In the Milky Way, a new member of a kind of star so uncommon that we can count the number of them on our fingers and toes has been discovered.
It's known as MAXI J1816-195, and it's around 30,000 light-years away. According to astronomer Alessandro Patruno's pulsar database, first observations and research imply it's an accreting X-ray millisecond pulsar, of which just 18 others are known.
When the numbers are so low, every new item is a thrilling discovery that can provide valuable statistical information on how the things create, evolve, and act.
The information is fresh off the press. On the 7th of June, the Japanese Space Agency's Monitor of All-sky X-ray Image (MAXI) equipment placed on the exterior of the ISS identified X-ray radiation emerging from the object for the first time.
A team led by astrophysicist Hitoshi Negoro of Nihon University in Japan announced on The Astronomer's Telegram (ATel) that they'd discovered a hitherto uncatalogued X-ray source on the galactic plane between the constellations of Sagittarius, Scutum, and Serpens. They stated it was blazing brilliantly, but they couldn't figure out what it was based on the MAXI data.
It didn't take long for other astronomers to join in. Astrophysicist Jamie Kennea of Pennsylvania State University and colleagues used the Neil Gehrels Swift Observatory, a space-based telescope, to zero in on the position and confirm the observation using an independent instrument.
Swift saw the object in X-rays, but not optical or ultraviolet light, in the MAXI observations' position.
"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 said in an ATel notification.
"In addition, archival observations by Swift/XRT of this region taken in 2017 June 22, do not reveal any point source at this location."
The Neutron Star Interior Composition Explorer (NICER), an X-ray NASA equipment also housed on the ISS, was the next target in an experiment headed by NASA's Goddard Space Flight Center astrophysicist Peter Bult.
And it was at this point that things began to become very intriguing. In addition to an X-ray thermonuclear explosion, NICER detected X-ray pulsations at 528.6 Hz, implying that the object is spinning at a rate of 528.6 times per second.
"This detection shows that MAXI J1816-195 is a neutron star and a new accreting millisecond X-ray pulsar," they said.
So, what exactly does that imply? Not all pulsars are created equal. A pulsar is a kind of neutron star, which is the compressed core of a dead massive star that has gone supernova at its most basic level. These objects are very tiny and dense, weighing up to 2.2 times the mass of the Sun and fitting within a sphere just 20 kilometers (12 miles) wide.
A neutron star must pulse in order to be categorized as a pulsar. Radiation beams are fired from the star's poles, and because of the way the star is pointed, these rays sweep over Earth like lighthouse beams. Pulsars that spin at such a high rate that they pulse hundreds of times per second are known as millisecond pulsars.
Some pulsars are driven only by rotation, whereas others are driven by accretion. The neutron star and another star are in a binary system, with their orbits so close that material from the companion star is sucked onto the neutron star. This material is funneled down the magnetic field lines of the neutron star to its poles, where it falls to the surface, forming hotspots that flash brilliantly in X-rays.
The accretion process can speed up the pulsar to millisecond spinning rates in some situations. The accreting X-ray millisecond pulsar is an uncommon type of pulsar, and MAXI J1816-195 looks to be one of them.
The thermonuclear X-ray burst discovered by NICER was most likely caused by the companion star's unstable thermonuclear burning of material.
Because the finding is so fresh, observations in a variety of wavelengths are still being made. Swift was used as a follow-up, and the 2m Liverpool Telescope on the Canary Island of La Palma in Spain was used to search for an optical analogue, but none was found. Other astronomers are welcome to join the MAXI J1816-195 train as well.
Meanwhile, Bult and his colleagues are doing a thorough pulsar timing study, which will be distributed when additional data becomes available, according to Bult and his team. You can keep up with the action on ATel.
