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Science

Unusual ‘revived’ pulsars could detect gravitational waves

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Paul M. Sutter (opens in new tab) is an astrophysicist at SUNY (opens in new tab) Stony Brook and the Flatiron Institute, host of “Ask a Spaceman (opens in new tab)“ and “Space Radio (opens in new tab),” and author of “How t (opens in new tab)o Die in Space.”

Astronomers hope to use pulsars scattered around the galaxy as a giant gravitational wave detector. But why do we need them, and how do they work?

Gravitational waves, or ripples in the fabric of space-time, from all sorts of sources constantly slosh throughout the universe. Right now, you are being slightly stretched and squeezed as wave after wave passes through you. Those waves come from merging black holes, the explosions of giant stars and even the earliest moments of the Big Bang.

On Earth, we’ve developed incredibly sensitive gravitational wave detectors that have been able to sense brief-but-loud events, such as black hole mergers, which last only a few seconds but generate such enormous signals that we can detect them. (“Enormous” is a relative term here; the distortion resulting from the passing wave is less than the width of an atomic nucleus.) 

Related: The first telescope of its kind will hunt for sources of gravitational waves

But ground-based detectors have a much harder time finding low-frequency gravitational waves, since those take weeks, months or even years to pass through Earth. Those kinds of low-frequency waves come from mergers of giant black holes, which take a lot longer to merge than their smaller cousins do. Our detectors simply don’t have the sensitivity to measure those small differences over such long time spans. For that, we need a much, much larger detector.

So, instead of using instruments on the ground, we can use distant pulsars to help us measure gravitational waves. This is the idea behind so-called pulsar timing arrays. 

Powering up the pulsars 

Pulsars are already fantastic objects, and that’s especially true for the kinds of pulsars used as gravitational wave detectors.

Pulsars are the leftover cores of giant stars and are among the most exotic objects ever known to inhabit the cosmos. They are ultradense balls made almost purely of neutrons, with some electrons and protons thrown in for good measure. Those spinning charges power up incredibly strong magnetic fields — in some cases, the most powerful magnetic fields in the universe.

Those intense magnetic fields also whip up strong electric fields. Together, they power beams of radiation (if you’re getting Death Star vibes here, you’re not far off) that blast out from the magnetic poles in each direction. Those magnetic poles don’t always line up with the rotational axis of the pulsar, in much the same way Earth’s North and South magnetic poles don’t line up with our planet’s rotational axis.

This forces the beams of radiation to sweep out circles in the sky. When those beams cross over Earth, we see them as periodic flashes of radio emission, putting the “pulse” in “pulsar.”

Related: Gravitational waves play with fast spinning stars, study suggests

Pulsars are incredibly regular. They are so heavy, and spin so quickly, that we can use their flashes as extremely precise clocks. But most pulsars are susceptible to random starquakes (when the star’s contents shift around, disturbing the pulsar’s rotation), glitches and slowdowns that change their regularity. That means most pulsars aren’t good for studying gravitational waves.

So instead, timing arrays rely on a subset of pulsars known as millisecond pulsars, which, as the name suggests, have rotational periods of a few milliseconds. Astronomers think millisecond pulsars are “revived” pulsars, spun up to incredible speeds after infalling material from a companion star accelerates them like a grown-up pushing a kid on a schoolyard merry-go-round.

Because of their ludicrous speed, millisecond pulsars can maintain fantastic precision over very long timescales. For example, one pulsar, PSR B1937+21, has a rotational period of 1.5578064688197945 +/- 0.0000000000000004 seconds. That’s the same level of precision as our best atomic clocks.

And those millisecond pulsars are perfect gravitational wave detectors.

Timing the array

Here’s how it works. First, astronomers observe the rotational periods of as many millisecond pulsars as possible. If a gravitational wave passes over Earth, over a pulsar or even between us, then as it passes, it will change the distance between Earth and the pulsar. As the wave moves, the pulsar will appear slightly closer, then slightly farther, then slightly closer, and so on until the wave has moved on.

That change in distance will appear to us as changes in the rotational period. One flash from the pulsar may arrive a bit too soon; then another may arrive a little too late. For a typical gravitational wave, the shift in the timings is incredibly tiny — a change of just 10 or 20 nanoseconds every few months. But the measurements of the millisecond pulsars are sensitive enough that those changes can be detected — at least in principle.

The “array” part of “pulsar timing array” comes from studying many pulsars at once and looking for correlated movements: If a gravitational wave passes over one region of space, then all the timings from the pulsars in that direction will shift in unison. 

Many collaborations across the world have used radio telescopes to study pulsar timing arrays for decades. So far, they’ve had limited success, finding shifts in timings from various pulsars but no hints of correlations. But every year, the techniques get better, and the hope is that soon, these arrays will unlock a huge part of the gravitational wave universe.

Learn more by listening to the “Ask a Spaceman” podcast, available on iTunes (opens in new tab) and askaspaceman.com. Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter.

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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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