SEPARATION OF TWO CONTRIBUTIONS TO THE HIGH ENERGY EMISSION OF CYGNUS X-1: POLARIZATION MEASUREMENTS WITH<i>INTEGRAL</i>SPI

Abstract

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