How do you find variables?

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Based on materials developed by Peter Plavchan, NExScI/IPAC/Caltech

Getting a light curve

There are many steps from an astronomical image to a science-quality light curve.

  • Reducing the image
    • Artifact corrections, biases, flats, etc.
  • Aperture or differential photometry
  • Generating image catalogs of photometry
  • Cross-correlating / rectifying images/catalogs
  • Extracting the light curve for each source
  • Ensemble Analysis
    • Zero point corrections
    • Detrending
    • Red noise filtering (e.g., removing long slow variations due to, for example, the source rising and the consequent changing airmass)

How do you find variables?

In a word, STATISTICS.

The NASA Exoplanet Archive provides a series of standardized variability statistics for all light curves it serves, including:

  • Mean, median
  • Standard Deviation (aka RMS) w/r/t the median
  • Chi-Squared
  • Fraction & # of data > 5-sigma deviant from median
  • Median Uncertainty
  • Median Absolute Deviation Statistic (or MADS)

This is all important, because if you find a characteristic time scale for variations in your light curve, you have a clue to the physical mechanism producing the variability.