X-ray astronomy : The instruments behind the science

If you follow science news regularly, you will come across news of the kind "Astronomers find brightest object of the early universe" or "Black hole-powered jets plow into galaxy". I would just like to point out that when they mean "brightest" object, they don't mean that it's the brightest object in the visible part of the electromagnetic (EM) spectrum. They mean that it's the brightest object in terms of energy emitted across the EM spectrum i.e X-ray, UV, visible, IR and microwave radiation all put together. (And just to throw in a bit of jargon, an object's brightness is quantified in terms of it's luminosity and the luminosity of an object measured across the EM spectrum is referred to as the "Bolometric" luminosity.)

Coming to the specific cases mentioned above, the quasars are in fact brightest in the X-ray, UV regime of the EM spectrum and astronomers use X-ray telescopes to look at them! At this point, I would like to point out the fact that X-rays are notorious for being able to pass through anything and that lead is one of the few materials which can prevent them from propagating further. Heck, all of us have come across X-rays at some point or the other in our personal lives when we fractured a bone and had to go to the radiologist to get an X-ray! So, I ask, how can one study X-ray emissions, something that's notorious for it's ability to penetrate anything and everything?

Well, a trivial answer that should've come to your mind is the use of photo-sensitive plates, like in the case of radiology. A photo-sensitive plate, reactive to X-rays, can be placed at focus of a telescope and will be able to picture the astronomical object of interest. While we are making progress, it should be noted that when we use photo-sensitive plates, we are losing information about the energy of the X-rays. Above a certain limit, the photo-sensitive plate will absorb all X-ray photons and therefore we are losing information about the emission spectrum of the astronomical object!

After CCDs were developed, photo-sensitive films got replaced by CCDs, which had better spatial resolution. Then again, do not assume that these CCDs are similar to those in normal cameras. If our cameras were exposed to X-ray radiation, the radiation would pass right through them without being detected and all we would see is white noise. Therefore, for astronomical purposes, scientists came up with CCDs made of wide bandgap semiconductors, which can absorb X-ray radiation.

X-ray emissions can also be studied using proportional counters (PCs). PCs are similar to Geiger counters, which most of us have heard of. Geiger counters are used to detect the presence of harmful gamma radiation in nuclear contaminated regions. Similarly, PCs are able to detect X-ray radiation. PCs are different from CCDs in the sense that they can measure the exact energy of the radiation but they do not possess the spatial resolution that CCDs do.

There you go, two ways in which X-ray detectors work. Now, before we get to the detector, we need a telescope to gather light from the astronomical object and focus it on the detector, right? Again, we run into the same problem as the one we ran into earlier. In telescopes used to study visible and IR radiation, astronomers use silver coated mirrors, mostly made of glass. If we use such spherical or parabolic mirrors, the X-ray radiation will just pass through it instead of being focused at the detector. On the other hand, for those of us who have performed X-ray experiments in our undergraduate physics labs, we know that X-rays can be reflected if they are incident very close to grazing angle. Therefore, we can nest multiple grazing angle reflectors, one inside another, to focus X-ray radiation onto the detectors of interest.

Astronomical instrumentation is an amazing field and in my opinion, astronomy is as much about the science as it is about the engineering. Rarely does the instrument behind the science get as popular as the science itself. On that note, I'll stop.

References :
[1] https://en.wikipedia.org/wiki/X-ray_astronomy_detector
[2] http://imagine.gsfc.nasa.gov/science/toolbox/xray_detectors1.html
[3] http://chandra.harvard.edu/xray_astro/history.html

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