SEPARATION OF TWO CONTRIBUTIONS TO THE HIGH ENERGY EMISSION OF CYGNUS X-1: POLARIZATION MEASUREMENTS WITHINTEGRALSPI

Abstract

Operational since 2002 on-board the INTEGRAL observatory, the SPI spectrometer can be used to perform polarization measurements in the hard Xray/softγ-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 • , 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.

Gamma rays detected from Cygnus X-1 with likely jet origin

“…When particles are accelerated along the relativistic jets, the seed photons can be either thermal photons from the accretion disk or synchrotron soft photons produced by the same population of electrons (Synchrotron-Self-Compton, SSC), or the photons from the companion star (with a black-body peak emission at 2.7×kT∼10 eV). The existence of synchrotron emission from the jet is supported by the hint of strong polarisation in the 0.2-1 MeV tail (Laurent et al 2011;Jourdain et al 2012;Rodriguez et al 2015), the luminosity of which is L MeV tail ∼ 7×10 35 erg s −1 . The inverse Compton scattering on stellar photons would be the dominant mechanism of high-energy radiation if the emission is not originated at the base of the jet.…”

“…This soft gamma-ray radiation was recently shown to be polarised with a polarisation fraction increasing with energies and an average value of (76±15)% at a position angle of (42±3) • for energies above 230 keV (Laurent et al 2011;Jourdain et al 2012). The most plausible explanation for this is that the jet synchrotron emission extends itself up to MeV energies (Jourdain et al 2012;Zdziarski et al 2012), requiring the existence of a parent population of ultra-relativistic electrons. In addition, such a high-level of polarisation would imply that the phenomenon is persistent on time-scales of weeks to months.…”

“…Another peculiarity of the Cygnus X-1 HS is that above the hard X-ray tail which cuts off at ∼100 keV (Wilms et al 2006), an additional harder (with a 1.6 photon index) non-thermal component emerges extending up to a few MeV (Cadolle Bel et al 2006;Rodriguez et al 2015). This soft gamma-ray radiation was recently shown to be polarised with a polarisation fraction increasing with energies and an average value of (76±15)% at a position angle of (42±3) • for energies above 230 keV (Laurent et al 2011;Jourdain et al 2012). The most plausible explanation for this is that the jet synchrotron emission extends itself up to MeV energies (Jourdain et al 2012;Zdziarski et al 2012), requiring the existence of a parent population of ultra-relativistic electrons.…”

Long term variability of Cygnus X-1

“…While the radio emission, and possibly also the gamma-ray emission above 400 keV (Laurent et al 2011;Jourdain et al 2012) originate in the jet, the origin of the X-rays is still unclear. As shown, e.g., by Nowak et al (2011), the combination of the best resolution and the most broadband X-ray spectra available today fails to enable us to statistically distinguish between jet models (Markoff et al 2005;Maitra et al 2009) and thermal and/or hybrid Comptonization in a corona (e.g., Coppi 1999Coppi , 2004.…”

The high-energy spectrum of Cygnus X–1: corona and jet contributions

“…[9]). We were motivated by the polarization measurements of the hard X-ray emission of Cyg X-1 performed with INTEGRAL, independently obtained with the two telescopes IBIS [7] and SPI [6], Laurent et al (2011) found that, while the 250-400 keV spectrum is consistent with emission dominated by Compton scattering by thermal electrons and are weakly polarized (Pf < 20%), the second spectral component seen in the 400 keV-2 MeV band is strongly polarized (Pf = 67± 30%). They have argued that the MeV excess is likely to be produced in jet through synchrotron emission in a very coherent magnetic field.…”