Accretion flows, X-ray states and jet launching

7 problems

• Why do observed disks transport angular momentum faster than simulations?

  Open

  The magnetorotational instability (magnetic stirring of orbiting gas) explains how disks move angular momentum outward, but simulations give about ten times less transport than outbursts of real disks imply.

• Is jet power set by black hole spin via Blandford-Znajek?

  Contested

  Theory predicts that magnetic fields threading a spinning black hole extract its rotational energy into jets, so faster-spinning holes should have stronger jets.

• Are low-frequency QPOs caused by Lense-Thirring precession?

  Contested

  The X-ray brightness of accreting black holes wobbles with a near-regular period of 0.03 to 10 s (a quasi-periodic oscillation, QPO), possibly because frame dragging by the spinning hole makes the inner gas flow precess like a tilted top.

• What produces quasi-periodic X-ray eruptions from galactic nuclei?

  Contested

  Since 2019 the centers of some galaxies have been seen to flare in X-rays every few hours to days, like a clock; a leading idea is a star or small black hole repeatedly crashing through the gas disk around the central supermassive black hole.

• Why do radiation-dominated accretion disks appear thermally stable?

  Open

  Theory from the 1970s predicts that disks held up mostly by the pressure of light should heat up and fall apart unstably, yet many bright disks look steady.

• Why do X-ray binary state transitions show hysteresis?

  Open

  During an outburst a black-hole binary switches from a hard X-ray state to a soft one at a higher brightness than the one at which it switches back, so the state is not set by the feeding rate alone.

• How do ultraluminous X-ray pulsars exceed the Eddington limit?

  Contested

  Some neutron stars shine hundreds of times brighter than the Eddington limit (the brightness at which the push of light on infalling gas balances gravity), and it is unclear whether they truly do or only beam their light toward us.