And then the universe said ‘Hah!’ NASA’s Swift satellite can’t believe its eyes when it spots the brightest X-ray glow from a gamma ray burst outside our galaxy
Three weeks ago, a distant point in space in another galaxy released a blast of X-rays so bright even the satellite that saw it first didn’t believe its eyes. Then Phil Evans came home from vacation and got very, very lucky.
“One of the things I personally like most about doing research,” he says, “is when you discover something brand new — even if it’s ‘just’ the brightest X-ray object that we think we’ve ever seen — there’s a moment when there is only one person in the universe who knows about this. And sometimes you get to be that one person.”
It went down this way: On June 21, NASA’s orbiting Swift observatory was on sentry duty for Earth’s astronomers, watching the universe for Gamma ray bursts. A GRB went off on June 21, later catalogued GRB 100621A. (GRBs are violent eruptions of energy from the explosion of a massive star turning into a black hole.)
Gamma ray bursts announce their appearance as, well, a burst of gamma rays. Since gammas are the most energetic form of electromagnetic radiation, GRBs are the most powerful beacons in the universe.
When Swift detects a burst, it radios the coordinates to Earth. Astronomers and robotic observatories scramble to aim their instruments at the GRB. Swift also slews its instruments, such as the X-Ray Telescope (XRT), to the target.
Ideally, astronomers want to observe both the immediate or “prompt” emissions and, as time passes, the fading afterglow of X-rays, ultraviolet light, and (rarely) visible light.
Swift beams to Earth a record of when it detected each photon, and then software on the ground turns this into a “light curve” — literally a record of how the GRB’s brightness changes over time in various wavelengths.
Meanwhile, back at the lab…
OK, enough about Swift; back to Phil. He’s is a post-doctoral research assistant in the X-ray and Observational Astronomy group at the University of Leicester in England, and part of the Swift team. He wrote the software that converts the photo detections from Swift into light curves.
So he got home from holiday on June 29. The next morning, he examined the light curves that his software had created while he was in the Lake District in North West England, camping with his wife and two young sons.
And he saw something very puzzling: For one event, GRB 100621A, the record of its earliest X-ray emission was missing. He’d also received an email from another astronomer who had also noticed the gap.
By noon, he had the data gap plugged. It took him a few more hours to check it, and to appreciate what had actually happened. The next day he announced it to the rest of the Swift community around the globe.
It turned out that GRB 100621A had been so bright early on, it had temporarily blinded Swift’s detectors. At the center of the image, which is the brightest part of the image, X-rays streamed in at a peak rate of 143,000 per second — well, for 0.2 seconds, anyway! But the X-ray camera literally could not count that fast. It was like a lone soccer goalkeeper being fired at by a dozen World Cup strikers.
Correct me if I’m wrong
Phil’s light-curve-making software has a way of dealing with this situation. It counts the X-ray photons streaming in around the edges of the image, where it’s not so bright and intense. Then it multiplies that by a correction factor to estimate how bright it must be in the glaring center of the image.
This correction was used in the famous “naked eye” GRB 080319B of 2008, which was so bright you could have seen it without a telescope, briefly, in a dark location on Earth. The correction: 32 times.
Correction for the June 21 GRB: 168!
Phil designed the software so that if the correction factor exceeds a certain expected threshold, the software just doesn’t report the data to astronomers on the ground. In a sense, Swift didn’t believe its own eyes.
“When it did the correction, it saw the size of it and said, no, that’s got to be nonsense, there’s got to be some sort of mistake,” Phil explains. “It just said, ‘This can’t be real. I’m not publishing this to the world because I’m going to look like an idiot.”
Or to put it more politely, Phil’s software refuses to report what it determines to be bad data. But it wasn’t bad; it was spot-on correct.
A new (X-ray) world record
“I didn’t totally register at first how bright it is, and then I mentioned it to a few people, and they went ‘What!’ And then we started to get a better feel for the fact that this was something to put in your record books.”
What kind of record are we talking about?
When the system was designed, astronomers weren’t expecting to see anything as bright as what Swift saw on June 21. The next brightest such object in X-rays was 2008’s naked-eye GRB 080319B.
This GRB was seven times fainter but twice as far away as the June 21 event. But move the new record holder to the same distance, and it would still be 1.5 times as bright as GRB 080319B.
Both of these events happened outside our Milky Way Galaxy. For example, the June 21 GRB happened 5 billion light years away, which means the light from it left about the time our solar system formed. There are brighter X-ray sources in our own galaxy, but that’s like comparing your next-door neighbor’s porch light to the blinking aircraft beacon atop a radio tower 10 miles away.
Relative to Swift, the latest GRB is a clear record-breaker. It is definitely the brightest thing, GRB or otherwise, Swift’s X-ray eye has ever seen.
And that is, perhaps, the most striking thing about this whole episode: the way in which this latest GRB has confounded expectations.
Barbara Kennedy said it best recently on a teleconference with Phil and a couple of other scientists, including Dave Burrows of Penn State University (PSU), the lead scientist for Swift’s X-ray telescope. Barbara is a press officer for PSU, which has issued a release about the event.
We — the scientists, Barbara, and a couple other media people who cover Swift — were discussing what approach to use to explain this GRB to the public. Biggest? Brightest? First?
I thought Barbara nailed it when she said: “The best scientists in the world thought ‘We’ll never see anything that bright, so we don’t have to design the software to handle it.’ And then the universe said ‘Hah! Look what I can throw at you!'”
Follow Phil Evans in his role as a Swift Scientist on Twitter: @Swift_Phil, where news of this discovery was first announced!]
See Barbara Kennedy’s press release on the X-ray GRB on the PSU website.
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.