Neutron stars are few kilometres sized objects that come to life when a massive star dies in a devastating supernova explosion. A few months ago a very peculiar neutron star has been discovered in an even more peculiar location of our galaxy. A magnetar is neutron star surrounded by a magnetic field so intense that it is billion times stronger than the strongest magnetic field ever produced in a physics lab on Earth. Place yourself in such a strong magnetic field and you won’t survive a whole second. One such magnetar has been discovered in the Galactic Center, very close to the supermassive black hole that exists there. This monster contains several million times the mass of the Sun and is slowly eating a tenuous cloud of gas that fills its environment. The magnetar is so close to the monster that it orbits around it, like the Earth around the Sun.
In an interesting paper posted today on arXiv (submitted to the Astrophysical Journal Letters), a team of scientists lead by Laura G. Spitler (from the Max Planck Insitute for Radio Astronomy, Bonn) used the magnetar to probe the cloud of gas around the supermassive black hole. By using radio waves emitted by the magnetar and detected by radio-telescopes on Earth, they measured how the signal is scattered and degraded when crossing the cloud of material that surrounds the monster. They found that the interstellar material is rather tenuous, an interesting result per-se, which however, also hides a surprising conundrum.
Indeed if the cloud around the black hole is tenuous, then the radio signal emitted by other neutron stars should not degrade too much when crossing it before reaching Earth. Therefore it should be possible to detect many other neutron stars around the supermassive black hole, assuming some more objects of this type truly exist in that region. Theoretical models do indeed predict that many neutron stars should be there.
However, astronomers have been searching for such neutron stars around the supermassive black hole for a long time and so far only this magnetar has popped-up and nothing else. So does this imply that there are not many other neutron stars around there? Magnetars are very rare types of neutron stars, so the odds are strongly against detecting a single magnetar rather than a more conventional neutron star. Laura Spitler and colleagues indeed notice that for each magnetar detected there should be a hidden population of many more neutron stars. The reason why we haven’t seen any other neutron star in previous radio searches is probably associated to the fact that the cloud is very inhomogeneous and patchy and we happened to see the magnetar shining through a hole in the cloud.
If we would find one more neutron star even closer to the black hole, especially one belonging to the so-called millisecond pulsar family, then we would have an amazing tool to understand gravity and test the theory of General Relativity. Now Laura Spitler & co-authors have shown us that many other neutron stars might be silently lurking in the darkness around the monster and one of them might just be waiting to be discovered through another hole in the cloud.
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