A new member of a category of stars that are so rare that we can count the known number of them on our fingers and toes has just been discovered in the Milky Way.
It is called MAXI J1816-195 and is not more than 30,000 light-years away. Preliminary observations and examinations indicate that it is an accreting X-ray millisecond pulse – of which only 18 others are known, according to a pulsar database compiled by astronomer Alessandro Patruno.
When the numbers are so low, each new object represents an extremely exciting find that can provide important statistical information about how these objects are formed, developed and behave.
The discovery is really hot from the press. X-rays from the object were first detected on June 7 by the Japanese Space Agency’s Monitor of All-sky X-ray Image instrument (MAXI) mounted on the outside of the ISS.
In a message to The Astronomer’s Telegram (ATel), a team led by astrophysicist Hitoshi Negoro of Nihon University in Japan, wrote that they had identified a previously uncataloged X-ray source, located in the galactic plane between the constellations Sagittarius, Scutum and Serpens. It flared, they said, relatively strongly, but they had not been able to identify it based on MAXI data.
It was not long before other astronomers piled on. Using Neil Gehrel’s Swift Observatory, a space-based telescope, astrophysicist Jamie Kennea of Pennsylvania State University and colleagues entered the site to confirm the discovery with an independent instrument and locate it.
Swift saw the object in X-rays, but not optical or ultraviolet light, at the location specified by the MAXI observations.
“This site is not located at the site of any known cataloged X-ray source, therefore we agree that this is a new transient source MAXI J1816-195,” they wrote in a message to ATel.
“In addition, archival observations by Swift / XRT of this region, taken on June 22, 2017, do not disclose any point source at this site.”
More and more curious.
Next up was Neutron star Interior Composition Explorer (MORE HANDSOME), a NASA X-ray instrument also mounted on the ISS, in a study led by astrophysicist Peter Bult from NASA’s Goddard Space Flight Center.
And this is where things started to get really interesting. NICER picked up X-ray pulsations at 528.6 Hz – which indicates that the thing is spinning at a speed of 528.6 times per second – in addition to a thermonuclear X-ray burst.
“This discovery,” they wrote“shows that MAXI J1816-195 is a neutron star and a new accreting millisecond X-ray pulses.”
So what does that mean? Well, not everyone pulsars are built the same. At the very basic level, a pulsar is a type of neutron star, which is the collapsed nucleus of a dead massive star that has become supernova. These objects are very small and very dense – up to about 2.2 times the mass of the sun, packed in a sphere just 20 kilometers (12 miles) or so across.
To be classified as a pulsar, a neutron star must … pulsate. Rays of radiation are emitted from its poles; due to the angle of the star, these rays sweep past the earth like the rays of a lighthouse. Millisecond pulsars are pulsars that spin so fast that they pulsate hundreds of times per second.
Some pulsars are driven purely by rotation, but another type is driven by accretion. The neutron star is in a binary system with another star, their orbit so close that the material is sucked from the companion star and into the neutron star. This material is channeled along the neutron star’s magnetic field lines to its poles, where it falls to the surface, producing hotspots that flare up strongly in X-rays.
In some cases, the accretion process can spin the pulsar up to millisecond rotational speeds. This is the increasing X-ray millisecond pulsar, and it seems that the MAXI J1816-195 belongs to this rare category.
The thermonuclear X-ray burst discovered by NICER was probably the result of the unstable thermonuclear combustion of materials accumulated by the accompanying star.
Because the discovery is so new, observations are underway in several wavelengths. Follow-up has already been done has been performed with Swiftand the 2m Liverpool Telescope on the Canary Island of La Palma in Spain were hired look for an optical equivalent, although no one was detected. Other astronomers are also encouraged to board the MAXI J1816-195 train.
At the same time, a complete pulse timing analysis is being carried out, and, said Bult and his team, will be circulated when more data becomes available. You can follow along ATel.
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