The brightest explosion of a star ever seen temporarily blinded a satellite set up to watch such events, astronomers said on Wednesday.
The gamma-ray burst and explosion of X-rays that followed came from a star that died 5 billion years ago, far beyond our own Milky Way galaxy, NASA and British scientists said. It took this long for the radiation to reach the Swift orbiting observatory.
The bright X-ray burst blinded Swift on June 21, and the observatory’s software ignored it as if it were an anomaly, the astronomers said.
“The intensity of these X-rays was unexpected and unprecedented,” Neil Gehrels, Swift’s principal investigator at NASA’s Goddard Space Flight Center in Maryland, said in a statement.
Gehrels said the burst, named GRB 100621A, is the brightest X-ray source that Swift has detected since it started looking for them in 2005.
“Just when we were beginning to think that we had seen everything that gamma-ray bursts could throw at us, this burst came along to challenge our assumptions about how powerful their X-ray emissions can be,” Gehrels said.
“The burst was so bright when it first erupted that our data-analysis software shut down,” said Phil Evans of Britain’s University of Leicester, who discovered the burst when he was going through some recorded data from Swift.
“So many photons were bombarding the detector each second that it just couldn’t count them quickly enough. It was like trying to use a rain gauge and a bucket to measure the flow rate of a tsunami.”
When a star explodes, radiation travels at the speed of light in all directions. Gamma rays reach Earth first, followed by X-rays.
This particular one was 140 times brighter than the brightest continuous X-ray source in the sky — a nearby neutron star.
It went down this way: On June 21, NASAâ€™s orbiting Swift observatory was on sentry duty for Earthâ€™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 â€œpromptâ€ 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 â€œlight curveâ€ â€” literally a record of how the GRBâ€™s brightness changes over time in various wavelengths.
Meanwhile, back at the labâ€¦
OK, enough about Swift; back to Phil. Heâ€™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. Heâ€™d also received an email from another astronomer who had also noticed the gap.
â€œI looked at this and thought thatâ€™s odd, Iâ€™ll have to come back to it. I was looking at it and thinking, this is very strange.â€
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 Swiftâ€™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 â€” well, for 0.2 seconds, anyway! But the X-ray camera literally could not count that fast. …
Philâ€™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 itâ€™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 â€œnaked eyeâ€ 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 doesnâ€™t report the data to astronomers on the ground. In a sense, Swift didnâ€™t believe its own eyes. …
Astronomer Don Alexander who co-discovered this says,”It triggered swift at 03:03:32 Universal Time” on June 21.
How frequent are these gamma-ray bursts?
Currently orbiting satellites detect an average of about one gamma-ray burst per day.Â … Measuring the exact rate is difficult, but for a galaxy of approximately the same size as the Milky Way, the expected rate (for long GRBs) is about one burst every 100,000 to 1,000,000 years. Only a few percent of these would be beamed towards Earth. Estimates of rates of short GRBs are even more uncertain because of the unknown beaming fraction….
Could this be dangerous? Yes.Â A gamma ray burst could wipe out life on Earth.
A gamma-ray burst in the Milky Way, if close enough to Earth and beamed towards it, could have significant effects on the biosphere. The absorption of radiation in the atmosphere would cause photodissociation of nitrogen, generating nitric oxide that would act as a catalyst to destroy ozone. According to a 2004 study, a GRB at a distance of about a kiloparsec (3,262 light-years) could destroy up to half of Earth’s ozone layer; the direct UV irradiation from the burst combined with additional solar UV radiation passing through the diminished ozone layer could then have potentially significant impacts on the food chain and potentially trigger a mass extinction. The authors estimate that one such burst is expected per billion years, and hypothesize that the Ordovician-Silurian extinction event could have been the result of such a burst, although there is no current evidence to support this idea. – wikipedia
Now that we’ve had a big one, keep an eye on the ozone layer. NOAA knows.
NOAA uses satellite, airborne and ground-based systems to continuously monitor stratospheric ozone as well as the chemical compounds and atmospheric conditions that affect its concentration. – http://www.ozonelayer.noaa.gov/
Assuming I’m reading this right, there has been no measurable change thus far. The ozone layer is doing what it has always done:
But wait, the problem with ozone depletion is that it will increase the amount of UV radiation reaching us, so we should really look at the UV radiation forcast… and according to this, we are getting cooked right now. I’m still looking for a chart of UV radiation that spans a few years to figure out if this is normal.