A new member of a category of star so rare 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 greater than 30,000 light-years away. Preliminary observations and investigations suggest that it's an accreting X-ray millisecond pulsar – of which only 18 others are known, according to a pulsar database compiled by astronomer Alessandro Patruno.
When numbers are that low, any new object represents an extremely exciting find that can yield important statistical information about how those objects form, evolve, and behave.
The discovery really is hot off the presses. X-ray light emanating from the object was first detected on 7 June 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 posted to The Astronomer's Telegram (ATel), a team headed by astrophysicist Hitoshi Negoro of Nihon University in Japan posted that they'd identified a previously uncatalogued X-ray source, located in the galactic plane between the constellations of Sagittarius, Scutum, and Serpens. It was, they said, flaring relatively brightly, but they hadn't been able to identify it based on the MAXI data.
It wasn't long before other astronomers piled on. Using the Neil Gehrels Swift Observatory, a space-based telescope, astrophysicist Jamie Kennea of Pennsylvania State University and colleagues homed in on the location to confirm the detection with an independent instrument, and localize it.
Swift saw the object in X-rays, but not optical or ultraviolet light, at the location specified by the MAXI observations.
"This location does not lie at the location of any known catalogued X-ray source, therefore we agree that this is a new transient source MAXI J1816-195," they wrote in a notice posted to ATel.
"In addition, archival observations by Swift/XRT of this region taken in 2017 June 22, do not reveal any point source at this location."
Curiouser and curiouser.
Next up was the Neutron Star Interior Composition Explorer (NICER), an X-ray NASA instrument also mounted to the ISS, in an investigation led by astrophysicist Peter Bult of 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 – suggesting that the thing is spinning at a rate of 528.6 times per second – in addition to an X-ray thermonuclear burst.
"This detection," they wrote, "shows that MAXI J1816-195 is a neutron star and a new accreting millisecond X-ray pulsar."
So what does that mean? Well, not all pulsars are built alike. At the very basic level, a pulsar is a type of neutron star, which is the collapsed core of a dead massive star that has gone supernova. These objects are very small and very dense – up to around 2.2 times the mass of the Sun, packed into a sphere just 20 kilometers (12 miles) or so across.
To be classified as a pulsar, a neutron star has to… pulse. Beams of radiation are launched from its poles; because of the way the star is angled, these beams sweep past Earth like the beams from a lighthouse. Millisecond pulsars are pulsars that spin so fast, they pulse hundreds of times a 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, their orbit so close that material is siphoned from the companion star and onto the neutron star. This material is channeled along the neutron star's magnetic field lines to its poles, where it falls down onto the surface, producing hotspots that flare brightly in X-rays.
In some cases, the accretion process can spin up the pulsar to millisecond rotational speeds. This is the accreting X-ray millisecond pulsar, and it appears that MAXI J1816-195 belongs to this rare category.
The thermonuclear X-ray burst detected by NICER was likely the result of the unstable thermonuclear burning of material accumulated by the companion star.
Since the discovery is so new, observations in multiple wavelengths are ongoing. Follow-up has already been conducted using Swift, and the 2m Liverpool Telescope on the Canary Island of La Palma in Spain was employed to look for an optical counterpart, although none was detected. Other astronomers are also encouraged to climb aboard the MAXI J1816-195 train.
Meanwhile, a full pulsar timing analysis is being conducted, and will, Bult and his team said, be circulated as more data becomes available. You can follow along on ATel.