One Astronomer's Noise

We’re heading back from an awesome time out here in Boulder for the URSI Radio Sciences meeting, and wow there was a lot more great stuff! It wasn’t too overwhelming after three days, and has left me with all kinds of motivation for research when I get back. Or at least, that’s the plan.

I’m going to jump a little out of order here, since I’m still thrilled by the last session. This was all about VLBI, which as I mention time and time again, I love. Very Long Baseline Interferometry is the method by which multiple radio telescopes are linked over large distances to act as one telescope, specifically an interferometer, that can be as big as the Earth itself. Or bigger, as we saw today.

First up was Dayton Jones from NASA’s Jet Propulsion Laboratory with VLBA observations of, not an astronomical object, but the Cassini spacecraft! Cassini (if you haven’t been gawking at all the fabulous Saturn pictures like you should) is a mission that has been studying the Saturnian system (planet and rings and moons… oh my!) But why look at a spacecraft? Because the VLBA is so large, and thus has spectacularly great angular resolution, it can measure positions on the sky very, very accurately. Cassini gives off radio waves to communicate with Earth, the VLBA can see Cassini as a point source on the sky and measure its position extremely well. Since the distance from Cassini to Saturn is known with incredible accuracy for the mission plan, they can then measure the position of Saturn with high accuracy. The goal is to understand its position and orbit good to 4 kilometers. We’re talking about a planet that is over 120,000 kilometers in diameter!

Okay, so, that just seems unnecessary. Who cares if we know the position of such a massive planet THAT accurately? Well, it turns out that an accurate ephemeris (position table) of Saturn is useful for solar system tests of general relativity, predicting future occultations and eclipses, and navigation of spacecraft. And, although the ephemerides of the inner planets are very well known, most likely due to accurate radar measurements in part, this is the first time that such precision is possible for an outer planet. Current measurements with the VLBA are showing positions good to 0.5 milli-arcseconds (0.00000014 of a degree!) on the sky, and they hope to extend the project through the rest of Cassini’s mission. If Cassini lasts til 2012, then they will have tracked Saturn for one quarter of its orbit, providing excellent data for the ephemeris.

Gratuitous picture of Saturn from Cassini. Also my iPod Touch background.

The next talk was on the ground-breaking work done with millimeter-wave VLBI and was given by Shep Doelman of MIT Haystack. I started writing about this before, and also should have written up what I learned when I presented on this work in journal club. So check out the earlier link, which includes an overview of VLBI and a basic introduction to the monster black hole at the center of our Milky Way galaxy. If there was any doubt that such a thing existed, it was wiped away by the observations of stellar orbits just 45 AU from this dark object, the fact that Sgr A* itself does not move, and the x-ray and infrared variability of the object on the order of hours. The new VLBI research resolved the radio emission to just 4 times the size of the black hole (or 4 Schwarzschild radii). However, due to relativistic effects next to such a dense object, the size of the black hole as seen from Earth was expected to be larger. So, radio astronomers are just seeing some part of the emission in the disk around the black hole, possibly just the approaching side of the disk. It is not yet known whether it would look like a point source or a donut, given this first bit of data. However, constraints on the black hole and disk can be placed already, such as that it cannot be face-on. With current and upcoming millimeter wave telescopes (ex: ALMA), astronomers will be able to explore why our black hole is so underluminous, whether it is spinning and how fast, how well general relativity holds up in such a strong-field regime. The observational challenges are extremely tough, but the science payoff is worth it! With clever analysis of the data (which cannot yet provide an image) much can be learned from the Milky Way’s 4 million solar mass black hole.

The next few talks were on VLBI techniques. Jon Romney of the NRAO gave an overview of the major sensitivity upgrade that is happening at the VLBA. The telescopes have already gotten new, more sensitive receivers at 22 GHz. This band is important for measuring lines from water vapor. Water masers (like lasers, but with microwaves!) are important for directly measuring the distances to other galaxies and helping to determine the nature of dark energy. (Another story for another time… sorry!) Also, the data rate and bandwidth of the array will be expanded, making it easier to see fainter and fainter objects. As Steve Duran, also of the NRAO, explained, how the data is sampled makes a big difference to the telescope performance. At the end of the session, Adam Deller, now of the NRAO and also affiliated with PAPER (soon anyway!) talked about his software correlator for the VLBA and other VLBI instruments. What is a correlator? That’s the huge computer at the back end that takes the data from all of the antennas and combines it in a way that the astronomers can calibrate and image. Usually these live in HUGE racks with LOTS of wiring, but more modern correlators are being built on programmable hardware which is more flexible. This correlator, however, is written in a high level programming language, like C++, and runs on a relatively off-the-shelf multi-core computer for a much, much lower cost than “traditional” correlators. It’s the radio astronomy of the future!

VLBA, rocking your world since 1993. Image courtesy NRAO/AUI.

The second to last talk, not to forget it, was by Hideyuki Kobayashi of the National Astronomical Observatory in Japan. They have a four-antenna VLBI array there, which is soon to be linked up with a Korean facility. Plans are underway to build a large array that even extends far into China. Interestingly, Japanese astronomers are also planning to launch a satellite radio telescope known as ASTRO-G (although the name VSOP-2 is sometimes cited). This makes a radio telescope much larger than the Earth! This is a follow-up of a previous, similar project, called VSOP. However, I don’t know much about it, now have I seen too much astronomical data that actually used it in my short time doing VLBI as an undergrad.

That’s all for now. I’ll need to sort through my notes (and everything else going on) in the next week or so. It was a great meeting and I’m glad to have had the chance to listen to all these great talks. I’m incredibly honored to have been among them and present our work on PAPER. Now it’s back to work and getting research done and getting this thesis rolling!