Vacuum birefringence and the X-ray polarization from black-hole accretion disks

Abstract

In the next decade, x-ray polarimetry will open a new window on the high-energy Universe, as several missions that include an x-ray polarimeter are currently under development. Observations of the polarization of x-rays coming from the accretion disks of stellar-mass and supermassive black holes are among the new polarimeters' major objectives. In this paper, we show that these observations can be affected by the quantum electrodynamic (QED) effect of vacuum birefringence: after an x-ray photon is emitted from the accretion disk, its polarization changes as the photon travels through the accretion disk's magnetosphere, as a result of the vacuum becoming birefringent in presence of a magnetic field. We show that this effect can be important for black holes in the energy band of the upcoming polarimeters, and has to be taken into account in a complete model of the x-ray polarization that we expect to detect from black-hole accretion disks, both for stellar mass and for supermassive black holes. We find that, for a chaotic magnetic field in the disk, QED can significantly decrease the linear polarization fraction of edge-on photons, depending on the spin of the hole and on the strength of the magnetic field. This effect can provide, for the first time, a direct way to probe the magnetic field strength close to the innermost stable orbit of black-hole accretion disks and to study the role of magnetic fields in astrophysical accretion in general.

11 pages, 3 figures

References in corpus (4)

• The magnetic nature of disk accretion onto black holes
• The Accretion Disk Wind in the Black Hole GRO J1655-40
• Evidence for vacuum birefringence from the first optical polarimetry measurement of the isolated neutron star RX\, J1856.5$-$3754
• Black-hole jets without large-scale net magnetic flux

Cited by in corpus (8)

• Vacuum birefringence in the head-on collision of XFEL and optical high-intensity laser pulses
• Polarisation of Accreting X-ray Pulsars. II. Hercules X-1
• Reducible contributions to quantum electrodynamics in external fields
• Probing Axions via Light Circular Polarization and Event Horizon Telescope
• The Relative Importance of Faraday Rotation and QED Birefringence for the Linear Polarization of X-rays from Mass Accreting Black Holes
• Probing Black Hole Magnetic Fields with QED
• Criticality and dominance of axion physics in highly magnetized vacuum
• Quantum refraction effects in pulsar radio emission