# 1D counts spectra¶

This section describes a data format for 1D counts spectra and associated reduced responses (ARF and RMF) that has been used for decades in X-ray astronomy, and is also used in VHE gamma-ray astronomy.

The EVENTS and 2D 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¶

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 and DEC-OBJ

## BKG¶

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)

## ARF¶

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¶

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.