May 15, 2014
The Magnetar Mystery
I am pretty sure at this point that if I go on too much about how wondrous and incredible our universe can be again, someone out there is going to smack me. Even so, it is hard not to be enthralled by all of the spectacular things we can discover beyond the border of our own world. Not that the wonders of Earth are anything to scoff at, but there is just something about the mysteries of the cosmos that continually draw us back to it. Every new finding brings us something unexpected and spectacular, bringing something to light that we might have never even imagined.
For example, look at the magnetar known as CXOU J164710.2-455216. Not the most imaginative name, I will grant you. It is part of the Westerlund 1 star cluster, which is found 16,000 light-years away in the southern constellation of Ara (the Altar) and is one of the only two magnetars known in all of the Milky Way galaxy. What is a magnetar, you ask? They are formed when a massive star collapses under its own gravitational force, going supernova. When this happens, the star will either form a neutron star or, more popularly known, a black hole. Magnetars are a very rare type of neutron star and, like all of these celestial objects, are incredibly dense. Just a teaspoon of a neutron star would have a mass of somewhere around one-billion tons. What makes magnetars so incredible is that they are also have incredibly powerful magnetic fields, much stronger than anything we have here on Earth. These unusual neutron stars release vast amounts of gamma rays when they undergo something known as a “starquake,” which certainly sounds very epic.
Until now, researchers have postulated that the magnetar in the Westerlund 1 cluster must have been created after the supernova of a star about 40 times more massive than our own Sun, but this theory always seemed a bit weak. Stars of that size, when they go supernova, typically collapse into black holes rather than neutron stars, so how did the Westerlund 1 magnetar come to be? One possible solution for this mystery was that the magnetar formed through the interactions of two massive stars that, at one time, orbited one another in a binary system so small that it would have fit within the orbit of Earth around the Sun. However, no such star was ever identified so a team of European astronomers decided to go looking for one using ESO‘s Very Large Telescope (VTL). What they were looking for were objects trying to escape the cluster at high velocities that may have been knocked out of their orbit by the supernova that created the magnetar, and so came the discovery of the Westerlund 1-5, a star that was trying to do exactly that. According to Ben Ritchie of Open University, co-author of the paper detailing the discovery of Westerlund 1-5, “Not only does this star have the high velocity expected if it is recoiling from a supernova explosion, but the combination of its low mass, high luminosity, and carbon-rich composition appear impossible to replicate in a single star – a smoking gun that shows it must have originally formed with a binary companion.”
The discovery of Westerlund 1-5 has allowed astronomers to gain a better understanding of how magnetars form and has led to the realization that being a part of a double star may be essential for the formation of these unique neutron stars. The rapid rotation that is created by the mass transfer between the two stars seems to be necessary in order to generate the strong magnetic field and a second mass transfer phase during the supernova is what allows the soon-to-be magnetar to thin out sufficiently enough so that it does not collapse into a black hole.
Yet another galactic mystery solved.
Image Credit: ESO / L. Calçada