Separation of two contributions to the high energy emission of Cygnus X-1: Polarization measurements with INTEGRAL SPI

TL;DR

This study measures the polarization of high energy emission from Cygnus X-1 using INTEGRAL SPI data, revealing a high polarization fraction above 230 keV that suggests jet-related synchrotron emission involving high energy electrons.

Contribution

Developed new tools to analyze polarization in INTEGRAL SPI data and applied them to 6.5 years of observations of Cygnus X-1, providing the first detailed polarization measurements at these energies.

Findings

High energy emission of Cyg X-1 is polarized with 76% +/- 15% above 230 keV.

Polarization fraction increases with energy, exceeding 75% between 370 and 850 keV.

Emission likely originates from jet-related synchrotron processes involving high energy electrons.

Abstract

Operational since 2002 on-board the INTEGRAL observatory, the SPI spectrometer can be used to perform polarization measurements in the hard X-ray/soft gamma-ray domain (~ 130 keV - 8 MeV). However, this phenomenon is complex to measure at high energy and requires high fluxes. Cyg X-1 appears as the best candidate amongst the X-ray binaries since it is one of the brightest persistent sources in this energy domain. Furthermore, a polarized component has recently been reported above 400 keV from IBIS data. We have therefore dedicated our efforts to develop the required tools to study the polarization in the INTEGRAL SPI data and have first applied them to 2.6 Ms of Cyg X-1 observations, covering 6.5 years of the INTEGRAL mission. We have found that the high energy emission of Cyg X-1 is indeed polarized, with a mean polarization fraction of 76 % +/- 15 % at a position angle estimated to 42 +/- 3 degrees, for energies above 230 keV. The polarization fraction clearly increases with energy. In the 130-230 keV band, the polarization fraction is lower than 20 %, but exceeds 75 % between 370 and 850 keV, with the (total) emission vanishing above this energy. This result strongly suggests that the emission originates from the jet structure known to emit in the radio domain. The same synchrotron process could be responsible for the emission from radio to MeV, implying the presence of high energy electrons. This illustrates why the polarization of the high energy emission in compact objects is an increasingly important observational objective.