Lots of Stuff

As noted by the title, lots of interesting stuff (at least to me) has been slamming down from the heavens recently. First we have the Mercury Messenger finally making its orbital insertion. This marks the first time we have ever placed a satellite in orbit around Mercury. Quite a feat of astronautical engineering and celestial navigation.

It's actually much harder than it seems to send a satellite to the inner Solar System. You have to expend quite a bit of energy to do so, as you have to match the orbital velocity of Mercury while lowering yourself into the potential well of the Sun. Very tricky stuff. For reference take a look at this video of the orbit that Messenger followed to get to Mercury:

http://www.youtube.com/watch?v=otF2FjpCyZk

The first images of Mercury with Messenger in orbit have been released and were today's Astronomy Picture of the Day (APOD):

http://apod.nasa.gov/apod/ap110331.html

Next we have this cool video of the Earth as it goes through its orbit around the Sun. You can watch as the seasons change on the Earth due to the tilt of the Earth relative to the Sun. The video takes out the rotation of the Earth in order to make it easier to see.

Finally we have something slightly more esoteric. The Fermi gamma-ray satellite has released its detection of RX J1713.7-3946. That may not mean much to most people but its a fairly big deal in my field. RX J1713 is the name (well not so much name as catalogue number and celestial position) of a supernova remnant (SNR). This SNR (also known as G347.3-0.5) is thought to be the remnant of a supernova that went of in the constellation of Scorpius in AD 393. It's actually pretty close to us at only 1 kiloparsec away towards the interior of the galaxy.

What makes RX J1713 so interesting is that it was the first SNR detected using TeV gamma-ray telescopes, specifically the HESS telescope. In order for an SNR to produce gamma-rays of that high energy, and these are extremely high energy gamma-rays, there has to be extremely high energy particles generating the radiation. We are talking 10-100 TeV particles. For a sense of scale the rest mass energy of the electron is 511 keV and proton is 938 MeV. This means that if these particles are electrons they have up to 100,000,000 times the energy of a electron just sitting at rest. That means the electron is moving at 99.99999999% the speed of light. Crazy fast.

However, its not the fact that particles were being accelerated up to those energies. Astrophysicists had expected that for years and this confirmed that suspicion. No, what made this object interesting was that we had an opportunity to settle a long standing issue in cosmic ray astrophysics. Astrophysicists knew for a long time that electrons were accelerated up to extremely high energies at SNRs due to the observed synchrotron light seen in the radio and X-ray.

However, we didn't know whether protons were accelerated at these remnants as well. Theorists, including myself, have long expected that SNRs would be efficient accelerators of galactic cosmic rays. To confirm this we would need to see emission that could be unequivocally tied to protons and not electrons. Unfortunately relativistic protons are much harder to detect via photon radiation than electrons. The only mechanism that produces large amounts of radiation is when two protons slam into each other producing neutral pions which then decay into gamma-rays. Thus if we saw gamma-ray emission and it had the right spectrum for this interaction we could say that we have detected proton acceleration at a SNR.

This was hoped to be the case for RX J1713. TeV emission though is not enough to construct the spectrum as the distinguishing features for a spectrum due to protons happen at MeV-GeV energies. This is where Fermi comes in. It could look at this energy range and confirm whether or not the TeV emission was due to pion decay from proton collisions or if it was inverse Compton (IC) which comes from relativistic electrons upscattering ambient photons.

Well the results are in for RX J1713 and sadly it looks like the gamma-ray emission spectrum is IC, not pion decays. Disappointing to us theorists as we were pretty convinced it would show us that protons were being accelerated at this remnant. Note that this doesn't disprove that protons are accelerated at SNR's. It could just be that in this particular case electrons are dominating the radiation output with the protons being sub-dominant. There are also many other SNR's that have the potential to provide us with evidence for proton acceleration. Several other SNR's have already shown promise from their Fermi spectrum, though none is unambiguous yet. Anyways for those interested in the article:

http://arxiv.org/abs/1103.5727

BTW, congratulations if you made it this far through this post. I know its been long but hopefully its been interesting. :)