Neutron Star Archives

May 20, 2018 by Matt Williams

The Pressure Inside Every Proton is 10x That Inside Neutron Stars

The first measurement of a subatomic particle’s mechanical property reveals the distribution of pressure inside the proton. Credit: DOE's Jefferson Lab

Neutron stars are famous for combining a very high-density with a very small radius. As the remnants of massive stars that have undergone gravitational collapse, the interior of a neutron star is compressed to the point where they have similar pressure conditions to atomic nuclei. Basically, they become so dense that they experience the same amount of internal pressure as the equivalent of 2.6 to 4.1 quadrillion Suns!

In spite of that, neutron stars have nothing on protons, according to a recent study by scientists at the Department of Energy’s Thomas Jefferson National Accelerator Facility. After conducting the first measurement of the mechanical properties of subatomic particles, the scientific team determined that near the center of a proton, the pressure is about 10 times greater than the pressure in the heart of a neutron star.

The study which describes the team’s findings, titled “The pressure distribution inside the proton“, recently appeared in the scientific journal Nature. The study was led by Volker Burkert, a nuclear physicist at the Thomas Jefferson National Accelerator Facility (TJNAF), and co-authored by Latifa Elouadrhiri and Francois-Xavier Girod – also from the TJNAF.

*Cross-section of a neutron star. Credit: Wikipedia Commons/Robert Schulze*

Basically , they found that the pressure conditions at the center of a proton were 100 decillion pascals – about 10 times the pressure at the heart of a neutron star. However, they also found that pressure inside the particle is not uniform, and drops off as the distance from the center increases. As Volker Burkert, the Jefferson Lab Hall B Leader, explained:

“We found an extremely high outward-directed pressure from the center of the proton, and a much lower and more extended inward-directed pressure near the proton’s periphery… Our results also shed light on the distribution of the strong force inside the proton. We are providing a way of visualizing the magnitude and distribution of the strong force inside the proton. This opens up an entirely new direction in nuclear and particle physics that can be explored in the future.”

Protons are composed of three quarks that are bound together by the strong nuclear force, one of the four fundamental forces that government the Universe – the other being electromagnetism, gravity and weak nuclear forces. Whereas electromagnetism and gravity produce the effects that govern matter on the larger scales, weak and strong nuclear forces govern matter at the subatomic level.

Previously, scientists thought that it was impossible to obtain detailed information about subatomic particles. However, the researchers were able to obtain results by pairing two theoretical frameworks with existing data, which consisted of modelling systems that rely on electromagnetism and gravity. The first model concerns generalized parton distributions (GDP) while the second involve gravitational form factors.

*Quarks inside a proton experience a force an order of magnitude greater than matter inside a neutron star. Credit: DOE’s Jefferson Lab*

Patron modelling refers to modeling subatomic entities (like quarks) inside protons and neutrons, which allows scientist to create 3D images of a proton’s or neutron’s structure (as probed by the electromagnetic force). The second model describes the scattering of subatomic particles by classical gravitational fields, which describes the mechanical structure of protons when probed via the gravitational force.

As noted, scientists previously thought that this was impossible due to the extreme weakness of the gravitational interaction. However, recent theoretical work has indicated that it could be possible to determine the mechanical structure of a proton using electromagnetic probes as a substitute for gravitational probes. According to Latifa Elouadrhiri – a Jefferson Lab staff scientist and co-author on the paper – that is what their team set out to prove.

“This is the beauty of it. You have this map that you think you will never get,” she said. “But here we are, filling it in with this electromagnetic probe.”

For the sake of their study, the team used the DOE’s Continuous Electron Beam Accelerator Facility at the TJNAF to create a beam of electrons. These were then directed into the nuclei of atoms where they interacted electromagnetically with the quarks inside protons via a process called deeply virtual Compton scattering (DVCS). In this process, an electron exchanges a virtual photon with a quark, transferring energy to the quark and proton.

*The bare masses of all 6 flavors of quarks, proton and electron, shown in proportional volume. Credit: Wikipedia/Incnis Mrsi*

Shortly thereafter, the proton releases this energy by emitting another photon while remaining intact. Through this process, the team was able to produced detailed information of the mechanics going on in inside the protons they probed. As Francois-Xavier Girod, a Jefferson Lab staff scientist and co-author on the paper, explained the process:

“There’s a photon coming in and a photon coming out. And the pair of photons both are spin-1. That gives us the same information as exchanging one graviton particle with spin-2. So now, one can basically do the same thing that we have done in electromagnetic processes — but relative to the gravitational form factors, which represent the mechanical structure of the proton.”

The next step, according to the research team, will be to apply the technique to even more precise data that will soon be released. This will reduce uncertainties in the current analysis and allow the team to reveal other mechanical properties inside protons – like the internal shear forces and the proton’s mechanical radius. These results, and those the team hope to reveal in the future, are sure to be of interest to other physicists.

“We are providing a way of visualizing the magnitude and distribution of the strong force inside the proton,” said Burkert. “This opens up an entirely new direction in nuclear and particle physics that can be explored in the future.”

Perhaps, just perhaps, it will bring us closer to understanding how the four fundamental forces of the Universe interact. While scientists understand how electromagnetism and weak and strong nuclear forces interact with each other (as described by Quantum Mechanics), they are still unsure how these interact with gravity (as described by General Relativity).

If and when the four forces can be unified in a Theory of Everything (ToE), one of the last and greatest hurdles to a complete understanding of the Universe will finally be removed.

Further Reading: Jefferson Lab, Cosmos Magazine, Nature

April 9, 2018 by Matt Williams

A Magnetar Just Woke Up After Three Years of Silence

Aerial image of the South African MeerKAT radio telescope, part of the Square Kilometer Array (SKA). Credit: SKA

When stars reach the end of their main sequence, they undergo a gravitational collapse, ejecting their outermost layers in a supernova explosion. What remains afterward is a dense, spinning core primarily made up of neutrons (aka. a neutron star), of which only 3000 are known to exist in the Milky Way Galaxy. An even rarer subset of neutron stars are magnetars, only two dozen of which are known in our galaxy.

These stars are especially mysterious, having extremely powerful magnetic fields that are almost powerful enough to rip them apart. And thanks to a new study by a team of international astronomers, it seems the mystery of these stars has only deepened further. Using data from a series of radio and x-ray observatories, the team observed a magnetar last year that had been dormant for about three years, and is now behaving somewhat differently.

The study, titled “Revival of the Magnetar PSR J1622–4950: Observations with MeerKAT, Parkes, XMM-Newton, Swift, Chandra, and NuSTAR“, recently appeared in The Astrophysical Journal. The team was led by Dr Fernando Camilo – the Chief Scientist at the South African Radio Astronomy Observatory (SARAO) – and included over 200 members from multiple universities and research institutions from around the world.

Magnetars are so-named because their magnetic fields are up to 1000 times stronger than those of ordinary pulsating neutron stars (aka. pulsars). The energy associated with these these fields is so powerful that it almost breaks the star apart, causing them to be unstable and display great variability in terms of their physical properties and electromagnetic emissions.

Whereas all magnetars are known to emit X-rays, only four have been known to emit radio waves. One of these is PSR J1622-4950 – a magnetar located about 30,000 light years from Earth. As of early 2015, this magnetar had been in a dormant state. But as the team indicated in their study, astronomers using the CSIRO Parkes Radio Telescope in Australia noted that it was becoming active again on April 26th, 2017.

At the time, the magnetar was emitting bright radio pulses every four seconds. A few days later, Parkes was shut down as part of a month-long planned maintenance routine. At about the same time, South Africa’s MeerKAT radio telescope began monitoring the star, despite the fact that it was still under construction and only 16 of its 64 radio dishes were available. Dr Fernando Camilo describes the discovery in a recent SKA South Africa press release:

“[T]he MeerKAT observations proved critical to make sense of the few X-ray photons we captured with NASA’s orbiting telescopes – for the first time X-ray pulses have been detected from this star, every 4 seconds. Put together, the observations reported today help us to develop a better picture of the behaviour of matter in unbelievably extreme physical conditions, completely unlike any that can be experienced on Earth”.

*Artist’s rendering of an outburst on an ultra-magnetic neutron star, also called a magnetar. Credit: NASA/Goddard Space Flight Center*

After the initial observations were made by the Parkes and MeerKAT observatories, follow-up observations were conducted using the XMM-Newton x-ray space observatory, Swift Gamma-Ray Burst Mission, the Chandra X-ray Observatory, and the Nuclear Spectroscopic Telescope Array (NuSTAR). With these combined observations, the team noted some very interesting things about this magnetar.

For one, they determined that PSR J1622-4950’s radio flux density, while variable, was approximately 100 times greater than it was during its dormant state. In addition, the x-ray flux was at least 800 times larger one month after reactivation, but began decaying exponentially over the course of a 92 to 130 day period. However, the radio observations noted something in the magnetar’s behavior that was quite unexpected.

While the overall geometry that was inferred from PSR J1622-4950’s radio emissions was consistent with what had been determined several years prior, their observations indicated that the radio emissions were now coming from a different location in the magnetosphere. This above all indicates how radio emissions from magnetars could differ from ordinary pulsars.

This discovery has also validated the MeerKAT Observatory as a world-class research instrument. This observatory is part of the Square Kilometer Array (SKA), the multi-radio telescope project that is building the world’s largest radio telescope in Australia, New Zealand, and South Africa. For its part, MeerKAT uses 64 radio antennas to gather radio images of the Universe to help astronomers understand how galaxies have evolved over time.

*Aerial image of the South African MeerKAT radio telescope in the Karoo, South Africa. Credit: SKA*

Given the sheer volume of data collected by these telescopes, MeerKAT relies on both cutting edge-technology and a highly-qualified team of operators. As Abbott indicated, “we have a team of the brightest engineers and scientists in South Africa and the world working on the project, because the problems that we need to solve are extremely challenging, and attract the best”.

Prof Phil Diamond, the Director-General of the SKA Organization leading the development of the Square Kilometer Array, was also impressed by the contribution of the MeerKAT team. As he stated in an SKA press release:

“Well done to my colleagues in South Africa for this outstanding achievement. Building such telescopes is extremely difficult, and this publication shows that MeerKAT is becoming ready for business. As one of the SKA precursor telescopes, this bodes well for the SKA. MeerKAT will eventually be integrated into Phase 1 of SKA-mid telescope bringing the total dishes at our disposal to 197, creating the most powerful radio telescope on the planet”.

When the SKA goes online, it will be one of the most powerful ground-based telescopes in the world and roughly 50 times more sensitive than any other radio instrument. Along with other next-generation ground-based and space-telescopes, the things it will reveal about our Universe and how it evolved over time are expected to be truly groundbreaking.

Further Reading: SKA Africa, SKA, The Astrophysical Journal

March 7, 2018 by Evan Gough

Astronomers See A Dead Star Come Back To Life Thanks To A Donor Star

The ESA INTEGRA observatory has witnessed a "zombie" neutron star being re-energized by the solar wind of its companion red giant star, and coming back to life in a burst of x-rays. Image: ESA

It’s not exactly an organ donor, but a star in the direction of the hyper-populated core of the Milky Way donating some of its mass to a dormant neighbor. The result? The dormant neighbor sprung back to life with an X-ray burst captured by the ESA‘s INTEGRAL (INTErnational Gamma-Ray Astrophysics Laboratory) space observatory.

“INTEGRAL caught a unique moment in the birth of a rare binary system” – Enrico Bozzo, University of Geneva.

The neighbors have likely been paired together for billions of years, which is not in itself noteworthy: stars often live in binary pairs. But the pair spotted by INTEGRAL on August 13th 2017 is very unusual. The donor star is a red giant, and the recipient is a neutron star. So far, astronomers only know of 10 of these pairs, called ‘symbiotic X-ray binaries’.

To understand what’s happening between these neighbors, we have to look at stellar evolution.

The donor star is in its red giant phase. That’s when a star in the same mass range as our star reaches the later stage of its life. As its mass is depleted, gravity can’t hold the star together in the same way it has for the early part of its life. The star expands outwards by millions of kilometers. As it does so, it sheds stellar material from its outer layers in a solar wind that travels several hundreds of km/sec.

The red giant and the neutron star may have traveled different evolutionary pathways, but proximity made them partners. Image: ESA

Its neighbor is in a different state. It’s a star that had an initial mass of about 25 to 30 times the Sun. When a star this big approaches the end of its life, it suffers a different fate. Stars this large live fast, and burn through their fuel quickly. Then, they explode as supernovae, in this case leaving a corpse behind. In the binary system captured by INTEGRAL, the corpse is a spinning neutron star with a magnetic field.

Neutron stars are dense. In fact, they’re some of the densest stellar objects we know of, packing as much mass as one-and-a-half of our Suns into an object that’s only about 10 km across.
When the red giant’s stellar wind met the neutron star, the neutron star slowed its rate of spin, and burst into life, emitting high-energy x-rays.

“INTEGRAL caught a unique moment in the birth of a rare binary system,” says Enrico Bozzo from University of Geneva and lead author of the paper that describes the discovery. “The red giant released a sufficiently dense slow wind to feed its neutron star companion, giving rise to high-energy emission from the dead stellar core for the first time.”

After INTEGRAL spotted the x-ray burst from the binary, other observations quickly followed. The ESA’s XMM Newton and NASA’s NuSTAR and Swift space telescopes got to work, along with ground-based telescopes. These observations confirmed what initial observations showed: this is a very peculiar pair of stars.

“…we believe we saw the X-rays turning on for the first time.” – Erik Kuulkers, ESA INTEGRAL Project Scientist.

The neutron star spins very slowly, taking about 2 hours to revolve, which is remarkable since other neutron stars can spin many times per second. The magnetic field of the neutron star was also much stronger than expected. But the magnetic field around a neutron star is thought to weaken over time, making this a relatively young neutron star. And a red giant is old, so this is a very odd pairing of old red giant with young neutron star.

One possible explanation is that the neutron star didn’t form from a supernova, but from a white dwarf. In that scenario, the white dwarf would’ve collapsed into a neutron star after a very long period of feeding on material from the red giant. That would explain the disparity in ages of the two stars in the system.

An artist’s illustration of ESA’s INTEGRAL space observatory. INTEGRAL was launched in 2002 to study some of the most energetic phenomena in the universe. Image: ESA.

“These objects are puzzling,” says Enrico. “It might be that either the neutron star magnetic field does not decay substantially with time after all, or the neutron star actually formed later in the history of the binary system. That would mean it collapsed from a white dwarf into a neutron star as a result of feeding off the red giant over a long time, rather than becoming a neutron star as a result of a more traditional supernova explosion of a short-lived massive star.”

The next question is how long will this process go on? Is it short-lived, or the beginning of a long-term relationship?

“We haven’t seen this object before in the past 15 years of our observations with INTEGRAL, so we believe we saw the X-rays turning on for the first time,” says Erik Kuulkers, ESA’s INTEGRAL project scientist. “We’ll continue to watch how it behaves in case it is just a long ‘burp’ of winds, but so far we haven’t seen any significant changes.”

The INTEGRAL space observatory was launched in 2002 to study some of the most energetic phenomena in the universe. It focuses on things like black holes, neutron stars, active galactic nuclei and supernovae. INTEGRAL is a European Space Agency mission in cooperation with the United States and Russia. Its projected end date is December, 2018.

December 19, 2017 by Matt Williams

Kilonova Neutron Star Collision Probably Left Behind a Black Hole

Artist's illustration of two merging neutron stars. The narrow beams represent the gamma-ray burst while the rippling spacetime grid indicates the isotropic gravitational waves that characterize the merger. Swirling clouds of material ejected from the merging stars are a possible source of the light that was seen at lower energies. Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonnet

In February of 2016, scientists from the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced the first-ever detection of gravitational waves. A little over a century after they were first predicted by Einstein’s Theory of General Relativity, we finally had proof that this phenomenon existed. In August of 2017, another major breakthrough occurred when LIGO detected waves that were believed to be caused by a neutron star merger.

Shortly thereafter, scientists at LIGO, Advanced Virgo, and the Fermi Gamma-ray Space Telescope were able to determine where in the sky the neutron star merger occurred. While many studies have focused on the by-products of this merger, a new study by researchers from Trinity University, the University of Texas at Austin and Eureka Scientific, has chosen to focus on the remnant, which they claim is likely a black hole.

For the sake of their study, which recently appeared online under the title “GW170817 Most Likely Made a Black Hole“, the team consulted data from the Chandra X-ray Observatory to examine what resulted of the supernova merger. This data was obtained during Director’s Discretionary Time observations that were made on December 3rd and 6th, 2017, some 108 days after the merger.

This data showed a light-curve increase in the X-ray band which was compatible to the radio flux increase that was reported by a previous study conducted by the same team. These combined results suggest that radio and X-ray emissions were being produced at the same source, and that the rising light-curve that followed the merger was likely due to an increase in accelerated charged particles in the external shock – the region where an outflow of gas interacts with the interstellar medium.

As they indicate in their study, this could either be explained as the result of a more massive neutron star being formed from the merger, or a black hole:

“The merger of two neutron stars with mass 1.48 ± 0.12 M and 1.26 ± 0.1 M — where the merged object has a mass of 2.74 +0.04-0.01 M… could result in either a neutron star or a black hole. There might also be a debris disk that gets accreted onto the central object over a period of time, and which could be source of keV X-rays.”

The team also ruled out various possibilities of what could account for this rise in X-ray luminosity. Basically, they concluded that the X-ray photons were not coming from a debris disk, which would have been left over from the merger of the two neutron stars. They also deduced that they would not be produced by a relativistic jet spewing from the remnant, since the flux would be much lower after 102 days.

*Collisions of neutron stars produce powerful gamma-ray bursts – and heavy elements like gold. Credit: Dana Berry, SkyWorks Digital, Inc.*

All of this indicated that the remnant was more likely to be a black hole than a hyper-massive neutron star. As they explained:

“We show next that if the merged object were a hyper-massive neutron star endowed with a strong magnetic field, then the X-ray luminosity associated with the dipole radiation would be larger than the observed luminosity 10 days after the event, but much smaller than the observed flux at t ~ 100 days. This argues against the formation of a hyper-massive neutron star in this merger.”

Last, but not least, they considered the X-ray and radio emissions that were present roughly 100 days after the merger. These, they claim, are best explained by continued emissions coming from the merger-induced shock (and the not remnant itself) since these emissions would continue to propagate in the interstellar medium around the remnant. Combined with early X-ray data, this all points towards GW170817 now being a black hole.

The first-ever detection of gravitational waves signaled the dawn of a new era in astronomical research. Since that time, observatories like LIGO, Advanced Virgo, and GEO 600 have also benefited from information-sharing and new studies that have indicated that mergers are more common than previously thought, and that gravity waves could be used to probe the interior of supernovae.

With this latest study, scientists have learned that they are not only able to detect the waves caused by black hole mergers, but even the creation thereof. At the same time, it shows how the study of the Universe is growing. Not only is astronomy advancing to the point where we are able to study more and more of the visible Universe, but the invisible Universe as well.

Further Reading: LIGO, arXiv

September 7, 2017 by Matt Williams

Second Fastest Pulsar Spins 42,000 Times a Minute

Artist's illustration of a rotating neutron star, the remnants of a super nova explosion. Credit: NASA, Caltech-JPL

Pulsars are what remains when a massive star undergoes gravitational collapse and explodes in a supernova. These remnants (also known as neutron stars) are extremely dense, with several Earth-masses crammed into a space the size of a small country. They also have powerful magnetic fields, which causes them to rotate rapidly and emit powerful beams of gamma rays or x-rays – which lends them the appearance of a lighthouse.

In some cases, pulsars spin especially fast, taking only milliseconds to complete a single rotation. These “millisecond pulsars” remain a source of mystery for astronomers. And after following up on previous observations, researchers using the Low Frequency Array (LOFAR) radio telescope in the Netherlands identified a pulsar (PSR J0952?0607) that spins more than 42,000 times per minute, making it the second-fastest pulsar ever discovered.

The study which described their findings, titled “LOFAR Discovery of the Fastest-spinning Millisecond Pulsar in the Galactic Field“, recently appeared in The Astrophysical Journal Letters. Led by Dr. Cees Bassa, an astrophysicist from the University of Utrecht and the Netherlands Institute for Radio Astronomy (ASTRON), the team conducted follow-up observations of PSR J0952?0607, a millisecond pulsar located 3,200 to 5,700 light-years away.

*An all-sky view in gamma ray light made with the Fermi gamma ray space telescope. Credit: NASA/DOE/International LAT Team*

This study was part of an ongoing LOFAR survey of energetic sources originally identified by NASA’s Fermi Gamma-ray space telescope. The purpose of this survey was to distinguish between the gamma-ray sources Fermi detected, which could have been caused by neutron stars, pulsars, supernovae or the regions around black holes. As Elizabeth Ferrara, a member of the discovery team at NASA’s Goddard Space Center, explained in a NASA press release:

“Roughly a third of the gamma-ray sources found by Fermi have not been detected at other wavelengths. Many of these unassociated sources may be pulsars, but we often need follow-up from radio observatories to detect the pulses and prove it. There’s a real synergy across the extreme ends of the electromagnetic spectrum in hunting for them.”

Their follow-up observations indicated that this particular source was a pulsar that spins at a rate of 707 revolutions (Hz) per second, which works out to 42,000 revolutions per minute. This makes it, by definition, a millisecond pulsar. The team also confirmed that it is about 1.4 Solar Masses and is orbited every 6.4 hours by a companion star that has been stripped down to less than 0.05 Jupiter masses.

The presence of this lightweight companion is a further indication of how the spin of this pulsar became so rapid. Over time, matter would have been stripped away from the star, gradually accreting onto PSR J0952?0607. This would not only raise its spin rate but also greatly increase its electromagnetic emissions. The process continues to this day, with the star becoming increasingly smaller as the pulsar becomes more energetic.

Because of the nature of this relationship (which can only be described as “cannibalistic”), systems like PSR J0952?0607 are often called “black widow” or “redback” pulsars. Most of these systems were found by following up on sources identified by the Fermi mission, since the process has been known to result in a considerable amount of electromagnetic radiation being released.

Beyond the discovery of this record-setting pulsar, the LOFAR discovery could also be an indication that there is a new population of ultra-fast spinning pulsars in our Universe. As Dr. Bassa explained:

“LOFAR picked up pulses from J0952 at radio frequencies around 135 MHz, which is about 45 percent lower than the lowest frequencies of conventional radio searches. We found that J0952 has a steep radio spectrum, which means its radio pulses fade out very quickly at higher frequencies. It would have been a challenge to find it without LOFAR.”

The fastest spinning pulsar known, PSR J1748-2446ad, spins just slightly faster than PSR J0952?0607 – reaching a rate of nearly 43,000 rpm (or 716 revolutions per second). But some theorists think that pulsars could spin as fast as 72,000 rpm (almost twice as fast) before breaking up. This remains a theory, since rapidly-spinning pulsars are rather difficult to detect.

But with the help of instrument like LOFAR, that could be changing. For instance, both PSR J1748-2446ad and PSR J0952?0607 were shown to have steep spectra – much like radio galaxies and Active Galactic Nuclei. The same was true of J1552+5437, another millisecond pular detected by LOFAR which spins at 25,000 rpm.

As Ziggy Pleunis – a doctoral student at McGill University in Montreal and a co-author on the study – indicated, this could be a sign that the fastest-spinning pulsars are just waiting to be found.

“There is growing evidence that the fastest-spinning pulsars tend to have the steepest spectra,” he said. “Since LOFAR searches are more sensitive to these steep-spectrum radio pulsars, we may find that even faster pulsars do, in fact, exist and have been missed by surveys at higher frequencies.”

As with many other areas of astronomical research, improvements in instrumentation and methodology are allowing for new and exciting discoveries. As expected, some of the things we are finding are forcing astronomers to rethink more than a few previously-held assumptions about the nature and limits of certain phenomena.

Be sure to enjoy this NASA video that explains “black widow” pulsars and the ongoing search to find them:

Further Reading: NASA, Astrophysical Journal Letters

June 18, 2017 by Fraser Cain

Gravitational Astronomy? How Detecting Gravitational Waves Changes Everything

Just a couple of weeks ago, astronomers from Caltech announced their third detection of gravitational waves from the Laser Interferometer Gravitational-Wave Observatory or LIGO.

As with the previous two detections, astronomers have determined that the waves were generated when two intermediate-mass black holes slammed into each other, sending out ripples of distorted spacetime.

One black hole had 31.2 times the mass of the Sun, while the other had 19.4 solar masses. The two spiraled inward towards each other, until they merged into a single black hole with 48.7 solar masses. And if you do the math, twice the mass of the Sun was converted into gravitational waves as the black holes merged.

On January 4th, 2017, LIGO detected two black holes merging into one. Courtesy Caltech/MIT/LIGO Laboratory

These gravitational waves traveled outward from the colossal collision at the speed of light, stretching and compressing spacetime like a tsunami wave crossing the ocean until they reached Earth, located about 2.9 billion light-years away.

The waves swept past each of the two LIGO facilities, located in different parts of the United States, stretching the length of carefully calibrated laser measurements. And from this, researchers were able to detect the direction, distance and strength of the original merger.

Seriously, if this isn’t one of the coolest things you’ve ever heard, I’m clearly easily impressed.

Now that the third detection has been made, I think it’s safe to say we’re entering a brand new field of gravitational astronomy. In the coming decades, astronomers will use gravitational waves to peer into regions they could never see before.

Being able to perceive gravitational waves is like getting a whole new sense. It’s like having eyes and then suddenly getting the ability to perceive sound.

This whole new science will take decades to unlock, and we’re just getting started.

As Einstein predicted, any mass moving through space generates ripples in spacetime. When you’re just walking along, you’re actually generating tiny ripples. If you can detect these ripples, you can work backwards to figure out what size of mass made the ripples, what direction it was moving, etc.

Even in places that you couldn’t see in any other way. Let me give you a couple of examples.

Black holes, obviously, are the low hanging fruit. When they’re not actively feeding, they’re completely invisible, only detectable by how they gravitational attract objects or bend light from objects passing behind them.

But seen in gravitational waves, they’re like ships moving across the ocean, leaving ripples of distorted spacetime behind them.

With our current capabilities through LIGO, astronomers can only detect the most massive objects moving at a significant portion of the speed of light. A regular black hole merger doesn’t do the trick – there’s not enough mass. Even a supermassive black hole merger isn’t detectable yet because these mergers seem to happen too slowly.

LIGO has already significantly increased the number of black holes with known masses. The observatory has definitively detected two sets of black hole mergers (bright blue). For each event, LIGO determined the individual masses of the black holes before they merged, as well as the mass of the black hole produced by the merger. The black holes shown with a dotted border represent a LIGO candidate event that was too weak to be conclusively claimed as a detection. Credit: LIGO/Caltech/Sonoma State (Aurore Simonnet)

This is why all the detections so far have been intermediate-mass black holes with dozens of times the mass of our Sun. And we can only detect them at the moment that they’re merging together, when they’re generating the most intense gravitational waves.

If we can boost the sensitivity of our gravitational wave detectors, we should be able to spot mergers of less and more massive black holes.

But merging isn’t the only thing they do. Black holes are born when stars with many more times the mass of our Sun collapse in on themselves and explode as supernovae. Some stars, we’ve now learned just implode as black holes, never generating the supernovae, so this process happens entirely hidden from us.

Is there a singularity at the center of a black hole event horizon, or is there something there, some kind of object smaller than a neutron star, but bigger than an infinitely small point? As black holes merge together, we could see beyond the event horizon with gravitational waves, mapping out the invisible region within to get a sense of what’s going on down there.

This illustration shows the merger of two black holes and the gravitational waves that ripple outward as the black holes spiral toward each other. In reality, the area near the black holes would appear highly warped, and the gravitational waves would be difficult to see directly. Credit: LIGO/T. Pyle

We want to know about even less massive objects like neutron stars, which can also form from a supernova explosion. These neutron stars can orbit one another and merge generating some of the most powerful explosions in the Universe: gamma ray bursts. But do neutron stars have surface features? Different densities? Could we detect a wobble in the gravitational waves in the last moments before a merger?

And not everything needs to merge. Sensitive gravitational wave detectors could sense binary objects with a large imbalance, like a black hole or neutron star orbiting around a main sequence star. We could detect future mergers by their gravitational waves.

Are gravitational waves a momentary distortion of spacetime, or do they leave some kind of permanent dent on the Universe that we could trace back? Will we see echoes of gravity from gravitational waves reflecting and refracting through the fabric of the cosmos?

Perhaps the greatest challenge will be using gravitational waves to see beyond the Cosmic Microwave Background Radiation. This region shows us the Universe 380,000 years after the Big Bang, when everything was cool enough for light to move freely through the Universe.

But there was mass there, before that moment. Moving, merging mass that would have generated gravitational waves. As we explained in a previous article, astronomers are working to find the imprint of these gravitational waves on the Cosmic Microwave Background, like an echo, or a shadow. Perhaps there’s a deeper Cosmic Gravitational Background Radiation out there, one which will let us see right to the beginning of time, just moments after the Big Bang.

And as always, there will be the surprises. The discoveries in this new field that nobody ever saw coming. The “that’s funny” moments that take researchers down into whole new fields of discovery, and new insights into how the Universe works.

The Laser Interferometer Gravitational-Wave Observatory (LIGO)facility in Livingston, Louisiana. The other facility is located in Hanford, Washington. Image: LIGO — The Laser Interferometer Gravitational-Wave Observatory (LIGO) facility in Livingston, Louisiana. The other facility is located in Hanford, Washington. Image: LIGO

The LIGO project was begun back in 1994, and the first iteration operated from 2002 to 2012 without a single gravitational wave detection. It was clear that the facility wasn’t sensitive enough, so researchers went back and made massive improvements.

In 2008, they started improving the facility, and in 2015, Advanced LIGO came online with much more sensitivity. With the increased capabilities, Advanced LIGO made its first discovery in 2016, and now two more discoveries have been added.

LIGO can currently only detect the general hemisphere of the sky where a gravitational wave was emitted. And so, LIGO’s next improvement will be to add another facility in India, called INDIGO. In addition to improving the sensitivity of LIGO, this will give astronomers three observations of each event, to precisely detect the origin of the gravitational waves. Then visual astronomers could do follow up observations, to map the event to anything in other wavelengths.

Current operating facilities in the global network include the twin LIGO detectors—in Hanford, Washington, and Livingston, Louisiana—and GEO600 in Germany. The Virgo detector in Italy and the Kamioka Gravitational Wave Detector (KAGRA) in Japan are undergoing upgrades and are expected to begin operations in 2016 and 2018, respectively. A sixth observatory is being planned in India. Having more gravitational-wave observatories around the globe helps scientists pin down the locations and sources of gravitational waves coming from space. Image made in February 2016. Credit: Caltech/MIT/LIGO Lab

A European experiment known as Virgo has been operating for a few years as well, agreeing to collaborate with the LIGO team if any detections are made. So far, the Virgo experiment hasn’t found anything, but it’s being upgraded with 10 times the sensitivity, which should be fully operational by 2018.

A Japanese experiment called the Kamioka Gravitational Wave Detector, or KAGRA, will come online in 2018 as well, and be able to contribute to the observations. It should be capable of detecting binary neutron star mergers out to nearly a billion light-years away.

Just with visual astronomy, there are a set of next generation supergravitational wave telescopes in the works, which should come online in the next few decades.

The Europeans are building the Einstein Telescope, which will have detection arms 10 km long, compared to 4 km for LIGO. That’s like, 6 more km.

There’s the European Space Agency’s space-based Laser Interferometer Space Antenna, or LISA, which could launch in 2030. This will consist of a fleet of 3 spacecraft which will maintain a precise distance of 2.5 million km from each other. Compare that to the Earth-based detection distances, and you can see why the future of observations will come from space.

The Laser Interferometer Space Antenna (LISA) consists of three spacecraft orbiting the sun in a triangular configuration. Credit: NASA

And that last idea, looking right back to the beginning of time could be a possibility with the Big Bang Observer mission, which will have a fleet of 12 spacecraft flying in formation. This is still all in the proposal stage, so no concrete date for if or when they’ll actually fly.

Gravitational wave astronomy is one of the most exciting fields of astronomy. This entirely new sense is pushing out our understanding of the cosmos in entirely new directions, allowing us to see regions we could never even imagine exploring before. I can’t wait to see what happens next.

May 1, 2017 by Matt Williams

An Aging Pulsar has Captured a new Companion, and it’s Spinning back up Again

When massive stars reach the end of their life cycle, they explode in a massive supernova and cast off most of their material. What’s left is a “milliscond pulsar”, a super dense, highly-magnetized neutron star that spins rapidly and emit beams of electromagnetic radiation. Eventually, these stars lose their rotational energy and begin to slow down, but they can speed up again with the help of a companion.

According to a recent study, an international team of scientists witnessed this rare event when observing an ultra-slow pulsar located in the neighboring Andromeda Galaxy (XB091D). The results of their study indicated that this pulsar has been speeding up for the past one million years, which is likely the result of a captured a companion that has since been restoring its rapid rotational velocity.

Typically, when a pulsars pairs with an ordinary star, the result is a binary system consisting of a pulsar and a white dwarf. This occurs after the pulsar pulls off the outer layers of a star, turning it into a white dwarf. The material from these outer layer then forms an accretion disk around the pulsar, which creates a “hot spot” that radiates brightly in the X-ray specturum and where temperatures can reach into the millions of degrees.

The team was led by Ivan Zolotukhin of the Sternberg Astronomical Institute at Lomonosov Moscow State University (MSU), and included astronomers from the University of Toulouse, the National Institute for Astrophysics (INAF), and the Smithsonian Astrophysical Observatory. The study results were published in The Astrophysical Journal under the title “The Slowest Spinning X-Ray Pulsar in an Extragalactic Globular Cluster“.

As they state in their paper, the detection of this pulsar was made possible thanks to data collected by the XMM-Newton space observatory from 2000-2013. In this time, XMM-Newton has gathered information on approximately 50 billion X-ray photons, which has been combined by astronomers at Lomosov MSU into an open online database.

This database has allowed astronomers to take a closer look at many previously-discovered objects. This includes XB091D, a pulsar with a period of seconds (aka. a “second pulsar”) located in one of the oldest globular star clusters in the Andromeda galaxy. However, finding the X-ray photos that would allow them to characterize XB091D was no easy task. As Ivan Zolotukhin explained in a MSU press release:

“The detectors on XMM-Newton detect only one photon from this pulsar every five seconds. Therefore, the search for pulsars among the extensive XMM-Newton data can be compared to the search for a needle in a haystack. In fact, for this discovery we had to create completely new mathematical tools that allowed us to search and extract the periodic signal. Theoretically, there are many applications for this method, including those outside astronomy.”

*The slowest spinning X-ray pulsar in a globular star cluster has been discovered in the Andromeda galaxy. Credit: A. Zolotov*

Based on a total of 38 XMM-Newton observations, the team concluded that this pulsar (which was the only known pulsar of its kind beyond our galaxy at the time), is in the earliest stages of “rejuvenation”. In short, their observations indicated that the pulsar began accelerating less than 1 million years ago. This conclusion was based on the fact that XB091D is the slowest rotating globular cluster pulsar discovered to date.

The neutron star completes one revolution in 1.2 seconds, which is more than 10 times slower than the previous record holder. From the data they observed, they were also able to characterize the environment around XB091D. For example, they found that the pulsar and its binary pair are located in an extremely dense globular cluster (B091D) in the Andromeda Galaxy – about 2.5 million light years away.

This cluster is estimated to be 12 billion years old and contains millions of old, faint stars. It’s companion, meanwhile, is a 0.8 solar mass star, and the binary system itself has a rotation period of 30.5 hours. And in about 50,000 years, they estimate, the pulsar will accelerate sufficiently to once again have a rotational period measured in the milliseconds – i.e. a millisecond pulsar.

*A diagram of the ESA XMM-Newton X-Ray Telescope. Delivered to orbit by a Ariane 5 launch vehicle in 1999. Credit: ESA/XMM-Newton*

Interestingly, XB910D’s location within this vast region of super-high density stars is what allowed it to capture a companion about 1 million years ago and commence the process “rejuvenation” in the first place. As Zolotukhin explained:

“In our galaxy, no such slow X-ray pulsars are observed in 150 known globular clusters, because their cores are not big and dense enough to form close binary stars at a sufficiently high rate. This indicates that the B091D cluster core, with an extremely dense composition of stars in the XB091D, is much larger than that of the usual cluster. So we are dealing with a large and rather rare object—with a dense remnant of a small galaxy that the Andromeda galaxy once devoured. The density of the stars here, in a region that is about 2.5 light years across, is about 10 million times higher than in the vicinity of the Sun.”

Thanks to this study, and the mathematical tools the team developed to find it, astronomers will likely be able to revisit many previously-discovered objects in the coming years. Within these massive data sets, there could be many examples of rare astronomical events, just waiting to be witnessed and properly characterized.

Further reading: The Astrophysical Journal, Lomonosov Moscow State University

April 26, 2017April 26, 2017 by Fraser Cain

What Are Fast Radio Bursts?

You might think you’re reading an educational website, where I explain fascinating concepts in space and astronomy, but that’s not really what’s going on here.

What’s actually happening is that you’re tagging along as I learn more and more about new and cool things happening in the Universe. I dig into them like a badger hiding a cow carcass, and we all get to enjoy the cache of knowledge I uncover.

Okay, that analogy got a little weird. Anyway, my point is. Squirrel!

Fast radio bursts are the new cosmic whatzits confusing and baffling astronomers, and now we get to take a front seat and watch them move through all stages of process of discovery.

Stage 1: A strange new anomaly is discovered that doesn’t fit any current model of the cosmos. For example, strange Boyajian’s Star. You know, that star that probably doesn’t have an alien megastructure orbiting around it, but astronomers can’t rule that out just yet?

Stage 2: Astronomers struggle to find other examples of this thing. They pitch ideas for new missions and scientific instruments. No idea is too crazy, until it’s proven to be too crazy. Examples include dark matter, dark energy, and that idea that we’re living in a

Stage 3: Astronomers develop a model for the thing, find evidence that matches their predictions, and vast majority of the astronomical community comes to a consensus on what this thing is. Like quasars and gamma ray bursts. YouTuber’s make their videos. Textbooks are updated. Balance is restored.

Today we’re going to talk about Fast Radio Bursts. They just moved from Stage 1 to Stage 2. Let’s dig in.

Fast radio bursts, or FRBs, or “Furbys” were first detected in 2007 by the astronomer Duncan Lorimer from West Virginia University.

He was looking through an archive of pulsar observations. Pulsars, of course, are newly formed neutron stars, the remnants left over from supernova explosions. They spin rapidly, blasting out twin beams of radiation. Some can spin hundreds of times a second, so precisely you could set your watch to them.

Lorimer’s archive of pulsar observations was captured at the Parkes radio dish in Australia. Credit: CSIRO (CC BY 3.0)

In this data, Lorimer made a “that’s funny” observation, when he noticed one blast of radio waves that squealed for 5 milliseconds and then it was gone. It didn’t match any other observation or prediction of what should be out there, so astronomers set out to find more of them.

Over the last 10 years, astronomers have found about 25 more examples of Fast Radio Bursts. Each one only lasts a few milliseconds, and then fades away forever. A one time event that can appear anywhere in the sky and only last for a couple milliseconds and never repeats is not an astronomer’s favorite target of study.

Actually, one FRB has been found to repeat, maybe.

The question, of course, is “what are they?”. And the answer, right now is, “astronomers have no idea.”

In fact, until very recently, astronomers weren’t ever certain they were coming from space at all. We’re surrounded by radio signals all the time, so a terrestrial source of fast radio bursts seems totally logical.

About a week ago, astronomers from Australia announced that FRBs are definitely coming from outside the Earth. They used the Molonglo Observatory Synthesis Telescope (or MOST) in Canberra to gather data on a large patch of sky.

Then they sifted through 1,000 terabytes of data and found just 3 fast radio bursts. Three.

Since MOST is farsighted and can’t perceive any radio signals closer than 10,000 km away, the signals had to be coming outside planet Earth. They were “extraterrestrial” in origin.

Right now, fast radio bursts are infuriating to astronomers. They don’t seem to match up with any other events we can see. They’re not the afterglow of a supernova, or tied in some way to gamma ray bursts.

In order to really figure out what’s going on, astronomers need new tools, and there’s a perfect instrument coming. Astronomers are building a new telescope called the Canadian Hydrogen Intensity Mapping Experiment (or CHIME), which is under construction near the town of Penticton in my own British Columbia.

CHIME under construction in Penticton, British Columbia. Credit: Mateus A. Fandiño (CC BY-SA 4.0)

It looks like a bunch of snowboard halfpipes, and its job will be to search for hydrogen emission from distant galaxies. It’ll help us understand how the Universe was expanding between 7 and 11 billion years ago, and create a 3-dimensional map of the early cosmos.

In addition to this, it’s going to be able to detect hundreds of fast radio bursts, maybe even a dozen a day, finally giving astronomers vast pools of signals to study.

What are they? Astronomers have no idea. Seriously, if you’ve got a good suggestion, they’d be glad to hear it.

In these kinds of situations, astronomers generally assume they’re caused by exploding stars in some way. Young stars or old stars, or maybe stars colliding. But so far, none of the theoretical models match the observations.

This artist’s conception illustrates one of the most primitive supermassive black holes known (central black dot) at the core of a young, star-rich galaxy. Image credit: NASA/JPL-Caltech

Another idea is black holes, of course. Specifically, supermassive black holes at the hearts of distant galaxies. From time to time, a random star, planet, or blob of gas falls into the black hole. This matter piles upon the black hole’s event horizon, heats up, screams for a moment, and disappears without a trace. Not a full on quasar that shines for thousands of years, but a quick snack.

The next idea comes with the only repeating fast radio burst that’s ever been found. Astronomers looked through the data archive of the Arecibo Observatory in Puerto Rico and found a signal that had repeated at least 10 times in a year, sometimes less than a minute apart.

Since the quick blast of radiation is repeating, this rules out a one-time collision between exotic objects like neutron stars. Instead, there could be a new class of magnetars (which are already a new class of neutron stars), that can release these occasional shrieks of radio.

An artist’s impression of a magnetar. Credit: ESO/L. Calçada

Or maybe this repeating object is totally different from the single events that have been discovered so far.

Here’s my favorite idea. And honestly, the one that’s the least realistic. What I’m about to say is almost certainly not what’s going on. And yet, it can’t be ruled out, and that’s good enough for my fertile imagination.

Avi Loeb and Manasvi Lingam at Harvard University said the following about FRBs:

“Fast radio bursts are exceedingly bright given their short duration and origin at distances, and we haven’t identified a possible natural source with any confidence. An artificial origin is worth contemplating and checking.”

Artificial origin. So. Aliens. Nice.

Loeb and Lingam calculated how difficult it would be to send a signal that strong, that far across the Universe. They found that you’d need to build a solar array with twice the surface area of Earth to power the radio wave transmitter.

And what would you do with a transmission of radio or microwaves that strong? You’d use it to power a spacecraft, of course. What we’re seeing here on Earth is just the momentary flash as a propulsion beam sweeps past the Solar System like a lighthouse.

But in reality, this huge solar array would be firing out a constant beam of radiation that would propel a massive starship to tremendous speeds. Like the Breakthrough Starshot spacecraft, but for million tonne spaceships.

In other words, we could be witnessing alien transportation systems, pushing spacecraft with beams of energy to other worlds.

And I know that’s probably not what’s happening. It’s not aliens. It’s never aliens. But in my mind, that’s what I’m imagining.

So, kick back and enjoy the ride. Join us as we watch astronomers struggle to understand what fast radio bursts are. As they invalidate theories, and slowly unlock one of the most thrilling mysteries in modern astronomy. And as soon as they figure it out, I’ll let you know all about it.

What do you think? Which explanation for fast radio bursts seems the most logical to you? I’d love to hear your thoughts and wild speculation in the comments.

November 29, 2016November 29, 2016 by Fraser Cain

What Are Multiple Star Systems?

When we do finally learn the full truth about our place in the galaxy, and we’re invited to join the Galactic Federation of Planets, I’m sure we’ll always be seen as a quaint backwater world orbiting a boring single star.

The terrifying tentacle monsters from the nightmare tentacle world will gurgle horrifying, but clearly condescending comments about how we’ve only got a single star in the Solar System.

The beings of pure energy will remark how only truly enlightened civilizations can come from systems with at least 6 stars, insulting not only humanity, but also the horrifying tentacle monsters, leading to another galaxy spanning conflict.

Yes, we’ll always be making up for our stellar deficit in the eyes of aliens, or whatever those creepy blobs use for eyes.

What we lack in sophistication, however, we make up in volume. In our Milky Way, fully 2/3rds of star systems only have a single star. The last 1/3rd is made up of multiple star systems.

The Milky Way as seen from Devil's Tower, Wyoming. Image Credit: Wally Pacholka — The Milky Way as seen from Devil’s Tower, Wyoming. Image Credit: Wally Pacholka

We’re taking binary stars, triple star systems, even exotic 7 star systems. When you mix and match different types of stars in various Odd Couple stellar apartments, the results get interesting.

Consider our own Solar System, where the Sun and planets formed together out a cloud of gas and dust. Gravity collected material into the center of the Solar System, becoming the Sun, while the rest of the disk spun up faster and faster. Eventually our star ignited its fusion furnace, blasting out the rest of the stellar nebula.

But different stellar nebulae can lead to the formation of multiple stars instead. What you get depends on the mass of the cloud, and how fast it’s rotating.

Check out this amazing photograph of a multiple star system forming right now.

ALMA image of the L1448 IRS3B system, with two young stars at the center and a third distant from them. Spiral structure in the dusty disk surrounding them indicates instability in the disk, astronomers said. Credit: Bill Saxton, ALMA (ESO/NAOJ/NRAO), NRAO/AUI/NSF

In this image, you can see three stars forming together, two at the center, about 60 astronomical units away from each other (60 times the distance from the Earth to the Sun), and then a third orbiting 183 AU away.

It’s estimated these stars are only 10,000 to 20,000 years old. This is one of the most amazing astronomy pictures I ever seen.

When you have two stars, that’s a binary system. If the stars are similar in mass to each other, then they orbit a common point of mass, known as the barycenter. If the stars are different masses, then it can appear that one star is orbiting the other, like a planet going around a star.

When you look up in the sky, many of the single stars you see are actually binary stars, and can be resolved with a pair of binoculars or a small telescope. For example, in a good telescope, Alpha Centauri can be resolved into two equally bright stars, with the much dimmer Proxima Centauri hanging out nearby.

The two bright stars are (left) Alpha Centauri and (right) Beta Centauri. The faint red star in the center of the red circle is Proxima Centauri. Credit: Skatebiker at English Wikipedia (CC BY-SA 3.0)

You have to be careful, though, sometimes stars just happen to be beside each other in the sky, but they’re not actually orbiting one another – this is known as an optical binary. It’s a trap.

Astronomers find that you can then get binary stars with a third companion orbiting around them. As long as the third star is far enough away, the whole system can be stable. This is a triple star system.

You can get two sets of binary stars orbiting each other, for a quadruple star system.

In fact, you can build up these combinations of stars up. For example, the star system Nu Scorpii has 7 stars in a single system. All happily orbiting one another for eons.

If stars remained unchanging forever, then this would be the end of our story. However, as we’ve discussed in other articles, stars change over time, bloating up as red giants, detonating as supernovae and turning into bizarre objects, like white dwarfs, neutron stars and even black holes. And when these occur in multiple star systems, well, watch the sparks fly.

There are a nearly infinite combinations you can have here: main sequence, red giant, white dwarf, neutron star, and even black holes. I don’t have time to go through all the combinations, but here are some highlights.

This artist’s impression shows VFTS 352 — the hottest and most massive double star system to date where the two components are in contact and sharing material. The two stars in this extreme system lie about 160 000 light-years from Earth in the Large Magellanic Cloud. This intriguing system could be heading for a dramatic end, either with the formation of a single giant star or as a future binary black hole. ESO/L. Calçada — VFTS 352 is the hottest and most massive double star system to date where the two components are in contact and sharing material. ESO/L. Calçada

For starters, binary stars can get so close they actually touch each other. This is known as a contact binary, where the two stars actually share material back and forth. But it gets even stranger.

When a main sequence star like our Sun runs out of hydrogen fuel in its core, it expands as a red giant, before cooling and becoming a white dwarf.

When a red giant is in a binary system, the distance and evolution of its stellar companion makes all the difference.

If the two stars are close enough, the red giant can pass material over to the other star. And if the red giant is large enough, it can actually engulf its companion. Imagine our Sun, orbiting within the atmosphere of a red giant star. Needless to say, that’s not healthy for any planets.

An even stranger contact binary happens when a red giant consumes a binary neutron star. This is known as a Thorne-Zytkow object. The neutron star spirals inward through the atmosphere of the red giant. When it reaches the core, it either becomes a black hole, gobbling up the red giant from within, or an even more massive neutron star. This is exceedingly rare, and only one candidate object has ever been observed.

A Type Ia supernova occurs when a white dwarf accretes material from a companion star until it exceeds the Chandrasekhar limit and explodes. By studying these exploding stars, astronomers can measure dark energy and the expansion of the universe. CfA scientists have found a way to correct for small variations in the appearance of these supernovae, so that they become even better standard candles. The key is to sort the supernovae based on their color. Credit: NASA/CXC/M. Weiss — A white dwarf accreting material from a companion star. Credit: NASA/CXC/M. Weiss

When a binary pair is a white dwarf, the dead remnant of a star like our Sun, then material can transfer to the surface of the white dwarf, causing novae explosions. And if enough material is transferred, the white dwarf explodes as a Type 1A supernova.

If you’re a star that was unlucky enough to be born beside a very massive star, you can actually kicked off into space when it explodes as a supernova. In fact, there are rogue stars which such a kick, they’re on an escape trajectory from the entire galaxy, never to return.

If you have two neutron stars in a binary pair, they release energy in the form of gravitational waves, which causes them to lose momentum and spiral inward. Eventually they collide, becoming a black hole, and detonating with so much energy we can see the explosions billions of light-years away – a short-period gamma ray burst.

The combinations are endless.

How Earth could look with two suns. Credit: NASA/JPL-Caltech/Univ. of Ariz.

It’s amazing to think what the night sky would look like if we were born into a multiple star system. Sometimes there would be several stars in the sky, other times just one. And rarely, there would be an actual night.

How would life be different in a multiple star system? Let me know your thoughts in the comments.

In our next episode, we try to untangle this bizarre paradox. If the Universe is infinite, how did it start out as a singularity? That doesn’t make any sense.

We glossed over it in this episode, but one of the most interesting effects of multiple star systems are novae, explosions of stolen material on the surface of a white dwarf star. Learn more about it in this video.

November 7, 2016November 11, 2016 by Matt Williams

A Pulsar and White Dwarf Dance Together In A Surprising Orbit

Artist’s impression of the exotic double object that consists of a tiny neutron star orbited every two and a half hours by a white dwarf star. Credit: ESO/L. Calçada

Searching the Universe for strange new star systems can lead to some pretty interesting finds. And sometimes, it can turn up phenomena that contradict everything we think we know about the formation and evolution of stars. Such finds are not only fascinating and exciting, they allow us the chance to expand and refine our models of how the Universe came to be.

For instance, a recent study conducted by an international team of scientists has shown how the recent discovery of binary system – a millisecond pulsar and a low-mass white dwarf (LMWD) – has defied conventional ideas of stellar evolution. Whereas such systems were believed to have circular orbits in the past, the white dwarf in this particular binary orbits the pulsar with extreme eccentricity!

To break it down, conventional wisdom states that LMWDs are the product of binary evolution. The reason for this is because that under normal circumstances, such a star – with low mass but incredible density – would only form after it has exhausted all its nuclear fuel and lost its outer layers as a planetary nebula. Given the mass of this star, this would take about 100 billion years to happen on its own – i.e. longer than the age of the Universe.

An artist's impression of an accreting X-ray millisecond pulsar. The flowing material from the companion star forms a disk around the neutron star which is truncated at the edge of the pulsar magnetosphere. Credit: NASA / Goddard Space Flight Center / Dana Berry — An artist’s impression of an accreting X-ray millisecond pulsar. The flowing material from the companion star forms a disk around the neutron star which is truncated at the edge of the pulsar magnetosphere. Credit: NASA/Goddard/Dana Berry

As such, they are generally believed to be the result of pairing with other stars – specifically, millisecond radio pulsars (MSPs). These are a distinct population of neutron stars that have fast spin periods and magnetic fields that are several orders of magnitude weaker than that of “normal” pulsars. These properties are thought to be the result of mass transfer with a companion star.

Basically, MSPs that are orbited by a star will slowly strip them of their mass, sucking off their outer layers and turning them into a white dwarf. The addition of this mass to the pulsar causes it to spin faster and buries its magnetic field, and also strips the companion star down to a white dwarf. In this scenario, the eccentricity of orbit of the LMWD around the pulsar is expected to be negligible.

However, when looking to the binary star system PSR J2234+0511, the international team noticed something entirely different. Here, they found a low-mass white dwarf paired with a millisecond pulsar which the white dwarf orbited with a period of 32 days and an extreme eccentricity (0.13). Since this defies current models of white dwarf stars, the team began looking for explanations.

As Dr. John Antoniadis – a researcher from the Dunlap Institute at University of Toronto and the lead author of the study – told Universe Today via email:

“Millisecond pulsar-LMWD binaries are very common. According to the established formation scenario, these systems evolve from low-mass X-ray binaries in which a neutron star accretes matter from a giant star. Eventually, this star evolves into a white dwarf and the neutron star becomes a millisecond pulsar. Because of the strong tidal forces during the mass-transfer episode, the orbits of these systems are extremely circular, with eccentricities of ~0.000001 or so.”

An artist's impression of a millisecond pulsar and its companion. The pulsar (seen in blue with two radiation beams) is accreting material from its bloated red companion star and increasing its rotation rate. Astronomers have measured the orbital parameters of four millisecond pulsars in the globular cluster 47 Tuc and modeled their possible formation and evolution paths. Credit: European Space Agency & Francesco Ferraro (Bologna Astronomical Observatory) — An artist’s impression of a millisecond pulsar and its companion. The pulsar (blue) is accreting material from its bloated red companion star and increasing its rotation rate. Credit: ESA/Francesco Ferraro (Bologna Astronomical Observatory)

For the sake of their study, which appeared recently in The Astrophysical Journal – titled “An Eccentric Binary Millisecond Pulsar with a Helium White Dwarf Companion in the Galactic Field” – the team relied on newly obtained optical photometry of the system provided by the Sloan Digital Sky Survey (SDSS), and spectroscopy from the Very Large Telescope from the Paranal Observatory in Chile.

In addition, they consulted recent studies that looked at other binary star systems that show this same kind of eccentric relationship. “We now know [of] 5 systems which deviate from this picture in that they have eccentricities of ~0.1 i.e. several orders of magnitude larger that what is expected in the standard scenario,” said Antoniadis. “Interestingly, they all appear to have similar eccentricities and orbital periods.”

From this, they were able to infer the temperature (8600 ± 190 K) and velocity ( km/s) of the white dwarf companion in the binary star system. Combined with constraints placed on the two body’s masses – 0.28 Solar Masses for the white dwarf and 1.4 for the pulsar – as well as their radii and surface gravity, they then tested three possible explanations for how this system came to be.

These included the possibility that neutrons stars (such as the millsecond pulsar being observed here) form through an accretion-induced collapse of a massive white dwarf. Similarly, they considered whether neutron stars undergo a transformation as they accrete material, which results in them becoming quark stars. During this process, the release of gravitational energy would be responsible for inducing the observed eccentricity.

Second, they considered the possibility – consistent with current models of stellar evolution – that LMWDs within a certain mass range have strong stellar winds when they are very young (due to unstable hydrogen fusion). The team therefore looked at whether or not these strong stellar winds could have been what disrupted the orbit of the pulsar earlier in the system’s history.

Last, they considered the possibility that some of the material released from the white dwarf in the past (due to this same stellar wind) could have formed a short-lived circumbinary disk. This disk would then act like a third body, disturbing the system and increasing the eccentricity of the white dwarf’s orbit. In the end, they deemed that the first two scenarios were unlikely, since the mass inferred for the pulsar progenitor was not consistent with either model.

However, the third scenario, in which interaction with a circumbinary disk was responsible for the eccentricity, was consistent with their inferred parameters. What’s more, the third scenario predicts how (within a certain mass range) that there should be no circular binaries with similar orbital periods – which is consistent with all known examples of such systems. As Dr. Antoniadis explained:

“These observations show that the companion star in this system is indeed a low-mass white dwarf. In addition, the mass of the pulsar seems to be too low for #2 and a bit too high for #1. We also study the orbit of the binary in the Milky way, and it looks very similar to what we find for low-mass X-ray binaries. These pieces of evidence together favor the disk hypothesis.”

Of course, Dr. Antoniadis and his colleagues admit that more information is needed before their hypothesis can be deemed correct. However, should their results be borne out by future research, then they anticipate that it will be a valuable tool for future astronomers and astrophysicists looking to study the interaction between binary star systems and circumbinary disks.

In addition, the discovery of this high eccentricity binary system will make it easier to measure the masses of Low-Mass White Dwarfs with extreme precision in the coming years. This in turn should help astronomers to better understand the properties of these stars and what leads to their formation.

As history has taught us, understanding the Universe requires a serious commitment to the process of continuous discovery. And the more we discover, the stranger it seems to become, forcing us to reconsider what we think we know about it.

Further Reading: The Astrophysical Journal