Month: January 2019

Astronomers Observe Evolution Of A Black Hole As It Wolfs Down Stellar Material

On March 11, an instrument aboard the International Space Station detected an enormous explosion of X-ray light that grew to be six times as bright as the Crab Nebula, nearly 10,000 light years away from Earth. Scientists determined the source was a black hole caught in the midst of an outburst — an extreme phase in which a black hole can spew brilliant bursts of X-ray energy as it devours an avalanche of gas and dust from a nearby star.

Now astronomers from MIT and elsewhere have detected “echoes” within this burst of X-ray emissions, that they believe could be a clue to how black holes evolve during an outburst. In a study published today in the journal Nature, the team reports evidence that as the black hole consumes enormous amounts of stellar material, its corona — the halo of highly-energized electrons that surrounds a black hole — significantly shrinks, from an initial expanse of about 100 kilometers (about the width of Massachusetts) to a mere 10 kilometers, in just over a month.

The findings are the first evidence that the corona shrinks as a black hole feeds, or accretes. The results also suggest that it is the corona that drives a black hole’s evolution during the most extreme phase of its outburst.

“This is the first time that we’ve seen this kind of evidence that it’s the corona shrinking during this particular phase of outburst evolution,” says Jack Steiner, a research scientist in MIT’s Kavli Institute for Astrophysics and Space Research. “The corona is still pretty mysterious, and we still have a loose understanding of what it is. But we now have evidence that the thing that’s evolving in the system is the structure of the corona itself.”

Steiner’s MIT co-authors include Ronald Remillard and first author Erin Kara.

X-ray echoes

The black hole detected on March 11 was named MAXI J1820+070, for the instrument that detected it. The Monitor of All-sky X-ray Image (MAXI) mission is a set of X-ray detectors installed in the Japanese Experiment Module of the International Space Station (ISS), that monitors the entire sky for X-ray outbursts and flares.

Soon after the instrument picked up the black hole’s outburst, Steiner and his colleagues started observing the event with NASA’s Neutron star Interior Composition Explorer, or NICER, another instrument aboard the ISS, which was designed partly by MIT, to measure the amount and timing of incoming X-ray photons.

“This boomingly bright black hole came on the scene, and it was almost completely unobscured, so we got a very pristine view of what was going on,” Steiner says.

A typical outburst can occur when a black hole sucks away enormous amounts of material from a nearby star. This material accumulates around the black hole, in a swirling vortex known as an accretion disk, which can span millions of miles across. Material in the disk that is closer to the center of the black hole spins faster, generating friction that heats up the disk.

“The gas in the center is millions of degrees in temperature,” Steiner says. “When you heat something that hot, it shines out as X-rays. This disk can undergo avalanches and pour its gas down onto the central black hole at about a Mount Everest’s worth of gas per second. And that’s when it goes into outburst, which usually lasts about a year.”

Scientists have previously observed that X-ray photons emitted by the accretion disk can ping-pong off high-energy electrons in a black hole’s corona. Steiner says some of these photons can scatter “out to infinity,” while others scatter back onto the accretion disk as higher-energy X-rays.

By using NICER, the team was able to collect extremely precise measurements of both the energy and timing of X-ray photons throughout the black hole’s outburst. Crucially, they picked up “echoes,” or lags between low-energy photons (those that may have initially been emitted by the accretion disk) and high-energy photons (the X-rays that likely had interacted with the corona’s electrons). Over the course of a month, the researchers observed that the length of these lags decreased significantly, indicating that the distance between the corona and the accretion disk was also shrinking. But was it the disk or the corona that was shifting in?

To answer this, the researchers measured a signature that astronomers know as the “iron line” — a feature that is emitted by the iron atoms in an accretion disk only when they are energized, such as by the reflection of X-ray photons off a corona’s electrons. Iron, therefore, can measure the inner boundary of an accretion disk.

When the researchers measured the iron line throughout the outburst, they found no measurable change, suggesting that the disk itself was not shifting in shape, but remaining relatively stable. Together with the evidence of a diminishing X-ray lag, they concluded that it must be the corona that was changing, and shrinking as a result of the black hole’s outburst.

“We see that the corona starts off as this bloated, 100-kilometer blob inside the inner accretion disk, then shrinks down to something like 10 kilometers, over about a month,” Steiner says. “This is the first unambiguous case of a corona shrinking while the disk is stable.”

“NICER has allowed us to measure light echoes closer to a stellar-mass black hole than ever before,” Kara adds. “Previously these light echoes off the inner accretion disk were only seen in supermassive black holes, which are millions to billions of solar masses and evolve over millions of years. Stellar black holes like J1820 have much lower masses and evolve much faster, so we can see changes play out on human time scales.”

While it’s unclear what is exactly causing the corona to contract, Steiner speculates that the cloud of high-energy electrons is being squeezed by the overwhelming pressure generated by the accretion disk’s in-falling avalanche of gas.

The findings offer new insights into an important phase of a black hole’s outburst, known as a transition from a hard to a soft state. Scientists have known that at some point early on in an outburst, a black hole shifts from a “hard” phase that is dominated by the corona’s energy, to a “soft” phase that is ruled more by the accretion disk’s emissions.

“This transition marks a fundamental change in a black hole’s mode of accretion,” Steiner says. “But we don’t know exactly what’s going on. How does a black hole transition from being dominated by a corona to its disk? Does the disk move in and take over, or does the corona change and dissipate in some way? This is something people have been trying to unravel for decades And now this is a definitive piece of work in regards to what’s happening in this transition phase, and that what’s changing is the corona.”

This research is supported, in part, by NASA through the NICER mission and the Astrophysics Explorers Program.

X-Ray Pulse Detected Near Event Horizon As Black Hole Devours Star

On Nov. 22, 2014, astronomers spotted a rare event in the night sky: A supermassive black hole at the center of a galaxy, nearly 300 million light years from Earth, ripping apart a passing star. The event, known as a tidal disruption flare, for the black hole’s massive tidal pull that tears a star apart, created a burst of X-ray activity near the center of the galaxy. Since then, a host of observatories have trained their sights on the event, in hopes of learning more about how black holes feed.

Now researchers at MIT and elsewhere have pored through data from multiple telescopes’ observations of the event, and discovered a curiously intense, stable, and periodic pulse, or signal, of X-rays, across all datasets. The signal appears to emanate from an area very close to the black hole’s event horizon — the point beyond which material is swallowed inescapably by the black hole. The signal appears to periodically brighten and fade every 131 seconds, and persists over at least 450 days.

The researchers believe that whatever is emitting the periodic signal must be orbiting the black hole, just outside the event horizon, near the Innermost Stable Circular Orbit, or ISCO — the smallest orbit in which a particle can safely travel around a black hole.

Given the signal’s stable proximity to the black hole, and the black hole’s mass, which researchers previously estimated to be about 1 million times that of the sun, the team has calculated that the black hole is spinning at about 50 percent the speed of light.

The findings, reported today in the journal Science, are the first demonstration of a tidal disruption flare being used to estimate a black hole’s spin.

The study’s first author, Dheeraj Pasham, a postdoc in MIT’s Kavli Institute for Astrophysics and Space Research, says that most supermassive black holes are dormant and don’t usually emit much in the way of X-ray radiation. Only occasionally will they release a burst of activity, such as when stars get close enough for black holes to devour them. Now he says that, given the team’s results, such tidal disruption flares can be used to estimate the spin of supermassive black holes — a characteristic that has been, up until now, incredibly tricky to pin down.

“Events where black holes shred stars that come too close to them could help us map out the spins of several supermassive black holes that are dormant and otherwise hidden at the centers of galaxies,” Pasham says. “This could ultimately help us understand how galaxies evolved over cosmic time.”

Pasham’s co-authors include Ronald Remillard, Jeroen Homan, Deepto Chakrabarty, Frederick Baganoff, and James Steiner of MIT; Alessia Franchini at the University of Nevada; Chris Fragile of the College of Charleston; Nicholas Stone of Columbia University; Eric Coughlin of the University of California at Berkeley; and Nishanth Pasham, of Sunnyvale, California.

A real signal

Theoretical models of tidal disruption flares show that when a black hole shreds a star apart, some of that star’s material may stay outside the event horizon, circling, at least temporarily, in a stable orbit such as the ISCO, and giving off periodic flashes of X-rays before ultimately being fed by the black hole. The periodicity of the X-ray flashes thus encodes key information about the size of the ISCO, which itself is dictated by how fast the black hole is spinning.

Pasham and his colleagues thought that if they could see such regular flashes very close to a black hole that had undergone a recent tidal disruption event, these signals could give them an idea of how fast the black hole was spinning.

They focused their search on ASASSN-14li, the tidal disruption event that astronomers identified in November 2014, using the ground-based All-Sky Automated Survey for SuperNovae (ASASSN).

“This system is exciting because we think it’s a poster child for tidal disruption flares,” Pasham says. “This particular event seems to match many of the theoretical predictions.”

The team looked through archived datasets from three observatories that collected X-ray measurements of the event since its discovery: the European Space Agency’s XMM-Newton space observatory, and NASA’s space-based Chandra and Swift observatories. Pasham previously developed a computer code to detect periodic patterns in astrophysical data, though not for tidal disruption events specifically. He decided to apply his code to the three datasets for ASASSN-14li, to see if any common periodic patterns would rise to the surface.

What he observed was a surprisingly strong, stable, and periodic burst of X-ray radiation that appeared to come from very close to the edge of the black hole. The signal pulsed every 131 seconds, over 450 days, and was extremely intense — about 40 percent above the black hole’s average X-ray brightness.

“At first I didn’t believe it because the signal was so strong,” Pasham says. “But we saw it in all three telescopes. So in the end, the signal was real.”

Based on the properties of the signal, and the mass and size of the black hole, the team estimated that the black hole is spinning at least at 50 percent the speed of light.

“That’s not super fast — there are other black holes with spins estimated to be near 99 percent the speed of light,” Pasham says. “But this is the first time we’re able to use tidal disruption flares to constrain the spins of supermassive black holes.”

Illuminating the invisible

Once Pasham discovered the periodic signal, it was up to the theorists on the team to find an explanation for what may have generated it. The team came up with various scenarios, but the one that seems the most likely to generate such a strong, regular X-ray flare involves not just a black hole shredding a passing star, but also a smaller type of star, known as a white dwarf, orbiting close to the black hole.

Such a white dwarf may have been circling the supermassive black hole, at ISCO — the innermost stable circular orbit — for some time. Alone, it would not have been enough to emit any sort of detectable radiation. For all intents and purposes, the white dwarf would have been invisible to telescopes as it circled the relatively inactive, spinning black hole.

Sometime around Nov. 22, 2014, a second star passed close enough to the system that the black hole tore it apart in a tidal disruption flare that emitted an enormous amount of X-ray radiation, in the form of hot, shredded stellar material. As the black hole pulled this material inward, some of the stellar debris fell into the black hole, while some remained just outside, in the innermost stable orbit — the very same orbit in which the white dwarf circled. As the white dwarf came in contact with this hot stellar material, it likely dragged it along as a luminous overcoat of sorts, illuminating the white dwarf in an intense amount of X-rays each time it circled the black hole, every 131 seconds.

The scientists admit that such a scenario would be incredibly rare and would only last for several hundred years at most — a blink of an eye in cosmic scales. The chances of detecting such a scenario would be exceedingly slim.

“The problem with this scenario is that, if you have a black hole with a mass that’s 1 million times that of the sun, and a white dwarf is circling it, then at some point over just a few hundred years, the white dwarf will plunge into the black hole,” Pasham says. “We would’ve been extremely lucky to find such a system. But at least in terms of the properties of the system, this scenario seems to work.”

The results’ overarching significance is that they show it is possible to constrain the spin of a black hole, from tidal disruption events, according to Pasham. Going forward, he hopes to identify similar stable patterns in other star-shredding events, from black holes that reside further back in space and time.

“In the next decade, we hope to detect more of these events,” Pasham says. “Estimating spins of several black holes from the beginning of time to now would be valuable in terms of estimating whether there is a relationship between the spin and the age of black holes.”

This research was supported, in part, by NASA.

Nature’s Magnifying Glass Reveals Unexpected Intermediate Mass Exoplanets

Astronomers have found a new exoplanet that could alter the standing theory of planet formation. With a mass that’s between that of Neptune and Saturn, and its location beyond the “snow line” of its host star, an alien world of this scale was supposed to be rare.

Aparna Bhattacharya, a postdoctoral researcher from the University of Maryland and NASA’s Goddard Space Flight Center (GSFC), led the team that made the discovery, which was announced today during a press conference at the 233rd Meeting of the American Astronomical Society in Seattle.

Using the Near-Infrared Camera, second generation (NIRC2) instrument on the 10-meter Keck II telescope of the W. M. Keck Observatory on Maunakea, Hawaii and the Wide Field Camera 3 (WFC3) instrument on the Hubble Space Telescope, the researchers took simultaneous high-resolution images of the exoplanet, named OGLE-2012-BLG-0950Lb, allowing them to determine its mass.

“We were surprised to see the mass come out right in the middle of the predicted intermediate giant planet mass gap,” said Bhattacharya. “It’s like finding an oasis in the middle of the exoplanet desert!”

“I was very pleased with how quickly Aparna completed the analysis,” said co-author David Bennett, a senior research scientist at the University of Maryland and GSFC. “She had to develop some new methods to analyze this data — a type of analysis that had never been done before.”

In an uncanny timing of events, another team of astronomers (which included Bhattacharya and Bennett) published a statistical analysis at almost the same time showing that such sub-Saturn mass planets are not rare after all.

“We were just finishing up the analysis when the mass measurements of OGLE-2012- BLG-0950Lb came in,” said lead author Daisuke Suzuki of Japan’s Institute of Space and Astronautical Science. “This planet confirmed our interpretation of the statistical study.”

The teams’ results on OGLE-2012-BLG-0950Lb are published in the December issue of The Astronomical Journal and the statistical study was published in the December 20th issue of the Astrophysical Journal Letters.

OGLE-2012-BLG-0950Lb was among the sub-Saturn planets in the statistical study; all were detected through microlensing, the only method currently sensitive enough to detect planets with less than Saturn’s mass in Jupiter-like orbits.

Microlensing leverages a consequence of Einstein’s theory of general relativity: the bending and magnification of light near a massive object like a star, producing a natural lens on the sky. In the case of OGLE-2012-BLG-0950Lb, the light from a distant background star was magnified by OGLE-2012-BLG-0950L (the exoplanet’s host star) over the course of two months as it passed close to perfect alignment in the sky with the background star.

By carefully analyzing the light during the alignment, an unexpected dimming with a duration of about a day was observed, revealing the presence of OGLE-2012-BLG-0950Lb via its own influence on the lensing.

METHODOLOGY

OGLE-2012-BLG-0950Lb was first detected by the microlensing survey telescopes of the Optical Gravitational Lensing Experiment (OGLE) and the Microlensing Observations in Astrophysics (MOA) collaborations.

Bhattacharya’s team then conducted follow-up observations using Keck Observatory’s powerful adaptive optics system in combination with NIRC2.

“The Keck observations allowed us to determine that the sub-Saturn or super-Neptune size planet has a mass of 39 times that of the Earth, and that its host star is 0.58 times the mass of the Sun,” said Bennett. “They measured the separation of the foreground planetary system from the background star. This allowed us to work out the complete geometry of the microlensing event. Without this data, we only knew the star-planet mass ratio, not the individual masses.”

For the statistical study, Suzuki’s team and MOA analyzed the properties of 30 sub-Saturn planets found by microlensing and compared them to predictions from the core accretion theory.

CHALLENGING THE THEORY

What is unique about the microlensing method is its sensitivity to sub-Saturn planets like OGLE-2012-BLG-0950Lb that orbit beyond the “snow line” of their host stars.

The snow line, or frost line, is the distance in a young solar system, (a.k.a. a protoplanetary disk) at which it is cold enough for water to condense into ice. At and beyond the snow line there is a dramatic increase in the amount of solid material needed for planet formation. According to the core accretion theory, the solids are thought to build up into planetary cores first through chemical and then gravitational processes.

“A key process of the core accretion theory is called “runaway gas accretion,” said Bennett. “Giant planets are thought to start their formation process by collecting a core mass of about 10 times the Earth mass in rock and ice. At this stage, a slow accretion of hydrogen and helium gas begins until the mass has doubled. Then, the accretion of hydrogen and helium is expected to speed up exponentially in this runaway gas accretion process. This process stops when the supply is exhausted. If the supply of gas is stopped before runaway accretion stops, we get “failed Jupiter” planets with masses of 10-20 Earth-masses (like Neptune).”

The runaway gas accretion scenario of the core accretion theory predicts that planets like OGLE-2012- BLG-0950Lb are expected to be rare. At 39 times the mass of the Earth, planets this size are thought to be continuing through a stage of rapid growth, ending in a much more massive planet. This new result suggests that the runaway growth scenario may need revision.

Suzuki’s team compared the distribution of planet-star mass ratios found by microlensing to distributions predicted by the core accretion theory.

They found that the core accretion theory’s runaway gas accretion process predicts about 10 times fewer intermediate mass giant planets like OGLE-2012- BLG-0950Lb than are seen in the microlensing results.

This discrepancy implies that gas giant formation may involve processes that have been overlooked by existing core accretion models, or that the planet forming environment varies considerably as a function of host star mass.

NEXT STEPS

This discovery has not only called into question an established theory, it was made using a new technique that will be a key part of NASA’s next big planet finding mission, the Wide Field Infra-Red Survey Telescope (WFIRST), which is scheduled to launch into orbit in the mid-2020s.

“This is exactly the method that WFIRST will use to measure the masses of the planets that it discovers with its exoplanet microlensing survey. Until WFIRST comes online, we need to develop this method with observations from our Keck Key Strategic Mission Support (KSMS) program as well as observations from Hubble,” said Bennett.

“It’s very exciting to see Keck and Hubble combine forces to provide this surprising new result,” said Keck Observatory Chief Scientist John O’Meara. “And it’s equally exciting to know that we can make these kind of advances today to help facilitate the best science from WFIRST and Keck’s partnership in the future.”

The NASA Keck KSMS program will continue to make follow-up observations of microlensing events detected by telescopes on the ground and in space.

Magnitude 6.1 Earthquake Near Adak, Tsunami Not Expected

A magnitude 6.1 earthquake struck 57 miles southwest of Adak, Alaska, at 9:47 Saturday morning. At this time, a tsunami is not expected, according to the National Weather Service Tsunami Warning Center.

The earthquake epicenter was some 37 miles south of Bobrof island, just 62 miles deep. The area is in the far western Aleutians, some 1,200 miles southwest of Anchorage.

At this time there are no reports of damage.

Strong 6.8-Magnitude Earthquake Hits Western Brazil

A strong earthquake with a preliminary magnitude of 6.8 has struck Brazil’s Amazon region, but damage is unlikely because it struck at a depth of nearly 600 kilometers (372 miles), seismologists say.

The earthquake, which struck at 2:25 p.m. local time on Saturday, was centered in the Amazon rainforest, about 89 kilometers (55 miles) west of Tarauacá in Acre state, or 739 kilometers (459 miles) northeast of Lima.

The U.S. Geological Survey measured the magnitude at 6.8 but said it struck at a depth of 575 kilometers (479 miles), making it a very deep earthquake. Peru’s seismological agency put the magnitude significantly higher, at 7.2.

Damage is unlikely because it struck far below the surface and in a remote area. Computer models from the UN estimate that nearly 5,300 people live within 50 kilometers (31 miles) of the earthquake epicenter.

Saturday’s earthquake was the strongest to hit Brazil since 2003, when a powerful 7.1-magnitude earthquake hit the same region. However, strong and deep earthquakes sometimes hit Peru in areas that are close to the border with western Brazil. They rarely cause damage.

How Climate Change Caused the World’s First Empire to Collapse

Not one smoke stack, vehicle, or petroleum of any form was mentioned in this scientific article. However, there does appear to be an assumption of rhythmic cycles.

Gol-e-Zard Cave lies in the shadow of Mount Damavand, which at more than 5,000 meters dominates the landscape of northern Iran. In this cave, stalagmites and stalactites are growing slowly over millennia and preserve in them clues about past climate events. Changes in stalagmite chemistry from this cave have now linked the collapse of the Akkadian Empire to climate changes more than 4,000 years ago.

Akkadia was the world’s first empire. It was established in Mesopotamia around 4,300 years ago after its ruler, Sargon of Akkad, united a series of independent city states. Akkadian influence spanned along the Tigris and Euphrates rivers from what is now southern Iraq, through to Syria and Turkey. The north-south extent of the empire meant that it covered regions with different climates, ranging from fertile lands in the north which were highly dependent on rainfall (one of Asia’s “bread baskets”), to the irrigation-fed alluvial plains to the south.

It appears that the empire became increasingly dependent on the productivity of the northern lands and used the grains sourced from this region to feed the army and redistribute the food supplies to key supporters. Then, about a century after its formation, the Akkadian Empire suddenly collapsed, followed by mass migration and conflicts. The anguish of the era is perfectly captured in the ancient Curse of Akkad text, which describes a period of turmoil with water and food shortages: “… the large arable tracts yielded no grain, the inundated fields yielded no fish, the irrigated orchards yielded no syrup or wine, the thick clouds did not rain.”

Drought and dust

The reason for this collapse is still debated by historians, archaeologists and scientists. One of the most prominent views, championed by Yale archaeologist Harvey Weiss (who built on earlier ideas by Ellsworth Huntington), is that it was caused by an abrupt onset of drought conditions which severely affected the productive northern regions of the empire.

Weiss and his colleagues discovered evidence in northern Syria that this once prosperous region was suddenly abandoned around 4,200 years ago, as indicated by a lack of pottery and other archaeological remains. Instead, the rich soils of earlier periods were replaced by large amounts of wind-blown dust and sand, suggesting the onset of drought conditions. Subsequently, marine cores from the Gulf of Oman and the Red Sea which linked the input of dust into the sea to distant sources in Mesopotamia, provided further evidence of a regional drought at the time.

Many other researchers viewed Weiss’s interpretation with skepticism, however. Some argued, for example, that the archaeological and marine evidence was not accurate enough to demonstrate a robust correlation between drought and societal change in Mesopotamia.

A new detailed climate record

Now, stalagmite data from Iran sheds new light on the controversy. In a study published in the journal PNAS, led by Oxford palaeoclimatologist Stacy Carolin, colleagues and I provide a very well dated and high resolution record of dust activity between 5,200 and 3,700 years ago. And cave dust from Iran can tell us a surprising amount about climate history elsewhere.

Gol-e-Zard Cave might be several hundred miles to the east of the former Akkadian Empire, but it is directly downwind. As a result, around 90% of the region’s dust originates in the deserts of Syria and Iraq.

That desert dust has a higher concentration of magnesium than the local limestone which forms most of Gol-e-Zard’s stalagmites (the ones which grow upwards from the cave floor). Therefore, the amount of magnesium in the Gol-e-Zard stalagmites can be used as an indicator of dustiness at the surface, with higher magnesium concentrations indicating dustier periods, and by extension drier conditions.

The stalagmites have the additional advantage that they can be dated very precisely using uranium-thorium chronology. Combining these methods, our new study provides a detailed history of dustiness in the area, and identifies two major drought periods which started 4,510 and 4,260 years ago, and lasted 110 and 290 years respectively. The latter event occurs precisely at the time of the Akkadian Empire’s collapse and provides a strong argument that climate change was at least in part responsible.

The collapse was followed by mass migration from north to south which was met with resistance by the local populations. A 180km wall – the “Repeller of the Amorites” – was even built between the Tigris and Euphrates in an effort to control immigration, not unlike some strategies proposed today. The stories of abrupt climate change in the Middle East therefore echo over millennia to the present day.

JUST IN: Researchers Find Deep Ocean Getting Colder

A pair of researchers, one with the Woods Hole Oceanographic Institution, the other Harvard University, has found evidence of deep ocean cooling that is likely due to the Little Ice Age. In their paper published in the journal Science, Jake Gebbie and Peter Huybers describe their study of Pacific Ocean temperatures over the past 150 years and what they found.

The model showed that the Pacific Ocean cooled over the course of the 20th century at depths of 1.8 to 2.6 kilometers. The amount is still not precise, but the researchers suggest it is most likely between 0.02 and 0.08° C. That cooling, the researchers suggest, is likely due to the Little Ice Age, which ran from approximately 1300 until approximately 1870. Prior to that, there was a time known as the Medieval Warm Period, which had caused the deep waters of the Pacific to warm just prior to the cooling it is now experiencing.

Prior research has suggested that it takes a very long time for water in the Pacific Ocean to circulate down to its lowest depths. This is because it is replenished only from the south, which means it takes a very long time for water on the surface to make its way to the bottom – perhaps as long as several hundred years. That is what Gebbie and Huber found back in 2012. That got them to thinking that water temperature at the bottom of the Pacific could offer a hint of what surface temperatures were like hundreds of years ago.

To find out if that truly was the case, the researchers obtained data from an international consortium called the Argo Program – a group of people who together have been taking ocean measurements down to depths of approximately two kilometers. As a comparative reference, the researchers also obtained data gathered by the crew of the HMS Challenger – they had taken Pacific Ocean temperatures down to a depth of two kilometers during the years 1872 to 1876. The researchers used the data from both projects to build a computer model meant to mimic the circulation of water in the Pacific Ocean over the past century and a half.