Selecting the relevant events

What you will learn

You will learn how to select the relevant events for a target and how to determine the useful energy range for a set of observations.

Now you are ready for your first H.E.S.S. data analysis. As first step you should create a folder that will contain your analysis results and any intermediate data products. Let’s call this folder crab since we explain in this example how you can analyse the data from the Crab nebula. Create and step into the folder using

$ mkdir crab
$ cd crab

The first step of your analysis consists in selecting the relevant events from the observations. In this step you can select a specific energy range, time range or region of interest. In the example below you will select events according to the save energy thresholds that are defined in the effective area component of the instrument response functions. You do this by setting the hidden parameter usethres=DEFAULT. In addition, you will select all events within 2 degrees of the pointing direction:

$ ctselect usethres=DEFAULT
Input event list or observation definition XML file [events.fits] $HESSDATA/obs/obs_crab.xml
Lower energy limit (TeV) [0.1] NONE
Radius of ROI around pointing or specified RA/DEC (degrees) (0-180) [3.0] 2.0
Start time (UTC string, JD, MJD or MET in seconds) [NONE]
Output event list or observation definition XML file [selected_events.fits] obs_crab_selected.xml

Warning

If you encounter the error

*** ERROR encounterted in the execution of ctselect. Run aborted ...
*** ERROR in ctselect::run(): Invalid value. Invalid RoI selection: the new RoI must be enclosed in the original RoI

then run the ctselect tool with the forcesel=yes option:

$ ctselect usethres=DEFAULT forcesel=yes

Below is an excerpt of the log file that was created by ctselect. You will find that the following energy ranges were applied for the observations:

• id=023523: 0.871 - 100 TeV
• id=023526: 0.708 - 100 TeV
• id=023559: 0.661 - 100 TeV
• id=023592: 0.871 - 100 TeV

2018-09-11T14:05:15: +=================+
2018-09-11T14:05:15: | Event selection |
2018-09-11T14:05:15: +=================+
2018-09-11T14:05:15: === HESS observation "Crab" (id=023523) ===
2018-09-11T14:05:15:  Input filename ............: /project-data/hess/hess_dl3_dr1/data/hess_dl3_dr1_obs_id_023523.fits.gz
2018-09-11T14:05:15:  Event extension name ......: EVENTS
2018-09-11T14:05:15:  GTI extension name ........: GTI
2018-09-11T14:05:16: === Event selection ===
2018-09-11T14:05:16:  Selected energy range .....: 0.870964 - 100 TeV
2018-09-11T14:05:16:  Requested RoI .............: Centre(RA,DEC)=(83.633333333333, 21.514444444444) deg, Radius=2 deg
2018-09-11T14:05:16:  RoI of data ...............: Centre(RA,DEC)=(83.633333333333, 21.514444444444) deg, Radius=2 deg
2018-09-11T14:05:16:  Selected RoI ..............: Centre(RA,DEC)=(83.633333333333, 21.514444444444) deg, Radius=2 deg
2018-09-11T14:05:16:  cfitsio selection .........: ENERGY >= 0.87096359 && ENERGY <= 100.00000000 && ANGSEP(83.633333,21.514444,RA,DEC) <= 2.000000
2018-09-11T14:05:16:  FITS filename .............: /var/tmp/tmp.0.QX6f0L[EVENTS][ENERGY >= 0.87096359 && ENERGY <= 100.00000000 && ANGSEP(83.633333,21.514444,RA,DEC) <= 2.000000]
2018-09-11T14:05:16: === HESS observation "Crab" (id=023526) ===
2018-09-11T14:05:16:  Input filename ............: /project-data/hess/hess_dl3_dr1/data/hess_dl3_dr1_obs_id_023526.fits.gz
2018-09-11T14:05:16:  Event extension name ......: EVENTS
2018-09-11T14:05:16:  GTI extension name ........: GTI
2018-09-11T14:05:16: === Event selection ===
2018-09-11T14:05:16:  Selected energy range .....: 0.707946 - 100 TeV
2018-09-11T14:05:16:  Requested RoI .............: Centre(RA,DEC)=(83.633333333333, 22.514444444444) deg, Radius=2 deg
2018-09-11T14:05:16:  RoI of data ...............: Centre(RA,DEC)=(83.633333333333, 22.514444444444) deg, Radius=2 deg
2018-09-11T14:05:16:  Selected RoI ..............: Centre(RA,DEC)=(83.633333333333, 22.514444444444) deg, Radius=2 deg
2018-09-11T14:05:16:  cfitsio selection .........: ENERGY >= 0.70794578 && ENERGY <= 100.00000000 && ANGSEP(83.633333,22.514444,RA,DEC) <= 2.000000
2018-09-11T14:05:16:  FITS filename .............: /var/tmp/tmp.1.IoZj2b[EVENTS][ENERGY >= 0.70794578 && ENERGY <= 100.00000000 && ANGSEP(83.633333,22.514444,RA,DEC) <= 2.000000]
2018-09-11T14:05:16: === HESS observation "Crab" (id=023559) ===
2018-09-11T14:05:16:  Input filename ............: /project-data/hess/hess_dl3_dr1/data/hess_dl3_dr1_obs_id_023559.fits.gz
2018-09-11T14:05:16:  Event extension name ......: EVENTS
2018-09-11T14:05:16:  GTI extension name ........: GTI
2018-09-11T14:05:16: === Event selection ===
2018-09-11T14:05:16:  Selected energy range .....: 0.660693 - 100 TeV
2018-09-11T14:05:16:  Requested RoI .............: Centre(RA,DEC)=(85.2533333381014, 22.014444444444) deg, Radius=2 deg
2018-09-11T14:05:16:  RoI of data ...............: Centre(RA,DEC)=(85.2533333381014, 22.014444444444) deg, Radius=2 deg
2018-09-11T14:05:16:  Selected RoI ..............: Centre(RA,DEC)=(85.2533333381014, 22.014444444444) deg, Radius=2 deg
2018-09-11T14:05:16:  cfitsio selection .........: ENERGY >= 0.66069345 && ENERGY <= 100.00000000 && ANGSEP(85.253333,22.014444,RA,DEC) <= 2.000000
2018-09-11T14:05:16:  FITS filename .............: /var/tmp/tmp.2.IAwBNH[EVENTS][ENERGY >= 0.66069345 && ENERGY <= 100.00000000 && ANGSEP(85.253333,22.014444,RA,DEC) <= 2.000000]
2018-09-11T14:05:16: === HESS observation "Crab" (id=023592) ===
2018-09-11T14:05:16:  Input filename ............: /project-data/hess/hess_dl3_dr1/data/hess_dl3_dr1_obs_id_023592.fits.gz
2018-09-11T14:05:16:  Event extension name ......: EVENTS
2018-09-11T14:05:16:  GTI extension name ........: GTI
2018-09-11T14:05:16: === Event selection ===
2018-09-11T14:05:16:  Selected energy range .....: 0.870964 - 100 TeV
2018-09-11T14:05:16:  Requested RoI .............: Centre(RA,DEC)=(82.0133333285646, 22.014444444444) deg, Radius=2 deg
2018-09-11T14:05:16:  RoI of data ...............: Centre(RA,DEC)=(82.0133333285646, 22.014444444444) deg, Radius=2 deg
2018-09-11T14:05:16:  Selected RoI ..............: Centre(RA,DEC)=(82.0133333285646, 22.014444444444) deg, Radius=2 deg
2018-09-11T14:05:16:  cfitsio selection .........: ENERGY >= 0.87096359 && ENERGY <= 100.00000000 && ANGSEP(82.013333,22.014444,RA,DEC) <= 2.000000
2018-09-11T14:05:16:  FITS filename .............: /var/tmp/tmp.3.879wq4[EVENTS][ENERGY >= 0.87096359 && ENERGY <= 100.00000000 && ANGSEP(82.013333,22.014444,RA,DEC) <= 2.000000]

You may use the csobsinfo script to display a summary of the selected events into the console:

$ csobsinfo debug=yes
Input event list, counts cube, or observation definition XML file [$HESSDATA/obs/obs_crab.xml] obs_crab_selected.xml
Output DS9 region file [ds9.reg]

Note

The debug=yes attribute instructs csobsinfo to direct the log file output also to the console. Duplication of log file output into the console using the debug=yes attribute works for any tool or script.

The output of csobsinfo is shown below. ctselect selected 9675 events within an energy interval of 0.66 - 100 TeV. This will be the energy range that you will from now on use for the data analysis.

2018-09-11T20:30:16: +=========+
2018-09-11T20:30:16: | Summary |
2018-09-11T20:30:16: +=========+
2018-09-11T20:30:16: === Observations ===
2018-09-11T20:30:16:  Unbinned observations .....: 4
2018-09-11T20:30:16:  Binned observations .......: 0
2018-09-11T20:30:16: === Events ===
2018-09-11T20:30:16:  Number of events ..........: 9675
2018-09-11T20:30:16:  Number of bins ............: 0
2018-09-11T20:30:16: === Pointings ===
2018-09-11T20:30:16:  Mean offset angle .........: Unknown
2018-09-11T20:30:16:  Mean zenith angle .........: 47.04 deg
2018-09-11T20:30:16:  Mean azimuth angle ........: 13.76 deg
2018-09-11T20:30:16: === Energy range ===
2018-09-11T20:30:16:  Minimum energy ............: 660.693448007596 GeV
2018-09-11T20:30:16:  Maximum energy ............: 100 TeV
2018-09-11T20:30:16: === Time range ===
2018-09-11T20:30:16:  MJD (days) ................: 53343.922 - 53347.933
2018-09-11T20:30:16:  UTC .......................: 2004-12-04T22:07:06 - 2004-12-08T22:22:02
2018-09-11T20:30:16:  Total ontime ..............: 6742.00 s = 112.37 min = 1.87 h
2018-09-11T20:30:16:  Total livetime ............: 6313.81 s = 105.23 min = 1.75 h