390 research outputs found
Effect of Binary Source Companions on the Microlensing Optical Depth Determination toward the Galactic Bulge Field
Currently, gravitational microlensing survey experiments toward the Galactic
bulge field utilize two different methods of minimizing blending effect for the
accurate determination of the optical depth \tau. One is measuring \tau based
on clump giant (CG) source stars and the other is using `Difference Image
Analysis (DIA)' photometry to measure the unblended source flux variation.
Despite the expectation that the two estimates should be the same assuming that
blending is properly considered, the estimates based on CG stars systematically
fall below the DIA results based on all events with source stars down to the
detection limit. Prompted by the gap, we investigate the previously
unconsidered effect of companion-associated events on determination.
Although the image of a companion is blended with that of its primary star and
thus not resolved, the event associated with the companion can be detected if
the companion flux is highly magnified. Therefore, companions work effectively
as source stars to microlensing and thus neglect of them in the source star
count could result in wrong \tau estimation. By carrying out simulations based
on the assumption that companions follow the same luminosity function of
primary stars, we estimate that the contribution of the companion-associated
events to the total event rate is ~5f_{bi}% for current surveys and can reach
up to ~6f_{bi}% for future surveys monitoring fainter stars, where f_{bi} is
the binary frequency. Therefore, we conclude that the companion-associated
events comprise a non-negligible fraction of all events. However, their
contribution to the optical depth is not large enough to explain the systematic
difference between the optical depth estimates based on the two different
methods.Comment: 4 pages, 1 figure, 1 table, ApJ, submitte
Design Knowledge for Virtual Learning Companions from a Value-centered Perspective
The increasing popularity of conversational agents such as ChatGPT has sparked interest in their potential use in educational contexts but undermines the role of companionship in learning with these tools. Our study targets the design of virtual learning companions (VLCs), focusing on bonding relationships for collaborative learning while facilitating students’ time management and motivation. We draw upon design science research (DSR) to derive prescriptive design knowledge for VLCs as the core of our contribution. Through three DSR cycles, we conducted interviews with working students and experts, held interdisciplinary workshops with the target group, designed and evaluated two conceptual prototypes, and fully coded a VLC instantiation, which we tested with students in class. Our approach has yielded 9 design principles, 28 meta-requirements, and 33 design features centered around the value-in-interaction. These encompass Human-likeness and Dialogue Management, Proactive and Reactive Behavior, and Relationship Building on the Relationship Layer (DP1,3,4), Adaptation (DP2) on the Matching Layer, as well as Provision of Supportive Content, Fostering Learning Competencies, Motivational Environment, and Ethical Responsibility (DP5-8) on the Service Layer
On the Nature and Location of the Microlenses
This paper uses the caustic crossing events in the microlens data sets to
explore the nature and location of the lenses. We conclude that the large
majority of lenses, whether they are luminous or dark, are likely to be
binaries. Further, we demonstrate that blending is an important feature of all
the data sets. An additional interpretation suggested by the data, that the
caustic crossing events along the directions to the Magellanic Clouds are due
to lenses located in the Clouds, implies that most of the LMC/SMC events to
date are due to lenses in the Magellanic Clouds. All of these conclusions can
be tested. If they are correct, a large fraction of lenses along the direction
to the LMC may be ordinary stellar binary systems, just as are the majority of
the lenses along the direction to the Bulge. Thus, a better understanding of
the larger-than-anticipated value derived for the Bulge optical depth may allow
us to better interpret the large value derived for the optical depth to the
LMC. Indeed, binarity and blending in the data sets may illuminate connections
among several other puzzles: the dearth of binary-source light curves, the
dearth of non-caustic-crossing perturbed binary-lens events, and the dearth of
obviously blended point-lens events.Comment: 15 pages, 2 figures. Submitted to the Astrophysical Journal Letters,
4 January 199
Mass distribution in our Galaxy
This article summarizes recent work on the luminosity and mass distribution of the Galactic bulge and disk, and on the mass of the Milky Way's dark halo. A new luminosity model consistent with the COBE NIR data and the apparent magnitude distributions of bulge clump giant stars has bulge/bar length of \simeq 3.5\kpc, axis ratios of 1:(0.3-0.4):0.3, and short disk scale-length (\simeq 2.1\kpc). Gas-dynamical flows in the potential of this model with constant M/L fit the terminal velocities in very well. The luminous mass distribution with this M/L is consistent with the surface density of known matter near the Sun, but still underpredicts the microlensing optical depth towards the bulge. Together, these facts argue strongly for a massive, near-maximal disk in our , Sbc spiral Galaxy. While the outer rotation curve and global mass distribution are not as readily measured as in similar spiral galaxies, the dark halo mass estimated from satellite velocities is consistent with a flat rotation curve continuing on from the luminous mass distribution
An inner ring and the micro lensing toward the Bulge
All current Bulge-Disk models for the inner Galaxy fall short of reproducing
self-consistently the observed micro-lensing optical depth by a factor of two
(). We show that the least mass-consuming way to increase the
optical depth is to add density roughly half-way the observer and the highest
micro-lensing-source density. We present evidence for the existence of such a
density structure in the Galaxy: an inner ring, a standard feature of barred
galaxies. Judging from data on similar rings in external galaxies, an inner
ring can contribute more than 50% of a pure Bulge-Disk model to the
micro-lensing optical depth. We may thus eliminate the need for a small viewing
angle of the Bar. The influence of an inner ring on the event-duration
distribution, for realistic viewing angles, would be to increase the fraction
of long-duration events toward Baade's window. The longest events are expected
toward the negative-longitude tangent point at -22\degr . A properly
sampled event-duration distribution toward this tangent point would provide
essential information about viewing angle and elongation of the over-all
density distribution in the inner Galaxy.Comment: 9 pages, 7(15) figs, LaTeX, AJ (accepted
The EROS2 search for microlensing events towards the spiral arms: the complete seven season results
The EROS-2 project has been designed to search for microlensing events
towards any dense stellar field. The densest parts of the Galactic spiral arms
have been monitored to maximize the microlensing signal expected from the stars
of the Galactic disk and bulge. 12.9 million stars have been monitored during 7
seasons towards 4 directions in the Galactic plane, away from the Galactic
center. A total of 27 microlensing event candidates have been found. Estimates
of the optical depths from the 22 best events are provided. A first order
interpretation shows that simple Galactic models with a standard disk and an
elongated bulge are in agreement with our observations. We find that the
average microlensing optical depth towards the complete EROS-cataloged stars of
the spiral arms is , a number that is
stable when the selection criteria are moderately varied. As the EROS catalog
is almost complete up to , the optical depth estimated for the
sub-sample of bright target stars with () is easier to interpret. The set of microlensing events
that we have observed is consistent with a simple Galactic model. A more
precise interpretation would require either a better knowledge of the distance
distribution of the target stars, or a simulation based on a Galactic model.
For this purpose, we define and discuss the concept of optical depth for a
given catalog or for a limiting magnitude.Comment: 22 pages submitted to Astronomy & Astrophysic
Risk estimators for choosing regularization parameters in ill-posed problems - Properties and limitations
This paper discusses the properties of certain risk estimators that recently regained popularity for choosing regularization parameters in ill-posed problems, in particular for sparsity regularization. They apply Stein’s unbiased risk estimator (SURE) to estimate the risk in either the space of the unknown variables or in the data space. We will call the latter PSURE in order to distinguish the two different risk functions. It seems intuitive that SURE is more appropriate for ill-posed problems, since the properties in the data space do not tell much about the quality of the reconstruction. We provide theoretical studies of both approaches for linear Tikhonov regularization in a finite dimensional setting and estimate the quality of the risk estimators, which also leads to asymptotic convergence results as the dimension of the problem tends to infinity. Unlike previous works which studied single realizations of image processing problems with a very low degree of ill-posedness, we are interested in the statistical behaviour of the risk estimators for increasing ill-posedness. Interestingly, our theoretical results indicate that the quality of the SURE risk can deteriorate asymptotically for ill-posed problems, which is confirmed by an extensive numerical study. The latter shows that in many cases the SURE estimator leads to extremely small regularization parameters, which obviously cannot stabilize the reconstruction. Similar but less severe issues with respect to robustness also appear for the PSURE estimator, which in comparison to the rather conservative discrepancy principle leads to the conclusion that regularization parameter choice based on unbiased risk estimation is not a reliable procedure for ill-posed problems. A similar numerical study for sparsity regularization demonstrates that the same issue appears in non-linear variational regularization approaches
The Black Hole Mass of NGC 4151: Comparison of Reverberation Mapping and Stellar Dynamical Measurements
We present a stellar dynamical estimate of the black hole (BH) mass in the
Seyfert 1 galaxy, NGC 4151. We analyze ground-based spectroscopy as well as
imaging data from the ground and space, and we construct 3-integral
axisymmetric models in order to constrain the BH mass and mass-to-light ratio.
The dynamical models depend on the assumed inclination of the kinematic
symmetry axis of the stellar bulge. In the case where the bulge is assumed to
be viewed edge-on, the kinematical data give only an upper limit to the mass of
the BH of ~4e7 M_sun (1 sigma). If the bulge kinematic axis is assumed to have
the same inclination as the symmetry axis of the large-scale galaxy disk (i.e.,
23 degrees relative to the line of sight), a best-fit dynamical mass between
4-5e7 M_sun is obtained. However, because of the poor quality of the fit when
the bulge is assumed to be inclined (as determined by the noisiness of the
chi^2 surface and its minimum value), and because we lack spectroscopic data
that clearly resolves the BH sphere of influence, we consider our measurements
to be tentative estimates of the dynamical BH mass. With this preliminary
result, NGC 4151 is now among the small sample of galaxies in which the BH mass
has been constrained from two independent techniques, and the mass values we
find for both bulge inclinations are in reasonable agreement with the recent
estimate from reverberation mapping (4.57[+0.57/-0.47]e7 M_sun) published by
Bentz et al.Comment: 20 pages, including 11 low-res figures. Accepted for publication in
ApJ. High resolution version available upon reques
The MACHO project: Microlensing Optical Depth towards the Galactic Bulge from Difference Image Analysis
We present the microlensing optical depth towards the Galactic bulge based on
the detection of 99 events found in our Difference Image Analysis (DIA) survey.
This analysis encompasses three years of data, covering ~ 17 million stars in ~
4 deg^2, to a source star baseline magnitude limit of V = 23. The DIA technique
improves the quality of photometry in crowded fields, and allows us to detect
more microlensing events with faint source stars. We find this method increases
the number of detection events by 85% compared with the standard analysis
technique. DIA light curves of the events are presented and the microlensing
fit parameters are given. The total microlensing optical depth is estimated to
be tau_(total)= 2.43^(+0.39/-0.38) x 10^(-6) averaged over 8 fields centered at
l=2.68 and b=-3.35. For the bulge component we find
tau_(bulge)=3.23^(+0.52/-0.50) x 10^(-6) assuming a 25% stellar contribution
from disk sources. These optical depths are in good agreement with the past
determinations of the MACHO Alcock et al. (1997) and OGLE Udalski et al. (1994)
groups, and are higher than predicted by contemporary Galactic models. We show
that our observed event timescale distribution is consistent with the
distribution expected from normal mass stars, if we adopt the stellar mass
function of Scalo (1986) as our lens mass function. However, we note that as
there is still disagreement about the exact form of the stellar mass function,
there is uncertainty in this conclusion. Based on our event timescale
distribution we find no evidence for the existence of a large population of
brown dwarfs in the direction of the Galactic bulge.Comment: Updated references and corrected optical depth values. tau_tot=
[2.91(+0.47/-0.45) -> 2.43^(+0.39/-0.38)] x 10^(-6) tau_bul =
[3.88(+0.63/-0.60) -> 3.23^(+0.52/-0.50)] x 10^(-6
Exploiting transient protein states for the design of small-molecule stabilizers of mutant p53
The destabilizing p53 cancer mutation Y220C creates an extended crevice on the surface of the protein that can be targeted by small-molecule stabilizers. Here, we identify different classes of small molecules that bind to this crevice and determine their binding modes by X-ray crystallography. These structures reveal two major conformational states of the pocket and a cryptic, transiently open hydrophobic subpocket that is modulated by Cys220. In one instance, specifically targeting this transient protein state by a pyrrole moiety resulted in a 40-fold increase in binding affinity. Molecular dynamics simulations showed that both open and closed states of this subsite were populated at comparable frequencies along the trajectories. Our data extend the framework for the design of high-affinity Y220C mutant binders for use in personalized anticancer therapy and, more generally, highlight the importance of implementing protein dynamics and hydration patterns in the drug-discovery process
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