| Presentations |
Nathan Stein (Harvard U)
4 Sep 2012 |
- Combining Computer Models to Account for Mass Loss in Stellar Evolution
- Abstract:
I will present a technique for inferring the so-called initial-final mass
relation (IFMR), the mapping between the initial mass of a Sun-like star
and its final mass as a white dwarf. Our model incorporates several
separate computer models for various phases of stellar evolution. We bridge
these computer models with a parameterized IFMR in order to embed them in a
Bayesian statistical model. In contrast to traditional techniques for
inferring the IFMR, which tend to be quite ad hoc, we can estimate the
uncertainty in our fit and ensure that our model components are internally
coherent. We analyze data from three star clusters: NGC 2477, the Hyades,
and M35. The results from NGC 2477 and M35 suggest different conclusions
about the IFMR in the mid- to high-mass range, raising questions for
further astronomical work. We also compare the results from two different
computer models for the primary hydrogen-burning stage of stellar
evolution. Through simulations, we show that misspecification at this stage
of modeling can sometimes have a severe effect on inferred white dwarf
masses. Encouragingly, our inferences on observed data are not particularly
sensitive to the choice of computer model for this stage of stellar
evolution.
- Presentation Slides [.pdf]
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Aneta Siemiginowska (CfA)
18 Sep 2012 |
- Bayesian Methods in High Energy Astrophysics
- Abstract:
- High energy astrophysics data from space-based X-ray and gamma-ray
missions such as Chandra or Fermi follow the Poisson distribution. In
most situations the modeling has to account for instrumental effects
characterized by a probability of detecting photons of a given energy
at a particular detector channel, or a particular location on the
detector. In addition, systematic uncertainties in the
characterization of the instruments have to be taken into account as
they often can exceed the statistical uncertainties in the analysis of
bright sources. Our group (ICHASC) has developed Bayesian methods for
many cases that are highly relevant to the analysis of high-energy
data. I will present and discuss some of our methods and their
applications.
- This is a dry run for the SAMSI talk.
- Presentation Slides [.ppt]
- Crab time lapse movie [.mpg]
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Dan Cervone (Harvard)
2 Oct 2012 |
- Real-Time Light Curve Classification
- Abstract:
-
Identifying and classifying variable light sources is a very active area of
research in astronomy and astrophysics, but limited resources demand
procedures that work well on relatively small numbers of observations. In
this talk, we propose a framework for scheduling future observations in
order to maximize classification information so that we can make better
decisions under material constraints. We describe the intuition and
implementation of our algorithm, and show preliminary results indicating an
increase in the probability of a correct classification as a function of
the number of observations made, based on simulated light curves from nine
different variable source types.
- Presentation slides [.pdf]
|
Rebekah Dawson (CfA)
16 Oct 2012
at 1 pm EDT |
- Applications of Bayesian statistics to understanding the origin
of "hot Jupiters"
- Abstract:
- One of the biggest surprises in the field of extra-solar planets was
the discovery of "hot Jupiters," a mysterious class of planets with
masses similar to Jupiter but orbiting closer to their stars than
Mercury. A Bayesian approaches allows us to characterize the orbits of
hot Jupiters and their possible progenitors, to distinguish between
planetary signals and false positives, and to assess which of several
models for the origins of hot Jupiters is most likely. Today I'll
present results from my thesis work, as well as work by my
collaborators, with an emphasis on the statistical methods we have
employed.
- Presentation slides [.pdf]
|
Group
27 Nov 2012
at 1 pm EST |
- BYOQ: Bring your own questions
- An Open Q&A session where statisticians can clear up any
residual niggling doubts about any aspect of astronomical data
analysis, data gathering techniques, instrumentation, data archives,
astronomical objects and their spatial, spectral, and temporal
behavior, etc.
|
Pavlos Protopapas (CfA)
29 Jan 2013
at 1:15pm EST |
- Automatic classification of astronomical variables in catalogs with missing data
- Abstract:
- I will present an automatic classification method for
astronomical catalogs with missing data a common issue arising
when combining multiple catalogs together. We used Bayesian
networks -- a probabilistic graphical model -- that performs
inferences to predict the missing values given the observed
data and the dependency relationships between the variables.
We used an iterative process where we performed expectation
maximization to estimate the missing values using the current
learned network, and learn the structure of the network that
uses the imputed data. We tested our model by creating a
classifier for variable stars from four astronomical catalogs:
MACHO, SAGE, 2MASS and UBVI and compared the results with the
results obtained with classifiers learned with a subset of
those catalogs. We found that using the catalogs with missing
data improved the classification performance by 15% in
efficiency and by 8% when comparing to traditional missing
data approaches while the computational cost remains the same.
- Presentation [.pdf]
|
Kathy Reeves (CfA)
19 Feb 2013 |
- X-ray and Extreme Ultraviolet Observations of GOES C8 Solar Flare Events
- Katharine K. Reeves, Trevor A. Bowen, Paola Testa
- We present an analysis of soft X-rays (SXR) and
extreme-ultraviolet (EUV) imaging and spectral observations
of solar flares with an approximate C8 GOES class. Our
constraint on peak GOES SXR flux allows for the investigation
of correlations between various flare parameters. We show
that the the duration of the decay phase is proportional
to the duration of its rise phase. Additionally, we show
significant correlations between the radiation emitted in
the rise and decay phases of a flare: the total radiated
energy of a given flare is proportional to the energy radiated
during the rise phase alone. This partitioning of radiated
energy between the rise and decay phases is observed in
both soft X-ray (SXR) extreme ultraviolet (EUV) wavelengths.
Though observations from the EUV Variability Experiment (EVE)
show significant variation in the behavior of individual EUV
spectral lines during different C8 events, we show that
the broadband EUV emission is well constrained. Furthermore,
using GOES and AIA data, we determine several thermal parameters
of these events: temperature, volume, density, and emission
measure. Analysis of these parameters demonstrate that the
longer duration solar flares are cooler events with larger
volumes capable of emitting vast amounts of radiation. The
shortest C8 flares are typically the hottest events, smaller
in physical size, and have lower associated total energies.
These relationships are directly comparable with several
sample scaling laws and flare loop models.
- Presentation Slides [.pdf]
- Movies: 2011-12-27 ; 2012-08-32 ; EVEspectrum-4 [.mov]
|
Xu Jin (UC Irvine)
26 Feb 2013 |
- Fully Bayesian Analysis of Calibration Uncertainty In High Energy Spectral Analysis
- Systematic instrumental uncertainties in astronomical
analyses have been generally ignored due to the lack of robust
principled method, though importance of incorporating instrumental
calibration uncertainty is widely realized by users and instrument
builders. Ignorance of calibration uncertainty can cause bias in
the estimate of source model parameters and underestimate the
variance. In this talk, we focus on incorporating uncertainty
of affective area curves to source model fitting. Principle
component analyses method is explored to efficiently represent
affective area curve and energy redistribution matrix, bringing
in significant advantage in computing and sampling. Then, the
application of Bayesian approach to incorporate the calibration
uncertainty into spectral analysis of high-energy data is
presented and three different sampling schemes are discussed in
detail. We demonstrate the comparison of results from these
three schemes using Chandra data. Here, we concentrate on
FullBayes Model, which has the internal advantage that data
itself provides not only the information of source parameters
but also the information of calibration uncertainty. It is
verified that implementing FullBayes Model can result in more
accurate and efficient estimate of source parameters.
- Presentation slides [.pdf]
|
Jeff Scargle (NASA/Ames)
5 Mar 2013
Phillips Auditorium
CfA, 60 Garden St.
1:30pm |
- Adventures in Modern Time Series Analysis: From the Sun to the Crab Nebula and Beyond.
- With the observations of long, precise, and finely sampled time
series the Age of Digital Astronomy is uncovering and elucidating
energetic dynamical processes throughout the Universe.
Fulfilling these opportunities requires effective data analysis
techniques that can rapidly and automatically implement
advanced concepts. With various colleagues I have developed
tools ranging from simple but optimal histograms to time and
frequency domain analysis for arbitrary data modes and time
sampling. Examples to be shown include 3+ cycles of solar
chromospheric variability, gamma-ray activity in the Crab Nebula,
active galactic nuclei and gamma-ray bursts.
- Presentation slides: [.pdf] ; [.ppt] ; [.pptx]
|
Min Shandong (UC Irvine)
2 Apr 2013 |
- Bayes Factors
- There is an important class of model selection problems in
astrophysics where the standard asymptotics of the likelihood ratio
test do not apply. This project will study in detail the use of the
Bayes Factor for emission line detection in spectral analysis. We
develop a method to quantify the typically strong prior dependency
of the Bayes Factor, compare the results with those obtained with
posterior predictive p-values and the traditional likelihood ratio
test in a simulation study, and give suggestions about how to set
up the prior in the context of line detection problem. We will also
talk about the efficiency and accuracy of the available methods to
calculate Bayes Factors and propose a new method based on parallel
MCMC.
- Presentation slides [.pdf]
|
AAS/HEAD 2013
8 Apr 2013
7:30pm-9:00pm PDT
Monterrey, CA |
- 131. Astrostatistics in High Energy Astrophysics - Session in memory of Alanna Connors
|
Lazhi Wang & David Jones (Harvard)
16 Apr 2013 |
- Separating Overlapping Astronomical Sources
- In astronomical observations, it is often the case that sources
are situated close enough together that they cannot be fully resolved
instrumentally and it is of interest to infer the number of individual
sources, their locations, and their respective intensities. The
resolution of the detector is characterized by the point spread
function (PSF) which describes the spatial distribution of observed
photons from a point source. Convolving a number of sources with
the PSF results in a finite mixture model. We further incorporate
spectral models, background contamination, and a latent Poisson
process for the number and positions of the sources. We fit the
resulting multilevel model with RJMCMC (Richardson and Green 1997)
to separate the sources and obtain posterior distributions for the
number of sources and their individual parameters. This overall
approach has the benefit of being able to incorporate further
complexities such as non-uniform background and asymmetric PSFs.
Overfitting problems are avoided because knowledge of the PSF means
the spread of the mixture components is determined making the
inference relatively insensitive to the choice of prior on the
number of sources.
- Presentation slides [.pdf]
- Dark Sources Detection
- The goal of source detection is often to obtain the luminosity
function, which specifies the relative number of sources at each
luminosity for a population. Of particular interest in my project
is the existence of dark sources in the population. In this talk,
I will first briefly review the problem and the Bayesian model, in
which a zero-inflated gamma distribution is used to model the
intensity of sources. Secondly, I will discuss the weakly informative
prior we use for the hyper-parameters in the model and show the
frequency coverage of the probability intervals in two different
simulation settings. Thirdly, model comparison using posterior
predictive p-values will be discussed in detail to identify the
existence of dark sources in the population. Finally, a generalized
model for dealing with overlapping sources will be introduced.
- Presentation slides [.pdf]
-
|
Jack Steiner (CfA)
23 Apr 2013 |
- Accretion Lags and X-ray Heating
- Accreting black hole binaries show strong correlation between X-ray
and optical variability. In two systems, we find that X-rays lag
behind the optical with a characteristic delay of weeks. This
behavior is most readily attributed to viscous delay as inflowing gas
traverses the disk from outer to inner annuli. By applying a model
primarily comprised of a slowly-varying alpha disk to describe the
luminosity fluctuations, we are able to successfully map between the
X-ray and the optical. Using this model, we measure alpha and explore
the possibility of its dependence on luminosity. Additionally, we
discover a strong dependence of X-ray heating upon the geometry of the
binary system. Meanwhile, by removing the optical variance tied to
the X-rays, this technique may be useful in recovering dynamical
information from outbursting black holes, a feature of particular
importance for those exceptionally X-ray bright systems which balk
traditional methods.
- Presentation slides [.pptx]
|
Group
May 8-10
venue var. |
- ICHASC/C-BAS Internal Workshop at Cambridge, MA
- Wednesday May 8 (Room M-340, 3rd floor, 160 Concord)
- 9-Noon: New projects, overlapping sources, and pre-existing catalogs
- 1:30-3:45pm: Calibration, joint spectro-temporal analysis
- 4:15-5:45pm: Bayes Factors, Statistical Computation
- Thursday May 9 (Room M-340, 3rd floor, 160 Concord)
- 9-Noon: writing groups
- 12:30-1:15pm: Hinode Mission Control Tour
- 2:30-5:45pm: real time classification, solar spatial segmentation, solar activity, sunspot classification
- Friday May 10 (Room 705, Science Center)
- 10am-1pm: logN-logS, adaptive smoothing, other
- Paul Baines slides [.pdf]
- afternoon: writing, wrap-up
|
Brandon Kelly (UCSB)
9 July 2013
Noon EDT
Phillips Auditorium
CfA, 60 Garden |
- Characterizing Lightcurves of AGN and other Stochastic Variables in
the Era of Time-Domain Astronomy
-
Current and future time-domain surveys will provide, for the first
time, well-sampled multi-wavelength lightcurves for large numbers of
variables objects, such as AGN and variable stars. Such lightcurves
will provide a valuable resource for studying the physics of such
objects and their demographics, leading to many new discoveries.
However, these data sets present methodological and computational
challenges, including irregular sampling of the lightcurves and
contamination by measurement errors, and the need to handle massive
amounts of data. In this talk I will discuss the use of continuous
time stochastic processes for quantifying the variability in AGN and
other variable sources, and compare with more traditional approaches.
Although I will focus on the methodological and computational aspects
of these models, I will present some results from applying these
statistical time series models to AGN optical and X-ray lightcurves,
briefly illustrating how they can be used to provide astrophysical
insight. I will conclude with a discussion of potential future
directions and applications in the era of LSST.
- Presentation slides [.pdf]
|
Katy McKeough (CMU/CfA-REU)
13 Aug 2013
1pm EDT
Library Space
CfA, 60 Garden |
- Effect of Cosmic Microwave Background on X-ray Radiation of High Redshift Jets
- Slides [.pdf]
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