This black hole is five times the mass of our Sun, and it's getting bigger all the time, thanks to a normal star that has the colossally bad luck of being the black hole's partner in a binary star system.
This particular star system is what's known as an X-ray binary system, which is any system where a normal star is paired with either a neutron star or a black hole. We've discovered about 5,000 of these systems in our galaxy, but this system, classified XTE J1859+226, is one of just twenty that has a black hole.
This particular system has been under careful observation since it entered an eruption phase in 1999, in which the black hole began sucking in mass from its companion star.
Astrophysicist Jesús Corral Santana explains that this system fits into a sub-category known as transient X-ray binary. And what's that, you ask? Well, he explains:
"Transient X-ray binaries are characterised for spending most of their life in a state of calmness, but occasionally entering eruption stages, during which the rhythm of acretion of matter toward the black hole is triggered."
He also touches on the significance of the system, and how we know that it's definitely a black hole instead of a neutron star:
"Measuring the mass of compact objects is essential to determine what kind of object it may be. If it's greater than three times the solar mass, it can only be a black hole. We found that XTE J1859+226 has a black hole more than 5.4 times greater than the mass of the Sun. It's the definitive confirmation of the existence of a black hole in this object. With this result we add a new piece to the study of the mass distribution of black holes. The shape of this distribution has very important implications for our knowledge about the death of massive stars, the formation of black holes, and the evolution of X-ray binary systems."