How do you find variables?

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.