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Coincidentally, about those exploding stars. . .

October 24, 2011 Leave a comment

An exploding star can release a massive amount of energy, an event called a gamma ray burst, as shown in this National Science Foundation illustration. But it's hard sometimes to tell the different between a statistical blip in data from the real thing.

An exploding star can release a massive amount of energy, an event called a gamma ray burst, as shown in this National Science Foundation illustration. But it's hard sometimes to tell the difference between a statistical blip in data from the real thing.


Phil Evans, an X-ray astronomer in England and frequent guest blogger for Geeked On Goddard, sends us this report on the fascinating nature of coincidence in science.


I have the power to make stars explode!

No, seriously. True, I can’t draw my sword and turn miraculously into a muscle-bound hero, like He-Man, nor can I turn my pet cat (Tinkabell) into Battle Cat, He-Man’s ferocious feline familiar.

But I really can make explosions at the other end of the universe. Skeptical? Here is the proof:

Last year, NASA’s Swift satellite (data from which I use in my work) was going through a bit of a lean observing period, with no gamma ray bursts (GRBs) detected for some time. GRBs are vast releases of energy from collapsing or collidign stars.

So, just as my duty week began at the University of Leicester, I tweeted, “Wake up universe!”

In the next 24 hours, Swift snagged four GRBs. Coincidence?

The only other time that we have had that many bursts in one day was the day celebrated science fiction author Arthur C. Clarke died? Coincidence?

Well, actually — yes. The thing is that coincidences happen all of the time.

A couple of years ago on her BBC Radio show, Sarah Kennedy asked people to send in their “coincidence” stories. Countless people mailed in about times they’d gone around the world on holiday, and met someone from three streets away. The response was continually, “Wow! Isn’t that amazing?” when what the was program actually demonstrating was that these “unlikely” events actually happen regularly.

In fact, when people respond to these stories by saying, “Small world,” they’ve got it totally wrong! It’s because it’s a big world that these things happen. Imagine something that only affects 1 in a million people. Pretty unlikely? Well, it will affect something like 300 Americans, and 60 Brits!

image of possible gamma ray burst

Image of possible gamma ray burst, or statistical blip? (click to make me big)

Coincidences happen. And this can be a real pain for astronomers. I’ve got some data, there’s a cluster of pixels close together. Is it a faint source, or just a coincidence that some background light has clustered? (See image at right.) This spectrum shows a blip. Is it a real feature, or just noise?

Fortunately, using statistics we can at least quantify how likely things are. Typically in astronomy we would only claim we’d found a source, for example, if there was less than a 0.3% chance that it was just a “lucky” fluctuation in the background. Even this happens, well, 0.3% of the time!

For Swift, we have to be even more conservative. When the Burst Alert Telescope (BAT) thinks it’s found a GRB, there has to be only a 0.0000000000008% chance that it’s just a fluctuation in the background [for us to interpret the observation as “probably real.” This threshold was carefully determined to minimize the number of false alarms, without losing real (possible) GRBs.]

Despite this, we do get a few false alarms every year, because of the number of times and ways the BAT looks for GRBs. We tried a “subthreshold” test a couple of years ago, where we triggered on things which were more likely to be spurious, that is, there was a 0.00000000006% chance of them being a random change in the background. We expected, and got, about 2 false alarms a day.

Overall, I’d say we get maybe 5 false alarms a year — but about 100 real GRBs. And the false alarms we usually identify within 20 minutes or so, so they take very little of our time.

So, next time someone tells you something unusual that’s happened, and asks if it could be coincidence, the best answer is probably, “Yes!”

Check out Phil’s twitter feed: @swift_phil

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OH AND DID I MENTION? All opinions and opinionlike objects in this blog are mine alone and NOT those of NASA or Goddard Space Flight Center. And while we’re at it, links to websites posted on this blog do not imply endorsement of those websites by NASA.

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Swift Detects Most Distant Object In The Universe! AGAIN!

May 25, 2011 2 comments

Now where have we heard THAT news before? For aficionados of NASA’s Swift satellite, or even space science and astronomy in general, this headline probably rings a few bells. Like this one for example, announced on April 28, 2009:

New Gamma-Ray Burst Smashes Cosmic Distance Record

But what many of you may not be aware of is that, within 24 hours of the April 28 headline, Swift detected yet another gamma-ray burst (the death-throes of a massive star), which was even more distant. Why didn’t you know? Well, because we didn’t either!

image of GRB 090429b
A Gemini Observatory color image of the afterglow of GRB 090429B, a candidate for the most distant object in the universe. This “izH” image has been constructed from three images taken at the Gemini Observatory North telescope through different optical and infrared filters. The red color results from the absence of all optical light, which has been absorbed by hydrogen gas in the distant universe. Without that absorption, the afterglow color would be bluer than any of the galaxies and stars seen here. (Credit: Credit: Gemini Observatory/AURA/NASA/ Levan, Tanvir, Cucchiara, Fox)

The explosion, termed GRB 090429B, was detected on April 29, 2009, by Swift. Nino Cucchiara and his then-PhD supervisor Derek Fox, along with collaborators including Nial Tanvir and Andrew Levan from the UK, observed the GRB with the 8-meter Gemini telescope in Hawai’i, and found that it was red. Very red.

Now this can mean two things: either it’s a really long way away, or it went off in a really dusty galaxy. So Nino and collaborators asked the Gemini operators to take a spectrum of the source, which would provide a measurement of the object’s distance.

Unfortunately, even on Hawai’i, astronomers are at the mercy of the weather. And just as Gemini prepared to take the spectrum, the weather turned and observing was impossible. By the next observing opportunity, the GRB was too faint to take a usable spectrum.

Fortunately, that’s not the end of the story, but it made the job much harder. Now, after two years of hard graft, and observations with Gemini and with the Hubble Space Telescope, Nino and collaborators have released their findings. And the cosmic record holder has fallen!

Well, probably. Their result shows, based on analysis of the images, that there is a 99.3 percent likelihood that this object was more distant that GRB 090423 — the object being trumpeted just before this star exploded. The precise distance is not known because of the lack of spectrum, but there is a 98.9 percent chance that is lies further away than a galaxy discovered in 2010 — 13.07 billion light years away — which surpassed April 2009’s GRB 090423 as the most distant known object. Whether it is the farthest object ever seen is not entirely clear: a galaxy detected in 2011 may lie a little further away…. or may actually not be a distant object at all.

Either way, this new result is another triumph for GRB science, for Swift and the optical and infrared facilities like Gemini, and above all for the hard-working determination of the scientists studying these enigmatic phenomena.

Follow Phil Evans on twitter: @swift_phil

Has-been: In 2008, GRB 080319b had it's 15 minutes of fame as the farthest known object in the universe.

A gamma-ray burst is a tremendous release of energy triggered by the collapse of a massive star.

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OH AND DID I MENTION? All opinions and opinionlike objects in this blog are mine alone and NOT those of NASA or Goddard Space Flight Center. And while we’re at it, links to websites posted on this blog do not imply endorsement of those websites by NASA.


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