Clumpy Outflows from Supercritical Accretion Flow

Abstract

A significant amount of matter in supercritical (or super-Eddington) accretion flow is blown away by radiation force, thus forming outflows; however, the properties of such radiation-driven outflows have been poorly understood. We have performed global two-dimensional radiaion-magnetohydrodynamic simulations of supercritical accretion flow onto a black hole with 10 or 10$^{8} M_{\odot}$ in a large simulation box of 514 $r_{\rm S} \times 514 r_{\rm S}$ (with $r_{\rm S}$ being the Schwarzschild radius). We confirm that uncollimated outflows with velocities of 10 percent of the speed of light emerge from the innermost part of the accretion flow at a wide angle of 10$^{\circ}$ –50$^{\circ}$ from the disk rotation axis. Importantly, the outflows exhibit clumpy structures above heights of $\sim 250 r_{\rm S}$. The typical size of the clumps is $\sim 10 r_{\rm S}$, which corresponds to one optical depth, and their shapes are slightly elongated along the outflow direction. Since clumps start to form in the layer above which the (upward) radiation is superior in force to the (downward) gravity, the Rayleigh–Taylor instability seems to be a primary cause. In addition, a radiation-hydrodynamic instability, which arises when radiation funnels through a radiation-pressure-supported atmosphere, may also help to form clumps of one optical depth. A magnetic photon bubble instability does not seem to be essential, since a similar clumpy outflow structure is obtained in nonmagnetic radiation-hydrodynamic simulations. Since the spatial covering factor of the clumps is estimated to be $\sim$ 0.3, and since they are marginally optically thick, they will explain at least some of the rapid light variations of active galactic nuclei. We further discuss a possibility of producing broad-line region clouds by the clumpy outflow.

From ultraluminous X-ray sources to ultraluminous supersoft sources: NGC 55 ULX, the missing link

“…The detection of such a wind is consistent with the predictions of super-Eddington accretion (Takeuchi et al 2013). The wind has a complex dynamical structure with an ionization state that increases with the outflow velocity, which indicates launching from different regions of the accretion disk.…”

A spectral-timing model for ULXs in the supercritical regime

“…where R c is the radius of the clump (∼5R g from simulations; Takeuchi et al 2013). For the same values as above, we can see that ∼1 per cent covering fraction is quite reasonable assuming only a single clump; in the limit of a greater number, this value will of course increase (dependent on the time-scales between clumps being launched).…”

“…The toy model of Middleton et al (2011a) ascribed the observed large amplitude, short time-scale (tens to thousands of seconds) variability seen in NGC 5408 X-1 to obscuration by individual clumps, generated through radiative-hydrodynamic instabilities in the wind (now reproduced in simulations; Takeuchi et al 2013Takeuchi et al , 2014. In the specific case of NGC 5408 X-1, this requires our line of sight to the inner regions to intercept the wind (see Middleton et al 2014a).…”

A 78 Day X-Ray Period Detected From NGC 5907 Ulx1 by Swift

“…NGC 5907 ULX1 has a hard spectrum (classified as a "hard ultraluminous state" by Sutton et al 2013b), and so would be expected to be viewed at a low inclination. However, it may still be possible to reconcile dipping and a hard spectrum within this framework if our viewing angle lies close to the opening angle of the accretion flow, such that we are viewing the innermost regions through the uppermost atmosphere of the outer regions, which superEddington simulations predict to be dominated by a clumpy outflow (Takeuchi et al 2013). If NGC 5907 ULX1 is a standard ∼10  M stellar remnant, its extreme luminosity would suggest the opening funnel for the accretion flow would likely be quite narrow, and so the wind could well be close to our line of sight.…”