From NASA Chandra Blog
Peter Edmonds, CXC
We make progress in astrophysics in a variety of ways. There is the sort that starts along a defined path, driven by meticulous proposals for telescope time or detailed science justifications for new missions. The plan is to advance knowledge by traveling further than others, or clearing a broader path. And then there are others.
A big mission like NASA’s Chandra X-ray Observatory begins with plans for investigation along a slew of different directions and lines of study. At the time of Chandra’s launch on July 23rd, 1999, scientists thought these paths would mainly follow studies of galaxy clusters, dark matter, black holes, supernovas, and young stars. Indeed, in the last 20 years we’ve learned about black holes ripping stars apart (reported eg in 2004, 2011 and 2017), about a black hole generating the deepest known note in the universe, about dark matter being wrenched apart from normal matter in the famous Bullet Cluster and similar objects, about the discovery of the youngest supernova remnant in our galaxy, and much more.
Bullet Cluster NASA Chandra NASA ESA Hubble
NASA/ESA Hubble Telescope
Progress in astrophysics can also be made when new paths of study suddenly open up. Three outstanding examples for Chandra are studies of gravitational wave events, dark energy and exoplanets. None of these fields existed before Chandra was conceived or built, but have now delivered some of our most exciting results.
Release: NASA Missions Catch FirstLight from a Gravitational-Wave Event
The newest example is the study of X-rays produced by the aftermath of gravitational wave events. In 1999 the detection of gravitational waves seemed like a distant or even impossible goal for many astronomers. But the LIGO scientists kept improving their remarkably sensitive observatory until September 2015, when they detected a burst of gravitational waves from the merger of two black holes. Two black holes that merge are not expected to produce electromagnetic radiation, but the mergers of two neutron stars are. That is exactly what was as observed for the first time in August 2017 with LIGO and a slew of telescopes.
VIRGO Gravitational Wave interferometer, near Pisa, Italy
Caltech/MIT Advanced aLigo Hanford, WA, USA installation
Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA
Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project
Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger
Gravity is talking. Lisa will listen. Dialogos of Eide
ESA/eLISA the future of gravitational wave research
Localizations of gravitational-wave signals detected by LIGO in 2015 (GW150914, LVT151012, GW151226, GW170104), more recently, by the LIGO-Virgo network (GW170814, GW170817). After Virgo came online in August 2018
Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)
UC Santa Cruz
UCSC All the Gold in the Universe
A UC Santa Cruz special report
Astronomer Ryan Foley says “observing the explosion of two colliding neutron stars” –the first visible event ever linked to gravitational waves–is probably the biggest discovery he’ll make in his lifetime. That’s saying a lot for a young assistant professor who presumably has a long career still ahead of him.
The first optical image of a gravitational wave source was taken by a team led by Ryan Foley of UC Santa Cruz using the Swope Telescope at the Carnegie Institution’s Las Campanas Observatory in Chile. This image of Swope Supernova Survey 2017a (SSS17a, indicated by arrow) shows the light emitted from the cataclysmic merger of two neutron stars. (Image credit: 1M2H Team/UC Santa Cruz & Carnegie Observatories/Ryan Foley)
Carnegie Institution Swope telescope at Las Campanas, Chile, 100 kilometres (62 mi) northeast of the city of La Serena. near the north end of a 7 km (4.3 mi) long mountain ridge. Cerro Las Campanas, near the southern end and over 2,500 m (8,200 ft) high, at Las Campanas, Chile
A neutron star forms when a massive star runs out of fuel and explodes as a supernova, throwing off its outer layers and leaving behind a collapsed core composed almost entirely of neutrons. Neutrons are the uncharged particles in the nucleus of an atom, where they are bound together with positively charged protons. In a neutron star, they are packed together just as densely as in the nucleus of an atom, resulting in an object with one to three times the mass of our sun but only about 12 miles wide.
“Basically, a neutron star is a gigantic atom with the mass of the sun and the size of a city like San Francisco or Manhattan,” said Foley, an assistant professor of astronomy and astrophysics at UC Santa Cruz.
These objects are so dense, a cup of neutron star material would weigh as much as Mount Everest, and a teaspoon would weigh a billion tons. It’s as dense as matter can get without collapsing into a black hole.
Like other stars, neutron stars sometimes occur in pairs, orbiting each other and gradually spiraling inward. Eventually, they come together in a catastrophic merger that distorts space and time (creating gravitational waves) and emits a brilliant flare of electromagnetic radiation, including visible, infrared, and ultraviolet light, x-rays, gamma rays, and radio waves. Merging black holes also create gravitational waves, but there’s nothing to be seen because no light can escape from a black hole.
Foley’s team was the first to observe the light from a neutron star merger that took place on August 17, 2017, and was detected by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO).
ALL THE GOLD IN THE UNIVERSE
It turns out that the origins of the heaviest elements, such as gold, platinum, uranium—pretty much everything heavier than iron—has been an enduring conundrum. All the lighter elements have well-explained origins in the nuclear fusion reactions that make stars shine or in the explosions of stars (supernovae). Initially, astrophysicists thought supernovae could account for the heavy elements, too, but there have always been problems with that theory, says Enrico Ramirez-Ruiz, professor and chair of astronomy and astrophysics at UC Santa Cruz.
The violent merger of two neutron stars is thought to involve three main energy-transfer processes, shown in this diagram, that give rise to the different types of radiation seen by astronomers, including a gamma-ray burst and a kilonova explosion seen in visible light. (Image credit: Murguia-Berthier et al., Science)
A theoretical astrophysicist, Ramirez-Ruiz has been a leading proponent of the idea that neutron star mergers are the source of the heavy elements. Building a heavy atomic nucleus means adding a lot of neutrons to it. This process is called rapid neutron capture, or the r-process, and it requires some of the most extreme conditions in the universe: extreme temperatures, extreme densities, and a massive flow of neutrons. A neutron star merger fits the bill.
Ramirez-Ruiz and other theoretical astrophysicists use supercomputers to simulate the physics of extreme events like supernovae and neutron star mergers. This work always goes hand in hand with observational astronomy. Theoretical predictions tell observers what signatures to look for to identify these events, and observations tell theorists if they got the physics right or if they need to tweak their models. The observations by Foley and others of the neutron star merger now known as SSS17a are giving theorists, for the first time, a full set of observational data to compare with their theoretical models.
According to Ramirez-Ruiz, the observations support the theory that neutron star mergers can account for all the gold in the universe, as well as about half of all the other elements heavier than iron.
RIPPLES IN THE FABRIC OF SPACE-TIME
Einstein predicted the existence of gravitational waves in 1916 in his general theory of relativity, but until recently they were impossible to observe. LIGO’s extraordinarily sensitive detectors achieved the first direct detection of gravitational waves, from the collision of two black holes, in 2015. Gravitational waves are created by any massive accelerating object, but the strongest waves (and the only ones we have any chance of detecting) are produced by the most extreme phenomena.
Two massive compact objects—such as black holes, neutron stars, or white dwarfs—orbiting around each other faster and faster as they draw closer together are just the kind of system that should radiate strong gravitational waves. Like ripples spreading in a pond, the waves get smaller as they spread outward from the source. By the time they reached Earth, the ripples detected by LIGO caused distortions of space-time thousands of times smaller than the nucleus of an atom.
The rarefied signals recorded by LIGO’s detectors not only prove the existence of gravitational waves, they also provide crucial information about the events that produced them. Combined with the telescope observations of the neutron star merger, it’s an incredibly rich set of data.
LIGO can tell scientists the masses of the merging objects and the mass of the new object created in the merger, which reveals whether the merger produced another neutron star or a more massive object that collapsed into a black hole. To calculate how much mass was ejected in the explosion, and how much mass was converted to energy, scientists also need the optical observations from telescopes. That’s especially important for quantifying the nucleosynthesis of heavy elements during the merger.
LIGO can also provide a measure of the distance to the merging neutron stars, which can now be compared with the distance measurement based on the light from the merger. That’s important to cosmologists studying the expansion of the universe, because the two measurements are based on different fundamental forces (gravity and electromagnetism), giving completely independent results.
“This is a huge step forward in astronomy,” Foley said. “Having done it once, we now know we can do it again, and it opens up a whole new world of what we call ‘multi-messenger’ astronomy, viewing the universe through different fundamental forces.”
IN THIS REPORT
A team from UC Santa Cruz was the first to observe the light from a neutron star merger that took place on August 17, 2017 and was detected by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO)
Graduate students and post-doctoral scholars at UC Santa Cruz played key roles in the dramatic discovery and analysis of colliding neutron stars.Astronomer Ryan Foley leads a team of young graduate students and postdoctoral scholars who have pulled off an extraordinary coup. Following up on the detection of gravitational waves from the violent merger of two neutron stars, Foley’s team was the first to find the source with a telescope and take images of the light from this cataclysmic event. In so doing, they beat much larger and more senior teams with much more powerful telescopes at their disposal.
“We’re sort of the scrappy young upstarts who worked hard and got the job done,” said Foley, an untenured assistant professor of astronomy and astrophysics at UC Santa Cruz.
David Coulter, graduate student
The discovery on August 17, 2017, has been a scientific bonanza, yielding over 100 scientific papers from numerous teams investigating the new observations. Foley’s team is publishing seven papers, each of which has a graduate student or postdoc as the first author.
“I think it speaks to Ryan’s generosity and how seriously he takes his role as a mentor that he is not putting himself front and center, but has gone out of his way to highlight the roles played by his students and postdocs,” said Enrico Ramirez-Ruiz, professor and chair of astronomy and astrophysics at UC Santa Cruz and the most senior member of Foley’s team.
“Our team is by far the youngest and most diverse of all of the teams involved in the follow-up observations of this neutron star merger,” Ramirez-Ruiz added.
Charles Kilpatrick, postdoctoral scholar
Charles Kilpatrick, a 29-year-old postdoctoral scholar, was the first person in the world to see an image of the light from colliding neutron stars. He was sitting in an office at UC Santa Cruz, working with first-year graduate student Cesar Rojas-Bravo to process image data as it came in from the Swope Telescope in Chile. To see if the Swope images showed anything new, he had also downloaded “template” images taken in the past of the same galaxies the team was searching.
Ariadna Murguia-Berthier, graduate student
“In one image I saw something there that was not in the template image,” Kilpatrick said. “It took me a while to realize the ramifications of what I was seeing. This opens up so much new science, it really marks the beginning of something that will continue to be studied for years down the road.”
At the time, Foley and most of the others in his team were at a meeting in Copenhagen. When they found out about the gravitational wave detection, they quickly got together to plan their search strategy. From Copenhagen, the team sent instructions to the telescope operators in Chile telling them where to point the telescope. Graduate student David Coulter played a key role in prioritizing the galaxies they would search to find the source, and he is the first author of the discovery paper published in Science.
Matthew Siebert, graduate student
“It’s still a little unreal when I think about what we’ve accomplished,” Coulter said. “For me, despite the euphoria of recognizing what we were seeing at the moment, we were all incredibly focused on the task at hand. Only afterward did the significance really sink in.”
Just as Coulter finished writing his paper about the discovery, his wife went into labor, giving birth to a baby girl on September 30. “I was doing revisions to the paper at the hospital,” he said.
It’s been a wild ride for the whole team, first in the rush to find the source, and then under pressure to quickly analyze the data and write up their findings for publication. “It was really an all-hands-on-deck moment when we all had to pull together and work quickly to exploit this opportunity,” said Kilpatrick, who is first author of a paper comparing the observations with theoretical models.
César Rojas Bravo, graduate student
Graduate student Matthew Siebert led a paper analyzing the unusual properties of the light emitted by the merger. Astronomers have observed thousands of supernovae (exploding stars) and other “transients” that appear suddenly in the sky and then fade away, but never before have they observed anything that looks like this neutron star merger. Siebert’s paper concluded that there is only a one in 100,000 chance that the transient they observed is not related to the gravitational waves.
Ariadna Murguia-Berthier, a graduate student working with Ramirez-Ruiz, is first author of a paper synthesizing data from a range of sources to provide a coherent theoretical framework for understanding the observations.
Another aspect of the discovery of great interest to astronomers is the nature of the galaxy and the galactic environment in which the merger occurred. Postdoctoral scholar Yen-Chen Pan led a paper analyzing the properties of the host galaxy. Enia Xhakaj, a new graduate student who had just joined the group in August, got the opportunity to help with the analysis and be a coauthor on the paper.
Yen-Chen Pan, postdoctoral scholar
“There are so many interesting things to learn from this,” Foley said. “It’s a great experience for all of us to be part of such an important discovery.”
Enia Xhakaj, graduate student
IN THIS REPORT
Scientific Papers from the 1M2H Collaboration
Coulter et al., Science, Swope Supernova Survey 2017a (SSS17a), the Optical Counterpart to a Gravitational Wave Source
Drout et al., Science, Light Curves of the Neutron Star Merger GW170817/SSS17a: Implications for R-Process Nucleosynthesis
Shappee et al., Science, Early Spectra of the Gravitational Wave Source GW170817: Evolution of a Neutron Star Merger
Kilpatrick et al., Science, Electromagnetic Evidence that SSS17a is the Result of a Binary Neutron Star Merger
Siebert et al., ApJL, The Unprecedented Properties of the First Electromagnetic Counterpart to a Gravitational-wave Source
Pan et al., ApJL, The Old Host-galaxy Environment of SSS17a, the First Electromagnetic Counterpart to a Gravitational-wave Source
Murguia-Berthier et al., ApJL, A Neutron Star Binary Merger Model for GW170817/GRB170817a/SSS17a
Kasen et al., Nature, Origin of the heavy elements in binary neutron star mergers from a gravitational wave event
Abbott et al., Nature, A gravitational-wave standard siren measurement of the Hubble constant (The LIGO Scientific Collaboration and The Virgo Collaboration, The 1M2H Collaboration, The Dark Energy Camera GW-EM Collaboration and the DES Collaboration, The DLT40 Collaboration, The Las Cumbres Observatory Collaboration, The VINROUGE Collaboration & The MASTER Collaboration)
Abbott et al., ApJL, Multi-messenger Observations of a Binary Neutron Star Merger
PRESS RELEASES AND MEDIA COVERAGE
Watch Ryan Foley tell the story of how his team found the neutron star merger in the video below. 2.5 HOURS.
UC Santa Cruz Press Release
UC Berkeley Press Release
Carnegie Institution of Science Press Release
LIGO Collaboration Press Release
National Science Foundation Press Release
The Atlantic – The Slack Chat That Changed Astronomy
Washington Post – Scientists detect gravitational waves from a new kind of nova, sparking a new era in astronomy
New York Times – LIGO Detects Fierce Collision of Neutron Stars for the First Time
Science – Merging neutron stars generate gravitational waves and a celestial light show
CBS News – Gravitational waves – and light – seen in neutron star collision
CBC News – Astronomers see source of gravitational waves for 1st time
San Jose Mercury News – A bright light seen across the universe, proving Einstein right
Popular Science – Gravitational waves just showed us something even cooler than black holes
Scientific American – Gravitational Wave Astronomers Hit Mother Lode
Nature – Colliding stars spark rush to solve cosmic mysteries
National Geographic – In a First, Gravitational Waves Linked to Neutron Star Crash
Associated Press – Astronomers witness huge cosmic crash, find origins of gold
Science News – Neutron star collision showers the universe with a wealth of discoveries
UCSC press release
First observations of merging neutron stars mark a new era in astronomy
Writing: Tim Stephens
Video: Nick Gonzales
Photos: Carolyn Lagattuta
Header image: Illustration by Robin Dienel courtesy of the Carnegie Institution for Science
Design and development: Rob Knight
Project managers: Sherry Main, Scott Hernandez-Jason, Tim Stephens
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
Gemini South telescope, Cerro Tololo Inter-American Observatory (CTIO) campus near La Serena, Chile, at an altitude of 7200 feet
Noted in the video but not in the article:
NRAO/Karl V Jansky VLA, on the Plains of San Agustin fifty miles west of Socorro, NM, USA
Prompt telescope CTIO Chile
NASA NuSTAR X-ray telescope
See the full article here
A Chandra observation two days after the merger failed to make a detection, but about a week later a source was discovered. These and additional observations taught us about the behavior and orientation of the jet that the neutron star merger produced.
The interest in this X-ray detection was so intense that there was a race between three different teams to publish the early Chandra observations first. This was accompanied by a rush to publicly announce the full set of results from LIGO and telescopes across the electromagnetic spectrum, before too many smart science writers dug out the news from Twitter and other publicly available information.
Release: All in the Family: Kin ofGravitational-Wave Source Discovered
Chandra detections of two likely neutron star mergers have been reported since August 2017 (in 2018 and 2019). These did not involve a detection of GWs, both because Advanced LIGO wasn’t yet operating, and because the events were likely too distant to be detectable even if it was. When Advanced LIGO and Virgo detect other neutron star mergers, and optical telescopes track them down, Chandra “Target of Opportunity” programs will kick in to study them. (TOOs, as they are called, are special cases made by scientists to interrupt the regularly scheduled observations in favor of one that is time sensitive and/or extremely important.) One is a large proposal and collaboration between three different teams, one aims to take a spectrum, another aims to observe a relatively nearby event, and a fourth involves joint observations with the VLA.
Those who work on Chandra and many in the wider science community were very excited about this detection because it marked the first time that gravitational waves and electromagnetic radiation were observed together, as a new type of “multi-messenger” astrophysics. (Multi-messenger astrophysics involves at least two of the following messengers: electromagnetic radiation, gravitational waves, neutrinos and cosmic rays.) However, it did not represent the first instance of multi-messenger astrophysics, because both electromagnetic radiation and neutrinos had already been observed from the Sun and from Supernova 1987A. Chandra may have already got into the act with the observation of a flare from material very close to the supermassive black hole in the center of our Galaxy, as reported in 2014. An energetic neutrino observed with the IceCube detector may have originated from this flare.
Another exciting new line of study has come from the discovery that the expansion of the universe is accelerating. The two key papers providing the first evidence for this surprising result were published in 1998 and 1999, just before Chandra launched. Both papers used distance measurements to supernova explosions over the last 5 billion or so years to follow the expansion. Since then a set of different techniques have been used to independently confirm and extend these results, including two involving Chandra observations of galaxy clusters. In one of them the distances to galaxy clusters were used to probe the expansion rate of the universe and another involved measuring the effects of accelerating expansion in slowing down the growth rate of galaxy clusters, in a type of cosmic arrested development. As explained in this article, if it wasn’t for accelerating expansion the universe would look very different from how it looks today.
The work measuring the growth rate of galaxy clusters has led to independent tests of Einstein’s General Theory of Relativity over distances that are much greater than those of Earth-orbiting satellites. The confirmation of GR has added to the evidence that a mysterious force called “dark energy” is causing cosmic acceleration.
Release: Astronomers Find Dark EnergyMay Vary Over Time
More recently, Chandra is being used with a new technique to probe cosmic expansion out to greater distances than are possible with supernova data. Astronomers have found tentative evidence that dark energy might be strengthening with time, but this result needs to be confirmed with more extensive use of Chandra data, a study that is currently underway, and independent work.
The recently-launched European mission eROSITA will be taking a sensitive X-ray survey of the complete sky and will discover a huge number of galaxy clusters for follow-up studies of both dark energy and dark matter with Chandra.
eRosita DLR MPG
Release: NASA’s Chandra Sees Eclipsing Planetin X-rays for First Time
Many think the field of exoplanets studies started in 1995 with the detection of a hot Jupiter around the star 51 Pegasus, acclaimed as the first exoplanet discovered around a Sun-like star. (This was about the time that the grinding and polishing of Chandra’s grazing-incidence mirrors was completed.) Chandra observations have shown cases of the tail wagging the dog, where a planet is affecting the star it is orbiting, in one case by making the star appear unusually old, and in others causing it to behave like a much younger star, as reported in 2011 and 2013.
Chandra observations have uncovered multiple examples of planets under assault by outside forces. They’ve found cases where radiation from the host star is evaporating the atmosphere of a close-in planet (from 2011 and 2013), where the powerful gravity of a white dwarf may have ripped a planet apart, a case of possible stellar or planetary cannibalism, and a case where a star may be devouring a young planet. Chandra data was also used to show that young stars much less massive than the Sun can unleash a torrent of X-ray radiation that may significantly shorten the lifetime of planet-forming disks surrounding these stars.
We look forward to reporting more results in these three new fields, along with discoveries from X-ray astronomy’s traditional specialities. We also hope to see new fields appear, for fresh exploration with NASA’s premier X-ray mission.
See the full article here .
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NASA’s Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program for NASA’s Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra’s science and flight operations from Cambridge, Mass.