In-flight calibration of the INTEGRAL/IBIS Compton mode: Application to the Crab Nebula polarization

TL;DR

This paper details the in-flight calibration of the INTEGRAL/IBIS Compton mode over twenty years, analyzing the Crab Nebula's spectroscopic and polarimetric properties, including the highest energy polarization measurement to date.

Contribution

It introduces a comprehensive calibration method for the IBIS Compton mode using long-term data and provides new polarization measurements of the Crab Nebula at energies above 400 keV.

Findings

Determined the spectroscopic and polarimetric properties of IBIS Compton mode.

Measured the evolution of Crab Nebula polarization over 20 years.

Achieved the highest energy polarization measurement above 400 keV.

Abstract

The INTEGRAL satellite explored the gamma-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, due to the complexity of analysis. We describe the latest in-flight calibration of the Compton mode of the INTEGRAL/IBIS telescope, taking into account more than twenty years of data. The spectroscopy and polarization of the standard candle that is the Crab Nebula is analyzed in detail. We operate the IBIS telescope as a Coded mask Compton telescope, using the Crab Nebula to refine the calibration, as is usually done for high-energy instruments. We have determined the spectroscopic and polarimetric properties of the IBIS Compton mode and their evolution along the entire duration of the mission. In addition, the long-term evolution of the Crab Nebula's polarization has been successfully measured and compared with other high-energy experiments. We could estimate the energy dependence of the Crab Nebula polarization in four 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.