Polarized x-rays constrain the disk-jet geometry in the black hole x-ray binary Cygnus X-1

TL;DR

This study uses x-ray polarization measurements of Cygnus X-1 to determine the geometry of the accretion disk and jet, revealing the disk's orientation and the spatial extension of hot plasma in the system.

Contribution

It provides the first polarimetric constraints on the disk-jet geometry in Cygnus X-1, linking polarization angle to jet direction and disk inclination.

Findings

Jet is launched from the inner x-ray emitting region.

The accretion disk is viewed closer to edge-on.

Hot x-ray plasma is extended perpendicular to the jet.

Abstract

A black hole x-ray binary (XRB) system forms when gas is stripped from a normal star and accretes onto a black hole, which heats the gas sufficiently to emit x-rays. We report a polarimetric observation of the XRB Cygnus X-1 using the Imaging x-ray Polarimetry Explorer. The electric field position angle aligns with the outflowing jet, indicating that the jet is launched from the inner x-ray emitting region. The polarization degree is (4.01+-0.20)% at 2 to 8 kiloelectronvolts, implying that the accretion disk is viewed closer to edge-on than the binary orbit. The observations reveal that hot x-ray emitting plasma is spatially extended in a plane perpendicular to the jet axis, not parallel to the jet.