May 31st, 2008| 11:59 pm | Posted by hlee

Eight astro-ph papers and two statistics paper are listed this week. One statistics paper discusses detecting filaments and the other talks about maximum likelihood estimation of satellite images (clouds). Continue reading ‘[ArXiv] 4th week, May 2008’ »

Tags:

AGN,

Bayes factor,

bootstrap,

confidence set,

cosmological constanct,

dark energy,

Exofit,

exoplanet,

filament,

jackknife,

KDE,

Model Selection,

time series,

Type Ia SNe,

unbiased,

wavelet Category:

arXiv,

Bayesian,

MCMC,

Stat |

Comment
May 28th, 2008| 01:00 pm | Posted by vlk

The most widely used tool for detecting sources in X-ray images, especially Chandra data, is the wavelet-based `wavdetect`, which uses the Mexican Hat (MH) wavelet. Now, the MH is not a very popular choice among wavelet aficianados because it does not form an orthonormal basis set (i.e., scale information is not well separated), and does not have compact support (i.e., the function extends to inifinity). So why is it used here?

Continue reading ‘Mexican Hat [EotW]’ »

Tags:

Chandra,

ciao,

convolution,

correlation,

EotW,

Equation,

Equation of the Week,

Fourier Transform,

gaussian,

MexHat,

Mexican Hat,

MH,

multiscale,

wavdetect,

wavelet Category:

Algorithms,

Astro,

Imaging,

Jargon |

1 Comment
May 26th, 2008| 02:59 pm | Posted by hlee

Tags:

clustering,

high dimension,

LF,

maximum likelihood,

multivariate,

Poisson,

Schechter,

zero count Category:

arXiv,

Bayesian,

Fitting,

MCMC,

Methods,

Stat |

Comment
May 23rd, 2008| 10:00 pm | Posted by chasc

*“It’s easy to get a good fit, which means that your fit doesn’t mean much…”*

Ariane Lancon (from proceedings of “Starbursts: from 30 Doradus to Lyman break galaxies”, 2005)

May 21st, 2008| 01:00 pm | Posted by vlk

There is a lesson that statisticians, especially of the Bayesian persuasion, have been hammering into our skulls for ages: __do not subtract__ background. Nevertheless, old habits die hard, and old codes die harder. Such is the case with X-ray aperture photometry. Continue reading ‘Background Subtraction [EotW]’ »

Tags:

aperture photometry,

background,

background marginalization,

background subtraction,

celldetect,

Chandra,

ciao,

EotW,

Equation,

error propagation,

ldetect,

local detect,

wavdetect,

X-ray Category:

Algorithms,

Astro,

Jargon |

6 Comments
May 20th, 2008| 12:10 am | Posted by vlk

Earlier this year, Peter Edmonds showed me a press release that the Chandra folks were, at the time, considering putting out describing the possible identification of a Type Ia Supernova progenitor. What appeared to be an accreting white dwarf binary system could be discerned in 4-year old observations, coincident with the location of a supernova that went off in November 2007 (SN2007on). An amazing discovery, but there is a hitch.

And it is a statistical hitch, and involves two otherwise highly reliable and oft used methods giving contradictory answers at nearly the same significance level! Does this mean that the chances are actually 50-50? Really, we need a bona fide statistician to take a look and point out the errors of our ways.. Continue reading ‘Did they, or didn’t they?’ »

Tags:

arXiv,

Chandra,

CXC,

Optical,

Peter Edmonds,

positional coincidence,

positional error,

Power,

progenitor,

question for statisticians,

significance,

Supernova,

Type Ia,

White Dwarf,

White Dwarf binary,

X-ray Category:

arXiv,

Astro,

Data Processing,

News,

Objects,

Optical,

Stat,

Uncertainty |

5 Comments
May 19th, 2008| 10:42 am | Posted by hlee

There’s no particular opening remark this week. Only I have profound curiosity about jackknife tests in [astro-ph:0805.1994]. Including this paper, a few deserve separate discussions from a statistical point of view that shall be posted. Continue reading ‘[ArXiv] 2nd week, May 2008’ »

Tags:

bimodality,

bootstrap,

calibration uncertainty,

CF,

Classification,

CMB,

dip,

exoplanet,

Fisher matrix,

flare,

GL,

jackknife,

KS test,

marked point,

maximum likelihood,

MLE,

poisson point process,

spatial data,

XLF Category:

arXiv,

Frequentist,

Uncertainty,

X-ray |

Comment
May 14th, 2008| 01:00 pm | Posted by vlk

Spectral lines are a ubiquitous feature of astronomical data. This week, we explore the special case of optically thin emission from low-density and high-temperature plasma, and consider the component factors that determine the line intensity. Continue reading ‘Line Emission [EotW]’ »

Tags:

abundance,

emission,

emission measure,

emissivity,

EotW,

Equation,

Equation of the Week,

flux,

ion balance,

line Category:

Astro,

Jargon,

Misc,

Physics,

Stars |

2 Comments
May 14th, 2008| 01:20 am | Posted by hlee

May 13th, 2008| 03:47 pm | Posted by hlee

The brackets could be filled with other languages but two are introduced today: **Perl** (perl.org) and **Python** (python.org). These two are widely used among astronomers and can be empowered by **R** (r-project.org). Continue reading ‘R-[{Perl,Python}] Interface’ »

May 11th, 2008| 10:42 pm | Posted by hlee

I think I have to review spatial statistics in astronomy, focusing on tessellation (void structure), point process (expanding 2 (3) point correlation function), and marked point process (spatial distribution of hardness ratios of X-ray distant sources, different types of galaxies -not only morphological differences but other marks such as absolute magnitudes and existence of particular features). When? Someday…

In addition to Bayesian methodologies, like this week’s astro-ph, studies on characterizing empirical spatial distributions of voids and galaxies frequently appear, which I believe can be enriched further with the ideas from stochastic geometry and spatial statistics. Click for what was appeared in arXiv this week. Continue reading ‘[ArXiv] 1st week, May 2008’ »

Tags:

Classification,

covariance,

FARIMA,

Fisher information,

GL,

GRB,

Levy,

light curve,

limb darkening,

ML,

Pareto distribution,

quasars,

solar flare,

standard candle,

tessellation,

time series,

VO,

void Category:

arXiv,

MCMC,

Uncertainty |

1 Comment
May 6th, 2008| 06:12 pm | Posted by vlk

The *gamma* function [not the Gamma -- note upper-case G -- which is related to the factorial] is one of those insanely useful functions that after one finds out about it, one wonders “why haven’t we been using this all the time?” It is defined only on the ~~positive~~ non-negative real line, is a highly flexible function that can emulate almost any kind of skewness in a distribution, and is a perfect complement to the Poisson likelihood. In fact, it is the conjugate prior to the Poisson likelihood, and is therefore a natural choice for a prior in all cases that start off with counts. Continue reading ‘*gamma* function (Equation of the Week)’ »

May 5th, 2008| 03:08 am | Posted by hlee

Since I learned Hubble’s tuning fork^{[1]} for the first time, I wanted to do classification (semi-supervised learning seems more suitable) galaxies based on their features (colors and spectra), instead of labor intensive human eye classification. Ironically, at that time I didn’t know there is a field of computer science called machine learning nor statistics which do such studies. Upon switching to statistics with a hope of understanding statistical packages implemented in IRAF and IDL, and learning better the contents of Numerical Recipes and Bevington’s book, the ignorance was not the enemy, but the accessibility of data was. Continue reading ‘[ArXiv] 5th week, Apr. 2008’ »

Tags:

ANN,

automation,

Classification,

correlation function,

denoising,

FFT,

gravitational wave,

lensing,

LISA,

machine learning,

missing data,

mock data,

morphology,

PCA,

power spectrum,

robust,

SDSS,

spectrum,

sunspots,

wavelet,

zoo Category:

arXiv,

Galaxies,

Imaging,

MCMC,

Physics,

Spectral |

Comment
May 2nd, 2008| 06:06 pm | Posted by vlk

Starting a new feature — highlighting some equation that is widely used in astrophysics or astrostatistics. Today’s featured equation: what instruments do to incident photons. Continue reading ‘Equation of the Week: Confronting data with model’ »

Tags:

ARF,

effective area,

EotW,

Equation,

Equation of the Week,

LRF,

point spread function,

PSF,

response matrix,

RMF,

source Category:

Astro,

Jargon |

Comment
May 1st, 2008| 02:00 pm | Posted by vlk

*Why is it that detection of emission lines is more reliable than that of absorption lines?*

That was one of the questions that came up during the recent AstroStat Special Session at HEAD2008. When you look at the iconic Figure 1 from Protassov et al (2002), which shows how the null distribution of the Likelihood Ratio Test (LRT) and how it holds up for testing the existence of emission and absorption lines. The thin vertical lines are the nominal F-test cutoffs for a 5% false positive rate. The nominal F-test is too conservative in the former case (figures a and b; i.e., actual existing lines will not be recognized as such), and is too anti-conservative in the latter case (figure c; i.e., non-existent lines will be flagged as real). Continue reading ‘The Flip Test’ »