THE POLISH DOUGHNUT MODEL FOR ULX SOURCES

Image: The magenta spots in this image indicate two black holes in the spiral galaxy called NGC 1313, the Topsy Turvy galaxy. Both black holes belong to a class called ultraluminous X-ray sources, or ULXs. The magenta X-ray data come from NASA's Nuclear Spectroscopic Telescopic Array (NuSTAR) and are overlaid on a visible image from the Digitized Sky Survey. ULXs consist of black holes actively accreting, or feeding, off material drawn in from a partner star. Astronomers are trying to figure out why ULXs shine so brightly with X-rays. NuSTAR's new high-energy X-ray data on NGC 1313 helped narrow down the masses of the black holes in the ULXs: the black hole closer to the center of the galaxy is about 70 to 100 times that of our sun. The other black hole is probably smaller, about 30 solar masses. The Topsy Turvy galaxy is located about 13 million light-years away in the Reticulum constellation.
Credit: NASA's Goddard Space Flight Center/JPL-Caltech/IRAP

An Ultra-Luminous X-ray source (ULX) is an astronomical source of X-rays that is less luminous than an active galactic nucleus but is more consistently luminous than any known stellar process, assuming that it radiates isotropically (the same in all directions). Some galaxies contain many ULXs. The Milky Way does not contain a ULX. The main interest in ULXs stems from the fact that their luminosity exceeds the Eddington luminosity of neutron stars and even stellar black holes.


The Eddington luminosity is the maximum luminosity a body (such as a star) can achieve when there is balance between the force of radiation acting outward and the gravitational force acting inward.

It is not known what powers ULXs; models include beamed emission of stellar mass objects, accreting intermediate-mass black holes, and super-Eddington emission.

One of the models is based on the presence of a Polish Doughnut (PD), a thick disk around the black hole rotating with super-Keplerian velocity in its innermost parts and with a long and narrow funnel along the rotation axis. This funnel should collimate the emerging radiation into beams.

In a recent paper (Wielgus et al 2016, A&A) the authors try to understand if PDs may explain the observed properties of ULXs. They investigate the conditions that maximize both geometrical thickness and radiative efficiency of the Polish Doughnuts.

They show that at high accretion rates the relative thickness of PDs is significantly reduced and the doughnuts may be considered as approximate models of slim disks. On the basis of their results, the authors cast doubts on whether collimation by a thick disk’s funnel is an adequate model for the ULXs.

▪Wielgus et al. 2016 (A&A) - Limits on thickness and efficiency of Polish Doughnuts in application to the ULX sources (arXiv)

▪Ultraluminous X-ray source --> (Wikipedia)
▪Eddington luminosity --> (Wikipedia)