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Accepted · first revision · record

Title: Why do radiation-dominated accretion disks appear thermally stable?
Plain: 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.
Precise: Standard alpha disks in which radiation pressure exceeds gas pressure, with stress proportional to total pressure, are thermally and viscously unstable (Lightman-Eardley), predicting limit cycles; most X-ray binaries in the soft state at $L \sim 0.1 \text{ to } 0.5\,L_{\mathrm{Edd}}$ show stable disk emission, with GRS 1915+105 and a few others as exceptions. Determine the stabilizing physics (magnetic pressure support, opacity effects, stress scaling with gas pressure). Answer: a mechanism and stability criterion matching which sources vary and which do not.
Settled by: Radiation-MHD simulations predicting stability boundaries that match the observed luminosity at which limit-cycle variability appears.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: open
Status note: (empty)
Posed since: 1974