The Coolest Things in the Universe...or, Impressing Your Co-Workers

Dr. David Pooley is an astrophysicist at the University of Wisconsin at Madison. Before landing at the home of the Badgers, Dave was at the University of California at Berkeley after getting his Ph.D. from MIT. He shares his thoughts on what's interesting in the Universe in this installment of the Chandra blog.

Taking stock of the coolest things in the universe, I find black holes are at the top of the list. Neutron stars are a damn close second. Supernovae are up there, too, since (1) they produce neutron stars and (presumably) black holes, and (2) they're enormous explosions.

As cool as these objects are, they're tough to study. Really tough. That's why it's useful to have a list like the one above: when you spend days or weeks or months trying to make sense of some numbers on a computer screen that came from a satellite that was observing such objects, you need to keep perspective. As rewarding as it is, astronomy can be a difficult and sometimes very frustrating detective game, often based on scant evidence. Why, if Sherlock Holmes were alive today, and not fictional, I think he'd love it. He might might wake up one day and say, "Today is a good day I think for X-ray Astrophysics."

The appeal for Holmes would lie in drawing conclusions from indirect evidence. We don't observe black holes directly; we detect radiation from matter around them. We don't observe the actual supernova explosion; we see its aftermath. Much the same way that Holmes could determine where someone had been by seeing the dirt on his boots and knowing the geology of London, we can figure out properties of black holes, neutron stars, and supernovae by observing
their X-rays and knowing the laws of Physics.

For example, my colleagues and I recently made indirect measurements of the sizes of black hole accretion disks on scales several orders of magnitude smaller than the best direct methods. We were able to do this by solving a mystery that was uncovered using the superb resolution of Chandra.

The systems that we were studying are called "quadruply gravitationally lensed quasars." The basic set up is that a very distant supermassive black hole (the quasar) is gravitationally lensed by an intervening galaxy, and its light is split up into four separate paths around the galaxy so that we actually see four separate images of the quasar on the sky. These four images are typically separated by about an arcsecond, which is about the width of a U.S. quarter at a distance of 3 miles. That's a very small separation, but several optical telescopes, including the Hubble Space Telescope, can easily resolve the four distinct images. It's a different story in the X-rays, which are much more difficult to focus. Chandra is the first and only X-ray telescope that has been able to resolve these sets of four images.

An aside: you can easily demonstrate how poor focusing affects the ability to resolve images. Draw four dots on a piece of paper about a half inch apart, and put it on your cubicle wall or other wall a few feet away so that you can still separate the dots. Then take of your glasses and see how much more difficult it is. If you don't wear glasses, put someone else's glasses on and see how difficult it is to make out the dots. This is a great way to make new friends and introduce yourself to people, like good-looking coworkers. You will impress them with your interest in the Universe.

Okay, back to the quasars! Because Chandra is able to resolve the four images in X-ray light, we were able to compare the X-ray and optical light, and we found that they were affected differently by the gravitational lensing happening in the intervening galaxy. However, we know from General Relativity that gravitational lensing is independent of wavelength so the X-rays and optical *should* be affected the same way. We had ourselves a conundrum.

The solution was in realizing that, although the X-rays and optical light are both coming from material around the black hole, they're not necessarily coming from the exact same region. The X-rays are originating very close to the black hole, and the optical light comes from further out. By characterizing the degree to which the optical and X-rays were affected differently, we were able to determine about how far away from the black hole the optical light was being produced. We were therefore able to indirectly resolve the regions around the black holes on *micro*arcsecond scales.

It was like being a detective, but with Physics! That might not be for everyone, but I enjoy it.

-Dave Pooley

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