Energy and arrival time photon by photon

[Ana Virginia Penacchioni]

Hi, I am using Enrico to make Blazar analysis, generating lightcurves and spectra, and I would like to know if it is possible to obtain information (in a .fits file, table, ecc) of the arrival time and the energy of each of the photons that came from the source I am analyzing. I have seen each of the .fits files that Enrico generates when producing the lightcurve and I saw the columns "Time" and "Energy", but the positions (RA,Dec) are within several degrees from the source coordinates, which makes me think those photons are not exclusively from the source (it's a .fits file similar to the one I get from the http://fermi.gsfc.nasa.gov/cgi-bin/ssc/LAT/LATDataQuery.cgi site, which contains all the photons from a ROI). How can I have a table for which I am sure that each line corresponds to a photon that came exclusively from the source and I can read the arrival time and the energy? 

Thank you,

Ana.

[Christoph Deil]

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Hi Ana,

what is the final analysis product you’re interested in?

There’s in principle (at least) two ways to analyse a Fermi-LAT source:

1. Choose a large ROI (e.g. 20 deg radius), model and fit all sources and diffuse emission components in that ROI.

This is what’s most often used for Fermi-LAT analysis: http://fermi.gsfc.nasa.gov/ssc/data/analysis/scitools/

If you want to make light-curves and have many time bins, it can be computationally very intensive to obtain the results.

2. Do aperture photometry

http://fermi.gsfc.nasa.gov/ssc/data/analysis/scitools/aperture_photometry.html

This is a simpler and faster analysis where you don’t model the spatial distribution of the photons.

It doesn’t work well at low energies (where the Fermi-LAT PSF is very large) and crowded regions.

But at high energies (where the Fermi-LAT PSF is much smaller) and for rather isolated sources, if they dominate the emission at their location, this can work nicely.

Another benefit with the aperture photometry is that you get a list of events for that region, whereas with method 1 you never try to make this decision about which photon belongs to your source or other emission components. Although you still could run http://fermi.gsfc.nasa.gov/ssc/data/analysis/scitools/help/gtsrcprob.txt and obtain such a probability for each event.

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If you do just want to do a region-based analysis and select photons of interest and work with them, you could also look at writing your own analysis script using astropy.table.Table.read to read the event list

http://astropy.readthedocs.io/en/latest/table/index.html

and then select the photons you’re interested in, and use the RA, DEC, ENERGY and TIME columns to get their measured properties.

I’m not sure if there’s a pre-baked function for that in Enrico or Gammapy, but some example code showing how to do a cone selection:

https://github.com/gammapy/fhee/blob/master/v04/fhee/event_list.py#L19

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Does this help a little?

Basically: think about the analysis you’d like to do first and which analysis method would be most appropriate, and then come back with questions about how to run the tools or write the scripts to implement that analysis procedure.

In any case: good luck!

Christoph