In a recent study published in the journal Nature Astronomy, astronomers at the University of Turku discovered that a well-known X-ray binary—the precise nature of which has eluded scientists up until now—is actually a covert ultraluminous X-ray source.
A view towards the black hole in an X-Ray binary and the X-Rays we see that are reflected from the inner surface of the powerful outflow surrounding the hole. Image Credit: Alexander Mushtukov
X-ray binaries are fascinating systems made up of two stars: a regular star and a compact, dead object like a neutron star or black hole that absorbs matter from its star companion. So far, only a few hundred of these sources have been found within the Milky Way galaxy. The release of gravitational energy in X-ray binary systems is one of the most effective processes in the Universe and one of the most potent phenomena.
Cygnus X-3 is one of the earliest known X-ray binary systems in the universe. This binary system has been well-known since the early 1970s for its ability to fluctuate between being one of the most intense radio sources for a few days and then dimming or disappearing completely. This unusual feature prompted early attempts to synchronize astronomical observations worldwide through phone calls.
R.M. Hjellming named the system Cygnus X-3 in 1973 due to its unusual behavior during these brief, intensely energetic events, which contrasted with its otherwise "normal" nature. Since then, many attempts have been made to comprehend its nature.
The system's study using the satellite Imaging X-Ray Polarimetry Explorer (IXPE), which was launched by NASA in December 2021, has proven instrumental in solving this puzzle. Alexandra Veledina, the study's lead author and an Academy Research Fellow at the University of Turku in Finland, claims that the application of X-ray polarized vision has shed light on the composition of matter surrounding the compact object closest to the black hole.
We have discovered that the compact object is surrounded by an envelope of a dense, opaque matter. The light that we observe is a reflection off the inner funnel walls formed by the surrounding gas, resembling a cup with a mirror interior.
Alexandra Veledina, Study Lead Author and Academy Research Fellow, University of Turku
As a result of this discovery, Cygnus X-3 has been identified as a member of the class of ultra-luminous X-ray sources (ULXs), which devour matter at such an enormous rate that a significant portion of the material that is falling out of the system rather than fitting inside the event horizon.
ULXs are typically observed as luminous spots in the images of distant galaxies, with their emissions amplified by the focusing effects of the compact object’s surrounding funnel, acting akin to a megaphone.
Juri Poutanen, Professor and Study Co-Author, Department of Physics and Astronomy, University of Turku
Poutanen said, “However, due to the vast distances to these sources, thousands of times beyond the span of the Milky Way, they appear relatively faint to X-ray telescopes. Our discovery has now unveiled a bright counterpart of these distant ULXs residing within our own Galaxy.”
This important discovery opens a new line of inquiry into this remarkable cosmic source and provides a means of investigating extreme matter consumption in greater detail.
Journal Reference:
Veledina, A., et al. (2024) Cygnus X-3 revealed as a Galactic ultraluminous X-ray source by IXPE. Nature Astronomy. doi.org/10.1038/s41550-024-02294-9