Light curves for Chandra

Question

I am working with Chandra data. I only have the corrected PI files for the sources. Is there a way to plot the light curve from this PI file. I wanted to extract the color magnitude ultimately.

Answer 1

PI files, or more commonly PHA files, are higher level data products derived by binning the (cleaned) event list by the registered PI/PHA channel of each event, which should correspond to their energy. Therefore, PHA/PI files contain the spectra of a given source.

This binning, however, removes the information when each event was detected, which makes it impossible to generate a light-curve from a PI/PHA file.

If you want the light-curve you have to go back to the event-list and perform a binning not by PI/PHA channel but by time (while accounting for GTI intervals).

Usually the different X-ray missions provide the necessary tools that perform these different binnings for you. In case of Chandra this would be e.g. dmextract.

Chandra provides a bunch guides how to do all that. And for anyone new to X-ray astronomy their primer is also highly recommended.

Alternatively, you can only avoid this if you already have, say, a PI-file for every time bin your are interested in. Say, a PI file for each 100s period of your observation. In that case you can get the count-rate for each PI file by first summing up all the counts in the individual spectral bins and dividing them by the total exposure time (which should be a FITS keyword in the header, or can be derived from the GTI file). Alternatively you could fit the spectrum of each PI-file and derive the flux over time, which might be physically more interesting, but also more work.