If you had X-ray vision, most of the objects in the sky - besides the Sun - would be too faint for you to see with the naked eye. But every once in a while - in some random corner of the universe - there would be a short burst of X-rays appearing almost as bright as the planet Venus is in visible light.
"It is like the sky is black, and then this explosive flash goes off," said George Ricker from the Kavli Institute of Astrophysics and Space Research at MIT.
Astronomers discovered these X-ray flashes (XRFs) in 2000. They typically last less than a minute and apparently originate in distant galaxies--traits that also belong to gamma ray bursts (GRBs), the biggest blasts in the sky. Not believing in coincidence, many scientists suspect XRFs and GRBs are part of the same family of events.
If so, then XRFs are the little brothers of GRBs. They peak in X-rays, which are a factor of 100 lower in energy than the gamma rays of GRBs. The XRFs are also typically 100 times fainter than GRBs.
"You might think that would make them less interesting," Ricker said. But in actuality, this weakness could be a real advantage in trying to sort out what is the mechanism behind these bursting events.
Several observatories, including the Keck telescope in Hawaii, have made follow-up observations of XRFs one of their high priorities.
"It's a reflection of how important X-ray flashes are to the community," said Don Lamb from the University of Chicago.
The potential for using XRFs to figure out GRBs was discussed at the meeting of the American Astronomical Society last month.
A burst of another name
If XRFs are related to gamma ray bursts, one might wonder why they are not called X-ray bursts.
Unfortunately that name was already taken. "X-ray bursts" happen in binary systems, where one companion is stealing material from a nearby star. At times, this matter accretion speeds up, causing a brief eruption in X-rays.
The accreting "thief" is usually thought to be a neutron star, which is a compact remnant of a dead star. X-ray bursts belong to a whole class of phenomena--including X-ray novae - that involve relatively nearby compact objects which erupt in X-rays, sometimes repeatedly.
In contrast, X-ray flashes--just like GRBs--are in far-off galaxies (which means they are intrinsically brighter than X-ray bursts), and they only strike once in the same place.
When XRFs were first identified, some people suggested they might be normal GRBs that were fainter only because they were further away. Although this idea has been proven wrong, XRFs are still thought to be a variant of GRBs.
"There were lots of models to explain GRBs, but then along came XRFs and most of them no longer worked," Lamb said.
The currently favored model for GRBs is that they originate from the death of a massive star. The star's central spinning core collapses--likely becoming a black hole--while the outer layers of the star explode in what is called a supernova.
In addition, theorists suspect that--with so much matter spiraling into the core--some of the material gets sprayed out in jets that poke out of the two poles.
These intense beams of fast-moving particles generate gamma ray radiation. If the Earth lies "downwind" of one of these jets, we see the event as a GRB.
"We think that regular gamma-ray bursts are all produced by the collapse of massive stars and probably the creation of black holes," Lamb said. "I personally think it's essentially a certainty that X-ray flashes are produced by the same kind of event."
The question is how to make them weaker than GRBs.
One idea is that all supernova jets look the same, but they become less intense as you look at them at a slight angle. Seen straight on, a jet looks like a GRB, but rotate 20 or 30 degrees and it appears to be an XRF. Rotate further, and all we see is the supernova explosion.
According to Lamb, this "universal jet" hypothesis predicts a particular spread in GRB and XRF energies, which has not been seen.
"You can try to accommodate the GRBs, but then you cannot accommodate the XRFs, or vice versa," Lamb said.
For this reason, he advocates getting rid of the "one-size-jet-fits-all" by letting the size of the jet vary with each supernovae. In this "variable opening angle" scenario, a narrow jet--say 10 degrees across--would be a GRB, while a broad jet--maybe 40 degrees wide--would be an XRF.
Lamb thinks that the focus of the jet could depend on how fast the collapsing core rotates. But he calls this a "story"--not qualifying as a theory just yet.
Seeing the bump
Ricker said that the universal jet and variable opening angle theories are the two leading models out there for tying XRFs to GRBs.
"There is a lot of controversy in the field over which one of these pictures is right," Ricker said. "I'm a little agnostic on this score."
The fainter XRFs could be the key to discriminate between the models.
"GRBs are too bright for their own good," Ricker said "They swamp the light from the supernova that emerges 8 to 10 days later."
After a GRB goes off, there is typically a glow in X-rays and visible light that shines for days to weeks. This afterglow can be so bright that it hides the signatures of the supernova explosion.
However, afterglows have also been detected following XRFs, and because they are intrinsically weaker, observers have a better chance seeing the light from the underlying explosion. In the last few years, this extra light--called the "supernova bump"--has been detected in one XRF, and tentatively in another.
With more XRF data, observers will be able to firm up the connection between short bursts and supernova. Moreover, they hope to uncover the jet structure that gives rise to both "big" GRBs and "little" XRFs.
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