Fig.1: Radio image of W50 nebula (see http://snrcat.physics.umanitoba.ca/SNRrecord.php?id=G039.7m02.0 and https://ui.adsabs.harvard.edu/abs/1998AJ....116.1842D/abstract for detailed description). The central bright spot is a hyperaccreting source SS433. The radio nebula (W50), features a quasi-spherical part (indicated by the dashed circle) and two extensions.
Fig. 2: Sketch of the W50 model as a combination of isotropic and “polar” winds coming from an accreting black hole. The central region represents a combination of an isotropic wind and a more collimated polar wind aligned with the orbital axis of the binary system. The isotropic wind passes through a termination shock, which converts its kinetic energy into heat, and after that, its increased pressure and density re-collimate in turn the polar wind. The resulting recollimation shocks are capable of accelerating particles to extremely high energies giving rise to the high-energy synchrotron emission from the axial flow.
Fig.3: Slices of gas density obtained in numerical simulations for different wind configurations. In response to changes in the density contrast between the wind components and/or the wind velocities, the morphology of the nebula changes too. W50 morphology is best reproduced by the two central plots.
Among many X-ray sources in our Galaxy, the one called SS 433 (as an entry number 433 in the catalog of Halpha emitters by Stephenson & Sanduleak 1977) is especially famous and peculiar. It is likely powered by a black hole in a massive binary system. The accretion rate on this black hole from its companion star is hundreds of times higher than the critical value known as the Eddington limit (when the pressure of produced radiation becomes so great that it can eject matter and form powerful “winds” of the accretion disk). The new model discusses the impact of such winds on the surrounding interstellar medium. In particular, this wind can inflate the giant W50 nebula, encompassing SS 433 and spanning tens of parsecs in size. A similar situation may occur for rapidly growing massive black holes at the dawn of the Universe, galaxies with extreme nucleus activity and star formation rates during the “Cosmic Noon” (when the Universe was about 2-3 billion years old), or in the most extreme ultraluminous X-ray sources in normal star-forming galaxies today.
By now, millions of X-ray sources are known throughout the sky. The names of several dozen of them are known to almost all astronomers and astrophysicists. Among them is the microquasar SS433 in our Galaxy, a unique object in all parts of the electromagnetic spectrum from radio waves to ultra-high-energy photons. Since the late 1970s, it was known that this microquasar ejects narrow jets of matter, the speed of which is approximately a quarter of the speed of light, and the direction strictly periodically changes in time like a precessing top! Although such a picture was predicted and considered in pioneering works on accretion theory (Shakura & Sunyaev 1973), and since its discovery, several thousand works have been devoted to the study of this source, there are still no generally accepted answers to numerous questions about the structure of the super-Eddington flow of matter onto a black hole and the mechanism for launching the jets.
Similarly impressive and puzzling is the radio nebula W50 surrounding SS433 (Fig.1), the origin of which also remains open. Its shape has made another name popular - “Manatee Nebula”. The formation of a close binary system of a massive star + a black hole is preceded by a supernova explosion, which accompanies the formation of a black hole. It is usually believed that the quasi-spherical part of the nebula was created by the expanding shell of the supernova. As for the elongated parts of W50, there is no generally accepted model, but it is often assumed that a certain role is played by those very narrow precessing sub-relativistic jets of matter, although global traces of their deceleration and violent interaction with the environment have not yet been found.
A completely different explanation was recently proposed. Given the gigantic accretion rate onto a black hole, most of the matter should be ejected by radiation pressure, forming a powerful wind with a speed of thousands and even tens of thousands of kilometers per second. The work suggests that this wind is anisotropic, and the entire W50 nebula was created just by the action of such a profiled wind. In this model, the density of the kinetic energy flux in the wind is higher in the direction perpendicular to the plane of the accretion disk and is almost the same in other directions. This explains the shape of the nebula - in the directions where the wind power is higher, the nebula has a more elongated shape (Fig. 2).
It is remarkable that this assumption also explains the mysterious structures inside the nebula observed at high energies, from keV to TeV. Moreover, the magnetohydrodynamic structure of the anisotropic wind indicates the possibility of efficient acceleration of relativistic cosmic ray protons with petaelectronvolt energies. It is well known how jets of matter behave when interacting with the environment. In astrophysical conditions, such jets arise near supermassive black holes, as well as stellar-mass black holes. When the density of matter in the jets becomes much less than the density of the surrounding gas, shock waves arise that can focus the jets and allow them to propagate over large distances. In the case of SS433, the isotropic part of the wind plays the role of the “environment” for the more collimated part of the wind. It focuses and heats the axial part of the wind, remaining invisible to the observer. As a result, at a large distance from the compact source, a bright structure in the X-ray and TeV range appears “out of nowhere”. Depending on the density/velocity contrast between the isotropic and “polar” winds, the morphology of the nebula can change the aspect ratio and its inner structure, but the recollimation shocks are almost always present (Fig.3).
If we extrapolate this model to other sources in which the regime of very fast accretion onto a compact object is realized, then we can expect that conditions for effective acceleration of particles in the anisotropic wind can naturally arise in them. As a result, a noticeable share of the released accretion energy is converted into cosmic rays and also stored in a hot multiphase cocoon inside the nebula. The total energy released at this stage of the source's life turns out to be greater than the energy of the explosion of the parent supernova.
By now, millions of X-ray sources are known throughout the sky. The names of several dozen of them are known to almost all astronomers and astrophysicists. Among them is the microquasar SS433 in our Galaxy, a unique object in all parts of the electromagnetic spectrum from radio waves to ultra-high-energy photons. Since the late 1970s, it was known that this microquasar ejects narrow jets of matter, the speed of which is approximately a quarter of the speed of light, and the direction strictly periodically changes in time like a precessing top! Although such a picture was predicted and considered in pioneering works on accretion theory (Shakura & Sunyaev 1973), and since its discovery, several thousand works have been devoted to the study of this source, there are still no generally accepted answers to numerous questions about the structure of the super-Eddington flow of matter onto a black hole and the mechanism for launching the jets.
Similarly impressive and puzzling is the radio nebula W50 surrounding SS433 (Fig.1), the origin of which also remains open. Its shape has made another name popular - “Manatee Nebula”. The formation of a close binary system of a massive star + a black hole is preceded by a supernova explosion, which accompanies the formation of a black hole. It is usually believed that the quasi-spherical part of the nebula was created by the expanding shell of the supernova. As for the elongated parts of W50, there is no generally accepted model, but it is often assumed that a certain role is played by those very narrow precessing sub-relativistic jets of matter, although global traces of their deceleration and violent interaction with the environment have not yet been found.
A completely different explanation was recently proposed. Given the gigantic accretion rate onto a black hole, most of the matter should be ejected by radiation pressure, forming a powerful wind with a speed of thousands and even tens of thousands of kilometers per second. The work suggests that this wind is anisotropic, and the entire W50 nebula was created just by the action of such a profiled wind. In this model, the density of the kinetic energy flux in the wind is higher in the direction perpendicular to the plane of the accretion disk and is almost the same in other directions. This explains the shape of the nebula - in the directions where the wind power is higher, the nebula has a more elongated shape (Fig. 2).
It is remarkable that this assumption also explains the mysterious structures inside the nebula observed at high energies, from keV to TeV. Moreover, the magnetohydrodynamic structure of the anisotropic wind indicates the possibility of efficient acceleration of relativistic cosmic ray protons with petaelectronvolt energies. It is well known how jets of matter behave when interacting with the environment. In astrophysical conditions, such jets arise near supermassive black holes, as well as stellar-mass black holes. When the density of matter in the jets becomes much less than the density of the surrounding gas, shock waves arise that can focus the jets and allow them to propagate over large distances. In the case of SS433, the isotropic part of the wind plays the role of the “environment” for the more collimated part of the wind. It focuses and heats the axial part of the wind, remaining invisible to the observer. As a result, at a large distance from the compact source, a bright structure in the X-ray and TeV range appears “out of nowhere”. Depending on the density/velocity contrast between the isotropic and “polar” winds, the morphology of the nebula can change the aspect ratio and its inner structure, but the recollimation shocks are almost always present (Fig.3).
If we extrapolate this model to other sources in which the regime of very fast accretion onto a compact object is realized, then we can expect that conditions for effective acceleration of particles in the anisotropic wind can naturally arise in them. As a result, a noticeable share of the released accretion energy is converted into cosmic rays and also stored in a hot multiphase cocoon inside the nebula. The total energy released at this stage of the source's life turns out to be greater than the energy of the explosion of the parent supernova.
Author:
Ildar Khabibullin (LMU)
and
Eugene Churazov
Scientific Staff
2219
echurazov@mpa-garching.mpg.de
Original Publication
Eugene Churazov, Ildar Khabibullin, Andrey Bykov
Minimalist model of the W50/SS433 extended X-ray jet: Anisotropic wind with recollimation shocks
DOI 10.1051/0004-6361/202449343
