Analysis #

I will be explaining how you can analyze the point sources detected by Fermi LAT.

Background model download #

We need to download the latest Galactic interstellar emission model and the Isotropic spectral template from this page. So download the gll_iem_v07.fits and iso_P8R3_SOURCE_V3_v1.txt for Event Selection of Pass 8 Source.

Additionally, I will suggest downloading the latest Fermi LAT catalog in FITS format from this link.

Keep all these files in a separate folder, so that we can use them for each analysis.

Data download #

Download the data for your specified source from the link.

For time conversion, you can use the xTime tool.

Conventionally, we choose a search radius of 20 $\degree$. Fermi LAT operates in the energy range ~20 MeV to >300 GeV, but we can generally choose 100 MeV to 500 GeV. We need the spacecraft file also for analysis.

Now download all the files in a separate folder. Some files will contain PH, and some will contain SC in their names. The PH files hold the photon data, and SC will hold the spacecraft data during that observation. Now, create a file that contains the list of PH files using the code in the terminal opened in the same folder

ls *PH* > ph_list.dat

It will create a ph_list.dat file, which will list the PH files.

Make sure that the folder you create does not contain any spaces; otherwise, your analysis will fail.

Configuration file #

You need to create a config.yaml file in that folder where the data is located. It is like a simple text file, which contains the configuration information. This file will be structured as below

data:
  evfile : ph_list.dat
  scfile : L250815080059DAC8B79833_SC00.fits

binning:
  roiwidth   : 20.0
  binsz      : 0.1
  binsperdec : 8

selection :
  emin : 100
  emax : 500000
  zmax    : 90
  evclass : 128
  evtype  : 3
  tmin : 757382405.0
  tmax : 776908805.0
  filter : 'DATA_QUAL>0 && LAT_CONFIG==1'
  target : '3C 138'

gtlike:
  edisp: true
  irfs: 'P8R3_SOURCE_V3'
  edisp_disable : ['isodiff','galdiff']

model:
  src_radius : 15.0
  catalogs : '/home/joysankar/fermi-catlog/gll_psc_v35.fit'
  galdiff  : '/home/joysankar/fermi-catlog/gll_iem_v07.fits'
  isodiff  : '/home/joysankar/fermi-catlog/iso_P8R3_SOURCE_V3_v1.txt'

The evfile is the name of the PH file list, which we have created previously. scfile is the spacecraft file you have already downloaded. Make sure to set emin (MeV), emax(MeV), tmin (MET), and tmax (MET). In target, put the object name, but better to use the 4FGL name. Make sure to give the appropriate path of catalogs, galdiff, and isodiff. binsperdec is a factor that determines the binning.

Notebook analysis #

Now open a terminal in that folder and activate the fermipy environment.

conda activate fermipy

Then open a Jupyter Notebook and run the code attached below step by step.

from fermipy.gtanalysis import GTAnalysis
gta = GTAnalysis('config.yaml',logging={'verbosity' : 3})
gta.setup()

It will take time and perform the initial analysis.

gta.optimize()

It will fit the spectral parameters of the source.

gta.print_roi()

It will show the sources with TS values Now we’ll free the sources within the 3$\degree$ of ROI.

gta.free_sources(free=False)
gta.free_sources(distance=3.0,pars='norm')
gta.free_source('4FGL J2321.9+2734')
gta.free_source('galdiff')
gta.free_source('isodiff')

You need to fit the parameters by running the code below

gta.fit()

SED analysis #

sed = gta.sed('4FGL J2321.9+2734',make_plots=True)

It will generate a file like 4fgl_j0521.2+1637_sed.fits which will be used for SED plotting.

Light curve analysis #

lc = gta.lightcurve('4FGL J2321.9+2734', binsz=21600.0,outdir='6hrs_dir', multithread=True, nthread=3, write_fits=True)

It will generate a file like 4fgl_j1310.5+3221_lightcurve.fits which will be used for light curve plotting. The binsz is the binning time in seconds. The multithread is for parallel calculation. The outdir is the directory where each bin file will be generated. The nthread is the number of parallel threads. It will take time based on the binning size.