OGIP 1D spectrum data formats¶
The IACT event lists and 2D IACT IRFs can be transformed into a 1D counts vector and 1D IRFs that can serve as input to general X-ray spectral analysis packages such as Sherpa. For an introduction to this so-called OGIP data format please refer to the official Documentation on HEASARC.
The following section only highlight differences and modifications made to the OGIP standard in order to meet the requirements of gamma-astronomy.
PHA file¶
The OGIP standard PHA file format is described here.
TODO: Should an EBOUNDS extension be added to the PHA file (channels -> energy)? In OGIP this info has to be extraced from the RMF file.
The values of following header keywords need some attention when using them for IACT analysis.
BACKSCAL
For now it is assumed that exposure ration between signal and background counts does not depend on energy, thus this keyword must be set
The BACKSCAL keywords in the PHA and the BKG file must be set so that
\[\alpha = \frac{\mathrm{PHA}_{\mathrm{backscal}}}{\mathrm{BKG}_{\mathrm{backscal}}}\]It is recommended to set the
BACKSCAL
keyword to 1 in the PHA file and to \(1/\alpha\) in the BKG file
Additional header keyword that can be stored in the PHA header for IACT analysis are listed below.
OFFSET
type: float, unit deg- Distance between observation position and target of a spectral analysis
MUONEFF
type: float- Muon efficiency of the observation
ZENITH
type: tbd, unit: deg- Zenith angle of the observation
ON-RAD
type: float, unit deg- Radius of the spectral extraction region
- Defines the spectral extraction region together with the standard keywords
RA-OBJ
andDEC-OBJ
BKG file¶
The values of following header keywords need some attention when using them for IACT analysis.
BACKSCAL
- It is recommended to set the
BACKSCAL
keyword to \(1/\alpha\) in the BKG file (see above)
- It is recommended to set the
ARF file¶
The OGIP standard ARF file format is described here.
Additional header keyword that can be stored in the ARF header for IACT analysis are listed below.
LO_THRES
type: tbd, unit: TeV- Low energy threshold of the analysis
HI_THRES
type: tbd, unit: TeV- High energy threshold of the analysis
RMF file¶
The OGIP standard RMF file format is described here.
How an RMF file can be extracted from a IACT 2D energy dispersion file is explained in Energy Dispersion.