A new member of a class of stars so rare that we count the known number on our fingers and toes has just been discovered in the Milky Way.
It is called MAXI J1816-195, located less than 30,000 light years away. According to a pulsar database compiled by astronomer Alessandro Patruno, preliminary observations and investigations suggest it is an accreting X-ray millisecond pulsar – of which only 18 others are known.
When the numbers are so low, any new object represents an extremely exciting discovery that can provide important statistical information about how these objects form, evolve and behave.
The discovery is really fresh off the press. X-ray light emanating from the object was first detected June 7 by the Japan Space Agency’s Monitor of All-sky X-ray Image (MAXI) instrument mounted outside the ISS.
In a notice published on The Astronomer’s Telegram (ATel), a team led by astrophysicist Hitoshi Negoro of Nihon University in Japan announced that they had identified a previously uncatalogued X-ray source, located in the galactic plane between the constellations of Sagittarius, Scutum, and Serpents. He was, they said, blazing relatively strong, but they had been unable to identify him based on the MAXI data.
It wasn’t long before other astronomers piled in. Using the Neil Gehrels Swift Observatory, a space telescope, Pennsylvania State University astrophysicist Jamie Kennea and his colleagues traveled to the scene to confirm the detection with an independent instrument and locate 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 at the location of any known cataloged x-ray source, so we agree it is a new MAXI J1816-195 transient source,” they wrote in a notice posted to ATel.
“Additionally, archival Swift/XRT observations of this region taken on June 22, 2017 reveal no point sources at this location.”
More and more curious.
Next, the Neutron Star Interior Composition Explorer (NICER), a NASA X-ray instrument also mounted on the ISS, as part of a survey 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 pulses at 528.6 Hz – suggesting the thing is spinning at a speed of 528.6 times per second – in addition to a thermonuclear burst of X-rays.
“This detection,” they wrote, “shows that MAXI J1816-195 is a neutron star and a new millisecond accretion X-ray pulsar.”
So what does this mean? Well, not all pulsars are built the same. At the most 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 about 2.2 times the mass of the Sun, packed into a sphere about 20 kilometers (12 miles) in diameter.
To be classified as a pulsar, a neutron star must…pulsate. Beams of radiation are launched from its poles; because of the way the star is tilted, these beams sweep across the Earth like beams from a lighthouse. Millisecond pulsars are pulsars that spin so fast that they pulse hundreds of times per second.
Some pulsars are powered only by rotation, but another type is powered by accretion. The neutron star is in a binary system with another star, their orbit is so close that matter is siphoned from the companion star and onto 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 flare up brightly in X-rays.
In some cases, the accretion process can spin the pulsar at rotational speeds on the order of a millisecond. This is the accreting X-ray millisecond pulsar, and MAXI J1816-195 appears to fall into this rare category.
The burst of thermonuclear X-rays detected by NICER was likely the result of unstable thermonuclear burning of material accumulated by the companion star.
Since the discovery is so new, observations in multiple wavelengths are ongoing. Tracking has already been carried out using Swift, and the 2m Liverpool Telescope on the Canary Island of La Palma in Spain has been used to search for an optical counterpart, although none have been detected. Other astronomers are also invited to board the MAXI J1816-195 train.
Meanwhile, a full analysis of pulsar timing is underway and, according to Bult and his team, will be released as more data becomes available. You can follow on ATel.