Due the charge loss in the CdTe crystal, for a given energy deposit, events which have interacted at different depths in the crystal get different measured amplitudes (PHA). The rise-time of the signal induced by these events is also different, and a clear quasi-linear correlation between the charge loss and the rise-time variation is observed (a deeper interaction induces a larger charge loss, giving the longer rise-time). It is then possible to correct this charge loss effect, by taking into account the rise-time information of the signal.
The executable ibis_isgr_energy performs a rise-time correction for each raw ISGRI event, using the ISGRI rise-time correction table (Section B.2). The corrected energy is given in keV (ISGRI_ENERGY), and the corrected rise-time (ISGRI_PI) is the row number used in the IC file. The correction is done by rescaling the measured spectra in accordance with the observed rise-time rt, so that the corrected event corresponds to the deposited energy.
Also the temperature and bias corrections are done at this step.
The ISGRI spectral gain has been observed to decrease with time. In OSA9 the description of the gain drift was based on IREM counters integrated over time, to take into account the solar flares. However, this correction proved to be not stable along the whole mission. In OSA10, in order to stabilize the instrument response across the whole mission, a refined dependence of the gains-offsets correction with the (measured) temperature of the different MDU has been introduced. A similar correction was performed also in OSA versions later than OSA 7 assuming, however, a constant T between the different modules, which is not a correct assumption along the mission.
The new OSA10 calibration results in an increased stability of the W and Na line positions along the mission, and in a minor dispersion of Crab and background spectra along the entire mission compared to previous OSA versions. More details on the new energy calibration can be found at [14]