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Science

An Ultra-Rare Cosmic Object Was Just Detected in The Milky Way, Astronomers Report

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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.

 

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Science

Something’s Glowing at The Galactic Core, And We Could Be Closer to Solving The Mystery

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Something deep in the heart of the Milky Way galaxy is glowing with gamma radiation, and nobody can figure out for sure what it might be.

Colliding dark matter has been proposed, ruled out, and then tentatively reconsidered.

 

Dense, rapidly rotating objects called pulsars were also considered as candidate sources of the high-energy rays, before being dismissed as too few in number to make the sums work.

A study by researchers from Australia, New Zealand and Japan could breathe new life into the pulsar explanation, revealing how it might be possible to squeeze some serious intense sunshine from a population of spinning stars without breaking any rules.

Gamma radiation isn’t your typical hue of sunlight. It requires some of the Universe’s most energetic processes to produce. We’re talking black holes colliding, matter being whipped towards light speed, antimatter combining with matter kinds of processes.

Of course, the center of the Milky Way has all of these things in spades. So when we gaze into the heavens and consider all of the crashing bits of matter, spiraling black holes, whizzing pulsars, and other astrophysical processes, we’d expect to see a healthy gamma glow.

But when researchers used NASA’s Fermi telescope to measure the intense shine within the heart of our galaxy about ten years ago, they found there was more of this high-energy light than they could account for: what’s known as the Galactic Centre Excess.

 

One exciting possibility involves unseen bits of matter bumping together in the night. These weakly interacting massive particles – a hypothetical category of dark matter commonly described as WIMPs – would cancel each other out as they smoosh together, leaving nothing but radiation to mark their presence.

It’s a fun explanation to consider, but is also light on evidence.

“The nature of dark matter is entirely unknown, so any potential clues garner a lot of excitement,” says astrophysicist Roland Crocker from the Australian National University.

“But our results point to another important source of gamma ray production.”

That source is the millisecond pulsar.

To make one, take a star much bigger than our own and let its fires die down. It will eventually collapse into a dense ball not much wider than a city, where its atoms pack together so tightly, many of its protons are slowly baked into neutrons.

This process generates super-strong magnetic fields that channel incoming particles into fast-flowing streams glowing with radiation.

Since the object is rotating, these streams swivel around from the star’s poles like the Universe’s biggest lighthouse beacons – so it appears to pulse with energy. Pulsing stars that spin hundreds of times a second are known as millisecond pulsars, and we know a lot about the conditions under which they’re likely to form.

 

“Scientists have previously detected gamma-ray emissions from individual millisecond pulsars in the neighborhood of the Solar System, so we know these objects emit gamma rays,” says Crocker.

To emit them, however, they’d need a generous amount of mass to feed on. Most pulsar systems in the center of the Milky Way are thought to be too puny to emit anything more energetic than X-rays, though.

That might not always be the case, however, especially if the dead stars they emerged from are of a particular variety of ultra-massive white dwarf.

According to Crocker, if enough of these heavyweights were to turn into pulsars and hold onto their binary partners, they would provide just the right amount of gamma radiation to match observations.

“Our model demonstrates that the integrated emission from a whole population of such stars, around 100,000 in number, would produce a signal entirely compatible with the Galactic Centre Excess,” says Crocker.

Being a purely theoretical model, it’s an idea that now needs a generous dose of empirical evidence. Unlike suggestions based on dark matter, however, we already know exactly what to look for.

This research was published in Nature Astronomy.  

 

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