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A Brief Introduction to X-Ray Astronomy

By raaymoose in Technology
Sun Mar 17, 2002 at 07:04:33 AM EST
Tags: Science (all tags)

As one may guess, x-ray astronomy is the branch of astronomy that deals with high-energy photons, or x-rays. While it tends to overlap somewhat with gamma ray astronomy, and consequently is called high energy astronomy or something analogous, our current technology is quite limited as to what can be seen in gamma, while technologies that allow us to observe the x-ray spectrum are starting to mature and are offering us glimpses into some of the more exotic phenomena in the universe - black holes, neutron stars and pulsars, quasars, star clusters, galaxy clusters, supernovae and remnants, galactic centres and much more.


X-rays are generally emitted by very high temperature gas (plasma), and in the astronomical context are in this state due to extreme gravity, magnetic fields, or explosive forces. X-Rays were first discovered by Wilhelm Conrad Röntgen, a German scientist who stumbled upon them while experimenting with vacuum tubes in the mid 1800s. He named them "X"-rays because it was an unknown radiation type at the time and the name has stuck with them, although occasionally they're referred to as Röntgen rays. X-Rays are a form of electromagnetic radiation with very high frequency (~10^17 to ~10^21 Hz) and short wavelength (~10^-8 to ~10^-12 m).

X-ray astronomy is a relatively young branch of astronomy, due to the fact that the Earth's atmosphere is effectively opaque to wavelengths at x-ray frequency. Because of this, it requires high-altitude balloons or orbiting observatories to collect enough x-ray photons to be of any use. For comparison, trillions of photons in the visible frequency reach a detector of an optical telescope on Earth's surface every second, while it requires hours, or even days for even one x-ray photon to be observed by a detector on the Earth's surface.

Another consequence of the high energy x-ray photons is that the photons cannot be easily reflected and focused. This makes it impossible to resolve x-ray images using conventional telescope designs, however, x-rays can be deflected off a surface if the angle of incidence is very small. This grazing incidence property has lead to designs that involve complex nests of mirrors designed to gently coax the x-ray photons into focus. Previous to this type of design, x-ray observing was limited to detectors that were not much more complex than putting a geiger counter at the end of a tube and forming crude 'images' by counting the number of x-rays detected coming from the part of the sky it was aimed at.

Past X-Ray Observatories:
There has been a steady precession of x-ray observatories being launched since the 1970s by agencies like NASA and the ESA. A list of these, their technical specifications and mission objectives can be found at theHigh Energy Astrophysics Science Archive Research Centre and European Space Agency Missions (look for X-ray). These past probes have been invaluable in our current understanding of the forementioned cosmic phenomena, but we'll focus on the two important current missions.

Present X-Ray Observatories:
The High-Throughput X-Ray Spectroscopy Mission. This is a large x-ray observatory put together by the European Space Agency launched in 1999. XMM-Newton is the most sensitive x-ray telescope that has ever been in operation, it utilises 15 EPIC CCDs capable of detecting tiny fluctuations of energy in the 0.5 to 10 keV band and resolving an image in between 40 and 80 milliseconds. The angular resolution isn't as fine, but comes in at a respectable 5 arcseconds (5/3600ths of a degree). This observatory also has the ability to monitor the UV spectrum simultaneously with x-ray spectrum and is expected to be operational until 2009.

The Advanced X-Ray Astrophysics Facility, renamed Chandra in honour of the Indian astrophysicist Subramanyan Chandrasekhar. This observatory was also launched in 1999 and has the finest angular resolution of any x-ray observatory ever operational at 0.5 arcseconds (10x better than XMM-Newton). Although it has a greater angular resolution than XMM, Chandra is not as sensitive.

Working together, XMM-Newton with its extreme sensitivity, and Chandra with its fine angular resolution are returning data with higher spatial and spectral resolutions, allowing us to produce more precise images (1, 2) and spectra of range of high-energy cosmic objects than ever before.

The Future:
There is a myriad of proposed x-ray missions for the future, including Constellation-X, which would involve a network of multiple spacecraft with moderate resolution x-ray optics, the NGST, SWIFT, and XEUS. One of the more fascinating future missions is MAXIM (Micro-Arcsecond X-Ray Imaging Mission) which promises angular resolutions of an astounding 10^-7 arcseconds - 300000 times finer than the Hubble space telescope inferometry, and 20 million times finer than Chandra's resolution. This would potentially allow us to resolve the event horizon of supermassive black holes, the equivalent of resolving an object the size of a dinner plate on the surface of our Sun.

X-Ray astronomy is just starting to enter its prime. Challenging our previous notions of the behavior of accretion discs around black holes and other exotic objects and giving us insights into a multitude of high-energy events, it is a great subject for anyone interested in astrophysics, astronomy, or just big explosions.


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Related Links
o black holes
o pulsars
o quasars
o star clusters
o galaxy clusters
o supernovae
o remnants
o galactic centres
o mirrors
o High Energy Astrophysics Science Archive Research Centre
o European Space Agency Missions
o Present X-Ray Observatories:
o XMM-Newton :
o AXAF/Chand ra:
o Subramanya n Chandrasekhar
o 1
o 2
o Constellat ion-X
o Also by raaymoose

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A Brief Introduction to X-Ray Astronomy | 17 comments (5 topical, 12 editorial, 0 hidden)
Great Article (4.33 / 3) (#3)
by whojgalt on Sun Mar 17, 2002 at 02:16:06 AM EST

And it looks like it is heading for FP. Well done.

One of my favorite daily websites is NASA's Astronomy Picture of the Day, which is a great sampling of astronomy pictures on a variety of subjects, including many X-Ray pictures

If you can't see it from the car, it's not really scenery.
Any code more than six months old was written by an idiot.

10^-7 arcsec!!!! (none / 0) (#12)
by xriso on Sun Mar 17, 2002 at 03:25:41 AM EST

Yay for pretty pictures!
*** Quits: xriso:#kuro5hin (Forever)
Wow (none / 0) (#14)
by zocky on Sun Mar 17, 2002 at 01:49:59 PM EST

If I'm not mistaken,

4*10^13 km / 1.5*10^8 km = 266,666; meaning that proxima is roughly 270,000 times further from earth than sun. If a plate is sized, say, 0.2 m, that means that one pixel on proxima would be 54 km.

Means that a planet of earth size orbiting proxima would have a radius of roughly 230 pixels?

Wow... gotta go look at that site. And I have to resize an image of earth to 230 pixels to see what it looks like. I'm sure it looks great.


I mean, if coal can be converted to energy, then couldn't diamonds?

Coming to think of it (none / 0) (#15)
by zocky on Sun Mar 17, 2002 at 01:55:18 PM EST

Actually, planets don't give up much in X-rays, so this won't really help us see the planets that well.

Do you think that we will in foreseeable future see visible light pictures of planets in other stellar systems with, say, 230 pixel radius?


I mean, if coal can be converted to energy, then couldn't diamonds?
[ Parent ]

Yes (none / 0) (#17)
by Rand Race on Mon Mar 18, 2002 at 12:47:40 PM EST

Yea, I think we'll see it someday with larger space-borne telescopes than we have now, although I doubt Proxima is the best candidate due to it's stepchild nature in the Alpha Centauri trinary system. This page (at the bottom) has some info on methods of detecting earthlike planets through both direct imaging and spectography; although they contend infra-red imaging would be best. At the moment, as far as I know, an image of Betelgeuse is the best visible light imagery we can do and Betelgeuse is freakin' huge (450-550 million km diameter or about .1 arcsecond as seen from earth).

"Question with boldness even the existence of God; because if there be one, He must approve the homage of Reason rather than that of blindfolded Fear." - Thomas Jefferson
[ Parent ]

A Brief Introduction to X-Ray Astronomy | 17 comments (5 topical, 12 editorial, 0 hidden)
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