The nature of the first black hole discovered, Cygnus-X1, clarified thanks to Ixpe

The X-ray source called Cygnus X-1 is a legend in the world of astrophysics and X-ray astronomy. It had been the subject of a bet in 1975 between nothing less than stephen hawking and the Nobel Prize in Physics Kip Thornebetting that Hawking will concede losing in 1990.

It is towards this source of X-rays that, from May 15 to 21, 2022, the astrophysicists of the XXI^e century turned the gaze of one of the eyes of the orbiting noosphere, the satellite Imaging X-Ray Polarimetry Explorer (IXPE). The X-ray photons he collected during this period supplemented the information that could already be gleaned from those collected in space with Neutron star Interior Composition Explorer (NICE) and Nuclear Spectroscopic Telescope Array (NuSTAR) in May and June 2022. This resulted in an article that can be viewed at arXiv.

But what makes Cygnus X-1 so interesting?

It is quite simply the first serious candidate for the title of stellar black hole detected! So, of course, the last word has not yet been said as to the existence of black holes but, today, it has become very difficult to deny their existence.

Let us recall that as an X source, Cygnus X-1 had already been discovered in 1964 using X-ray detectors, in this case in the form of a Geiger counter, on board a sounding rocket launched from White Sands Missile Range in New Mexico. We were then still at the dawn of the development of X-ray astronomy, which began about 70 years ago when a V2 rocket carried a detector to observe the Sun’s radiation in this band of wavelengths.

X-rays being easily stopped by the atmosphere (but not by the interstellar medium, X-rays with wavelengths below a nanometer being able to cross right through the Milky Way), we knew that we could not observe this radiation predicted theoretically from the determination of the very high temperature of the solar corona plasma (10⁶ K) without leaving the atmosphere.

A presentation of the History of astronomy X. To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. The English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose “French”. © University of California Television (UCTV)

The rise of astronomy X

Astrophysicists will be able to focus on the case of Cygnus X-1 from the year 1970 with the launch of Uhuru (“freedom” in Swahili), named to thank Kenya which hosted the platform in its continental waters. where the satellite was launched on December 12, Kenya’s independence day.

It was a revolution with the production in a few years of the first map of the celestial vault in X-rays and the discovery, or more precise study, of several hundred sources, some of which have become famous such as Centaurus X-3, the first pulsar X discovered. Behind this success was the group of the Italian-American Nobel Prize in Physics Riccardo Giacconi — died Dec. 9, 2018, aged 87 — who discovered Scorpius X-1 in 1962, the first known X-ray source outside the Solar System.

In April-May 1971, radiostronomers Luc Braes and George K. Miley of the Leiden Observatory, and independently Robert M. Hjellming and Campbell Wade of the National Radio Astronomy Observatory, succeeded in pinpointing the position on the celestial vault of Cygnus X-1 thanks to its radio emissions. We then discover that the source X is clearly associated with a blue supergiant, the star HDE 226868 located in the constellation Cygnus about 6,000 light-years from the Solar System. The surface of such a star is not hot enough to produce the observed X-radiation, and since the observations of Uhuru it is known that this radiation fluctuates so rapidly that the laws of physics imply that its source must be in a celestial body whose size can hardly exceed a hundred kilometers.

Explanations on the polarization of light studied with Ixpe. To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. The English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose “French”. © NASA Marshall Space Flight Center

A black hole of 21 solar masses

Louise Webster and Paul Murdin, at the Royal Observatory in Greenwich, and Charles Thomas Bolton, working independently at the David Dunlap Observatory at the University of Toronto, then showed in 1972 that HDE 226868 oscillates under the effect of the attraction gravity of a compact star like a star with an exoplanet in orbit, which translates into a variable Doppler effect making it possible to trace the mass of the invisible body other than in X-rays.

We finally arrive for Cygnus X-1 at the following diagram. It is a binary star with HDE 226868 which is only about 5 million years old according to stellar evolution theory. Material is torn from it by tidal forces from a compact star containing about 21 solar masses orbiting at 20% of the Earth-Sun distance of HDE 226868 and which must be the product of the supernova explosion of a star It too is 5 million years old at the most, which implies that it must have contained, before its gravitational collapse and the ejection of matter caused by its explosion, more than 40 solar masses. No known law of physics can describe an object of 21 solar masses, contained in a volume of 100 km, without leading to the conclusion that it is a black hole described by the equations of general relativity of Einstein.

Hawking and Thorne were already 80% certain that it must be so already in 1975 but, having devoted so much of his time to the development of the theory of black holes, Hawking wanted to be sure that he had not lost everything. if we ended up discovering that black holes did not exist after all.

A laboratory micro-quasar

Cygnus X-1, as Source X, must be the accretion disk of material from the blue supergiant spiraling down toward the black hole’s event horizon, heating up from viscous friction between the spirals of the disc and therefore becoming a plasma at several million degrees and therefore capable of emitting X-rays. A sort of equivalent of the solar corona must also exist above the disc and it is the geometry of this plasma which is today clarified by the international collaboration between NASA and the Italian Space Agency (ASI) behind the mission Imaging X-Ray Polarimetry Explorer (IXPE).

The polarization of the light is sensitive to the state of the magnetic field in the region where it propagates and the X-radiation of the accretion thus makes it possible to trace the state of the magnetic field and therefore of the currents and of the plasma which generate it . In particular, valuable information is obtained on the mechanism of acceleration of the particles forming the jets of matter associated with Cygnus X-1.

Since there are similarities between the X-ray emissions from Cygnus X-1 and those from the active nuclei of galaxies with these jets, Cygnus X-1 is a member of a class of objects called micro-quasars, an analogue of quasars. Any study of the HDE 226868/Cygnus X-1 binary system can therefore lead to new insights into the mechanisms behind active galaxies.

In this case, it now appears that the new observations already indicate that the disk of Cygnus X-1 is seen much more edge-on than previously thought and that the plasma corona is in fact a kind of sandwich surrounding the accretion disk, or that it instead occupies the central region between the disk and the black hole’s event horizon.