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

TL;DR

The study delivers a comprehensive in-flight calibration of the INTEGRAL/IBIS Compton mode across two decades, enabling robust spectroscopy and polarization measurements of the Crab Nebula from 200 keV to 1 MeV and beyond. By integrating Geant4-based response modeling, updated PICsIT energy calibration, and spurious-flux corrections, the authors extract reliable polarimetric parameters and compare them with SPI and other missions, revealing both temporal and energy-dependent polarization behavior. A key finding is the detection of polarized emission above 400 keV, marking the highest-energy Crab polarization measurement to date, with a polarization fraction around 50% and a polarization angle near the pulsar axis. The work also provides a public Python library (comibis) to analyze processed data, supporting broader use and future high-energy polarimetry developments.

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.

Figures (19)

• Figure 1: Sketch of a Compton interaction in a Compton telescope. The shown quantities are the initial photon energy ($E_0$), recoil electron energy ($E_1$), scattered photon energy ($E_2$), polar angle ($\theta$), and azimuthal angle ($\phi$).
• Figure 2: IBIS/Compton response matrix (RMF), with a saturation value for better contrast.
• Figure 3: IBIS/Compton ancillary response (ARF).
• Figure 4: $a_{100}$ value between 200 and 3000 keV, determined through Geant4 simulations.
• Figure 5: Example of a polarigram fitted by Eq. \ref{['eq:polarigram']}. The data used (in black) are from Crab observations in the 200 -- 400 keV, summed over the 2003--2016 period (see Section \ref{['sec:evo_time']}).