantum key distribution (QKD) at GHz clock
rates with InGaAs avalanche photodiodes (APDs) operating in a self-differencing
mode. Such a mode of operation allows detection of extremely weak avalanches so
that the detector afterpulse noise is sufficiently suppressed. The system is
characterized by a secure bit rate of 2.37 Mbps at 5.6 km and 27.9 kbps at 65.5
km when the fiber dispersion is not compensated. After compensating the fiber
dispersion, the QKD distance is extended to 101 km, resulting in a secure key
rate of 2.88 kbps. Our results suggest that InGaAs APDs are very well suited to
GHz QKD applications.Comment: 4 pages, 4 figure
Improved fidelity of triggered entangled photons from single quantum dots
We demonstrate the on-demand emission of polarisation-entangled photon pairs
from the biexciton cascade of a single InAs quantum dot embedded in a GaAs/AlAs
planar microcavity. Improvements in the sample design blue shifts the wetting
layer to reduce the contribution of background light in the measurements.
Results presented show that >70% of the detected photon pairs are entangled.
The high fidelity of the (|HxxHx>+|VxxVx>)/2^0.5 state that we determine is
sufficient to satisfy numerous tests for entanglement. The improved quality of
entanglement represents a significant step towards the realisation of a
practical quantum dot source compatible with applications in quantum
information.Comment: 9 pages. Paper is available free of charge at
http://www.iop.org/EJ/abstract/1367-2630/8/2/029/, see also 'A semiconductor
source of triggered entangled photon pairs', R. M. Stevenson et al., Nature
439, 179 (2006
High Carbon in I Zwicky 18: New Results from Hubble Space Telescope Spectroscopy
We present new measurements of the gas-phase C/O abundance ratio in both the
NW and SE components of the extremely metal-poor dwarf irregular galaxy I Zw
18, based on ultraviolet spectroscopy of the two H II regions using the Faint
Object Spectrograph on the Hubble Space Telescope. We determine values of log
C/O = -0.63 +/- 0.10 for the NW component and log C/O = -0.56 +/- 0.09 for the
SE component. In comparison, log C/O = -0.37 in the sun, while log C/O = -0.85
+/- 0.07 in the three most metal-poor irregular galaxies measured by Garnett et
al. (1995a). Our measurements show that C/O in I Zw 18 is significantly higher
than in other comparably metal-poor irregular galaxies, and above predictions
for the expected C/O from massive star nucleosynthesis. These results suggest
that carbon in I Zw 18 has been enhanced by an earlier population of lower-mass
carbon producing stars; this idea is supported by stellar photometry of I Zw 18
and its companion, which demonstrate that the current bursts of massive stars
were not the first. Despite its very low metallicity, it is likely that I Zw 18
is not a ``primeval'' galaxy.Comment: 14 pages including 4 figures; uses aaspp4.sty. Accepted for
publication in ApJ. Postscript version also available by e-mail request to
author at [email protected]
MicroRNA-125b transforms myeloid cell lines by repressing multiple mRNA
Background: We previously described a t(2;11)(p21;q23) chromosomal translocation found in patients with myelodysplasia or acute myeloid leukemia that leads to over-expression of the microRNA miR-125b, and we showed that transplantation of mice with murine stem/progenitor cells overexpressing miR-125b is able to induce leukemia. In this study, we investigated the mechanism of myeloid transformation by miR-125b.
Design and Methods: To investigate the consequences of miR-125b over-expression on myeloid differentiation, apoptosis and proliferation, we used the NB4 and HL60 human promyelocytic cell lines and the 32Dclone3 murine promyelocytic cell line. To test whether miR-125b is able to transform myeloid cells, we used the non-tumorigenic and interleukin-3-dependent 32Dclone3 cell line over-expressing miR-125b, in xenograft experiments in nude mice and in conditions of interleukin-3 deprivation. To identify new miR-125b targets, we compared, by RNA-sequencing, the transcriptome of cell lines that do or do not over-express miR-125b.
Results: We showed that miR-125b over-expression blocks apoptosis and myeloid differentiation and enhances proliferation in both species. More importantly, we demonstrated that miR-125b is able to transform the 32Dclone3 cell line by conferring growth independence from interleukin-3; xenograft experiments showed that these cells form tumors in nude mice. Using RNA-sequencing and quantitative real-time polymerase chain reaction experiments, we identified multiple miR-125b targets. We demonstrated that ABTB1, an anti-proliferative factor, is a new direct target of miR-125b and we confirmed that CBFB, a transcription factor involved in hematopoiesis, is also targeted by miR-125b. MiR-125b controls apoptosis by down-regulating genes involved in the p53 pathway including BAK1 and TP53INP1.
Conclusions: This study demonstrates that in a myeloid context, miR-125b is an oncomiR able to transform cell lines. miR-125b blocks myeloid differentiation in part by targeting CBFB, blocks apoptosis through down-regulation of multiple genes involved in the p53 pathway, and confers a proliferative advantage to human and mouse myeloid cell lines in part by targeting ABTB1.Leukemia & Lymphoma Society of AmericaNational Institutes of Health (U.S.) (NIH grant DK068348)National Institutes of Health (U.S.) (NIH grant 5P01 HL066105
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