This star rotates on its axis at 10% of the speed of light, and it is not a pulsar!
Illustration of the neutron star 4U 1820–30 and its white dwarf companion from which it steals matter and its rotation speed. © Generated on Bing Creator by Brice Haziza
When a very massive star – between 8 and 40 times the mass of the Sun – collapses on itself due to a lack of thermonuclear fuel, its core is compressed into a material made of neutrons and it reaches an incredible density. The rest is blown away during the supernova explosion. It becomes a neutron star whose diameter is only approximately 25 to 40 km. But the craziest part is yet to come…
As the star was rotating before collapsing and its diameter reduced by a factor of one to two million, then its rotation speed increases accordingly. This is what physicists call the conservation of angular momentum and which is perfectly illustrated in this video:
A neutron star therefore rotates very, very quickly. The fastest are pulsars, they also emit very strong electromagnetic radiation in the direction of their poles. The fastest rotational speed – or angular momentum – measured so far was 707 revolutions per second, which is absolutely dizzying.
Artist's impression of a pulsar. © NASA's Goddard Space Flight Center (converted to gif by ezgif.com)
But what is perhaps most astonishing here is that the star in question is not a pulsar!
This neutron star steals matter from a white dwarf and this accelerates its rotation to unprecedented values!
It belongs to a binary system: it is in a relationship with a white dwarf and their orbital period is only 11 min. This is the couple of all records… And this is precisely where it gets really interesting: our neutron star steals matter from its white dwarf companion, and in doing so, it also monopolizes its moment angular (or kinetic).
A “vampire” neutron star…
Astronomers realized this thanks to an X-ray emission at 716 Hz, which strongly suggests that this is precisely the rotation speed of the “vampire” neutron star.
X-ray burst showing 716 Hz oscillation. © Gaurava K. Jaisawal et al (The Astrophysical Journal)
If we do a quick calculation, we arrive at a rotation speed of around 35,000 km/s, or 10% of the speed of light (denoted “C” in physics). And precisely, it does not seem possible to go much faster than this 10% of C, because the ultra-dense structure of the star would shatter under the centrifugal effect…
