A new member of a category of stars 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’s called MAXI J1816-195, located no more than 30,000 light-years away. Preliminary observations and investigations suggest it is an accreting X-ray millisecond pulsar — of which only 18 others are known, according to a pulsar database compiled by astronomer Alessandro Patruno.
When the numbers are that low, any new object represents an extremely exciting discovery that could provide important statistical information about how these objects form, evolve and behave.
Discovery is really hot in the presses. The X-ray light emanating from the object was first detected on June 7 by the Japanese Space Agency’s Monitor of All-sky X-ray Image (MAXI) instrument mounted on the outside of the ISS.
In a notice published in The Astronomer’s Telegram (ATel), a team led by astrophysicist Hitoshi Negoro of Nihon University in Japan posted that they had identified a previously uncataloged X-ray source located in the galactic plane between the constellations of Sagittarius. , Scutum. , and Serpens. It was, they said, glowing relatively brightly, but they were not able to identify it based on the MAXI data.
It didn’t take long for other astronomers to pile in. Using the Neil Gehrels Swift Observatory, a space telescope, astrophysicist Jamie Kennea of Pennsylvania State University and colleagues focused on the site to confirm the detection with an independent instrument and pinpoint it.
Swift saw the object in X-rays, but not in optical or ultraviolet light, at the location specified by the MAXI observations.
“This location is not in the location of any known cataloged X-ray source, therefore we agree that this is a new transient MAXI J1816-195 source,” they wrote in a notice sent to ATel.
“Furthermore, Swift/XRT archival observations of this region taken on June 22, 2017 do not reveal any point sources at this location.”
Curious and curious.
Next up was the Neutron Star Interior Composition Explorer (NICER), a NASA X-ray instrument also mounted on the ISS, in an investigation led by astrophysicist Peter Bult of NASA’s Goddard Space Flight Center.
And that’s when things started to get really interesting. NICER picked up X-ray pulsations at 528.6 Hz – suggesting the thing is spinning at a rate of 528.6 times per second – plus a thermonuclear X-ray burst.
“This detection,” they wrote, “shows that MAXI J1816-195 is a neutron star and a new millisecond X-ray pulsar.”
So what does this mean? Well, not all pulsars are built the same. At the very basic level, a pulsar is a type of neutron star, which is the collapsed core of a massive dead star that has gone supernova. These objects are very small and very dense – up to about 2.2 times the mass of the Sun, packed into a sphere just 20 kilometers (12 miles) in diameter.
To be classified as a pulsar, a neutron star has to… pulsar. Radiation beams are launched from their poles; because of the angle of the star, these rays pass through 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 powered purely by rotation, but another type is powered by accretion. The neutron star is in a binary system with another star, its orbit so close that material is deflected from the companion star to the neutron star. This material is funneled along the neutron star’s magnetic field lines to its poles, where it falls to the surface, producing hot spots that glow brightly in X-rays.
In some cases, the accretion process can spin the pulsar to rotational speeds of milliseconds. This is the millisecond X-ray pulsar of accretion, and it looks like MAXI J1816-195 belongs in this rare category.
The X-ray thermonuclear explosion detected by NICER was likely the result of unstable thermonuclear burning of material accumulated by the companion star.
Because the discovery is so new, observations at multiple wavelengths are ongoing. Follow-up has already been carried out using Swift, and the Liverpool 2m Telescope on the Canary Islands of La Palma, Spain, has been employed to look for an optical counterpart, although none have been detected. Other astronomers are also encouraged to board the MAXI J1816-195 train.
In the meantime, a full pulsar timing analysis is being conducted and, according to Bult and his team, will be released as more data becomes available. You can follow along on ATel.