Thursday, November 08, 2007

Comets, X-rays, and Pumpkins



I've been rather busy lately writing a NSF grant proposal, so I haven't written any posts for a while. Despite the time-crunch, I feel like taking a few minutes to briefly mention Comet Holmes which I managed to see with the naked eye (even from light-polluted Baltimore suburbia) last week and the week before.

As everyone is probably now aware, Comet 17P/Holmes is currently visible to the naked eye (to find it check out this S&T article, or the easier-to-use directions from SPACE.com), after undergoing a dramatic and unexpected increase in luminosity believed to be associated with some form of explosive out-gassing. Note is only through binoculars or a telescope that the truly diffuse nature of the comet is apparent, but even with small binoculars it was pretty impressive. Far more impressive to my mind than Halley was in 1986, so if you haven't seen it yet please go look for it.

Indeed, this one of rare astronomical phenomena that is more impressive to see yourself than when viewed as a picture taken with a big telescope (Astronomy Picture of the Day has a whole series of Comet Holmes images: 1, 2, 3, 4, 5).

I have a soft spot for comets, I worked very briefly (for a few hours) on trying to explain the X-ray emission from comets. The material evaporating off a comet is very cold (only about T~50 K), so it was a great surprise when X-ray emission was discovered coming from Comet Hyakatuke in 1996 using the ROSAT X-ray telescope (see e.g. Glanz, 1996, Science, 272, 194). Many explanations were advanced at the time, the vast majority of which did not work out. As the Solar wind has a velocity of several hundred km/s, one hypothesis was that a shock wave caused by the interaction of the Solar wind with the cometary halo caused X-ray emission by thermal bremsstrahlung. Ian Stevens, my thesis advisor at the time, performed hydrodynamical simulations of this, and my job was to take the simulations and calculate the expected X-ray luminosity. Which turned out to be orders of magnitude less than the observed emission, hence disproving the hypothesis. We didn't even bother considering to publish the results.

The real explanation for the cometary X-ray emission turned out to be charge exchange with the Solar wind. The material in the Solar wind is highly ionized, while the material out-gassed from the comet is largely neutral. A highly ionized ion interacts with a neutral atom, basically stealing one or more electron from the neutral atom. The formerly neutral atom is now ionized, and is left in an excited state. This excited state decays to a ground state by the emission of one or more photons. Dennis Bodewits PhD thesis "Cometary X-rays : solar wind charge exchange in cometary atmospheres" (2007, The University of Groningen) deals with many aspects of X-ray emission from comets, and is available chapter by chapter in PDF form.

Ironically solar wind charge exchange (SWCX) has now been recognized as a process than almost all X-ray astronomers must worry about (see Snowden et al 2004, ApJ, 610, 1182), even those like me who study distant galaxies. SWCX is now recognized as a major contributor to the soft X-ray background that affects all X-ray observations, and which makes observing faint diffuse X-ray emission difficult. Worse still, the SWCX can be time variable, further complicating background estimation and removal.


In acknowledgement of this link my Halloween pumpkin this year was a comet, which looked quite good until our local deer ate it.

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