Improvements in calibration of GSO scintillators in the Suzaku Hard X-ray Detector

TL;DR

This paper details improved in-orbit calibration methods for GSO scintillators on Suzaku, addressing energy scale changes through thorough re-analyses and laboratory experiments, leading to more accurate spectral data.

Contribution

It introduces a refined calibration approach for GSO scintillators that eliminates the need for artificial corrections, enhancing spectral accuracy.

Findings

In-orbit data now aligns with ground measurements within ~5%.

Updated calibration improves spectral analysis of the Crab Nebula.

Response files now accurately reproduce observed spectra.

Abstract

Improvements of in-orbit calibration of GSO scintillators in the Hard X-ray Detector on board Suzaku are reported. To resolve an apparent change of the energy scale of GSO which appeared across the launch for unknown reasons, consistent and thorough re-analyses of both pre-launch and in-orbit data have been performed. With laboratory experiments using spare hardware, the pulse height offset, corresponding to zero energy input, was found to change by ~0.5 of the full analog voltage scale, depending on the power supply. Furthermore, by carefully calculating all the light outputs of secondaries from activation lines used in the in-orbit gain determination, their energy deposits in GSO were found to be effectively lower, by several percent, than their nominal energies. Taking both these effects into account, the in-orbit data agrees with the on-ground measurements within ~5%, without employing the artificial correction introduced in the previous work (Kokubun et al. 2007). With this knowledge, we updated the data processing, the response, and the auxiliary files of GSO, and reproduced the HXD-PIN and HXD-GSO spectra of the Crab Nebula over 12-300 keV by a broken powerlaw with a break energy of ~110 keV.