radio astronomy

Something’s cooking

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As it sometimes happen in astronomy, very peculiar signals that could potentially represent a groundbreaking discovery turns out to come not from the far reaches of space, but from a much more local source, like the case of the potassium flaring stars. These incidents illustrate the rigorous testing and investigations that occurs when unexpected signals are present in the data, before a solid discovery can be claimed.

Another example of such a story involves the Parkes Observatory Radio Telescope and the search for fast radio bursts. Fast radio bursts are high-energy astrophysical signals lasting only a few milliseconds, and they are believed to originate from outside of the Milky Way. They were first observed in 2001 and to date only around 20 of these bursts have been detected. Their origin is still a mystery, but astronomers have proposed merging black holes, merging neuron stars, flaring magnetars or collapsing pulsars as the source of these bursts, but these explanations are all somewhat speculative. It is naturally of great interest to discover more of these bursts, to hopefully learn more about their origin.

When observing at radio frequencies, especially when looking for something as rare as fast radio bursts, it is important to rule out any kind of radio interference originating from Earth that could potentially mimic the astrophysical signal. A certain type of interference observed at Parkes has been labelled “perytons” after the mythological Peryton creature. They are short-lived burst of radiation at around 1.4GHz, with a shape that is somewhat similar to a bona fide fast radio burst signal.

While it was pretty clear to the astronomers that the perytons arose from some source on Earth (they were observed over a large field of view), it was not evident what was causing them. The radio astronomers at Parkes Observatory began a meticulous investigation to pin this down. One hint to the source was that they only appeared when the telescope was pointed in certain directions (towards the Visitors Center and the staff building). Furthermore, they mostly appeared between 9am and 6pm, clustering just around lunchtime.

The hunt then began for equipment localized in those two buildings that might emit signals in the right frequency range. Immediately, microwave ovens were suspected as the source, as the magnetron inside these operates at 2.4GHz. This is not too far from the 1.4GHz perytons detected with the radio dish. Several tests were performed, to see if there were malfunctioning microwave ovens that would emit at the right frequency. This initial effort did not produce any perytons, but by further experimentation it was found that if the door of the microwave was opened prematurely, a short 1.4GHz radio burst would escape from the oven. So something had definitely been cooking in the data. Take-away lesson: Save those microwave popcorn for when you’re not observing.

While this may seem like a silly investigation, it nevertheless underlines an important scientific principle, namely that when seeing a strange new signal, it is important to rule out sources of terrestrial origin, before a discovery can be claimed. This particular case also has the advantage that perytons have now been properly characterized and this can be used when trying to identify proper astrophysical signals. While peryton signals looks similar to fast radio bursts, they are not identical. Thus investigations like these helps astronomers in their search for more of these mysterious signals.

For the interested readers, a full paper describing the investigation of the perytons can be found here.

The world’s largest bug zapper

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The 305m diameter radio dish of the Arecibo Observatory in Puerto Rico. Photo credit: NAIC

The 305m diameter radio dish of the Arecibo Observatory in Puerto Rico. Photo credit: NAIC

There are big telescopes, and then there are the truly humongous telescopes, like some of the radio telescopes. These bad boys are so big that the largest of them takes up an entire valley. This is the well-known Arecibo Observatory in Puerto Rico, that a lot of people likely know from Golden Eye, X-files or Contact, to name a few times it has been used in popular culture.

The observatories are, of course, mainly used to do astronomical observations, and not as fancy movie sets. The planetary radar transmitter here, and at the Goldstone Deep Space Network site in California are used extensively to observe asteroids, the terrestrial planets, and the larger satellites of Jupiter and Saturn.  To do this, they run hundreds of kilowatts of UHF signal out through each telescope.  By the time the beam is distributed across the many thousands of square meters of the primary telescope reflector, it’s diluted to the point that it doesn’t pose a hazard to anything.  However, along the beam path from the transmitter feed to the tertiary and then to the secondary reflectors, it is significantly more concentrated. This means that every now and then, the telescopes turn into something very different from instruments for peacefully observing the Universe.

The Gregorian dome of the Arecibo Observatory. Photo credit: NAIC

The Gregorian dome of the Arecibo Observatory. Finding your way out is not as easy as it seems. Photo credit: NAIC

At Arecibo, the transmitters, receivers, tertiary, and secondary are all contained inside a Gregorian dome. Birds tend to fly in and get confused about how to exit again. As interesting as it may be to inspect the inside of the world’s largest radio telescope, this is not without risk! If the birds happen to be between the transmitter and the tertiary reflector when the transmitter goes on, they are very rapidly microwaved. The birds’ remains may then land on the tertiary, where they get cooked into char. They can be removed from the tertiary’s surface from the access platform by using sophisticated tools, like a large wad of sticky tape on the end of a stick.

At Goldstone, birds can fly out of the beam line more easily, since the transmitter is not contained within a dome. But on one occasion, a swarm of bees were in the beam when the radar started transmitting. The telescope briefly acted as the world’s most expensive bug zapper. The resulting cloud of steam and fried bees caused a dramatic back-reflection of the beam until it dispersed.

There are no reports (yet) of larger things being fried by any of these instruments, and, admittedly, it would take quite some work to get anything without wings to be in the right place. But you could host a rather impressive and efficient BBQ party there. Just be mindful of  where you are, once the beam goes off. We don’t want any accidents!

Thanks to Michael Busch for providing this anecdote.

One of the Goldstone Deep Space Network Antennas. Photo courtesy NASA/JPL-Caltech

One of the Goldstone Deep Space Network Antennas. Photo courtesy NASA/JPL-Caltech

Blame the animals, it wasn’t our fault!

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When doing radio interferometry observations, one needs to combine observations from a number of different telescopes, at different locations, to get good quality data. This of course means that there is a potential for a lot of different things going wrong, as one needs to deal with multiple sites. Some of the problems that may come up, are completely out of your hands. However, even if one is having problems with the observations for these reasons, as the good scientists we are, we investigate the problems in detail. This is to make sure that it was indeed out of our hands, and not just us overlooking some obvious step we could have taken to avoid the issue in the first place. A situation that occurs more frequently than one would imagine.  Such inspection can also allow us to at least put the blame for loss of observations in the right place. Whether this means it will not happen again, is a completely different story. A beautiful example of this is the following excerpt from a paper by N. Bartel and colleagues on radio observations of the galaxy M82:

“No data were taken at station D during the period 0830 to 1630 GST due to the presence of a red racer snake (Coluber constrictor) draped across the high-tension wires (33,000 V) serving the station. However, even though this snake, or rather a three-foot section of its remains, was caught in the act of causing an arc between the transmission lines, we do not consider it responsible for the loss of data. Rather we blame the incompetence of a red tailed hawk (Buteo borealis) who had apparently built a defective nest that fell off the top of the nearby transmission tower, casting her nestlings to the ground, along with their entire food reserve consisting of a pack rat, a kangaroo rat, and several snakes, with the exception of the above-mentioned snake who had a somewhat higher density. No comparable loss of data occurred at the other antenna sites.”

At the very least, that snake will not be interrupting the observations again in the future. If you are interested in the actual outcome of the observations, the paper can be read in full here.

Signal lost!

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When doing observations, it sometimes happens that things just won’t collaborate, one way or the other, making it impossible to get any good data. This is true for all areas of astronomy, where you can have clouds, high water vapor, light pollution, radio frequency interference or equipment malfunction of various sorts. Sometimes the reasons for not getting observations are tricky, but in other cases they are fairly obvious… Or so you would think.

Doing a pulsar survey one night at the Parkes Observatory, an undergrad student observer doesn’t seem to be getting any good data from the 64m radio telescope. In fact, he’s not even getting bad data, he’s getting no data at all. As much as it is annoying getting bad data, getting zero data is just immensely frustrating. The bad data would at least get you something that could potentially be useful. But having gone through all the planning and work associated with observations, for zero gain, is just painful.

After long hours that no doubt involved pulling out hairs and being frustrated, the relief, which happens to be the supervisor of the undergrad student, enters the observing tower at 4am. He’d apparently been keeping an eye out for things on the ground on the way there, as he had found a receiver lying on the ground outside. His immediate reaction was along the lines of “Huh, I wonder what that’s doing here? I better pick that up and bring it in with me.” The receiver was then brought to the control room, after which the supervisor continued to try and get some observations going, with very limited success.

At sunrise, one of the staff members arrive, to meet what was no doubt a very frustrated astronomer, who had not gotten any useful data at all that night. Upon seeing the receiver in the control room, he casually pointed out that there was a very nice receiver-shaped hole in the radio dish. In fact, it looked suspiciously like the receiver the supervisor had picked up during the night, would fit exactly there. Which of course provided an immediate explanation as to why no data was received. One can only wonder, why this had not come to mind earlier. I suppose it’s like when you are looking for your glasses while already wearing them. You just don’t see what’s right in front of you.

The Parkes Radio Telescope from the air Credit: Shaun Amy, CSIRO

The Parkes Radio Telescope from the air. Credit: Shaun Amy, CSIRO

Apparently, the receiver had recently been changed, and whoever was putting the receiver in, had not tightened the bolts sufficiently, so as the telescope was turning, the receiver had gotten loose and fallen to the ground. I suppose that the lesson here is: If you find a fairly essential piece of your instrument in a weird place, that might be a good place to start, when trying to figure out why nothing is working. In addition to that, some safety advice; Always wear a hard hat when observing. Even if in the middle of nowhere, with no tall things around, you never know what might hit you.