In-flight calibration of the INTEGRAL/IBIS Compton mode

Application to the Crab Nebula polarization

¹ AIM, CEA/CNRS/Université Paris-Saclay, Université Paris Cité, F-91191 Gif-sur-Yvette, France
² Julius-Maximilians-Universität Würzburg, Fakultät für Physik und Astronomie, Institut für Theoretische Physik und Astrophysik, Lehrstuhl für Astronomie, Emil-Fischer-Str. 31, 97074 Würzburg, Germany
³ APC, Université Paris Cité/CNRS/CEA, 75013 Paris, France

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 8 December 2025
Accepted: 10 February 2026

Abstract

Context. The INTEGRAL satellite explored the γ-ray sky since its launch on October 17, 2002, and until the end of its scientific operation on February 28, 2025. A large fraction of the available data is still largely untouched because the analysis is complex.

Aims. We describe the latest in-flight calibration of the Compton-mode of the INTEGRAL/IBIS telescope, which takes more than 20 years of data into account. The spectroscopy and polarization of the standard candle that is the Crab Nebula is analyzed in detail.

Methods. We operated the IBIS telescope as a coded-mask Compton telescope and used the Crab Nebula to refine the calibration, as is usually done for high-energy instruments.

Results. We determined the spectroscopic and polarimetric properties of the IBIS Compton mode and their evolution for the entire duration of the mission. In addition, the long-term evolution of the Crab Nebula polarization was successfully measured and compared with other high-energy experiments. We were able to estimate the energy dependence of the Crab Nebula polarization in four energy bands between 200 keV and 1 MeV. In particular, the detection of polarized emissions strictly above 400 keV makes it the highest-energy measurement ever performed for the Crab Nebula. A Python library was also made publicly available to analyze processed data.