With the help of an automated supernova-hunting pipeline and a galaxy sitting 2 billion light years away from Earth that’s acting as a “magnifying glass,’’ astronomers have captured multiple images of a Type Ia supernova—the brilliant explosion of a star—appearing in four different locations on the sky. So far this is the only Type Ia discovered that has exhibited this effect.
This animation shows the phenomenon of strong gravitational lensing. This effect caused the supernova iPTF16geu to appear 50 times brighter than under normal circumstances and to be visible on the sky four times. (Credit: ESA/Hubble, L. Calçada)
This phenomenon called ‘gravitational lensing’ is an effect of Einstein’s Theory of Relativity—mass bends light. This means that the gravitational field of a massive object—like a galaxy—can bend light rays that pass nearby and refocus them somewhere else, causing background objects to appear brighter and sometimes in multiple locations. Astrophysicists believe that if they can find more of these magnified Type Ia’s, they may be able to measure the rate of the universe’s expansion to unprecedented accuracy and shed some light on the distribution of matter in the cosmos.
Fortunately, by taking a closer look at the properties of this rare event, two Lawrence Berkeley National Laboratory (Berkeley Lab) researchers have come up with a method—a pipeline— for identifying more of these so-called “strongly lensed Type Ia supernovae” in existing and future wide-field surveys. A paper describing their approach was recently published in Astrophysical Journal Letters. Meanwhile, a paper detailing the discovery and observations of the 4 billion year old Type Ia supernova, iPTF16geu, was published in Science on April 21.
“It is extremely difficult to find a gravitationally lensed supernova, let alone a lensed Type Ia. Statistically, we suspect that there may be approximately one of these in every 50,000 supernovae that we identify,” says Peter Nugent, an astrophysicist in Berkeley Lab’s Computational Research Division (CRD) and an author on both papers. “But since the discovery of iPTF16geu, we now have some thoughts on how to improve our pipeline to identify more of these events.”
Cosmic Surprise Sheds New Light on Cosmology
For many years, the transient nature of supernovae made them extremely difficult to detect. Thirty years ago, the discovery rate was about two per month. But thanks to the Intermediate Palomar Transient Factory (iPTF), a new survey with an innovative pipeline, these events are being detected daily, some within hours of when their initial explosions appear.
The process of identifying transient events, like supernovae, begins every night at the Palomar Observatory in Southern California, where a wide-field camera mounted on the robotic Samuel Oschin Telescope scans the sky. As soon as observations are taken, the data travel more than 400 miles to the Department of Energy’s (DOE’s) National Energy Research Scientific Computing Center (NERSC), which is located at Berkeley Lab. At NERSC, machine learning algorithms running on the facility’s supercomputers sift through the data in real-time and identify transients for researchers to follow up on.
On September 5, 2016, the pipeline identified iPTF16geu as a supernova candidate. At first glance, the event didn’t look particularly out of the ordinary. Nugent notes that many astronomers thought it was just a typical Type Ia supernova sitting about 1 billion light years away from Earth.
Like most supernovae that are discovered relatively early on, this event got brighter with time. Shortly after it reached peak brightness (19th magnitude) Stockholm University Professor in Experimental Particle Astrophysics Ariel Goobar decided to take a spectrum—or detailed light study—of the object. The results confirmed that the object was indeed a Type Ia supernova, but they also showed that, surprisingly, it was located 4 billion light years away. A second spectrum taken with the OSIRIS instrument on the Keck telescope on Mauna Kea, Hawaii, showed without a doubt that the supernova was 4 billion light years away, and also revealed its host galaxy and another galaxy located about 2 billion light years away that was acting as a gravitational lens, which amplified the brightness of the supernova and caused it to appear in four different places on the sky.
“I’ve been looking for a lensed supernova for about 15 years. I looked in every possible survey, I’ve tried a variety of techniques to do this and essentially gave up, so this result came as a huge surprise,” says Goobar, who is lead author of the Science paper. “One of the reasons I’m interested in studying gravitational lensing is that it allows you to measure the structure of matter—both visible and dark matter—at scales that are very hard to get.”
This composite image shows the gravitationally lensed Type Ia supernova iPTF16geu, as seen with different telescopes. The background image shows a wide-field view of the night sky as seen with the Palomar Observatory located on Palo-mar Mountain, California. Far Left Image: Captured by the Sloan Digital Sky Survey, this optical light observation shows the lens galaxy and its surrounding environment in the sky. Center Left Image: Captured by the Hubble Space Telescope, this is a 20x zoom infrared image of the lens galaxy. Center Right Image: Captured by the Hubble Space Telescope, this 5x optical light zoom reveals the four gravitationally lensed images of iPTF16geu. Far Right Image: Captured by the Keck Telescope, this infrared observation features the four gravitationally lensed images of iPTF16geu and the gravitational “arc” of its host galaxy. (Credit: Joel Johansson/Stockholm University)
According to Goobar, the survey at Palomar was set up to look at objects in the nearby universe, about 1 billion light years away. But finding a distant Type Ia supernova in this survey allowed researchers to follow up with even more powerful telescopes that resolved small-scale structures in the supernova host galaxy, as well as the lens galaxy that is magnifying it.
“There are billions of galaxies in the observable universe and it takes a tremendous effort to look in a very small patch of the sky to find these kind of events. It would be impossible to find an event like this without a magnified supernova directing you where to look,” says Goobar. “We got very lucky with this discovery because we can see the small scale structures in these galaxies, but we won’t know how lucky we are until we find more of these events and confirm that what we are seeing isn’t an anomaly.”
Another benefit of finding more of these events is that they can be used as tools to precisely measure the expansion rate of the universe. One of the keys to this is gravitational lensing. When a strong gravitational lens produces multiple images of a background object, each image’s light travels a slightly different path around the lens on its way to Earth. The paths have different lengths, so light from each image takes a different amount of time to arrive at Earth.
“If you measure the arrival times of the different images, that turns out to be a good way to measure the expansion rate of the universe,” says Goobar. “When people measure the expansion rate of the universe now locally using supernovae or Cepheid stars they get a different number from those looking at early universe observations and the cosmic microwave background. There is tension out there and it would be neat if we could contribute to resolving that quest.”
New Methods Sniff Out Lensed Supernovae
According to Danny Goldstein, a UC Berkeley astronomy graduate student and an author of the Astrophysical Journal letter, there have only been a few gravitationally lensed supernovae of any type ever discovered, including iPTF16geu, and they’ve all been discovered by chance.
“By figuring out how to systematically find strongly lensed Type Ia supernovae like iPTF16geu, we hope to pave the way for large-scale lensed supernova searches, which will unlock the potential of these objects as tools for precision cosmology,” says Goldstein, who worked with Nugent to devise a method for finding them in existing and upcoming wide-field surveys.
The key idea of their technique is to use the fact that Type Ia supernovae are “standard candles”—objects with the same intrinsic brightness—to identify ones that are magnified by lensing. They suggest starting with supernovae that appear to go off in red galaxies that have stopped forming stars. These galaxies only host Type Ia supernovae and make up the bulk of gravitational lenses. If a supernova candidate that appears to be hosted in such a galaxy is brighter than the “standard” brightness of a Type Ia supernova, Goldstein and Nugent argue that there is a strong chance the supernova does not actually reside in the galaxy, but is instead a background supernova lensed by the apparent host.
“One of the innovations of this method is that we don’t have to detect multiple images to infer that a supernova is lensed,” says Goldstein. “This is a huge advantage that should enable us to find more of these events than previously thought possible.”
Using this method, Nugent and Goldstein predict that the upcoming Large Synoptic Survey Telescope should be able to detect about 500 strongly lensed Type Ia supernovae over the course of 10 years—about 10 times more than previous estimates. Meanwhile, the Zwicky Transient Facility, which begins taking data in August 2017 at Palomar, should find approximately 10 of these events in a three-year search. Ongoing studies show that each lensed Type Ia supernova image has the potential to make a 4 percent, or better, measurement of the expansion rate of the universe. If realized, this could add a very powerful tool to probe and measure the cosmological parameters.
“We are just now getting to the point where our transient surveys are big enough, our pipelines are efficient enough, and our external data sets are rich enough that we can weave through the data and get at these rare events,” adds Goldstein. “It’s an exciting time to be working in this field.”
IPTF is a scientific collaboration between Caltech; Los Alamos National Laboratory; the University of Wisconsin, Milwaukee; the Oskar Klein Centre in Sweden; the Weizmann Institute of Science in Israel; the TANGO Program of the University System of Taiwan; and the Kavli Institute for the Physics and Mathematics of the Universe in Japan. NERSC is a DOE Office of Science User Facility.
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Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel Prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more, visit www.lbl.gov.
DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.
Quelle: Berkeley National Laboratory
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Rare Brightening of a Supernova's Light found by Palomar Observatory
An international team of astronomers has, for the first time, seen a cosmic magnification of the light from a class of supernova called Type Ia. Type Ia supernovas—often referred to as "standard candles" because of their well-known intrinsic brightness—are frequently used by astronomers to accurately measure the expansion rate of our universe, as well as the amount of dark energy, which is thought to be accelerating this expansion.
Finding a magnified, or "gravitationally lensed," Type Ia supernova is like discovering a brighter candle with which to view the universe. The researchers say this discovery is the first of many to come, and that having a whole collection of similarly lensed Type Ia supernovas will lead to more precise measurements of our universe's most fundamental traits.
Gravitational lensing occurs when the gravity of a cosmic object, such as a galaxy, bends and magnifies the light of a more distant object. The effect can cause galaxies to appear strangely twisted, and even produce multiple images of the same object. While this phenomenon of gravitational lensing has been observed many times since the early 20th century, when it first was predicted by Albert Einstein, imaging a lensed Type Ia supernova has proven formidably difficult, until now.
In the new study, published April 21 in the journal Science, the researchers imaged the Type Ia supernova called iPTF16geu and found it duplicated into four different images.
"Resolving, for the first time, multiple images of a strongly lensed 'standard candle' supernova is a major breakthrough," says Ariel Goobar, a professor with the Oskar Klein Centre at the University of Stockholm, Sweden, and a lead author of the study. "Normally, when we view a lensed object, we don't know the intrinsic brightness of that object, but with Type Ia supernova, we do. This will allow us to better quantify and understand the phenomenon of gravitational lensing."
Goobar and his group are partners in two Caltech-led international scientific collaborations— the intermediate Palomar Transient Factory (iPTF) and the Global Relay of Observatories Watching Transients Happen (GROWTH) project. The iPTF takes advantage of the Palomar Observatory and its unique capabilities to scan the skies and discover, in near real-time, fast-changing cosmic events such as supernovas. GROWTH manages a global network of researchers and telescopes that can swiftly perform follow-up observations to study these transient events in detail.
"I was baffled when I saw the initial data for iPTF16geu from the Palomar Observatory. It looked like a normal Type Ia supernova but it was much brighter than it should have been given its distance from us. The rapid follow up with more powerful facilities confirmed that we had stumbled upon an extremely interesting and rare event," says coauthor Mansi Kasliwal (MS '07, PhD '11), the principal investigator of GROWTH and an assistant professor of astronomy at Caltech.
Within two months of detection, the team observed the iPTF16geu supernova with the NASA/ESA Hubble Space Telescope; the adaptive-optics instruments on the W.M. Keck Observatory atop Mauna Kea, Hawaii; and the VLT telescopes in Chile. Apart from producing a striking visual effect, capturing the image of a strongly lensed Type Ia supernova such as iPTF16geu is extremely useful scientifically. Astronomers can measure very accurately how much time it takes for the light from each of the multiple images of the supernova to reach us. The difference in the time of arrival can then be used to estimate with a high precision the expansion rate of the universe, known as the Hubble Constant.
Another unique advantage of lensed Type Ia supernovas is that they can be identified with relatively small telescopes, such as the 48-inch Samuel Oschin Telescope at Palomar Observatory, which was used to image the iPTF16geu supernova. Larger telescopes are in high demand, and equipped with narrow-field cameras that take too much time to routinely scan the sky. The iPTF project scanned one-fifteenth of the visible sky every night. Its successor, the Zwicky Transient Facility (ZTF), set to begin observing this summer, will scan the skies even faster, and is capable of covering the entire accessible sky every night. By scanning large swaths of the sky, astronomers can sift through thousands of cosmic objects to find rare events such as the lensing of a Type Ia supernova.
"I am blown away. When iPTF was conceived, we only dreamed of discovering such events. We knew they existed but we honestly did not expect to detect one! It bodes well for the iPTF's successor, ZTF," says Shri Kulkarni, John D. and Catherine T. MacArthur Professor of Astronomy and Planetary Science, who is the principal investigator of ZTF as well as director of the Caltech Optical Observatories.
"What's more, while ZTF is 10 times faster than iPTF, new facilities such as the national flagship Large Synoptic Survey Telescope (LSST) are 10 times faster than ZTF. Clearly, the discovery of iPTF16geu suggests a wealth of new science that will be made possible with the LSST," adds Kulkarni.
The study of iPTF16geu is already delivering interesting results. Using data from Keck and Hubble the team calculated that the lensing matter in the galaxy magnifying iPTF16geu has a mass up to 10 billion times that of the sun and a radius of nearly 3,000 light-years. Compared to other lensing objects, this is relatively tiny. Studies of unusual lensed objects like this give astronomers a new peek into gravitational lensing and may redefine what we know about the factors, such as dark matter and Einstein's general theory of relativity, that contribute to lensing.
"The discovery of iPTF16geu is truly like finding a somewhat weird needle in a haystack. It reveals to us a bit more about the universe, but mostly triggers a wealth of new scientific questions. That's why I love science and astronomy," says Rahman Amanullah, a research scientist at Stockholm University and a coauthor on the study.
The Science study is titled, "iPTF16geu: A multiply-imaged gravitationally lensed Type Ia supernova." Other Caltech authors include Chuck Steidel (PhD '90), Lee A. DuBridge Professor of Astronomy; postdoctoral scholars Nadia Blagorodnova, Thomas Kupfer, and Ragnhild Lunnan; Frank Masci, IPAC scientific research/task lead; Research Scientist Don Neill; Vikram Ravi, R A and G B Millikan Postdoctoral Scholar in Astronomy; Richard Walters, assistant research engineer for Palomar; and IPAC Staff Scientist Lin Yan (PhD '96).
The intermediate Palomar Transient Factory (iPTF) is based at the Palomar Observatory, operated by Caltech. The iPTF project is a scientific collaboration among Caltech; Los Alamos National Laboratory; the University of Wisconsin, Milwaukee; the Oskar Klein Centre; the Weizmann Institute of Science in Israel; the TANGO Program of the University System of Taiwan; and the Kavli Institute for the Physics and Mathematics of the Universe at the University of Tokyo.
GROWTH is a partnership of 13 international academic institutions that jointly pursue science at the frontier of time-domain astronomy. GROWTH is jointly funded by the National Science Foundation (Partnership for International Research and Education program) and partners in Japan, Taiwan, and India.
Quelle: Caltech