Jimmy Irwin wasn't looking to get a paper published in Nature when he gave three of his University of Alabama undergraduate students an assignment.
He told them to comb through archived Chandra and XMM-Newton telescope data for examples of bright X-ray emissions coming from galaxies beyond the Milky Way. The catch was to find examples emanating from globular cluster galaxies, a type of very old galaxy.
"It seemed like a suitably straightforward project for undergraduates who can only dedicate a few hours a week to work on," Irwin wrote in an email to Seeker.
Chandra had picked up a puzzling signal once before.
In 2005, astronomers detected a flare from the general direction of galaxy NGC4697, a globular cluster located some 40 million to 50 million light-years away in the constellation Virgo.
"Two brief flares were seen, separated by four years," Irwin writes in this week's Nature.
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Both times, the flares grew 90 times brighter in less than a minute, and there were no associated optical emissions in the position of the flares.
"It is still a bit of a mystery," Irwin said. "It is possible that (the flare) lies far beyond NGC4697, along the same line of sight, making it much more luminous than is believed, or the opposite case -- it is much closer to us, within our own Milky Way galaxy in fact, in which case it would be much less luminous than we think."
So the students went hunting.
"We were only hoping to find objects that varied by modest factors of three – to five or so on time scales of an hour or so," Irwin said.
But the searching software Irwin wrote for them to use could search for variability on all time scales, and the kids pulled in two whoppers: X-ray flares that brightened 100- to 200 percent in less than a minute.
"We were quite surprised," Irwin said.
At its peak, one source, located in Virgo galaxy NGC4636, was 30 times brighter than the X-ray luminosity of the entire Milky Way, he said.
The second source, located near the Centaurus A galaxy NGC 5181, had a big surprise too. So far, it has flared six times in X-ray light, with some pulses as short as 22 seconds. All flares peaked in less than a minute and lasted for about an hour.
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"The peak luminosity of the flares is intriguing, because it is even greater than the luminosity that can be attained by a neutron star under normal circumstances," astronomer Sergio Campana, with Italy's National Institute of Astrophysics, wrote in a related essay in Nature.
Scientists aren't sure what is producing the X-rays. One idea is that a neutron star, black hole or other extremely compact object is stealing material from a stellar companion, triggering flares as the star comes close. The flares recede as the star pulls away.
Irwin and his colleagues have submitted proposals for telescope time for follow-on observations. Meanwhile, the search through the archives continues.
Image: Chandra X-ray Observatory's image of the galaxy NGC5128, with its hot gas and multiple X-ray point sources. The circled source flared by a factor of 200 in less than a minute on multiple occasions. Credit: NASA/CXC/U.Birmingham/M.Burke et al.
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