X-ray panorama of the “Manatee Nebula” by SRG/eROSITA

Figure 1 shows a composite X-ray image of the radio nebula W50 taken with the eROSITA telescope. The surface brightness of the X-ray emission is colour-coded in the 0.5–1 keV (red), 1–2 keV (green), and 2–4 keV (blue) energy bands. The white arrows depict the projection of the SS433 jets' precession cone extrapolated to distances of more than 100 pc. The hard and soft X-ray diffuse emission can be convincingly split into two components: softer filamentary emission (red-yellow) and harder (green-blue) emission from EXJs. Additionally, there are numerous nearby compact sources, such as active stars and accreting white dwarfs, as well as distant compact sources, mostly AGN. For distant sources, absorption by Milky Way gas suppresses emission below 1 or 2 keV, giving them a blue colour. © MPA; eROSITA/SRG

Figure 2 shows a schematic summary of the W50 nebula superimposed on a composite X-ray (red and green) and radio (VLA at 1.4 GHz, blue) image. Radio emission most likely arises at the outer, shell-like boundary of the nebula, while soft X-ray emission (0.3–0.9 keV) traces shock-heated interstellar medium (ISM) gas behind it, filling almost the entire interior of the nebula. The harder X-ray emission (0.9–2.7 keV in this case) is of a non-thermal (synchrotron) nature and may be produced by ultrarelativistic electrons that are accelerated at the shocks in the axial outflows from the system. The central part of the nebula, within 25 pc of SS433 (dashed circle), is likely to be of very low density and could be a wind-blown cavity created by an almost spherically symmetric outflow with a kinetic luminosity close to the Eddington limit. © MPA; eROSITA/SRG

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Rare or unusual astrophysical objects are used to test the limits of theoretical models because of their extreme properties. The bright X-ray source SS433 in our galaxy undoubtedly belongs to this category. Initially identified as an Hα emitter, it was later recognised as a black hole in a binary system. Since then, SS433, which emits strongly in the radio and X-ray bands, has been targeted by almost every space- and ground-based observatory, leading to a flurry of discoveries.  In contrast, the surrounding huge W50 nebula, spanning more than two degrees, is much fainter and difficult to study. The complete radio image earned W50 the nickname 'Manatee Nebula', while X-ray maps were mostly patches from different observatories or lacked spatial or energy resolution. This shortcoming has finally been overcome by the recently published SRG/eROSITA map of W50 in multiple X-ray colours, which reveals a beautiful blend of thermal and non-thermal processes within an elongated cocoon.

At the core of the W50 nebula lies a compact source (most likely a stellar-mass black hole) that accretes matter from a companion star at an astonishingly high rate — thousands of times greater than the amount the black hole can digest. This limiting rate (known as the Eddington accretion rate) arises due to the pressure exerted by the radiation produced by the infalling gas. This configuration has an immediate impact on the observational appearance of the compact source and its large-scale environment. The key prediction of the accretion theory is that most of the gas supplied to the black hole will be expelled from the system, depositing a large amount of energy into the ambient medium in the process (see Highlight September 2024).

The W50 nebula is well known in radio astronomy for its croissant-like shape. Mapping this large nebula in X-rays used to be problematic due to the limited field of view of space telescopes. Additionally, strong and inhomogeneous absorption by gas and dust occurs in the direction of W50, which is located just two degrees away from the Galactic Plane. These problems can be resolved by using a telescope with a large field of view and high sensitivity to diffuse emission — the very characteristics of the eROSITA telescope on board the SRG observatory.

The full-size X-ray map of the W50 nebula is shown in Fig. 1. The central bright spot is the black hole that powers the entire nebula. It appears extended because it is much brighter than the nebula emission, causing the central part of the image to become saturated.

The 'X-ray colours' in this figure serve the same role as red, green, and blue colours in visible light. Specifically, red corresponds to X-ray photons with a longer wavelength, while green and blue correspond to progressively shorter wavelengths. Remarkably, this simple approach immediately reveals the nature of the X-ray emission: red and yellow colours dominate where thermal plasma with a temperature of 2–10 million degrees is present. Conversely, in the bluer regions, non-thermal emission from relativistic particles dominates.

The nebula is clearly asymmetric, most likely due to a gradient in the ambient gas density surrounding it. The most remarkable feature is the so-called 'Extended X-ray Jets' (EXJs), which have sharp inner edges located around 25 parsecs from the central black hole SS433. Their spectra do not have the emission lines characteristic of thermal plasma. Rather, they must be due to the emission of relativistic particles accelerated by shocks powered by SS433’s outflows. These structures have recently been detected at TeV energies; each TeV photon carries a billion times more energy than a soft X-ray photon at keV energies.

These new X-ray data support the idea that the energy flow from SS433 evolves through three distinct stages:

1) an invisible 'dark' flow of energy between the black hole and the EXJs, presumably carried by a cold wind from the binary system;

2) a 'non-thermal' flow of energy over some 30 pc in the form of EXJs; and

3) a thermal flow (i.e., shock-heated interstellar medium (ISM)) that envelops the EXJs.

The thermal part of the W50 X-ray emission can be reasonably well described by a shock-heated plasma that has not yet reached temperature and ionisation equilibrium. Such emission is typical of middle-aged or old supernova remnants (SNRs). The outer radio boundary of the nebula also resembles SNR shocks (see Fig. 2).

In contrast, the 'extended X-ray jets' are the most remarkable features of this system on tens-of-pc scales. Their sharp inner edges plausibly correspond to extreme shocks that accelerate particles and power the X-ray (synchrotron) and TeV emission, which is 9–10 orders of magnitude more energetic. The W50/SS433 system clearly illustrates the important role that hyper-Eddington accretors might play in the energetics of the interstellar medium in galaxies at different redshifts, as well as in the production of ultra-high-energy particles.

Source: Max Planck Institute for Astrophysics/Research Highlights

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Authors:

Rashid Sunyaev
Emeritus Director
Tel: 2244
Email: rsunyaev@mpa-garching.mpg.de

Eugene Churazov
Scientific Staff
Tel: 2219
Email: echurazov@mpa-garching.mpg.de

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Original publication

Sunyaev R., Khabibullin I., Churazov E., Gilfanov M., Medvedev P., Sazonov S.
X-ray panorama of the SS433/W50 complex by SRG/eROSITA
A&A, in press

DOI

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How hyper-accreting black holes shape their environment with anisotropic winds

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.

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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.

Source: Max Planck Institute for Astrophysics/All News

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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

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