eSASS task: ebarycen

Scope

This document provides a description of the algorithm and instructions for the correct usage of the EBARYCEN task, which is part of eSASS (the eROSITA Science Analysis Software System).

Summary:

The task EBARYCEN is a wrapper script invoking the HEASOFT barycen tool. The task can be used for data from the early data release (EDR) and for all sky survey data (e.g. DR1).

Algorithms:

The script ebarycen is mostly a wrapper script to call the HEASOFT tool barycen. Barycen applies a barycentric correction to the time values in the event list and the corresponding GTI extensions. Please consult the HEASOFT documentation for a description of the full algorithm.

The script ebarycen can be applied to any eROSITA events file containing at least the FITS extensions EVENTS and GTIn, where n is the index of the respective eROSITA TM.

The scripts detects all relevant FITS extensions and complements the respective FITS headers of the input file with keywords required by barycen. The GTIn and FLAREGTInextension is temporarily renamed to GTI, as required by barycen.

The actual computation is done by a call of HEASOFT barycen, which corrects the values in all TIME columns detected in the FITS extensions. Also START and STOP values in the GTIn and FLAREGTIn If the environment variable SASS_VERBOSITY is set to "CHATTY", barycen will be set higher verbosity (chatter=3).

The corrected event file is written to the file specified by parameter "$3" . The following keywords are added to the headers of the relevant FITS extensions:

TIMEREF="SOLARSYSTEM", PLEPHEM, RA_TDB, DEC_TBD, CHECKSUM, DATASUM, HISTORY.

Input parameters:

• "$1" (string) REQUIRED
  • Input event file
• "$2" (string) REQUIRED
  • Camera index
• "$3" (string) REQUIRED
  • Output event file
• "$4" (string) REQUIRED
  • name of SRG orbit FITS file or "CALDB"
• "$5" (string) REQUIRED
  • Object RA [decimal deg]
• "$7" (string) REQUIRED
  • Object DEC [decimal deg]

Input files:

• Input event list

  Single camera eSASS standard event file. The file name is defined by task parameter "$1" .

• SRG orbit file

  SRG orbit file in FITS format. The file srg_orbit_201908_202201.fits contains all currently available orbit data.

Output files:

• Output event list

  Output event file with barycentrically correct times in EVENTS and GTI extensions.

Examples:

1. Run ebarycen on event file events_tm1.fits, use orbit file srg_orbit_202001.fits:

   ebarycen events_tm1.fits 1 events_tm1_bary.fits srg_orbit_201908_202201.fits 180.000 -12.5000

2. Run ebarycen on event file events_tm1.fits, use orbit file from CALDB

   ebarycen events_tm1.fits 1 events_tm1_bary.fits CALDB 180.000 -12.5000