Basic Principles of X-ray Source Detection

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Basic Principles of X-ray Source
Detection Or Who Stole All Our Photons?.....
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• Overview
• Optical sources
• Comparison to X-ray images
• Why so few X-rays?
• What can we do about it?
• The theory of low count levels
• Application to X-ray sources
• Calculating flux and luminosity.

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Optical Sources
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X-ray Images
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Why so few X-rays? - Mirrors PSF

• Mirrors
• Made from Zerodur
• Coated with Iridium
• 100 reflectivity at 0.1keV
• 10 reflectivity at 10keV
• 10m focal length
• Optical/UV filters remove 20 of low energy
  X-rays.

• PSF
• Angular resolution of 0.5 on axis
• PSF increases rapidly off axis
• Also a function of energy

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Why so few X-rays? - Background
99.7 of detections are or could be background
that is of a non cosmic origin.

• Distant Sources
• Solar wind 11 year cycle
• Solar flares short time scale
• Radiation belt
• (Cosmic rays)

• Types of Particle
• Mostly protons magnets deflect those with
  lowest momentum
• Heavy ions
• Neutrons
• Gamma rays
• X-rays

• Local Sources
• Flaring pixels
• Detector radiation gold, aluminium, silicon
• Trailing (only for bright sources)

In a normal image, these would completely swamp
the sources, but in X-ray astronomy we have one
major advantage
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Why so few X-rays? CCDs
Position - x, y - obvious
Can obtain x, y, E, t and grade

• Energy
• 1 electron per 3.7eV
• Typically 1000 electrons
• Read noise 2 electrons
• E/ E 50
• Energy is summed over the surrounding pixels
  which share the signal.

• Time of arrival
• Low count rate
• 3.2 sec frame time
• 0.04 sec readout time
• Fine for most sources (no pileup unless very
  bright)
• Removes flickering pixels

• Grade
• Triggering one pixel will effect those around it
  in a way that depends on the type of particle and
  its incident angle.
• Therefore looking at the 3x3 region around a peak
  can distinguish between true X-rays and
  background events.

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Why so few X-rays? Grades and Filters
The 3x3 region around a peak can predict the
cause of the event. If all pixels with a value
above a given threshold are included, the
resulting shape determines the grade. Rejecting
certain grades greatly increases the signal to
noise ratio.
Rejected grades gt Accepted grades gt

• Probable background events are removed by
  filtering for grade.
• Events received during times of particularly high
  background are removed.
• Events with Elt0.5keV or Egt8keV are removed.

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Source Detection

• Detect using Wavelets
• Apertures and annulus defined by wavelet
• Calculate counts and background
• Find errors

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Calculating Flux and Luminosity

• Select all significant sources
• Find source counts and error
• Assume a source spectrum
• Convert counts to ergs using assumed spectrum
• Create Exposure Map allowing for Quantum
  Efficiency and source spectrum
• Convert ergs to ergs/cm2/sec using exposure map
• Convert to luminosity using distance (requires
  optical astronomers!)
• Finished!