Stellar mass black holes are most easily identified when they are accompanied by a companion. The gravitational effect of a black hole on it's companion star can help give us a location where the black hole might be hiding. If the black hole is actively feeding on the companion star, we will be able to see this clearly in the X-ray portion of the spectrum. In fact, because these systems are so bright in X-rays, they are often referred to as X-ray binaries. We should note however, that this term refers to systems containing a star and a compact object. As we're aware, compact objects can be either neutron stars or black holes. As such, it is important that when we observe these systems we try and find the mass of the compact object. If the mass of the compact object is more than three solar masses, it must be a black hole. If it's lighter than that, it's likely a neutron star. In some cases though, it can be very hard to tell the mass of the compact object. When astronomers are unsure of the characteristics of systems like this, they list them as a black hole candidate. There are two types of X-ray binaries: high-mass X-ray binaries and low-mass X-ray binaries. But this classification is not based on the mass of the compact object. It may seem strange to you at first, but X-ray binaries are classified by the mass of the companion star, not the compact object. The companion stars in low-mass X-ray binaries have masses that are the mass of the Sun or smaller. High-mass X-ray binaries have companion stars that are at least 10 times more massive than the Sun. Any time astronomers come up with a classification like this, you'll find that some objects don't quite fit. So, we also have an in-between group that is sometimes called intermediate-mass X-ray binaries. But their properties are usually pretty similar to the low-mass X-ray binary group. Why would astronomers choose to classify binary systems based on the type of companion star, rather than the type of compact object? The reason for this classification is that the properties of the system depend more on the type of donor star than the type of compact object. What this means is that observations of these systems vary more dramatically if you compare high-mass and low-mass X-ray binaries than if you were to compare stellar-mass black holes and neutron stars that are both feeding on, say, a low-mass star. When their companion is a low-mass star such as in a low-mass X-ray binary, the gas from the companion star flows to the black hole via Roche lobe overflow that we studied in an earlier module. Also, recall high-mass stars tend to have larger outflows of material in the form of powerful stellar winds. In high-mass X-ray binaries the mass loss through wind ends up being accreted onto the black hole. This is called wind fed accretion. However, we should note that high-mass stars can also feed black holes via Roche lobe overflow. Typically, low-mass companions are small in size, while high-mass companions are large. Small stars can orbit closer to the black hole than large stars can. Kepler's laws of motion tells us that stars with small orbital separations orbit with faster speeds and take shorter amount of time to orbit. Low-mass X-ray binaries typically have short orbital periods that can range from less than an hour to many hours. Meanwhile, the larger companions in high-mass X-ray binaries orbit further away from the center of mass of the system and can take a few days to complete one orbit. This means that the feeding or mass transfer mechanism, and so the rate of mass transfer along with the orbital period, can be greatly impacted by the type of companion star. Stars are usually classified by observing their color in visible light, since this is the portion of the spectrum where they are usually the brightest. We've already learned that accretion disks around black holes will also emit some visible light. This means that if we want to view the companion star of the black hole, we'll have to wait until a black hole was finished eating a major meal so that the disk isn't emitting light which would otherwise pollute our image. When astronomers want to classify a star, they look at it using different filters to determine the star's properties. Low-mass stars with masses less than the Sun's mass are dim and have colors that range from yellow to orange to red. High-mass stars are bright and are blue in color. Since low-mass stars are dim, they can be difficult to detect. So, sometimes we have trouble detecting the companion in a low-mass X-ray binary, and the binary is classified based on its X-ray emission instead. The companion stars in high-mass X-ray binary systems are usually easier to see since they are so bright, meaning that in many cases we can also obtain a detailed spectrum of the star. So, it isn't at all surprising that the first confirmed black hole, Cygnus X-1 has a bright blue high-mass companion star. However, accretion disks can also look very blue, bluer in fact than hot blue stars. This means that when the disk is bright, it can be incredibly hard to work out what kind of star is feeding the compact object. X-ray images of black holes are not quite as impressive to look at as some of the other types of images we've seen in this course. They can be fairly featureless with just a series of dots scattered in a black section of the sky, except, of course, when they suddenly change. The left-hand image shows an X-ray image of the sky near our old friend, Cygnus X-1. In the left image taken before June 2015, we see full bright, X-ray point sources. Cygnus X-1 is the brightest X-ray source in Cygnus and a high-mass X-ray binary. Cygnus X-3 was the third X-ray source discovered in Cygnus and is a low-mass X-ray binary. At this moment, it is unknown whether there is a neutron star or a black hole in Cygnus X-3. 3A 1954+319 is also a low-mass X-ray binary, most likely harboring a neutron star. Cygnus A is a supermassive black hole, but it looks dim because it's in a galaxy far, far away, while the other sources are in our own galaxy. A small X marks the spot where the low-mass X-ray binary, V404 Cyg, suddenly became as bright as Cygnus X-1 and Cygnus X-2 in June 2015. V404 Cyg is close to 8,000 light-years away from us. The companion is a type K star, which means that it's orange in color and has a mass that is just 40 percent of our Sun's mass. The black hole has a mass that is seven times our Sun's mass. So, there is no danger that this could be a neutron star masquerading as a black hole. In this movie, the black hole and it's accretion disk are the bluish white light at the center of the image. The accretion disk suddenly erupted on June 26th, 2015, emitting X-rays in all directions. These X-rays form a spherical shell front that expands and collides with dust clouds far away from the black hole. The red rings are X-rays that are reflected off the dust that lies between the black hole and the Earth. Although the wave front is a sphere, we see circles since the dust clouds are a series of surfaces between the black hole and the Earth. Another example of a low-mass X-ray binary is the X-ray source, X9 in the globular cluster named 47 Tuc. A globular cluster is a dense star cluster that can have many millions of stars. Since the stars are closer to each other than in part of the galaxy where we live, the stars can easily hook up with other stars to form binary systems through dynamical formation. So, if you were to randomly choose a globular cluster to look at with an X-ray telescope, you'd have a good chance of finding an X-ray binary. The X-ray binary X9 is still classified as a candidate black hole, since its mass has not yet been measured, but the orbital period is very small, only 25 minutes, and the companion star is most likely a white dwarf. The companion star to the black hole Cygnus X-1 is easily seen as the bright star in the very center of this visible light image of the constellation, Cygnus. Since this is a visible light image, we can't see the accretion disk of the black hole. The red light is coming from glowing hydrogen gas in a nearby star forming region. Cygnus X-1's companion star is named HDE 226868. But for obvious reasons, we normally call it Cygnus X-1's companion star. The companion is a type O supergiant that has a larger mass than the black hole. The companion star's mass is 19 times larger than the Sun while the black hole's mass is 15 times the Sun's mass. The two objects orbit their common center of mass, which is closer to the companion once every 5.6 days. Both high-mass and low-mass X-ray binaries are spotted scattered throughout galaxies. They're relatively easy to spot because the black holes have their dinner sitting right there next to them in the binary system. What happens when we switch up to other size scales? What are the alternative diets for supermassive black holes?
