Accretion geometry of the black-hole binary Cygnus X-1 from X-ray polarimetry

Abstract

2 Black-hole binary (BHB) systems comprise a stellar-mass black hole and a closely orbiting companion star. Matter is transferred from the companion to the black hole, forming an accretion disk, corona and jet structures. The resulting release of gravitational energy leads to emission of X-rays (1). The radiation is affected by special/general relativistic effects, and can serve as a probe of the properties of the black hole and surrounding environment, if the accretion geometry is properly identified. Two competing models describe the diskcorona geometry for the hard spectral state of BHBs, based on spectral and timing measurements (e.g., 2, 3). Measuring the polarization of hard X-rays reflected from the disk allows the geometry to be determined. The extent of the corona differs between the two models, affecting the strength of relativistic effects (e.g., enhancement of polarization fraction and rotation of polarization angle). Here, we report observational results on linear polarization of hard X-ray (19-181 keV) emission from a BHB, Cygnus X-1 (e.g., 4), in the hard state. The low polarization fraction, <8.6% (upper limit at 90% confidence level), and the alignment of the polarization angle with the jet axis show that the dominant emission is not influenced by strong gravity. When considered together with existing spectral and timing data, our result reveals that the accretion corona is either an extended structure, or is located far from the black hole in the hard state of Cygnus X-1.The two competing models describe the location of the hot corona and the accretion disk in the hard state of BHBs -the 'lamp-post corona' model (e.g., 5) and the 'extended corona' model (e.g., 6), as illustrated in Figure 1. In the lamp-post corona model, the size of the corona is small and it is located on the rotation axis of the black hole, close to the event horizon. The emission near the black hole is influenced by strong general/special relativistic effects. The extended

PoGO+ polarimetric constraint on the synchrotron jet emission of Cygnus X-1

“…This energy range is suitable to study the corona emission reflected by the disk. We have previously shown that the corona geometry is extended rather than compact (Chauvin et al 2018). Such discrimination has not been possible from previous observations in soft X-rays or soft gamma-rays (Long et al 1980;Laurent et al 2011;Jourdain et al 2012).…”

“…The source was in the typical hard state, based on light curves by the MAXI (Matsuoka et al 2009) and Swift/BAT (Krimm et al 2013) instruments. Using a previous Suzaku observation at similar MAXI and Swift fluxes (Mitsuda et al 2007), the 20-180 keV flux is estimated as 2.6 × 10 −8 erg s −1 cm −2 (Chauvin et al 2018). At the distance of 1.86 kpc, the luminosity is 1.1 × 10 37 erg s −1 , which corresponds to 0.6% of the 15M ⊙ Eddington luminosity.…”

“…PoGO+ is a hard X-ray polarimeter which performed Cyg X-1 observations in the 19-181 keV range (median en-ergy of 57 keV) during July 12-18 in 2016 (Chauvin et al 2018). The source was in the typical hard state, based on light curves by the MAXI (Matsuoka et al 2009) and Swift/BAT (Krimm et al 2013) instruments.…”

“…The point-estimate and the uncertainty correspond to the peak and the region of highest posterior density containing 68.3% probability content, respectively. Details of the PoGO+ polarization analysis are described in Chauvin et al (2018).…”

“…1 shows the results from Fig. 2 of Chauvin et al (2018) represented in the Stokes QU plane. The red cross is the MAP estimate and the red circle corresponds to the 90% credibility region.…”