Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse
Object: Xi-1 Canis Majoris
Superpower: Surprise X-ray pulses
Sometimes monsters hide in plain sight, and it takes a hero with X-ray vision to find them.
A European space telescope has spotted a massive star that is pulsing with brilliant X-rays. Studying the bizarre beast might yield important insights into stellar evolution, for instance, why some stars have strong magnetic fields while others don't, and what controls their powerful winds of charged particles.
The European Space Agency's XMM-Newton satellite has been zooming overhead since 1999, revealing details about objects such as galaxies, stars and black holes based on the X-rays they emit. Lidia Oskinova at the University of Potsdam in Germany and her colleagues were using the telescope to take a closer look at Xi-1 Canis Majoris, a large blue star near the constellation Canis Major.
Giant's pulse
The team got more than they bargained for. Although the star is well-known for emitting X-rays, it was a surprise to find these emissions were pulsing. Until now, almost all stars known to throb with such radiation were rapidly spinning neutron stars, the leftover cores of massive stars that have exploded. Some of these stellar corpses give off jets of high-energy particles from their poles. As the stars spin, we only detect the jets when they are aimed at Earth, so we see them as periodic pulses of light.
But Xi-1 Canis Majoris is still very much alive. It is between 13 and 15 times as massive as our sun and is visible in the winter sky roughly to the southeast of the bright "dog star" Sirius. Xi-1 Canis Majoris is known to vary in brightness over the course of a few hours, because heat fluctuations from the nuclear furnace at its core cause it to regularly expand and contract.
This shouldn't affect the star's X-ray emissions, however. The X-rays are produced by interactions in its stellar wind, so its slight size changes should not interfere with this flow of charged particles.
Magnetic mystery
Oddly, Oskinova and her team found that the X-ray pulses are in time with the star's brightness changes. The reason is still a mystery, but the star's strong magnetic field might be a clue – the astronomers were first interested in Xi-1 Canis Majoris because its magnetic field is thousands of times stronger than our sun's. It could be that the field is interacting with the star's wind in a way that creates the pulses, but it is too early to know for sure.
Gregg Wade at the Royal Military College of Canada says that astronomers had previously seen pulsing X-rays in only one other large object. Called Beta Crucis, this blue star is about 14 times as massive as the sun and marks the left point of the constellation Crux. However, it seems to have a much weaker magnetic field than Xi-1 Canis Majoris. A team that studied Beta Crucis with the Chandra X-ray Observatory in 2008 wasn't able to detect its magnetic field. The Beta Crucis team instead suggest this star's X-ray pulses are being influenced by oddities in its stellar wind.
"In principle it isn't hard to produce X-ray pulsations," says Wade. "The trick is to figure out what the mechanism actually is, and we haven't done that yet."
Journal reference: Nature Communications, DOI: 10.1038/ncomms5024
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