660 research outputs found
A Robust Determination of the size of quasar accretion disks using gravitational microlensing
Using microlensing measurements from a sample of 27 image-pairs of 19 lensed
quasars we determine a maximum likelihood estimate for the accretion disk size
of an {{\em}average} quasar of light days at rest
frame \AA\ for microlenses with a mean mass of
. This value, in good agreement with previous results from
smaller samples, is roughly a factor of 5 greater than the predictions of the
standard thin disk model. The individual size estimates for the 19 quasars in
our sample are also in excellent agreement with the results of the joint
maximum likelihood analysis.Comment: 6 pages, 3 figures, submitted to Ap
Alternative mechanism for bacteriophage adsorption to the motile bacterium Caulobacter crescentus
2D and 3D cryo-electron microscopy, together with adsorption kinetics assays of ϕCb13 and ϕCbK phage-infected Caulobacter crescentus, provides insight into the mechanisms of infection. ϕCb13 and ϕCbK actively interact with the flagellum and subsequently attach to receptors on the cell pole. We present evidence that the first interaction of the phage with the bacterial flagellum takes place through a filament on the phage head. This contact with the flagellum facilitates concentration of phage particles around the receptor (i.e., the pilus portals) on the bacterial cell surface, thereby increasing the likelihood of infection. Phage head filaments have not been well characterized and their function is described here. Phage head filaments may systematically underlie the initial interactions of phages with their hosts in other systems and possibly represent a widespread mechanism of efficient phage propagation
I. Flux and color variations of the quadruply imaged quasar HE 0435-1223
aims: We present VRi photometric observations of the quadruply imaged quasar
HE 0435-1223, carried out with the Danish 1.54m telescope at the La Silla
Observatory. Our aim was to monitor and study the magnitudes and colors of each
lensed component as a function of time. methods: We monitored the object during
two seasons (2008 and 2009) in the VRi spectral bands, and reduced the data
with two independent techniques: difference imaging and PSF (Point Spread
Function) fitting.results: Between these two seasons, our results show an
evident decrease in flux by ~0.2-0.4 magnitudes of the four lensed components
in the three filters. We also found a significant increase (~0.05-0.015) in
their V-R and R-i color indices. conclusions: These flux and color variations
are very likely caused by intrinsic variations of the quasar between the
observed epochs. Microlensing effects probably also affect the brightest "A"
lensed component.Comment: 10 pages, 8 figure
Reverberation Mapping Measurements of Black Hole Masses in Six Local Seyfert Galaxies
We present the final results from a high sampling rate, multi-month,
spectrophotometric reverberation mapping campaign undertaken to obtain either
new or improved Hbeta reverberation lag measurements for several relatively
low-luminosity AGNs. We have reliably measured thetime delay between variations
in the continuum and Hbeta emission line in six local Seyfert 1 galaxies. These
measurements are used to calculate the mass of the supermassive black hole at
the center of each of these AGNs. We place our results in context to the most
current calibration of the broad-line region (BLR) R-L relationship, where our
results remove outliers and reduce the scatter at the low-luminosity end of
this relationship. We also present velocity-resolved Hbeta time delay
measurements for our complete sample, though the clearest velocity-resolved
kinematic signatures have already been published.Comment: 52 pages (AASTeX: 29 pages of text, 8 tables, 7 figures), accepted
for publication in the Astrophysical Journa
Quantitative assessment of chronic lung disease of infancy using computed tomography
The aims of this study were to determine whether infants and toddlers with chronic lung disease of infancy (CLDI) have smaller airways and lower lung density compared with full-term healthy controls. Multi-slice computed tomography (CT) chest scans were obtained at elevated lung volumes during a brief respiratory pause in sedated infants and toddlers;38 CLDI were compared with 39 full-term controls. For CLDI subjects, gestational age at birth ranged from 25 to 29 weeks. Airway size was measured for the trachea and the next three to four generations into the right lower lobe;lung volumes and tissue density were also measured. The relationship between airway size and airway generation differed between the CLDI and full-term groups;the sizes of the first and second airway generations were larger in the shorter CLDI than in the shorter full-term subjects. The increased size in the airways in the CLDI subjects was associated with increasing mechanical ventilation time in the neonatal period. CLDI subjects had a greater heterogeneity of lung density compared with full-term subjects. Our results indicate that quantitative analysis of multi-slice CT scans at elevated volumes provides important insights into the pulmonary pathology of infants and toddlers with CLDI
Microlens OGLE-2005-BLG-169 Implies Cool Neptune-Like Planets are Common
We detect a Neptune mass-ratio (q~8e-5) planetary companion to the lens star
in the extremely high-magnification (A~800) microlensing event
OGLE-2005-BLG-169. If the parent is a main-sequence star, it has mass M~0.5
M_sun implying a planet mass of ~13 M_earth and projected separation of ~2.7
AU. When intensely monitored over their peak, high-magnification events similar
to OGLE-2005-BLG-169 have nearly complete sensitivity to Neptune mass-ratio
planets with projected separations of 0.6 to 1.6 Einstein radii, corresponding
to 1.6--4.3 AU in the present case. Only two other such events were monitored
well enough to detect Neptunes, and so this detection by itself suggests that
Neptune mass-ratio planets are common. Moreover, another Neptune was recently
discovered at a similar distance from its parent star in a low-magnification
event, which are more common but are individually much less sensitive to
planets. Combining the two detections yields 90% upper and lower frequency
limits f=0.37^{+0.30}_{-0.21} over just 0.4 decades of planet-star separation.
In particular, f>16% at 90% confidence. The parent star hosts no Jupiter-mass
companions with projected separations within a factor 5 of that of the detected
planet. The lens-source relative proper motion is \mu~7--10 mas/yr, implying
that if the lens is sufficiently bright, I<23.8, it will be detectable by HST
by 3 years after peak. This would permit a more precise estimate of the lens
mass and distance, and so the mass and projected separation of the planet.
Analogs of OGLE-2005-BLG-169Lb orbiting nearby stars would be difficult to
detect by other methods of planet detection, including radial velocities,
transits, or astrometry.Comment: Submitted to ApJ Letters, 9 text pages + 4 figures + 1 tabl
Revealing the Structure of an Accretion Disk Through Energy Dependent X-ray Microlensing
We present results from monitoring observations of the gravitationally lensed
quasar RX J1131-1231 performed with the Chandra X-ray Observatory. The X-ray
observations were planned with relatively long exposures that allowed a search
for energy-dependent microlensing in the soft (0.2-2 keV) and hard (2-10 keV)
light curves of the images of RX J1131-1231. We detect significant microlensing
in the X-ray light-curves of images A and D, and energy-dependent microlensing
of image D. The magnification of the soft band appears to be larger than that
in the hard band by a factor of ~ 1.3 when image D becomes more magnified. This
can be explained by the difference between a compact, softer-spectrum corona
that is producing a more extended, harder spectrum reflection component off the
disk. This is supported by the evolution of the fluorescent iron line in image
D over three consecutive time-averaged phases of the light curve. In the first
period, a Fe line at E = 6.36(-0.16,+0.13) keV is detected (at > 99%
confidence). In the second period, two Fe lines are detected, one at E =
5.47(-0.08,+0.06) keV (detected at > 99% confidence) and another at E =
6.02(-0.07,+0.09) keV (marginally detected at > 90% confidence), and in the
third period, a broadened Fe line at 6.42(-0.15,+0.19) keV is detected (at >
99% confidence). This evolution of the Fe line profile during the microlensing
event is consistent with the line distortion expected when a caustic passes
over the inner disk where the shape of the fluorescent Fe line is distorted by
General Relativistic and Doppler effects.Comment: 20 pages, includes 10 figures, submitted to Ap
The Race Between Stars and Quasars in Reionizing Cosmic Hydrogen
The cosmological background of ionizing radiation has been dominated by
quasars once the Universe aged by ~2 billion years. At earlier times (redshifts
z>3), the observed abundance of bright quasars declined sharply, implying that
cosmic hydrogen was reionized by stars instead. Here, we explain the physical
origin of the transition between the dominance of stars and quasars as a
generic feature of structure formation in the concordance LCDM cosmology. At
early times, the fraction of baryons in galaxies grows faster than the maximum
(Eddington-limited) growth rate possible for quasars. As a result, quasars were
not able to catch up with the rapid early growth of stellar mass in their host
galaxies.Comment: 5 pages, 1 figure, Accepted for publication in JCA
A Revised Broad-Line Region Radius and Black Hole Mass for the Narrow-Line Seyfert 1 NGC 4051
We present the first results from a high sampling rate, multi-month
reverberation mapping campaign undertaken primarily at MDM Observatory with
supporting observations from telescopes around the world. The primary goal of
this campaign was to obtain either new or improved Hbeta reverberation lag
measurements for several relatively low luminosity AGNs. We feature results for
NGC 4051 here because, until now, this object has been a significant outlier
from AGN scaling relationships, e.g., it was previously a ~2-3sigma outlier on
the relationship between the broad-line region (BLR) radius and the optical
continuum luminosity - the R_BLR-L relationship. Our new measurements of the
lag time between variations in the continuum and Hbeta emission line made from
spectroscopic monitoring of NGC 4051 lead to a measured BLR radius of R_BLR =
1.87 (+0.54 -0.50) light days and black hole mass of M_BH = 1.73 (+0.55 -0.52)
x 10^6 M_sun. This radius is consistent with that expected from the R_BLR-L
relationship, based on the present luminosity of NGC 4051 and the most current
calibration of the relation by Bentz et al. (2009a). We also present a
preliminary look at velocity-resolved Hbeta light curves and time delay
measurements, although we are unable to reconstruct an unambiguous
velocity-resolved reverberation signal.Comment: 38 pages, 7 figures, accepted for publication in ApJ, changes from v1
reflect suggestions from anonymous refere
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