A binary system containing a massive star and a likely black hole, which together are the source of intense X-rays, has turned out to be a smaller example of some of the most luminous quasars in the universe.
New insights from an international team that used NASA‘with Imaging X-ray Polarimetry Explorer spacecraft (IXPE), describe how the X-ray binary system is located about 24,000 light years away in our Milky way galaxy it amplifies its X-ray emission in a funnel-shaped cavity that surrounds the probable Black hole.
The system, Cygnus X-3, was discovered in the early 1970s when radio telescopes picked up powerful jets radiating from it in nearly the speed of light. The radio emissions from these jets take several days to turn off, only to turn on again later.
The origin of the jets was a mystery at the time. The system has been described as an “astronomical conundrum”, not helped by the fact that we can’t even see Cygnus X-3 in visible light; it is blocked by dense dust in the plane of our galaxy. During the 1970s, radio astronomers at observatories around the world coordinated by telephone to try to catch Cygnus X-3 as it powered on or off.
Related: A new map of the universe reveals an amazing X-ray view of the universe
Over the years, further observations at radio, infrared, and X-ray wavelengths have allowed astronomers to determine that Cygnus X-3 is an X-ray binary system involving mass transfer between a massive star and a compact object orbiting a common center. of gravitation. A compact object is either a neutron star or more likely a black hole with a mass of about five times the mass the mass of our sun. The massive star is a Wolf-Rayet star – a rare phase supergiant stars undergo, emitting strong stellar winds that begin to lift large chunks of their outer shell inward space. It is the material blown in by the wind from this Wolf-Rayet star that powers the accretion disk that spirals around the compact object.
However, the Cygnus X-3 luminosity it is hardly believable. The flow of matter onto a compact object such as a black hole is governed by a property known as the Eddington limit. If the accretion rate is high enough, the accretion disk becomes a jam—mass eventually backs up, the disk thickens and heats up enough that the amount of outgoing radiation can stop the inflow of fresh material. In this way, black holes can regulate their own growth, and some of the material is spit back in radio jets.
However, some of the most luminous quasars — galaxies with extremely active supermassive black holes at their cores—they appear to violate the Eddington limit because their luminosities are extremely high, yet they appear to be accreting mass. And Cygnus X-3 seems to fall into that category, albeit on a smaller scale.
Now, a team led by Alexandra Veledina of the University of Turku in Finland has used IXPE to measure the degree of polarization in X-ray light coming from Cygnus X-3. They found that the degree of polarization is high enough that it can only be explained by X-rays scattering inside the funnel-like cavity at the heart of the accretion disk.
“We discovered that the compact object is surrounded by an envelope of dense, opaque matter,” said Veledina v declaration. “The light we observe is a reflection from the inner walls of the funnel formed by the surrounding gas, resembling a cup with a mirrored interior.”
The opaque envelope, which is elevated by a funnel-shaped cavity, is typical of quasars, which are described as “ULX’ — ultraluminous X-ray sources. The range of gain due to X-ray scattering from inside the funnel cavity is also analogous to ULX.
“ULXs are typically observed as bright points in images of distant galaxies whose emissions are amplified by the focusing effects of the surrounding compact object funnel, which acts like a megaphone,” said study team member Juri Poutanen from the University of Turku. “However, due to the vast distances to these sources … they appear relatively faint to X-ray telescopes.”
Learning about ULXs in quasars has therefore proven difficult, but astronomers can now use the much closer Cygnus X-3 as a model to better understand these distant ULXs.
“Our discovery has now revealed a clear counterpart to these distant ULXs residing in our own galaxy,” said Poutanen.
Cygnus X-3’s outbursts are intermittent due to the Wolf-Rayet star’s elliptical orbit around the compact object, meaning it is occasionally closer and more material in the wind falls on the likely black hole. IXPE was able to see that when Cygnus X-3 is in the ULX phase – when the amount of infalling material is greatest – the degree of polarization reaches 24.9%, but when the system is less active, the polarization drops to 10.4%. . This suggests that the structure of the funnel changes in response to greater or lesser amounts of accretion. If the accretion rate drops too low, the funnel may collapse completely, only to rebuild when accretion picks up again, Veledin’s team predicts.
The team is now planning further observations to try to capture this collapse, which would be signaled by a drop in polarization to near zero, suggesting that the X-ray emission is coming directly from the hot gas on the surface of the accretion disk and not indirectly. through dispersion within the funnel.
The findings were published June 21 in the journal Astronomy of nature.