# Observations of Cygnus X-1 in the MeV band by the INTEGRAL imager

## TL;DR

This paper presents an independent analysis of Cygnus X-1's MeV spectrum using INTEGRAL IBIS data, resolving previous discrepancies and confirming models of the MeV tail related to Comptonisation or synchrotron emission.

## Contribution

It introduces a new analysis software for IBIS data and calibrates detector electronics, providing spectra consistent with prior observations and models.

## Key findings

- Spectra from IBIS agree with SPI and CGRO results.
- The MeV tail is consistent with hybrid Comptonisation or synchrotron models.
- Analysis software improvements enable more accurate spectral measurements.

## Abstract

The spectrum of the MeV tail detected in the black-hole candidate Cygnus X-1 remains controversial as it appeared much harder when observed with the INTEGRAL Imager IBIS than with the INTEGRAL spectrometer SPI or CGRO. We present an independent analysis of the spectra of Cygnus X-1 observed by IBIS in the hard and soft states. We developed a new analysis software for the PICsIT detector layer and for the Compton mode data of the IBIS instrument and calibrated the idiosyncrasies of the PICsIT front-end electronics. The spectra of Cygnus X-1 obtained for the hard and soft states with the INTEGRAL imager IBIS are compatible with those obtained with the INTEGRAL spectrometer SPI, with CGRO, and with the models that attribute the MeV hard tail either to hybrid thermal/non-thermal Comptonisation or to synchrotron emission.