Monday, March 24, 2014

Cygnus X-1 Undergoing Changes in Its Emission

First off, to all of my Rush fan readers (Alan, I'm looking at you in particular), this is not about either of the Cygnus X-1 songs. Sorry.

Actually, what I'm going to talk about is much cooler. It started when I came across a Tweet from the Astronomer's Telegram (@astronomerstel) which led to a "telegram" titled "Cygnus X-1 is entering its hard X-ray state". Of course, these are no longer actual telegrams. Rather, they are shared public messages from astronomers telling other astronomers that something exciting is happening in space and that people should point the appropriate telescopes at it.

To start, Cygnus X-1 is a binary system (two objects orbiting one another) that contains a massive blue supergiant star and a stellar-mass, that is to say, not supermassive, black hole. For simplicity, we just call these black hole X-ray binaries (abbreviated as BHXB). It is one of roughly 20 known systems to have a star orbiting a black hole. The image below, Figure 1 from Remillard & McClintock (2006) schematically shows 16 of these systems. Note that not all of these systems contain blue supergiants, though most are at least giant stars. In these systems, the black holes steal matter from their companion stars. The stolen gas doesn't fall right into the black hole though. Instead, because the gas is moving, it starts orbiting the black hole as an accretion disk. The material from the inner edge of the disk can eventually fall into the black hole itself, but it doesn't do so fast enough to get rid of the disk.
Schematic diagram of 16 black hole X-ray binary systems. The sizes of all objects are shown to scale with the average distance between Mercury and the Sun. The round/teardrop shaped objects are the stars, and the disks are the accretion disks surrounding the respective black holes. Cygnus X-1 is shown at the top. The color of the star indicates its temperature. Redder stars are cooler, while bluer stars are hotter.
Black hole X-ray binaries are ideal for studying stellar-mass black holes for a number of reasons. First and foremost, the BEST way to measure the mass of a object is to put something else in orbit around it. This becomes slightly more complicated when you don't know the mass of the object, like a star. However, one can figure out the total mass of the system, and then put reasonable constraints on the mass of each object individually based on the characteristics of the orbit, the nature of the star itself, and so forth. The mass measurements are how we are able to confirm that the objects at the center of these accretion disks are, in fact, black holes.

Now that I've covered the "black hole" and "binary" parts of the name, where are the X-rays coming from? As I mentioned previously, material stolen from the star forms an accretion disk around the black hole. As the gas falls toward the black hole, it gets heated up. The heating occurs because the gravity of the black hole causes the stolen material to speed up as it falls into the disk. When this material encounters the rest of the accretion disk, it starts to experience friction from the interactions with the rest of the gas and plasma surrounding it. This causes the material in the disk to heat up a lot. What do I mean by a lot? I mean tens of millions of Kelvin hot. Hot enough that the disk is thermally emitting X-ray radiation, the same way the Sun emits visible light.

However, these systems do a lot more than just emit thermal X-rays. Because you have an accretion disk, you can inevitably get jets of beamed radiation and particles being emitted perpendicular to the accretion disk itself. I cannot, for the life of me, tell you why this is the case. Usually astronomers will answer this question by waving their hands at you and chanting "angular momentum" and "magnetic fields", which is code for "we have no clue". What matters, however, is that we actually have observed these jets with radio telescopes. One such example is shown below for the black hole candidate (could still be a neutron star in this case) SS 433, in the constellation Aquila.
Radio observations of X-ray binary system SS 433 made with the Very Large Array in 2004. Image courtesy of the National Radio Astronomy Observatory. Image link:
Such a jet is not a constant feature of X-ray binaries though. In fact, neither is strong thermal emission, necessarily. The emission appears to go through a cycle based on what the matter in the accretion disk is actually doing at a given time. The thermal spectrum of the X-ray binary is strongest when the inner edge of the accretion disk (the hottest region of the accretion disk) is closer to the black hole. I don't know exactly what causes the location of the inner edge to change. If I had to guess, I'd say that the inner edge moves closer and closer to the black hole itself until something drives that material to eventually fall into the black hole itself.

Jets seem to occur when the material is accreted directly onto the black hole itself. If we go back to our hand-wavy picture of where these jets come from, the infalling material needs to lose angular momentum. Some of this matter is blasted out of the disk at very high speeds. When an X-ray binary has particularly strong jets, we see less of a thermal spectrum from the accretion disk itself, but rather, we see another strange type of spectrum that is described by what we call a "power law". I've talked a bit about power laws before. A power law spectrum is any spectrum whose intensity depends on the frequency of the radiation, as in the equation below. Here, Iν is the intensity, the amount of radiation given off per second at a given frequency, which is for some reason represented by the Greek letter ν (Nu). Alpha is what we call the "power law index", which is just fancy speak for saying it's a number that describes the shape of the spectrum.
This type of spectrum results in more high-energy X-rays being emitted from the X-ray binary than you get from a purely thermal spectrum. For some reason, higher-energy X-rays are referred to as "hard" X-rays, while the lower-energy X-rays are called "soft". I don't know where this nomenclature came about, but we're stuck with it now. Power law spectra come from phenomena like synchrotron radiation (charged particle spinning in a magnetic field gives off radiation) and inverse Compton scattering (super high-energy electrons smack into lower energy photons, giving them a major kick), both of which are consistent with the existence of jets. 

Returning to the title of the telegram that sparked this whole post, "Cygnus X-1 is entering its hard X-ray state" means that Cygnus X-1's jet is becoming stronger, likely because the black hole is gobbling up hot gas and such. What should happen is that the jet will continue to strengthen, and the region surrounding the black hole will get even brighter in X-rays for a time. This should go until there's no more material to eat up, so the black hole and its accretion disk return to their normal relatively quiet lives.

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