[Paper Review] Ergomagnetosphere, Ejection Disc, Magnetopause in M87. I Global Flow of Mass, Angular Momentum, Energy and Current

R. D. Blandford, Noémie Globus|arXiv (Cornell University)|Apr 25, 2022

Astrophysical Phenomena and Observations82 references30 citations

TL;DR

This paper proposes that the 1.3 mm ring observed by the Event Horizon Telescope in M87 is not a conventional accretion torus but a magnetically dominated ergomagnetosphere that extracts rotational energy from the spinning black hole via electromagnetic torque and negative-energy waves. This mechanism powers both the relativistic jets and a large-scale MHD wind that drives mass outflow, collimates the jets, and explains the system's low radiative efficiency despite high mass supply rates.

ABSTRACT

We interpret the 1.3mm VLBI observations made by the Event Horizon Telescope of the black hole in M87. It is proposed that, instead of being a torus of accreting gas, the observed annular ring is a rotating, magnetically-dominated ergomagnetosphere that can transmit electromagnetic angular momentum and energy outward to the disc through a combination of large scale magnetic torque and small scale instabilities. It is further proposed that energy can be extracted by magnetic flux threading the ergosphere through the efficient emission of long wavelength electromagnetic disturbances onto negative energy orbits, when the invariant $B^2-E^2$ becomes negative. In this way, the spinning black hole and its ergosphere not only power the jets but also the ejection disc so as to drive away most of the gas supplied near the Bondi radius. This outflow takes the form of a MHD wind, extending over many decades of radius, with a unidirectional magnetic field, that is collimated by the infalling gas across a magnetopause. This wind, in turn, collimates the relativistic jets and the emission observed from the jet sheath may be associated with a return current. A model for the global flow of mass, angular momentum, energy and current, on scales from the horizon to the Bondi radius, is presented and discussed.

Motivation & Objective

• To explain the low radiative efficiency of M87's accretion despite high mass supply rates.
• To resolve the discrepancy between high Bondi accretion rates and low observed luminosity.
• To explain how the jet power (6×10⁴³ erg s⁻¹) exceeds the ring luminosity (10⁴¹ erg s⁻¹) by identifying a mechanism for energy extraction from black hole spin.
• To propose a global model of mass, angular momentum, energy, and current flow from the horizon to the Bondi radius.
• To identify the ergomagnetosphere and magnetopause as key interfaces enabling energy and momentum transfer.

Proposed method

• Proposes a magnetically dominated ergomagnetosphere extending from the event horizon to ϖem ∼5×10¹⁵ cm, where electromagnetic torque transmits angular momentum and energy.
• Introduces the extraction of rotational energy via negative-energy electromagnetic waves on orbits where B² − E² < 0, enabling efficient power transfer.
• Models the outflow as a MHD wind driven centrifugally from the ejection disc, collimated by infalling gas across a magnetopause at ϖed ∼10²⁰ cm.
• Identifies four key interfaces: the magnetopause, the corona-disc boundary, the ergomagnetosphere, and the jet-sheath boundary.
• Uses force-free and MHD descriptions in different regions, with a transition at ϖem.
• Incorporates electromagnetic dissipation in the jet sheath and Faraday rotation effects in the wind for observational comparison.

Experimental results

Research questions

• RQ1 How can the jet power in M87 (6×10⁴³ erg s⁻¹) exceed the observed ring luminosity (10⁴¹ erg s⁻¹) by a factor of 600?
• RQ2 Why is the accretion flow in M87 so radiatively inefficient despite a Bondi mass supply rate of ∼10²⁵ g s⁻¹?
• RQ3 What mechanism enables the ejection of most inflowing gas as a collimated MHD wind while maintaining low radiative efficiency?
• RQ4 How is the jet collimated over scales of ∼30–105 gravitational radii without a bright, thick torus?
• RQ5 What role does the magnetopause play in separating inflow from outflow and enabling global feedback?

Key findings

• The ergomagnetosphere extracts ∼10% of the total jet power from the black hole's spin, estimated at ∼0.15×10⁶³ erg, to power the disc wind and collimate the jet.
• The observed ring at 1.3 mm is interpreted as emission from the ergomagnetosphere, not a standard accretion torus, with a brightness temperature of ∼6 GK.
• Energy is extracted via electromagnetic disturbances on negative-energy orbits when B² − E² < 0, enabling efficient spin energy transfer.
• The MHD wind carries away most of the mass supplied at the Bondi radius (rBondi ∼250 pc), explaining the low radiative efficiency.
• The magnetopause at ϖed ∼10²⁰ cm separates the inward infall from the outward ejection disc, with potential observability via Faraday rotation.
• The jet sheath, observed at de-projected distances ≥5×10¹⁷ cm, is an electromagnetic boundary layer with generalized viscous and resistive dissipation, enabling particle acceleration and radio emission.