Limits on thickness and efficiency of Polish doughnuts in application to the ULX sources

TL;DR

This paper establishes upper limits on the thickness and efficiency of Polish doughnuts, a model for super-Eddington accretion disks, and discusses their implications for ultraluminous X-ray sources, considering effects like advective cooling.

Contribution

It constructs extremal models of Polish doughnuts to determine maximum geometrical thickness and radiative efficiency, providing new limits relevant to ULX sources.

Findings

Upper limits on PD thickness and efficiency derived

Advective cooling reduces PD thickness and luminosity

Beamed radiation scales linearly with accretion rate

Abstract

Polish doughnuts (PDs) are geometrically thick disks that rotate with super-Keplerian velocities in their innermost parts, and whose long and narrow funnels along rotation axes collimate the emerging radiation into beams. In this paper we construct an extremal family of PDs that maximize both geometrical thickness and radiative efficiency. We then derive upper limits for these quantities and subsequently for the related ability to collimate radiation. PDs with such extreme properties may explain the observed properties of the ultraluminous X-ray sources without the need for the black hole masses to exceed ~ 10 solar masses. However, we show that strong advective cooling, which is expected to be one of the dominant cooling mechanisms in accretion flows with super-Eddington accretion rates, tends to reduce the geometrical thickness and luminosity of PDs substantially. We also show that the beamed radiation emerging from the PD funnels corresponds to "isotropic" luminosities that linearly scale with the mass accretion rate, and do not obey the familiar and well-known logarithmic relation.