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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<!--AstroStatistics Home page-->
<title>CASt: Chandra Star Flares dataset</title>
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<h2 style="text-align: center;">Chandra Orion star flares</h2>
<h3>The CASt datasets</h3>
<a href="COUP263.dat" target="ImageView">Faint_constant.dat</a><br>
<a href="COUP551.dat" target="ImageView">Faint_flaring.dat</a><br>
<a href="COUP554.dat" target="ImageView">Bright_fiaring.dat</a><br>
<br>
<span style="font-weight: bold;">Astronomical background</span>
<p>Our Sun and ordinary stars we see at nighttime appear
constant in brightness to our naked eyes. But at other
wavelengths of light where the stars are much fainter (such as radio,
X-ray and gamma-rays), the stars are violently variable on rapid
timescales. These emission arises from explosive releases of
energy at the stellar surface associated with magnetic fields.
These solar or stellar flares occur when magnetic fields generated in
the stellar interior erupt on the surface and, due to convective
motions, are twisted until a sudden reconfiguation occurs. The
process resembles the spark appearing when two wires are crossed in an
electric circuit. The result is a sudden acceleration of
particles to extremely high energies, and the heating of gas trapped in
magnetic loops to X-ray emitting temperatures (10<sup>6</sup>-10<sup>8</sup>
degrees Kelvin). </p>
<p>Ordinary stars exhibit their highest levels of magnetic
activity, and
their strongest flares, during the youngest pre-main sequence phases
(<a
href="http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1999ARA%26A..37..363F">Feigelson
& Montmerle 1999</a>). X-ray telescopes such as the <a
href="http://chandra.harvard.edu">Chandra X-ray Observatory</a>
provide an excellent view of these flares. The most extensive
dataset obtained in this field was the <a
href="http://www.astro.psu.edu/coup">Chandra Orion Ultradeep Project</a>
(COUP), where Chandra observed the Orion Nebula region of young stars
for nearly two weeks continuously. The X-ray sources are
sometimes very faint with only a handful of photons arriving over the
observation, and sometimes orders of magnitude brighter. The
arrival times of these photons can be viewed as a Poisson
process. These are stationary time series if the source does not
flare and non-stationary time series if flares are present. </p>
<br>
<span style="font-weight: bold;">Dataset</span><br>
<p>We have extracted time series from three of the 1616 COUP
sources:<br>
<br>
</p>
<div style="text-align: center;"><img
style="width: 500px; height: 210px;" src="COUP263.jpg"
alt="Constant source"><br>
</div>
<div style="margin-left: 40px;">COUP #263: Binned
lightcurve (histogram) of the photon arrival times. An apparently
constant X-ray source with 209 photons. We believe this is an
extragalactic quasar which cannot exhibit rapid flares <a
href="http://arxiv.org/abs/astro-ph/0504370">(Getman et al.
2005b)</a>. Here the blue and red curves give the hard and soft
energy lightcurves. <br>
<br>
<div style="text-align: center;"><img
style="width: 500px; height: 212px;" src="COUP551.jpg"
alt="Faint flaring Orion star"><br>
</div>
COUP # 551: A faint flaring newly discovered Orion star with 678
photons (<a href="http://arxiv.org/abs/astro-ph/0504370">Getman et al.
2005b</a>). <br>
<br>
<div style="text-align: center;"><img
style="width: 500px; height: 255px;" src="COUP554.jpg"
alt="Bright flaring Orion star"><br>
</div>
COUP #554: A bright extremely variable protostar deeply embedded in the
OMC-1 South molecular cloud (<a
href="http://arxiv.org/abs/astro-ph/0504204">Grosso et al. 2005</a>).
Here,
14,258 individual photons are shown as dots with their energies
indicated by
the right-hand ordinate. <br>
<br>
</div>
<p>Our datasets are obtained from the extracted Chandra event
files
(source.evt.fits) with two simplifications. First, only two
columns are provided: photon arrival time in seconds (from 0 to 850,000
seconds); and photon
energy in keV (from 0.5 to 8.0 keV). Second, 5 gaps associated
with the Chandra orbit
(readily seen in the lightcurves above) are removed. Thus, the
arrival times do not exactly correspond to clock time, but the
de-gapped time series is simpler to analyze and scientific information
is not seriously compromised. </p>
<br>
<span style="font-weight: bold;">Statistical exercises<br>
</span>
<ul>
<li>Establish statistical tests of the hypothesis that the
sources are variable which can be applied both low-signal and
high-signal sources. <a
href="http://arxiv.org/abs/astro-ph/0410136">Getman et al. (2005a)</a>
use the
Kolmogorov-Smirnov 1-sample test.</li>
<li>Segment the dataset into a sequence of constant brightness
levels assuming a Poisson process. This is done by <a
href="http://arxiv.org/abs/astro-ph/0410136">Getman et al.
(2005a)</a> using the Bayesian Blocks method of <a
href="http://arxiv.org/abs/astro-ph/9711233">Scargle (1998)</a>.
Here is the Bayesian Blocks segmentation obtained for COUP 551:</li>
<div style="text-align: center;"><img
style="width: 630px; height: 498px;" src="COUP551bb.gif"
alt="Bayesian Blocks segmentation for COUP 551" title=""><br>
<li style="text-align: left;">Develop more realistic models or
characterizations of the
variations; e.g. density estimation, wavelet decomposition and
denoising, autoregressive models.<br>
</li>
<div style="text-align: left;"> </div>
<li style="text-align: left;">Consider the photon energies as a
covariate of the
brightness level. Look for hardening of the spectrum (i.e.
higher energy values) at the peak of flares, and cooling (i.e.
decreasing energy values) as the flares decay. Search for
spectral variability in the inter-flare emission.<br>
</li>
</div>
</ul>
<span style="font-style: italic;">The COUP datasets were
generously provided by Konstantin Getman (Penn State University)</span><br>
<ul>
<div style="text-align: center;"> </div>
</ul>
<br>
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