AAS Reports: The Allen Telescope Array

The first official day of the conference, my collaborator and I spent quite a bit of time hanging out at the poster group for the Allen Telescope Array. Although they work at a different frequency regime than we do (check out my PAPER episode at 365 Days of Astronomy for more about our project!), we do share some personnel and technology and, as it turns out, science goals.

The ATA is a centimeter-band array of 42 six-meter dishes with the central goal of pushing the Search for Extraterrestrial Intelligence further out into the galaxy. However, SETI is not the only goal for this array, as it is poised to become a general survey instrument and do all kinds of astrophysics along the way.

The SETI search has begun with a Galactic Center Survey. This survey looks for very narrowband signals, as one might expect to come from a non-natural source. Of course, there are intelligent beings emitting narrowband signals right from our own planet, so the initial survey picked up 421,432 signals! Scientists keep track of man-made radio frequency interference (called RFI) and can avoid obvious, known signals, dropping the count to 10,060 candidates. These move on to the second phase of processing. The ATA uses two beams on the sky (that is, it looks at two places at once.) If the signal is seen in both beams, then it must be local. This knocks out about 75% of these candidates. Further schemes involve immediate re-observation of candidates and a finer search for known, man-made signals. As you can expect, since we haven’t heard about it on the news, these methods knocked out all the remaining signals, so ET has not yet been found. But the survey has just begun!

ET signals are not the only time-dependent, or transient, signals of interest to astronomers. While naturally occurring signals are rejected from ET searches, they are scooped up for scientific study. Slow radio transients are sources that occasionally burst with emission, not as often or regularly as a normal pulsar (a rotating neutron star with “hotspots”). These were first detected serendipitously, but more systematic searches are underway as radio surveys become more standard. These sources can be intermittent pulsars, X-ray binaries, and other interesting astrophysical phenomena. Scientists at the ATA have already overcome a number of calibration hurdles in order to begin their science observations. (Just getting the data is only the first step… a lot needs to happen before the data can tell us anything interesting!)

The ATA can also be used to probe the Galactic magnetic field. Measuring magnetic fields is notoriously difficult in astronomy, as we can’t directly go out and probe various regions. One important tool is the polarization (or orientation of the EM fields) of radio emission. Quite roughly, as polarized light passes through a magnetic field, it is “spun around” or re-oriented, based on the frequency of the light in an effect called Faraday Rotation. By measuring the polarization of a source at multiple frequencies, information about the magnetic field can be extracted. A recent study using this technique with the Very Large Array significantly constrained models of our Galaxy’s magnetic field, and produced a very cool rotation measure map.

Taylor et al. 2009, ApJ, 702, 1230. The Galactic Plane goes through the center of the projection.

The astronomers at the ATA have been able to confirm some of these results and plan to extend them with the telescope’s multi-frequency capability. My collaborator, Danny, was especially excited about this since PAPER will have to deal with polarized emission from the Galaxy and beyond as well!

There were a bunch more posters in the ATA section, so be sure to browse their abstracts and the official websites at the SETI Institute and Berkeley’s Radio Astronomy Laboratory.

Related: AAS Wrap-up: Personal Perspective and the Big Stories

3 comments for “AAS Reports: The Allen Telescope Array

  1. Phil
    February 28, 2010 at 12:13

    “Microwave window” that comes through Earth’s atmosphere clearly, from about 0.5 to 11 gigahertz?

    Shouldn’t they be looking at lower freqs?
    Even here on Earth we know Microwaves don’t travel as far as compared to lower band freqs.

    • February 28, 2010 at 12:39

      Good question. On Earth, the EM waves have to travel through a medium and around obstacles, and these are not a factor in interstellar space. We can, after all, see microwave radiation from billions of light years away!

      I’ve heard the argument for searching at L-band since hydrogen, the most common element in the universe, emits a spectral line at 1.4 GHz. So a radio-savvy civilization that wanted to be heard would transmit at that frequency, knowing that others are already using that band for studying astrophysics. (On Earth, there’s actually a small, protected band for radio astronomy around that line.)

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