Speed limits for radiation-driven SMBH winds

Abstract

Context. Ultra-fast outflows (UFOs) have become an established feature in analyses of the X-ray spectra of active galactic nuclei (AGN). According to the standard picture, they are launched at accretion disc scales with relativistic velocities, up to 0.3−0.4 times the speed of light. Their high kinetic power is enough to induce an efficient feedback on a galactic scale, possibly contributing to the co-evolution between the central supermassive black hole (SMBH) and the host galaxy. It is, therefore, of paramount importance to gain a full understanding of UFO physics and, in particular, of the forces driving their acceleration and the relation to the accretion flow from which they originate. Aims. In this paper, we investigate the impact of special relativity effects on the radiative pressure exerted onto the outflow. The radiation received by the wind decreases for increasing outflow velocity, v, implying that the standard Eddington limit argument has to be corrected according to v. Due to the limited ability of the radiation to counteract the black hole gravitational attraction, we expect to find lower typical velocities with respect to the non-relativistic scenario. Methods. We integrated the relativistic-corrected outflow equation of motion for a realistic set of starting conditions. We concentrated on a range of ionisations, column densities, and launching radii consistent with those typically estimated for UFOs. We explore a one-dimensional, spherical geometry and a three-dimensional setting with a rotating, thin accretion disc. Results. We find that the inclusion of special relativity effects leads to sizeable differences in the wind dynamics and that v is reduced up to 50% with respect to the non-relativistic treatment. We compare our results with a sample of UFOs from the literature and we find that the relativistic-corrected velocities are systematically lower than the reported ones, indicating the need for an additional mechanism, such as magnetic driving, to explain the highest velocity components. Finally, we note that these conclusions, derived for AGN winds, are generally applicable.

Qwind3: UV line-driven accretion disc wind models for AGN feedback

“…Thus UV line-driven winds can reach UFO velocities even when special relativistic corrections (radiation drag) are included. This contrasts with Luminari et al (2021), who conclude that UV line-driving is not capable of generating such high velocity winds. We caution that the two codes make different assumptions about the initial conditions and ray tracing (both of which Q does more accurately and selfconsistently) as well as the opacity (which their code does better), so it is premature to rule out UV line-driving in favour of other mechanisms such as magnetic driving (Blandford & Payne 1982;Fukumura et al 2017) as the origin of UFOs until these factors are all incorporated together.…”

“…We find that the final velocity of the trajectory is independent of the initial velocity and that the wind is able to drastically accelerate (up to 6 orders of magnitude in velocity) over a very small distance ( 1𝑅 g ), which suggests that line-driving can be very effective even when the X-ray shadowed region is very small. We thus find that UV line-driving is sufficient to reproduce the range of observed UFO velocities, as opposed to the findings of Luminari et al (2021). We note that their code assumes an initial density (similar to Q20) rather than calculating it from first principles, and does not include the full ray tracing of both UV and X-rays that are considered here.…”

“…This is explained in detail in section 5. Lastly, we include the relativistic corrections from Luminari et al (2020) in the calculation of the radiation force, solving the issue of superluminal winds, and we later compare our findings to Luminari et al (2021). Readers interested only in the results should skip to Section 9.…”

“…We ignore the contribution from the angular velocity component, 𝑣 𝜙 for simplicity, as its inclusion would break our assumption that angular momentum is conserved along a gas blob trajectory. The correction Ψ is given by (Luminari et al 2021),…”

Wind-luminosity evolution in NLS1 AGN 1H 0707-495

“…It has been recently reported that the special relativistic effects are of importance for UFO modelling (Luminari et al 2020(Luminari et al , 2021, because the radiation received by the ultra-fast outflows would decrease for increasing outflow velocity. They argued that most of UFO velocities from the literature cannot be reproduced in a radiative-driven scenario within the relativistic treatment, unless the winds are launched at a radii > 50 g , with a velocity up to 0.15 , in a Eddington or Super-Eddington luminosity case.…”