From Harvard-Smithsonian Center for Astrophysics: “Scientists Observe the Explosion of a Monster Star Requiring New Supernova Mechanism”

Harvard Smithsonian Center for Astrophysics


From Harvard-Smithsonian Center for Astrophysics

August 15, 2019
Amy Oliver
Public Affairs Officer
Fred Lawrence Whipple Observatory
Center for Astrophysics | Harvard & Smithsonian
amy.oliver@cfa.harvard.edu
+1 520-879-4406
mobile: +1-801-783-9067

CfA Whipple Observatory, located near Amado, Arizona on the slopes of Mount Hopkins, Altitude 2,606 m (8,550 ft)

1
Artist’s conception of the explosion of SN2016iet’s host star within a dense stellar environment. Credit: Joy Pollard/Gemini

Scientists at the Center for Astrophysics | Harvard & Smithsonian have announced the discovery of the most massive star ever known to be destroyed by a supernova explosion, challenging known models of how massive stars die and providing insight into the death of the first stars in the universe.

First noticed in November 2016 by the European Space Agency’s (ESA) Gaia satellite, three years of intensive follow up observations of the supernova SN2016iet revealed characteristics—incredibly long duration and large energy, unusual chemical fingerprints, and an environment poor in metals—for which there are no analogues in the existing astronomical literature.

ESA/GAIA satellite

“When we first realized how thoroughly unusual SN2016iet is my reaction was ‘whoa – did something go horribly wrong with our data?'” said Mr. Sebastian Gomez, Harvard University graduate student and lead author of the paper. “After a while we determined that SN2016iet is an incredible mystery, located in a previously uncatalogued galaxy one billion light years from Earth.”

The team used a variety of telescopes, including the CfA | Harvard & Smithsonian’s MMT Observatory located at the Fred Lawrence Whipple Observatory in Amado, AZ, and the Magellan Telescopes at the Las Campanas Observatory in Chile to show that SN2016iet is different than the thousands of supernovas observed by scientists for decades.

CfA U Arizona Fred Lawrence Whipple Observatory Steward Observatory MMT Telescope at the summit of Mount Hopkins near Tucson, Arizona, USA, Altitude 2,616 m (8,583 ft)

Carnegie 6.5 meter Magellan Baade and Clay Telescopes located at Carnegie’s Las Campanas Observatory, Chile. over 2,500 m (8,200 ft) high

“Everything about this supernova looks different—its change in brightness with time, its spectrum, the galaxy it is located in, and even where it’s located within its galaxy, said Dr. Edo Berger, Professor of Astronomy at Harvard University and an author on the paper. “We sometimes see supernovas that are unusual in one respect, but otherwise are normal; this one is unique in every possible way.”

The observations and analysis show that SN2016iet began as an incredibly massive star 200 times the mass of Earth’s Sun that mysteriously formed in isolation roughly 54,000 light years from the center of its host dwarf galaxy. The star lost about 85 percent of its mass during a short life of only a few million years, all the way up to its final explosion and demise. The collision of the explosion-debris with the material shed in the final decade before explosion led to SN2016iet’s unusual appearance, providing scientists with the first strong case of a pair-instability supernova.

“The idea of pair-instability supernovas has been around for decades,” said Berger. “But finally having the first observational example that puts a dying star in the right regime of mass, with the right behavior, and in a metal-poor dwarf galaxy is an incredible step forward. SN2016iet represents the way in which the most massive stars in the universe, including the first stars, die.”

The team will continue to observe and study SN2016iet for years, watching for additional clues as to how it formed, and how it will evolve. “Most supernovas fade away and become invisible against the glare of their host galaxies within a few months. But because SN2016iet is so bright and so isolated we can study its evolution for years to come,” said Gomez. “These observations are already in progress and we can’t wait to see what other surprises this supernova has in store for us.”

The results of the study are published in The Astrophysical Journal. In addition to Gomez and Berger, the study involved scientists from CfA | Harvard & Smithsonian—Peter K. Blanchard, V. Ashley Villar, Locke Patton, Joel Leja, and Griffin Hosseinzadeh; along with scientists from the University of Edinburgh—Matt Nicholl; Ohio University—Ryan Chornock; and, The Observatories of the Carnegie Institution for Science—Philip S. Cowperthwaite.

See the full article here .


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The Center for Astrophysics combines the resources and research facilities of the Harvard College Observatory and the Smithsonian Astrophysical Observatory under a single director to pursue studies of those basic physical processes that determine the nature and evolution of the universe. The Smithsonian Astrophysical Observatory (SAO) is a bureau of the Smithsonian Institution, founded in 1890. The Harvard College Observatory (HCO), founded in 1839, is a research institution of the Faculty of Arts and Sciences, Harvard University, and provides facilities and substantial other support for teaching activities of the Department of Astronomy.

#astronomy, #astrophysics, #basic-research, #cfa-harvard-smithsonian-center-for-astrophysics, #cosmology, #supernovae, #the-idea-of-pair-instability-supernovas-has-been-around-for-decades, #the-supernova-sn2016iet

From AAS NOVA: “Exploring an Odd Stellar Death”

AASNOVA

From AAS NOVA

17 July 2019
Susanna Kohler

1
Artist’s impression of a supernova explosion. A new study explores whether the merger of two massive stars could lead to a unique kind of supernova. [ESO/M. Kornmesser]

Massive stars can die in a lot of different ways! A new study explores one possible channel in more detail.

Detectives Are on the Case

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Artist’s illustration of a star exploding in a supernova at the end of its lifetime. [NASA/CXC/M. Weiss]

NASA/Chandra X-ray Telescope

Studying supernovae is a little like being a detective in an odd sort of murder mystery. You’ve witnessed the death of a massive star — and from this evidence, you must determine what type of star died, how it died, and even what interactions it had before its death.

As we enter the era of ever more expansive sky surveys, we can expect to amass not just evidence of typical stellar deaths, but also some more unusual ones. In the process, piecing together the evidence to solve each mystery becomes progressively more challenging — but also more intriguing!

In a recent study, a team of scientists led by Alejandro Vigna-Gómez (U. of Birmingham, UK; Monash U., Australia; U. of Copenhagen, Denmark) have explored one particular oddball type of theorized stellar death: pulsational pair-instability supernovae (PISNe).

Gravity (Usually) Wins

According to theory, PISNe occur when a very massive (hundreds of solar masses) star gets hot enough to start producing pairs of electrons and positions. This process saps the star’s internal energy, leading to its sudden collapse as the force of gravity triumphs.

This collapse can end in the dramatic explosion of a PISN, or it may lead to a smaller eruption that only sheds some of the star’s mass. In the latter case, the star may go through multiple rounds of smaller eruptions before eventually running out of nuclear fuel and undergoing a final explosion — as a pulsational PISN.

3
Schematic showing three possible ways massive stars can die; click to enlarge. Top and bottom panels describe outcomes of single-star evolution, depending on the star’s mass. The center channel depicts the merger of two evolved, massive stars to form an object with a large envelope of hydrogen. This can lead to a hydrogen-rich pulsational PISN. [Vigna-Gómez et al. 2019]

Starting with a Merger

If this weren’t complicated enough, Vigna-Gómez and collaborators propose one further twist on this stellar death scenario: the object exploding in a pulsational PISN needn’t simply be a massive star. Instead, it might be the product of the merger of two massive stars.

Vigna-Gómez and collaborators argue that this type of merger is expected to be common, and it would produce a very massive object with a large outer hydrogen shell. By running a series of simulations using the Modules for Experiments in Stellar Astrophysics (MESA), the authors demonstrate that such a merger product could undergo a pulsational PISN and still retain a significant portion of its hydrogen shell up to the final explosion, leaving the fingerprint of hydrogen in the supernova spectrum.

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The light curve of iPTF14hls is extremely unusual, featuring multiple apparent explosions. [Adapted from Las Cumbres Observatory/S. Wilkinson]

LCO_map_2017. Map of the Las Cumbres Observatory global network of robotic telescopes

Explanation for a Zombie Star?

Why does this particular theorized death matter? Stellar detectives are currently working to explain the deaths in a number of especially weird observed supernovae, and this model might match some of them. One example is iPTF14hls, the “zombie star” that’s made headlines for apparently erupting multiple times and defying explanation — in part because of the unexpected hydrogen signatures in its spectra.

We can’t yet say for sure whether iPTF14hls is an example of a stellar-merger-turned-pulsational-PISN — that will require more extensive modeling and analysis of observations — but Vigna-Gómez and collaborators think it’s a good candidate! And while we wait on the verdict of that mystery, we can be sure that transient surveys are busy finding many more examples of stellar deaths for us to puzzle over.

Citation

“Massive Stellar Mergers as Precursors of Hydrogen-rich Pulsational Pair Instability Supernovae,” Alejandro Vigna-Gómez et al 2019 ApJL 876 L29.
https://iopscience.iop.org/article/10.3847/2041-8213/ab1bdf

See the full article here .


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1

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

#exploring-an-odd-stellar-death, #aas-nova, #astronomy, #astrophysics, #basic-research, #cosmology, #supernovae

From Science News: “The highest-energy photons ever seen hail from the Crab Nebula”

From Science News

June 24, 2019
Emily Conover

Some of the supernova remnant’s gamma rays have more than 100 trillion electron volts of energy.

1
CRAB FISHING Scientists hunting for high-energy photons raining down on Earth from space have found the most energetic light yet detected. It’s from the Crab Nebula, a remnant of an exploded star (shown in an image combining light seen by multiple telescopes).

Physicists have spotted the highest-energy light ever seen. It emanated from the roiling remains left behind when a star exploded.

This light made its way to Earth from the Crab Nebula, a remnant of a stellar explosion, or supernova, about 6,500 light-years away in the Milky Way. The Tibet AS-gamma experiment caught multiple particles of light — or photons — from the nebula with energies higher than 100 trillion electron volts, researchers report in a study accepted in Physical Review Letters. Visible light, for comparison, has just a few electron volts of energy.

Tibet AS Gamma Expeiment

“This energy regime has not been accessible before,” says astrophysicist Petra Huentemeyer of Michigan Technological University in Houghton, who was not involved with the research. For physicists who study this high-energy light, known as gamma rays, “it’s an exciting time,” she says.

In space, supernova remnants and other cosmic accelerators can boost subatomic particles such as electrons, photons and protons to extreme energies, much higher than those achieved in the most powerful earthly particle accelerators (SN: 10/1/05, p. 213). Protons in the Large Hadron Collider in Geneva, for example, reach a comparatively wimpy 6.5 trillion electron volts. Somehow, the cosmic accelerators vastly outperform humankind’s most advanced machines.

“The question is: How does nature do it?” says physicist David Hanna of McGill University in Montreal.

In the Crab Nebula, the initial explosion set up the conditions for acceleration, with magnetic fields and shock waves plowing through space, giving an energy boost to charged particles such as electrons. Low-energy photons in the vicinity get kicked to high energies when they collide with the speedy electrons, and ultimately, some of those photons make their way to Earth.

When a high-energy photon hits Earth’s atmosphere, it creates a shower of other subatomic particles that can be detected on the ground. To capture that resulting deluge, Tibet AS-gamma uses nearly 600 particle detectors spread across an area of more than 65,000 square meters in Tibet. From the information recorded by the detectors, researchers can calculate the energy of the initial photon.

But other kinds of spacefaring particles known as cosmic rays create particle showers that are much more plentiful. To select photons, cosmic rays, which are mainly composed of protons and atomic nuclei, need to be weeded out. So the researchers used underground detectors to look for muons — heavier relatives of electrons that are created in cosmic ray showers, but not in showers created by photons.

Previous experiments have glimpsed photons with nearly 100 TeV, or trillion electron volts. Now, after about three years of gathering data, the researchers found 24 seemingly photon-initiated showers above 100 TeV, and some with energies as high as 450 TeV. Because the weeding out process isn’t perfect, the researchers estimate that around six of those showers could have come from cosmic rays mimicking photons, but the rest are the real deal.

Researchers with Tibet AS-gamma declined to comment for this story, as the study has not yet been published.

Looking for photons of ever higher energies could help scientists nail down the details of how the particles are accelerated. “There has to be a limit to how high the energy of the photons can go,” Hanna says. If scientists can pinpoint that maximum energy, that could help distinguish between various theoretical tweaks to how the particles get their oomph.

See the full article here .


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#the-highest-energy-photons-ever-seen-hail-from-the-crab-nebula, #astronomy, #astrophysics, #basic-research, #cosmology, #particle-physics, #physics, #science-news, #supernovae, #the-tibet-as-gamma-experiment, #when-a-high-energy-photon-hits-earths-atmosphere-it-creates-a-shower-of-other-subatomic-particles-that-can-be-detected-on-the-ground

From AAS NOVA: “Supernovae, Dark Energy, and the Fate of Our Universe”

AASNOVA

From AAS NOVA

5 April 2019
Susanna Kohler

Dark Energy Survey


Dark Energy Camera [DECam], built at FNAL


NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile, housing DECam at an altitude of 7200 feet

What’s the eventual fate of our universe? Is spacetime destined to continue to expand forever? Will it fly apart, tearing even atoms into bits? Or will it crunch back in on itself? New results from Dark Energy Survey supernovae address these and other questions.

Uncertain Expansion

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The evolution of the scale of our universe. Measurements suggest that the universe is currently expanding, but does dark energy behaves like a cosmological constant, resulting in continued accelerating expansion like now? Or might we instead be headed for a Big Rip or Big Crunch? [NASA/CXC/M. Weiss]

At present, the fabric of our universe is expanding — and not only that, but the its expansion is accelerating. To explain this phenomenon, we invoke what’s known as dark energy — an unknown form of energy that exists everywhere and exerts a negative pressure, driving the expansion.

Since this idea was first proposed, we’ve conducted decades of research to better understand what dark energy is, how much of it there is, and how it influences our universe.

In particular, dark energy’s still-uncertain equation of state determines the universe’s ultimate fate. If the density of dark energy is constant in time, our universe will continue its current accelerating expansion indefinitely. If the density increases in time, the universe will end in the Big Rip — space will expand at an ever-increasing acceleration rate until even atoms fly apart. And if the density decreases in time, the universe will recollapse in the Big Crunch, ending effectively in a reverse Big Bang.

Which of these scenarios is correct? We’re not sure yet. But there’s a project dedicated to finding out: the Dark Energy Survey (DES).

The Hunt for Supernovae

DES was conducted with the Dark Energy Camera at the Cerro Tololo Inter-American Observatory in Chile. After six years taking data, the survey officially wrapped up observations this past January.

One of DES’s several missions was to make detailed measurements of thousands of supernovae. Type Ia supernovae explode with a prescribed absolute brightness, allowing us to determine their distance from observations. DES’s precise measurements of Type Ia supernovae allow us to calculate the expansion of the space between us and the supernovae, probing the properties of dark energy.

Though DES scientists are still in the process of analyzing the tens of terabytes of data generated by the project, they recently released results from the first three years of data — including the first DES cosmology results based on supernovae.

Refined Measurements

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Constraints on the dark energy equation of state w from the DES supernova survey. Combining this data with constraints from the cosmic microwave background radiation suggest an equation of state consistent with a constant density of dark energy (w = –1). [Abbott et al. 2019]

Using a sample of 207 spectroscopically confirmed DES supernovae and 122 low-redshift supernovae from the literature, the authors estimate the matter density of a flat universe to be Ωm = 0.321 ± 0.018. This means that only ~32% of the universe’s energy density is matter (the majority of which is dark matter); the remaining ~68% is primarily dark energy.

From their observations, the DES team is also able to provide an estimate for the dark-energy equation of state w, finding that w = –0.978 ± 0.059. This result is consistent with a constant density of dark energy (w = –1), which would mean that our universe will continue to expand with its current acceleration indefinitely.

These results are exciting, but they use only ~10% of the supernovae DES discovered over the span of its 5-year survey. This means that we can expect even further refinements to these measurements in the future, as the DES collaboration analyzes the remaining data!

Citation

“First Cosmology Results using Type Ia Supernovae from the Dark Energy Survey: Constraints on Cosmological Parameters,” T. M. C. Abbott et al 2019 ApJL 872 L30.
https://iopscience.iop.org/article/10.3847/2041-8213/ab04fa/meta

Lambda-Cold Dark Matter, Accelerated Expansion of the Universe, Big Bang-Inflation (timeline of the universe) Date 2010 Credit: Alex Mittelmann Cold creation

See the full article here .


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1

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

#aas-nova, #and-the-fate-of-our-universe, #astronomy, #astrophysics, #basic-research, #cosmology, #dark-energy, #des-dark-energy-survey, #supernovae

From AAS NOVA: “A Rare Double-Detonation Supernova Caught in the Act”

AASNOVA

From AAS NOVA

29 March 2019
Kerry Hensley

1
This representative-color X-ray and infrared image shows supernova remnant G299, which is all that’s left after a massive explosion roughly 4,500 years ago. Like the supernova studied in today’s paper, G299 met its end when a white dwarf underwent a thermonuclear detonation. [X-ray: NASA/CXC/U.Texas/S.Post et al, Infrared: 2MASS/UMass/IPAC-Caltech/NASA/NSF]

NASA/Chandra X-ray Telescope


Caltech 2MASS Telescopes, a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center (IPAC) at Caltech, at the Whipple Observatory on Mt. Hopkins south of Tucson, AZ, Altitude 2,606 m (8,550 ft) and at the Cerro Tololo Inter-American Observatory at an altitude of 2200 meters near La Serena, Chile.

There’s more than just one way for a star to explode. Supernovae — perhaps the most dramatic form of star death — come in many flavors, and astronomers are still learning about the vast diversity of these stellar explosions.

When Stars Steal Mass

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This artist’s rendering depicts one kind of Type Ia supernova mechanism: the singly degenerate model, in which a white dwarf siphons mass from its companion, exceeds the Chandrasekhar mass, and explodes. [NASA/CXC/M. Weiss]

When a white dwarf accretes gas from a binary companion and gains enough mass to exceed the Chandrasekhar limit, it can ignite in a cataclysmic explosion. This is the typical scenario for a Type Ia supernova, a common curtain call for low- to intermediate-mass stars in binary systems.

However, this isn’t the only way a Type Ia supernova can happen. In the double-detonation model, the explosion of the white dwarf is triggered by the ignition of an accreted helium shell. In this case, the white dwarf can be far less massive than the Chandrasekhar limit, leading to unexpectedly dim explosions.

Past studies have explored the minimum helium shell mass necessary (~0.01 solar mass) for this process and found that helium-shell detonations can efficiently cause core detonations, but there’s still plenty we don’t know about these events. The best way to learn about supernovae — double-detonation or otherwise — is to spot them soon after they happen.

3
A comparison of ZTF 18aaqeasu’s optical light curve (red circles) to normal (orange hexagons) and sub-luminous Type Ia supernovae. [Adapted from De et al. 2019]

A Survey Spies a Supernova

In May 2018, an unusual supernova was detected by the Zwicky Transient Facility, an optical survey that hunts for fleeting events like stellar flares, fast-rotating asteroids, and the visible-light counterparts of gravitational-wave events.

Zwicky Transient Facility (ZTF) instrument installed on the 1.2m diameter Samuel Oschin Telescope at Palomar Observatory in California. Courtesy Caltech Optical Observatories

Edwin Hubble at Caltech Palomar Samuel Oschin 48 inch Telescope, (credit: Emilio Segre Visual Archives/AIP/SPL)

Caltech Palomar Samuel Oschin 48 inch Telescope, located in San Diego County, California, United States, altitude 1,712 m (5,617 ft)

Within days of its detection, a team led by Kishalay De (Caltech) began to collect photometric observations and spectra of the object.

The photometry revealed that the object, ZTF 18aaqeasu, was unusually red and less luminous than a typical Type Ia supernova, making it a good candidate for the double-detonation scenario.

Its spectra were unusual even for a sub-luminous supernova, taking much longer to develop the silicon absorption feature typically seen in this type of event. Even stranger, the spectra exhibited a never-before-seen cutoff in the flux at short wavelengths, likely due to the presence of metals like iron and titanium.

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Comparison of observed spectra (black) to helium-shell double-detonation models (green and orange). [Adapted from De et al. 2019]

An Unusual Event

In order to derive the properties of ZTF 18aaqeasu, De and collaborators compared their photometric and spectroscopic data to models, finding that the event was likely caused by the ignition of a 0.15 solar mass helium shell, which led to the explosion of a 0.76 solar mass white dwarf.

The combination of a massive helium shell with a low-mass white dwarf makes ZTF 18aaqeasu unique among Type Ia supernovae; SN 2016jhr (one of the only supernovae previously linked to a helium-shell detonation event) featured a much more massive white dwarf with a less massive helium shell.

Can we expect to find more supernovae like ZTF 18aaqeasu? Similarly luminous supernovae should be detectable out to about 1.3 billion light-years, but so far there have been none reported with similar spectral features and unusually red color. This may indicate that double-detonation events featuring massive helium shells might be rare — adding an elusive new member to the Type Ia supernova family.

Citation

“ZTF 18aaqeasu (SN2018byg): A Massive Helium-shell Double Detonation on a Sub-Chandrasekhar-mass White Dwarf,” Kishalay De et al 2019 ApJL 873 L18.
https://iopscience.iop.org/article/10.3847/2041-8213/ab0aec

See the full article here .


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1

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

#aas-nova, #astronomy, #astrophysics, #basic-research, #cosmology, #supernovae

From CERN Courier: “Mysterious burst confounds astrophysicists”


From CERN Courier

8 March 2019

1
Holy cow!

On 16 June 2018, a bright burst of light was observed by the Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope in Hawaii, which automatically searches for optical transient events.

ATLAS is an asteroid impact early warning system of two telescopes being developed by the University of Hawaii and funded by NASA


ATLAS telescope, First Asteroid Terrestrial-impact Last Alert system (ATLAS) fully operational 8/15/15 Haleakala , Hawaii, USA, Altitude 4,205 m (13,796 ft)

The event, which received the automated catalogue name “AT2018cow”, immediately received a lot of attention and acquired a shorter name: “the Cow”. While transient objects are observed on the sky every day – caused, for example, by nearby asteroids or supernovae – two factors make the Cow intriguing. First, the very short time it took for the event to reach its extreme brightness and fade away again indicates that this event is nothing like anything observed before. Second, it took place relatively close to Earth, 200 million light years away in a star-forming arm of a galaxy in the Hercules constellation, making it possible to study the event in a wide range of wavelengths.

Soon after the ATLAS detection, the object was observed by more than 20 different telescopes around the world, revealing it to be 10–100 times brighter than a typical supernova. In addition to optical measurements, the object was observed for several days by space-based X- and gamma-ray telescopes such as NuSTAR, XMM-Newton, INTEGRAL and Swift, which also observed it in the UV energy range, as well as by radio telescopes on Earth.

NASA/DTU/ASI NuSTAR X-ray telescope

ESA/XMM Newton

ESA/Integral

NASA Neil Gehrels Swift Observatory

The IceCube observatory in Antarctica also identified two possible neutrinos coming from the Cow, although the detection is still compatible with a background fluctuation.

U Wisconsin ICECUBE neutrino detector at the South Pole

The combination of all the data – demonstrating the power of multi-messenger astronomy – confirmed that this was not an ordinary supernova, but potentially something completely different.

Right spark

While standard supernovae take several days to reach maximum brightness, the Cow did so in just 1.5 days, after which the brightness also started to decrease much faster than a typical supernova. Another notable feature was the lack of heavy-element decays. Normally, elements such as 56Ni produced during the explosion are the main source of supernovae brightness, but the Cow only revealed signs of lighter elements such as hydrogen and helium. Furthering the event’s mystique is the variability of the X-ray emission several days after its discovery, which is a clear sign of an energy source at its centre. Half a year after its discovery, two opposing theories aim to explain these features.

The first theory states that an unlucky compact object was destroyed when coming too close to a black hole – a phenomenon called a tidal disruption event. The fast increase in brightness excludes normal stars. On the other hand, a smaller object (such as a neutron star, a very dense star consisting of neutron matter) cannot explain the hydrogen and helium observed in the remnant, since it contains no proper elements. The remaining possibility is a white dwarf, a dense star remaining after a normal star has ceased fusion but kept from gravitational collapse into a neutron star or black hole by the electron-degeneracy pressure in its core. The observed emission from the Cow could be explained if a white dwarf was torn apart by tidal forces in the vicinity of a massive black hole. One problem with this theory, however, is the event’s location, since black holes with the sizes required for such an event are normally not found in the spiral arms of galaxies.

The opposing theory is that the Cow was a special type of supernova in which either a black hole or a quickly rotating highly magnetic neutron star, a magnetar, is produced. While the bright emission in the optical and UV bands are produced by the supernova-like event, the variable X-ray emission is produced by radiating gas falling into the compact object. Normally the debris of a supernova blocks most of the light from reaching us, but the progenitor of the Cow was likely a relatively low-mass star that caused little debris. A hint of its low mass was also found in the X-ray data. If so, the observations would constitute the first observation of the birth of a compact object, making these data very valuable for further theoretical development. Such magnetar sources could also be responsible for ultra-high-energy cosmic rays as well as high-energy neutrinos, two of which might have been observed already. The debate on the nature of the Cow continues, but the wealth of information gathered so far indicates the growing importance of multi-messenger astronomy.

Further reading

R Margutti et al. 2018 arXiv:1810.10720.

K Fang et al. 2018 arXiv:1812.11673.

N Paul and M Kuin 2018 arXiv:1808.08492.

See the full article here .


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THE FOUR MAJOR PROJECT COLLABORATIONS

ATLAS

CERN/ATLAS detector

ALICE

CERN/ALICE Detector


CMS
CERN CMS New

LHCb
CERN LHCb New II

LHC

CERN map

CERN LHC Grand Tunnel

CERN LHC particles

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From insideHPC: “Supercomputing Neutron Star Structures and Mergers”

From insideHPC

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This image of an eccentric binary neutron star system’s close encounter is an example of the large surface gravity wave excitations, which are similar to ocean waves found in very deep water. Credit: William East, Perimeter Institute for Theoretical Physics


Perimeter Institute in Waterloo, Canada

Over at XSEDE, Kimberly Mann Bruch & Jan Zverina from the San Diego Supercomputer Center write that researchers are using supercomputers to create detailed simulations of neutron star structures and mergers to better understand gravitational waves, which were detected for the first time in 2015.

SDSC Dell Comet* supercomputer

During a supernova, a single massive star explodes – some die and form black holes while others survive, depending on the star’s mass. Some of these supernova survivors are stars whose centers collapse and their protons and electrons form into a neutron star, which has an average gravitational pull that is two billion times the gravity on Earth.

Researchers from the U.S., Canada, and Brazil have been focusing on the construction of a gravitational wave model for the detection of eccentric binary neutron stars. Using Comet* at the San Diego Supercomputer Center (SDSC) and Stampede2 at the Texas Advanced Computing Center (TACC), the scientists performed simulations of oscillating binary neutron stars to develop a novel model to predict the timing of various pericenter passages, which are the points of closest approach for revolving space objects.

Texas Advanced Computer Center

TACC DELL EMC Stampede2 supercomputer

Their study, Evolution of Highly Eccentric Binary Neutron Stars Including Tidal Effects was published in Physical Review D. Frans Pretorius, a physics professor at Princeton University, is the Principal Investigator on the allocated project.

“Our study’s findings provide insight into binary neutron stars and their role in detecting gravitational waves,” according to co-author Huan Yang, with the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, Canada. “We can see that the oscillation of the stars significantly alters the trajectory and it is important to mention the evolution of the modes. For this case, during some of the later close encounters, the frequency of the orbit is larger when this evolution is tracked – compared to when it is not – as energy and angular momentum are taken out of the neutron star oscillations and put back into orbit.”

In other words, probing gravitational waves from eccentric binary neutron stars provides a unique opportunity to observe neutron star oscillations. Through these measurements, researchers can infer the internal structure of neutron stars.

“This is analogous to the example of ‘hearing the shape on a drum,’ where the shape of a drumhead can be determined by measuring frequencies of its modes,” said Yang. “By ‘hearing’ the modes of neutron stars with gravitational waves, the star’s size and internal structure will be similarly determined, or at least constrained.”

“In particular, our dynamical space-time simulations solve the equations of Einstein’s theory of general relativity coupled to perfect fluids,” said co-author Vasileios Paschalidis, with the University of Arizona’s Theoretical Astrophysics Program. “Neutron star matter can be described as a perfect fluid, therefore the simulations contain the necessary physics to understand how neutron stars oscillate due to tidal interactions after every pericenter passage, and how the orbit changes due to the excited neutron star oscillations. Such simulations are computationally very expensive and can be performed only in high-performance computing centers.”

“XSEDE resources significantly accelerated our scientific output,” noted Paschalidis, whose group has been using XSEDE for well over a decade, when they were students or post-doctoral researchers. “If I were to put a number on it, I would say that using XSEDE accelerated our research by a factor of three or more, compared to using local resources alone.”

Neutron Star Mergers Form the Cauldron that Brews Gravitational Waves

Merging neutron stars. Image Credit: Mark Garlick, University of Warwick.

The merger of two neutron stars produces a hot (up to one trillion degrees Kelvin), rapidly rotating massive neutron star. This remnant is expected to collapse to form a black hole within a timescale that could be as short as one millisecond, or as long as many hours, depending on the sum of the masses of the two neutron stars.

Featured in a recent issue of the Monthly Notices of the Royal Astronomical Society, Princeton University Computational and Theoretical Astrophysicist David Radice and his colleagues presented results from their simulations of the formation of neutron star merger remnants surviving for at least one tenth of a second. Radice turned to XSEDE for access to Comet, Stampede2, and Bridges, which is based at the Pittsburgh Supercomputing Center (PSC).

Pittsburgh Supercomputer Center

Bridges supercomputer at PSC

It has been long thought that this type of merger product would be driven toward solid-body rotation by turbulent angular momentum transport, which acts as an effective viscosity. However, Radice and his collaborators discovered that the evolution of these objects is actually more complex.

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The massive neutron star shown in this three-dimensional rendition of a Comet-enabled simulation shows the emergence of a wind driven by neutrino radiation. The star is surrounded by debris expelled during and shortly after the merger. Credit: David Radice, Princeton University

“We found that long-lived neutron star merger remnants are born with so much angular momentum that they are unable to reach solid body rotation,” said Radice. “Instead, they are viscously unstable. We expect that this instability will result in the launching of massive neutron rich winds. These winds, in turn, will be extremely bright in the UV/optical/infrared bands. The observation of such transients, in combination with gravitational-wave events or short gamma-ray bursts, would be ‘smoking gun’ evidence for the formation of long-lived neutron star merger remnants.”

If detected, the bright transients predicted in this study could allow astronomers to measure the threshold mass below which neutron star mergers do not result in rapid black hole formation. This insight would be key in the quest to understand the properties of matter at extreme densities found in the hearts of neutron stars.

Radice’s research used 35 high-resolution, general-relativistic neutron star merger simulations, which calculated the geometry of space-time as predicted by Einstein’s equations and simulated the neutron star matter using sophisticated microphysical models. On average, one of these simulations required about 300,000 CPU-hours.

“My research would not be possible without XSEDE,” said Radice, who has used XSEDE resources since 2013, and for this study collaborated with Lars Koesterke at TACC to run his code efficiently on Stampede2. Specifically, this work was conducted in the context of an XSEDE Extended Collaborative Support Services (ECSS) project, which will be of benefit to future research.”

“The cost can be up to a factor of three times higher for the selected models that were run at even higher resolution and depending on the detail level in the microphysics,” added Radice. “Because of the unique requirements of this study, which included a large number of intermediate-size simulations and few larger calculations, a key enabler was the availability of a combination of capability and capacity supercomputers including Comet and Bridges.”

See the full article here .

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Founded on December 28, 2006, insideHPC is a blog that distills news and events in the world of HPC and presents them in bite-sized nuggets of helpfulness as a resource for supercomputing professionals. As one reader said, we’re sifting through all the news so you don’t have to!

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