Megan's Blog

Data tsunami

While radio telescope arrays make some of the highest resolution maps of the sky, one thing they are not generally very good at is making images that cover large areas. But new telescopes that are planned or currently under construction (such as the Australian SKA Pathfinder, ASKAP, being built here in Western Australia) are going to make wide field images as standard. This is exciting stuff for radio astronomy as, for one thing, it allows surveys to be conducted much faster. If you can only look at very small patches of sky at a time, it's going to take you a long time to cover the whole sky, but if you can widen your field of view then you can survey the same total area in a much shorter time (it's kinda like doing a jigsaw - the smaller the pieces, the longer it takes). This means that with new telescopes we will be able to do both more sensitive surveys (staring at patches of sky for longer to detect fainter sources), and more efficient searches for transient sources such as radio supernovae and gamma ray bursts (and other things we probably don't even know about yet).

It is possible to produce wide field images from existing radio arrays, but it's not exactly "normal operating procedure". With any kind of telescope, the resolution is determined by the wavelength you're observing at and the diameter of your telescope. For an interferometer, the diameter used is the largest separation between your antennas, which can be several thousand kilometres for large arrays. This makes them capable of very high resolution images but, due (amongst other things) to averaging effects, the standard data outputs from a correlator are only usable over a relatively small field of view around the pointing centre, the point on the sky your telescopes were aimed directly at during the observation. In principle though, the observations are sensitive to sources over a much larger area, it's just that using normal methods we can't make reliable images far from the pointing centre.

Well, earlier this month we observed two fields in the nearby Andromeda galaxy (M31), one using the US-based Very Long Baseline Array, the other using the European VLBI Network. The data are on the way to us, and when we get our hands on them we are going to have some fun. Instead of just using the correlated data around the pointing centre, we are going to use a software correlator, running on a computer cluster here, to re-correlate the baseband (raw) data wherever we like over the area the observations are sensitive to, with the ultimate goal of imaging the entire primary beam of the array. This means we will be able to make reliable images of sources across the primary beam of the array, essentially the area covered by the beam size of the individual telescopes in the array - in the case of the VLBA observation, we will be able to make images of sources with a resolution of 5 mas (1 mas = 1/1000 arcseconds = 0.0000003 degrees) over an area about half a degree in diameter.

Think about that for a moment. Half a degree - that's the size of the full Moon. If we were looking at the Moon, this is equivalent of being able to resolve features less than nine metres in size. We'd almost be able to spot the lunar landers! Imagine mapping the surface of the Moon at a resolution of nine metres. That's a lot of pixels.

So what are we hoping to see in our terabytes of data? Lots! Supernova remnants, star forming regions, planetary nebulae, and heaps of background quasars... may be something unexpected. This is going to be sooo cool!

In the news this month: a drop in eta Carinae's stellar wind

Supernovae are the violent explosions of massive stars, so bright that the events can be seen in distant galaxies. But not all apparent explosions are genuine supernovae. Some fall into the category of supernova impostors, the sudden increase in brightness of a star without the terminal explosion. One such impostor event was the great eruption of eta Carinae, a star which is amongst the most massive known in the Milky Way. Located 7,500 light years away in the constellation of Carina, the star is five million times more luminous than the Sun and an estimated 100 times as massive. Eta Carinae underwent a massive but non-terminal explosion 150 years ago, allowing the close-up study of a supernova impostor. During the eruption, the star lost about ten percent of its mass, throwing off the outer layers in the surrounding nebula. Since then, the star has been enshrouded in a thick cloud of dusty debris and has been losing material at the rate of one Jupiter-mass per year in a strong stellar wind.

Now, a team of researchers, led by Andrea Mehner at the University of Minnesota, have observed dramatic changes in the star's spectrum. Observations over the last decade have shown an increase in the star's magnitude, but with no major long-term changes in its spectrum, something that might be expected following an event causing a major change in brightness. New observations carried out with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope by Mehner's team show a substantial change in the emission lines, caused by specific elements in the star's atmosphere. According to the team, the sudden rapid decrease in the brightness of the emission lines (dropping to a third of their original strength in just ten years) suggests a decrease in the strength of the stellar wind, possibly signifying a much more rapid return to the pre-explosion state than was previously anticipated. With the wind density decreasing, the nebula should begin to thin and the star itself may become visible to modern telescopes for the first time, possibly within the next decade.

There are other explanations which may account for the unusual spectroscopic developments, including a change in the latitude dependence of the wind, but the complicated nature of the surrounding nebula and the difficulties constructing accurate models make an accurate assessment problematic.

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This blog post is a news story from the Jodcast, aired in the July 2010 edition.

Mehner, A., Davidson, K., Humphreys, R., Martin, J., Ishibashi, K., Ferland, G., & Walborn, N. (2010). A SEA CHANGE IN ETA CARINAEThe Astrophysical Journal, 717 (1) DOI: 10.1088/2041-8205/717/1/L22

The awesomeness of Astronomy.FM

I finally found time to test the updated version of Marzipan, but I can't get it to work yet. So, having rolled back to the old version, here's what I was going to post anyway:

Occasionally you come across something that just makes you stop and say "wow, that is a really neat idea". Not much on the internet gets that reaction out of me, but occasionally I'm surprised. One such gem is Astronomy.FM, an online radio station mainly devoted to astronomy but with plenty of other juicy bits of science thrown in for good measure.

Much of the content is made up of various astronomy and science podcasts such as the 365 Days of Astronomy, Naked Science, Slacker Astronomy, the Jodcast, and so on, but there are a number of original programmes as well, usually transmitted live and then replayed later the same day for listeners in other time zones. The schedule is currently arranged in four-hour programme blocks which repeat throughout the day.

I first came across AFM during 2009 when I took part in a live on-air discussion with Adrian West, Elias Jordan and Michael Foerster during #moonwatch. I'm pretty sure I'd heard about it before that, but it's the first time I had any actual involvement. Earlier this year Michael asked if the Jodcast could be included in their schedule, and it has been airing weekly on AFM for a few months now. In May I was a guest on Event Horizon, one of AFM's original shows broadcast live at 0200 GMT on Saturdays, and have popped up in the chat room regularly ever since. There's even talk of having a live show from down under, but that's a story for another day.

Anyway, there's heaps of interesting content and some entertaining hosts. It's all run by volunteers, and some of them put in a seriously amazing amount of their own time and effort to keep it running. So, next time it's a cloudy night and you've run out of Patrick Moore books to read, give it a try.

Spam spam spam spam

Apologies to anyone (human) who has made a genuine comment here recently, I may well have accidentally deleted it while trying to clean up the torrent of comment spam that has been plaguing this site for quite a while now. A lot of it is coming from IP addresses in China, and contains some very child-unfriendly text and links. Up to now, I've been deleting it from the comments file every few days, but quite frankly it's getting ridiculous and I've had enough. If I had the time I'd try and shoe-horn reCAPTCHA or something similar into the code, but I haven't what with everything else that's going on. So, as it's unlikely to stop, I may have to find another solution. That may involve switching to different software, but it might just end up with me disabling comments altogether, possibly just closing the blog down completely.

If you've got any suggestions, please do let me know.

In the news this month... and finally: Trans-Tasman VLBI

Astronomers have connected up the largest ever array of radio telescopes in the Southern hemisphere and made the highest resolution image of the core of the nearby active galaxy Centaurus A. The project linked up new telescopes in New Zealand and in mid-west Western Australia, with the existing long baseline array, including the Parkes radio telescope in New South Wales, to form an array more than 5,500 kilometres across, the first time telescopes have have been connected over such large distances in the southern hemisphere.

At 14 million light years from Earth, Centaurus A is the nearest example of a galaxy containing an active black hole at its core. Observations show two enormous jets moving out from the core at close to the speed of light, but probing the physics of the core itself requires very high resolution observations, only possible by linking up radio telescopes over many thousands of kilometres.

The new telescope at Warkworth is the first research-quality radio telescope in New Zealand, while the new antenna in Western Australia is the first of many that will make up the Australia Square Kilometre Array Pathfinder. Using the same technique, the Square Kilometre Array will consist of radio telescopes spread out over many thousands of kilometres and will be located either in Australia or Southern Africa.

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This blog post is a news story from the Jodcast, aired in the June 2010 edition.