The emission of Cygnus X-1: observations with INTEGRAL SPI from 20 keV to 2 MeV

TL;DR

This study analyzes over six years of INTEGRAL SPI data to examine the spectral variability and emission characteristics of Cygnus X-1 in the hard X-ray to MeV range, revealing spectral shape changes and flux limits.

Contribution

First detailed spectral analysis of Cygnus X-1 over multiple periods up to MeV energies using INTEGRAL SPI data, highlighting spectral variability and flux constraints.

Findings

Spectral shapes vary significantly between 20 and 200 keV.

Emission sharply decreases above 700 keV, with flux below previous reports.

No significant annihilation features detected in the 500 keV region.

Abstract

We report on Cyg X-1 observations performed by the SPI telescope onboard the INTEGRAL mission and distributed over more than 6 years. We investigate the variability of the intensity and spectral shape of this peculiar source in the hard X-rays domain, and more particularly up to the MeV region. We first study the total averaged spectrum which presents the best signal to noise ratio (4 Ms of data). Then, we refine our results by building mean spectra by periods and gathering those of similar hardness. Several spectral shapes are observed with important changes in the curvature between 20 and 200 keV, even at the same luminosity level. In all cases, the emission decreases sharply above 700 keV, with flux values above 1 MeV (or upper limits) well below the recently reported polarised flux (Laurent et al. 2011), while compatible with the MeV emission detected some years ago by CGRO/COMPTEL (McConnell et al., 2002). Finally, we take advantage of the spectroscopic capability of the instrument to seek for spectral features in the 500 keV region with negative results for any significant annihilation emission on 2 ks and days timescales, as well as in the total dataset.