# What exactly is it that is being magnified 50 times in this gravitational lensing observation?

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In the Los Angeles Times news item Scientists get a rare view of a type Ia supernova magnified 50 times what exactly is magnified 50 times?

This supernova is really very far away. Is it somehow imaged by the gravitational lensing and being resolved - the actual supernova itself?

edit: I believe this is it, but it's behind a paywall (will confirm in due course): http://science.sciencemag.org/content/356/6335/291

above: From the Los Angeles Times. Photo credit: Joel Johansson

above: "This schematic image represents how light from a distant galaxy is distorted by the gravitational effects of a nearer foreground galaxy, which acts like a lens and makes the distant source appear distorted, but brighter, forming characteristic rings of light, known as Einstein rings. An analysis of the distortion has revealed that some of the distant star-forming galaxies are as bright as 40 trillion Suns, and have been magnified by the gravitational lens by up to 22 times. (Credit: ALMA ESO/NRAO/NAOJ, L. Calçada (ESO), Y. Hezaveh et al., edited and modified by Joel Johansson)" From the Los Angeles Times.

The magnification refers to the increase in angular extent (expressed as a solid angle) of the background source, but is also the factor by which its total brightness is increased.

The reason for this is that the flux received per unit solid angle is unchanged by gravitational lensing. So if the source area increases by a factor of $M$, then so does the overall brightness.

This increase in total flux observed applies whether the magnified source is resolved by the telescope or not, since that is merely an instrumental issue.

It appears to just be a regular photo. It could've been taken by Hubble, and someone just magnified the photo. It doesn't appear to be gravitational lensing. If it was you would be able to see space being bent a bit in the photo, read here

Reading through the full article, I see that the authors write

we estimated the lensing amplification to be $musim52$.

This refers to the magnification factor. Essentially, it describes the solid angle of the image, as related to the solid angle of the source1: $$muequivfrac{ heta}{eta}frac{d heta}{deta}$$ Here's a diagram of the magnitudes of the angles:

Image courtesy of Wikipedia user Falcorian under the Creative Commons Attribution-Share Alike 3.0 Unported license.

## Distant supernova split four ways by gravitational lens

In this Hubble Space Telescope image, the many red galaxies are members of the massive MACS J1149.6+2223 cluster, which creates distorted and highly magnified images of the galaxies behind it. A large cluster galaxy (center of the box) has split the light from an exploding supernova in a magnified background galaxy into four yellow images (arrows) to form an Einstein Cross. Image credit: NASA, ESA, and S. Rodney (JHU) and the FrontierSN team T. Treu (UCLA), P. Kelly (UC Berkeley) and the GLASS team J. Lotz (STScI) and the Frontier Fields Team M. Postman (STScI) and the CLASH team and Z. Levay (STScI)

Over the past several decades, astronomers have come to realise that the sky is filled with magnifying glasses that allow the study of very distant and faint objects barely visible with even the largest telescopes.

A University of California, Berkeley, astronomer has now found that one of these lenses — a massive galaxy within a cluster of galaxies that are gravitationally bending and magnifying light — has created four separate images of a distant supernova.

The so-called “Einstein cross” will allow a unique study of a distant supernova and the distribution of dark matter in the lensing galaxy and cluster.

“Basically, we get to see the supernova four times and measure the time delays between its arrival in the different images, hopefully learning something about the supernova and the kind of star it exploded from, as well as about the gravitational lenses,” said UC Berkeley postdoctoral scholar Patrick Kelly, who discovered the supernova while looking through infrared images taken November 10th, 2014, by the Hubble Space Telescope (HST). “That will be neat.”

Kelly is a member of the Grism Lens-Amplified Survey from Space (GLASS) team led by Tommaso Treu at UCLA, which has worked in collaboration with the FrontierSN team organised by Steve Rodney at Johns Hopkins University to search for distant supernovae.

“It’s a wonderful discovery,” said Alex Filippenko, a UC Berkeley professor of astronomy and a member of Kelly’s team. “We’ve been searching for a strongly lensed supernova for 50 years, and now we’ve found one. Besides being really cool, it should provide a lot of astrophysically important information.”

One bonus is that, given the peculiar nature of gravitational lensing, astronomers can tune in for a supernova replay within the next five years. This is because light can take various paths around and through a gravitational lens, arriving at Earth at different times. Computer modelling of this lensing cluster shows that the researchers missed opportunities to see the exploding star 50 years ago and again 10 years ago, but images of the explosion will likely repeat again in a few years.

“The longer the path length, or the stronger the gravitational field through which the light moves, the greater the time delay,” noted Filippenko.

Kelly is first author of a paper reporting the supernova appearing this week in a special March 6th issue of Science magazine to mark the centenary of Albert Einstein’s general theory of relativity.

Kelly, Filippenko and their collaborators have dubbed the distant supernova SN Refsdal in honour of Sjur Refsdal, the late Norwegian astrophysicist and pioneer of gravitational lensing studies. It is located about 9.3 billion light-years away (redshift = 1.5), near the edge of the observable universe, while the lensing galaxy is about 5 billion light-years (redshift = 0.5) from Earth.

Einstein Cross
Einstein’s general theory of relativity predicts that dense concentrations of mass in the universe will bend light like a lens, magnifying objects behind the mass when seen from Earth. The first gravitational lens was discovered in 1979. Today, lensing provides a new window into the extremely faint universe shortly after its birth 13.8 billion years ago.

“These gravitational lenses are like a natural magnifying glass. It’s like having a much bigger telescope,” Kelly said. “We can get magnifications of up to 100 times by looking through these galaxy clusters.”

Illustration showing how the powerful gravity of a massive galaxy cluster bends and focuses the light from a supernova behind it – gravitational lensing – resulting in multiple images of the exploding star, an Einstein Cross. Image credit: NASA, ESA and A. Feild/STScI

When light from a background object passes by a mass, such as an individual galaxy or a cluster of galaxies, the light is bent. When the path of the light is far from the mass, or if the mass is not especially large, “weak lensing” will occur, barely distorting the background object. When the background object is almost exactly behind the mass, however, “strong lensing” can smear extended objects (like galaxies) into an “Einstein ring” surrounding the lensing galaxy or cluster of galaxies. Strong lensing of small, point-like objects, on the other hand, often produces multiple images — an Einstein cross — arrayed around the lens.

“We have seen many distant quasars appear as Einstein crosses, but this is the first time a supernova has been observed in this way,” Filippenko said. “This short-lived object was discovered only because Pat Kelly very carefully examined the HST data and noticed a peculiar pattern. Luck comes to those who are prepared to receive it.”

The galaxy that is splitting the light from the supernova into an Einstein cross is part of a large cluster, called MACS J1149.6+2223, that has been known for more than 10 years.

In 2009, astronomers reported that the cluster created the largest known image of a spiral galaxy ever seen through a gravitational lens. The new supernova is located in one of that galaxy’s spiral arms, which also appears in multiple images around the foreground lensing cluster. The supernova, however, is split into four images by a red elliptical galaxy within the cluster.

“We get strong lensing by a red galaxy, but that galaxy is part of a cluster of galaxies, which is magnifying it more. So we have a double lensing system,” Kelly said.

Looking for Transients
After Kelly discovered the lensed supernova November 10th while looking for interesting and very distant supernova explosions, he and the team examined earlier HST images and saw it as early as November 3rd, though it was very faint. So far, the HST has taken several dozen images of it using the Wide Field Camera 3 Infrared camera as part of the Grism survey. Astronomers using the HST plan to get even more images and spectra as the telescope focuses for the next six months on that area of sky.

“By luck, we have been able to follow it very closely in all four images, getting data every two to three days,” he said.

Kelly hopes that measuring the time delays between the phases of the supernova in the four images will enable constraints on the foreground mass distribution and on the expansion and geometry of the universe. If the spectrum identifies it as a Type Ia supernova, which is known to have a relatively standard brightness, it may be possible to put even stronger limits on both the matter distribution and cosmological parameters.

## Einstein&rsquos Spacetime Telescope

According to Einstein&rsquos general relativity, gravitational lenses form because galaxies and galaxy clusters noticeably warp spacetime. If a galaxy lies between Earth and some distant object, then that galaxy behaves like a lens, curving spacetime to magnify that object&rsquos light as seen from Earth. Gravitational waves also must follow curved spacetime&mdashso they too can be lensed and magnified by gravitational lenses. Furthermore, the greater an object&rsquos distance from Earth, the greater the chance its light&mdashor gravitational waves&mdashwill be gravitationally lensed by an intervening galaxy. All together, these circumstances yield a recipe for Smoot and colleagues&rsquo claim LIGO&ndashVirgo must be seeing gravitationally lensed black hole mergers. &ldquoWe are saying two thirds of their events are lensed,&rdquo Smoot says, of LIGO&ndashVirgo&rsquos catalogue of detections.

Daniel Holz, a member of the LIGO collaboration at the University of Chicago is entirely unconvinced. He and his colleagues predicted well before LIGO and Virgo made their detections the observatories would see mergers of black holes of about 30 solar masses each. He agrees greater numbers of low-metallicity stars would have formed in the early universe compared with today&rsquos universe&mdashand hence more 30-solar-mass black holes would have formed then compared with now. But despite most of these bulky black holes forming in earlier cosmic epochs he remains confident LIGO and Virgo are detecting their mergers now, in the relatively local universe, because the gravitational dance that ultimately leads to the coalescence of two orbiting black holes is a process that unfolds over billions of years.

Also, Holz adds, ground-based surveys have shown that some low-metallicity regions do in fact exist in the local universe, all of which could harbor such black hole binaries with 30 solar masses each. &ldquoYou put all that together and you make a prediction for what you should see with LIGO,&rdquo he says. And the detections are in line with the predictions, he adds, making it highly unlikely that any of the LIGO&ndashVirgo events are lensed events. &ldquoThe current theoretical underpinnings of star formation and evolution, and black hole binary formation and evolution, seem to account for all LIGO observations to date reasonably well. There is no need to go to extremely speculative models.&rdquo

## General relativity in brief

Before we can understand gravitational lensing in detail, we need to grasp how gravity can affect light. As many of you know, general relativity replaced the old concept of force – influences extending through space as formulated by Isaac Newton – with a geometrical picture. To see how this works, imagine a smooth globe with the latitude lines and meridians marked. Two tiny travelers set out northwards from the equator at different longitudes since their imaginary world is smooth, they can keep marching in straight lines until they reach the North Pole. In other words, they start off in parallel walking in straight lines, yet they both end up at exactly the same point.

If you slice the globe along both of their trajectories, then connect their starting points by cutting along the equator, and lay the shape you get flat on a tabletop, you would end up with a triangle. (In practice this is most easily done with an orange, with its thick peel!) Two sides of this triangle are not actually straight, and the angles add up to more than 180º, unlike the triangles you studied in high school geometry. In fact, this view of the two travelers makes it look like they were attracted to each other by a force – but there was no force! Each traveler believed they were following a straight line, but because of the curvature of the globe on which they walked, the effect was exactly the same as if they were attracted to each other. The effect is the same the cause is different. This is the essence of general relativity, though the geometry of many gravitational systems ends up being a lot more complicated than a sphere.

Another consequence of this geometrical view is that light will also follow curved paths under the influence of gravity. The paths won’t be the same as those of massive objects like electrons or planets, but Einstein recognized that this effect would be a good test of his theory. The deflection of light by the Sun’s gravity was first measured by English astrophysicist Arthur Stanley Eddington, who led an observatory expedition to the African island of Principe in 1919. The challenge is that starlight isn’t nearly as bright as the Sun’s light, so to see how light from stars is deflected by the Sun required performing the observation during a total solar eclipse. He measured the apparent position of stars and compared them to their positions when the Sun is in a different part of the sky, and found the difference in apparent position to be in agreement with Einstein’s predictions. Later observations have continued to confirm Eddington’s results.

## Distant supernova split four ways by gravitational lens

Over the past several decades, astronomers have come to realize that the sky is filled with magnifying glasses that allow the study of very distant and faint objects barely visible with even the largest telescopes.

In this Hubble Space Telescope image, the many red galaxies are members of the massive MACS J1149.6+2223 cluster, which creates distorted and highly magnified images of the galaxies behind it. A large cluster galaxy (center of the box) has split the light from an exploding supernova in a magnified background galaxy into four yellow images (arrows) to form an Einstein Cross. Image credit:
NASA, ESA, and S. Rodney (JHU) and the FrontierSN team T. Treu (UCLA), P. Kelly (UC Berkeley) and the GLASS team J. Lotz (STScI) and the Frontier Fields Team M. Postman (STScI) and the CLASH team and Z. Levay (STScI)

A University of California, Berkeley, astronomer has now found that one of these lenses – a massive galaxy within a cluster of galaxies that are gravitationally bending and magnifying light – has created four separate images of a distant supernova.

The so-called “Einstein cross” will allow a unique study of a distant supernova and the distribution of dark matter in the lensing galaxy and cluster.

“Basically, we get to see the supernova four times and measure the time delays between its arrival in the different images, hopefully learning something about the supernova and the kind of star it exploded from, as well as about the gravitational lenses,” said UC Berkeley postdoctoral scholar Patrick Kelly, who discovered the supernova while looking through infrared images taken Nov. 10, 2014, by the Hubble Space Telescope (HST). “That will be neat.”

Kelly is a member of the Grism Lens-Amplified Survey from Space (GLASS) team led by Tommaso Treu at UCLA, which has worked in collaboration with the FrontierSN team organized by Steve Rodney at Johns Hopkins University to search for distant supernovae.

“It’s a wonderful discovery,” said Alex Filippenko, a UC Berkeley professor of astronomy and a member of Kelly’s team. “We’ve been searching for a strongly lensed supernova for 50 years, and now we’ve found one. Besides being really cool, it should provide a lot of astrophysically important information.”

One bonus is that, given the peculiar nature of gravitational lensing, astronomers can tune in for a supernova replay within the next five years. This is because light can take various paths around and through a gravitational lens, arriving at Earth at different times. Computer modeling of this lensing cluster shows that the researchers missed opportunities to see the exploding star 50 years ago and again 20 years ago, but images of the explosion will likely repeat again in a few years.

“The longer the path length, or the stronger the gravitational field through which the light moves, the greater the time delay,” noted Filippenko.

Kelly is first author of a paper reporting the supernova appearing this week in a special March 6 issue of Science magazine to mark the centenary of Albert Einstein’s General Theory of Relativity.

Kelly, Filippenko and their collaborators have dubbed the distant supernova SN Refsdal in honor of Sjur Refsdal, the late Norwegian astrophysicist and pioneer of gravitational lensing studies. It is located about 9.3 billion light years away (redshift = 1.5), near the edge of the observable universe, while the lensing galaxy is about 5 billion light years (redshift = 0.5) from Earth.

Einstein cross

Einstein’s General Theory of Relativity predicts that dense concentrations of mass in the universe will bend light like a lens, magnifying objects behind the mass when seen from Earth. The first gravitational lens was discovered in 1979. Today, lensing provides a new window into the extremely faint universe shortly after its birth 13.8 billion years ago.

Illustration showing how the powerful gravity of a massive galaxy cluster bends and focuses the light from a supernova behind it – gravitational lensing – resulting in multiple images of the exploding star, an Einstein Cross. (Courtesy of NASA, ESA and A. Feild/STScI)

“These gravitational lenses are like a natural magnifying glass. It’s like having a much bigger telescope,” Kelly said. “We can get magnifications of up to 100 times by looking through these galaxy clusters.”

When light from a background object passes by a mass, such as an individual galaxy or a cluster of galaxies, the light is bent. When the path of the light is far from the mass, or if the mass is not especially large, “weak lensing” will occur, barely distorting the background object. When the background object is almost exactly behind the mass, however, “strong lensing” can smear extended objects (like galaxies) into an “Einstein ring” surrounding the lensing galaxy or cluster of galaxies. Strong lensing of small, point-like objects, on the other hand, often produces multiple images – an Einstein cross – arrayed around the lens.

“We have seen many distant quasars appear as Einstein crosses, but this is the first time a supernova has been observed in this way,” Filippenko said. “This short-lived object was discovered only because Pat Kelly very carefully examined the HST data and noticed a peculiar pattern. Luck comes to those who are prepared to receive it.”

The galaxy that is splitting the light from the supernova into an Einstein cross is part of a large cluster, called MACS J1149.6+2223, that has been known for more than 10 years.

In 2009, astronomers reported that the cluster created the largest known image of a spiral galaxy ever seen through a gravitational lens. The new supernova is located in one of that galaxy’s spiral arms, which also appears in multiple images around the foreground lensing cluster. The supernova, however, is split into four images by a red elliptical galaxy within the cluster.

“We get strong lensing by a red galaxy, but that galaxy is part of a cluster of galaxies, which is magnifying it more. So we have a double lensing system,” Kelly said.

Looking for transients

After Kelly discovered the lensed supernova Nov. 10 while looking for interesting and very distant supernova explosions, he and the team examined earlier HST images and saw it as early as Nov. 3, though it was very faint. So far, the HST has taken several dozen images of it using the Wide Field Camera 3 Infrared camera as part of the Grism survey. Astronomers using the HST plan to get even more images and spectra as the telescope focuses for the next six months on that area of sky.

“By luck, we have been able to follow it very closely in all four images, getting data every two to three days,” he said.

Kelly hopes that measuring the time delays between the phases of the supernova in the four images will enable constraints on the foreground mass distribution and on the expansion and geometry of the universe. If the spectrum identifies it as a Type Ia supernova, which is known to have a relatively standard brightness, it may be possible to put even stronger limits on both the matter distribution and cosmological parameters.

UC Berkeley co-authors of the paper, in addition to Kelly and Filippenko, are postdoctoral scholars Melissa Graham and Bradley Tucker. Other contributing authors are Steven A. Rodney, Tommaso Treu, Ryan J. Foley, Gabriel Brammer, Kasper B. Schmidt, Adi Zitrin, Alessandro Sonnenfeld, Louis-Gregory Strolger, Or Graur, Saurabh W. Jha, Adam G. Riess, Marusa Bradac, Benjamin J. Weiner, Daniel Scolnic, Matthew A. Malkan, Anja von der Linden, Michele Trenti, Jens Hjorth, Raphael Gavazzi, Adriano Fontana, Julian C. Merten, Curtis McCully, Tucker Jones, Marc Postman, Alan Dressler, Brandon Patel and S. Bradley Cenko.

The UC Berkeley work was supported by the Christopher R. Redlich Fund, the TABASGO Foundation and the National Science Foundation.

RELATED INFORMATION

## Cosmic illusion revealed: Gravitational lens magnifies supernova

A team of researchers led by Robert Quimby at the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) has announced the discovery of a galaxy that magnified a background, Type Ia supernova thirtyfold through gravitational lensing. This is the first example of strong gravitational lensing of a supernova confirms the team's previous explanation for the unusual properties of this supernova.

The team has further shown how such discoveries of supernovae of Type Ia (SNIa) can be made far more common than previously thought possible. SNIa　seen through gravitational lenses can be used to make a direct measurement of the universe's expansion rate (the Hubble parameter), so this discovery may have a significant impact on how cosmic expansion is studied in the future.

Supernovae of Type Ia (SNIa) are tremendously useful to understand the mysterious components of the Universe such as dark energy and dark matter. SNIa have strikingly similar peak luminosities, regardless of where they happen in the Universe. This property allows astronomers to use SNIa as standard candles to measure cosmological distance independent of the Universe's expansion. Distance measurement with SNIa was key to the discovery of accelerating expansion of the Universe (2011 Nobel Prize in Physics).

In 2010, a supernova named PS1-10afx was found that demonstrated the same color and light curve (the change in brightness over time) as a Type Ia supernova, but its peak brightness was 30 times greater than expected. This discovery was made using the Panoramic Survey Telescope & Rapid Response System 1 (Pan-STARRS1, a telescope located in Hawai'i that can image the entire visible sky several times each month). This anomaly led some to conclude that it was a completely new type of superluminous supernova. "PS1-10afx looked a lot like a Type Ia supernova, " says Quimby, "but it was just too bright."

The physics of Type Ia supernovae have been studied in detail over the past three decades, and there is no known way to produce a Type Ia supernova with normal colors and a normal light curve but a substantially higher luminosity.

"Generally, the rare supernovae that have been found to shine brighter than Type Ia usually have higher temperatures (bluer colors) and larger physical sizes (and thus slower light curves)," Quimby continues, "New physics would thus be required to explain PS1-10afx as an intrinsically luminous supernova."

"We found a second explanation," says Marcus Werner, Mathematical Physicist at the Kavli IPMU, "and it required only well demonstrated physics: gravitational lensing. If there was a massive galaxy in front of PS1-10afx, it could warp space-time to form magnified images of the supernova."

"Although the available observations were consistent with the hypothesis of our team, we needed to answer the question: Where was the lens galaxy?" says Anupreeta More, Astronomer at the Kavli IPMU, "The existing data clearly showed the presence of the supernova's host galaxy, but there was no evidence for the necessary foreground galaxy. We then tried to find the evidence."

In September 2013, Quimby's team set out to find the hidden lens. Using the Low-Resolution Imaging Spectrograph on the 10 meter Keck-I telescope located in Hawai'i, they spent 7 hours collecting light at the location of PS1-10afx, which had by then faded away itself.

"After carefully extracting the signal from the data, we had confirmation." More continues, "Buried in the glare of the relatively bright host galaxy, we found a second, foreground galaxy. This second galaxy was faint enough to have previously gone unnoticed. But our analysis showed that it was still the right size to explain the gravitational lensing of PS1-10afx."

"We had existing predictions of what a gravitationally lensed Type Ia supernova would look like," says Masamune Oguri from the Department of Physics at the University of Tokyo, "But the small size of this lens galaxy and the large magnification it produced was not exactly what we were expecting for the first discovery." Oguri continues, "However, this system may very well prove typical of discoveries to come. Because more distant supernovae are more likely to be gravitationally lensed, lensed supernovae are typically highly magnified and located in the distant universe."

A consequence of this is that most of the gravitationally lensed Type Ia supernovae that will be found with future surveys using instruments such as the coming Large Synoptic Survey Telescope can be identified by their colors the higher redshift, gravitationally lensed supernovae being redder than the more nearby, un-lensed objects. "Our new approach allows us to find unresolved strong lensing events produced by such low-mass galaxies. Thus, the expected number of gravitationally lensed Type Ia supernovae to be found in future surveys increases by an order of magnitude," says Oguri.

"In the future, when a target is identified as a possible lensed Type Ia supernova," says Quimby, "high-resolution follow-up observations can be taken to resolve the individual image components." Each image comes from the same source but travels a different path length on its way to the observer, so there is an arrival time difference between these multiple supernova images. If this "time delay" can be measured, a direct test of cosmic expansion is possible, faster expansion leads to shorter time delays. By timing the delays precisely and comparing these to the delay expected from the geometry of the lens, expansion history of the Universe can be directly inferred. Quimby continues, "The discovery and selection method we have crafted may thus soon improve our understanding of our expanding universe."

## Hubble peers through cosmic lens to capture most distant star ever seen

Thanks to a rare cosmic alignment, astronomers have captured the most distant normal star ever observed, some 9 billion light years from Earth.

A massive cluster (left) magnified a distant star more than 2,000 times, making it visible from Earth (lower right) even though it is 9 billion light years away, far too distant to be seen individually with current telescopes. It was not visible in 2011 (upper right). Credits: NASA, ESA, and P. Kelly (University of Minnesota)

While astronomers routinely study galaxies much farther away, they’re visible only because they glow with the brightness of billions of stars. And a supernova, often brighter than the galaxy in which it sits, also can be visible across the entire universe. Beyond a distance of about 100 million light years, however, the stars in these galaxies are impossible to make out individually.

But a phenomenon called gravitational lensing – the bending of light by massive galaxy clusters in the line of sight – can magnify the distant universe and make dim, far away objects visible. Typically, lensing magnifies galaxies by up to 50 times, but in this case, the star was magnified more than 2,000 times. It was discovered in NASA Hubble Space Telescope images taken in late April of 2016 and as recently as April 2017.

“You can see individual galaxies out there, but this star is at least 100 times farther away than the next individual star we can study, except for supernova explosions,” said former UC Berkeley postdoctoral scholar Patrick Kelly, now on the faculty at the University of Minnesota, Twin Cities. Kelly is first author of a paper about the discovery appearing online this week in advance of publication in the journal Nature Astronomy.

The discovery of the star, which astronomers often refer to as Icarus rather than by its formal name, MACS J1149 Lensed Star 1 (LS1), kicks off a new technique for astronomers to study individual stars in galaxies formed during the earliest days of the universe. These observations can provide a rare look at how stars evolve, especially the most luminous ones.

“For the first time ever we’re seeing an individual normal star – not a supernova, not a gamma ray burst, but a single stable star – at a distance of nine billion light years,” said Alex Filippenko, a professor of astronomy at UC Berkeley and one of many co-authors of the report. “These lenses are amazing cosmic telescopes.”

The astronomy team also used Icarus to test and reject one theory of dark matter – that it consists of numerous primordial black holes lurking inside galaxy clusters – and to probe the make-up of normal matter and dark matter in the galaxy cluster.

Einstein ring

Kelly noticed the star while monitoring a supernova he had discovered in 2014 while using Hubble to peer through a gravitational lens in the constellation Leo. That supernova, dubbed SN Refsdal in honor of the late Norwegian astrophysicist Sjur Refsdal, a pioneer of gravitational lensing studies, was split into four images by the lens, a massive galaxy cluster called MACS J1149+2223, located about 5 billion light years from Earth.

Suspecting that Icarus might be more highly magnified than SN Refsdal, Kelly and his team analyzed the colors of the light coming from it and discovered it was a single star, a blue supergiant. This B-type star is much larger, more massive, hotter and possibly hundreds of thousands of times intrinsically brighter than our Sun, though still much too far away to see without the amplification of gravitational lensing.

By modeling the lens, they concluded that the tremendous apparent brightening of Icarus was probably caused by a unique effect of gravitational lensing. While an extended lens, like a galaxy cluster, can only magnify a background object up to 50 times, smaller objects can magnify much more. A single star in a foreground lens, if precisely aligned with a background star, can magnify the background star thousands of times. In this case, a star about the size of our sun briefly passed directly through the line of sight between the distant star Icarus and Hubble, boosting its brightness more than 2,000 times.

In fact, if the alignment was perfect, that single star within the cluster turned the light from the distant star into an “Einstein ring”: a halo of light created when light from the distant star bends around all sides of the lensing star. The ring is too small to discern from this distance, but the effect made the star easily visible by magnifying its apparent brightness.

Kelly saw a second star in the Hubble image, which could either be a mirror image of Icarus, or a different star being gravitationally lensed.

“There are alignments like this all over the place as background stars or stars in lensing galaxies move around, offering the possibility of studying very distant stars dating from the early universe, just as we have been using gravitational lensing to study distant galaxies,” Filippenko said. “For this type of research, nature has provided us with a larger telescope than we can possibly build!”

As for Icarus, the astronomers predict that it will be magnified many times over the next decade as cluster stars move around, perhaps increasing its brightness as much as 10,000 times.

The research by Kelly and Filippenko was supported by funds from NASA, the Christopher R. Redlich Fund, TABASGO Foundation and Miller Institute for Basic Research in Science at UC Berkeley.

## Lensing: The Gravitational Imperative

We usually think of gravitational lenses in terms of massive objects. When light from a distant galaxy is magnified by a galactic cluster between us and that galaxy, we get all kinds of interesting magnifications and distortions useful for astronomical purposes. But gravitational lensing isn’t just about galaxies. It happens around stars as well, as we saw recently with the discovery of a solar system with planets analogous to Jupiter and Saturn in our own system. That find was made with the help of a single star crossing in front of another, the resulting magnification allowing the signature of two planets around the closer star to be seen.

Interestingly enough, some of the earliest work on solar sails in interstellar environments came out of the attraction of taking advantage of the Sun’s own gravitational lens. Push some 550 AU out and you reach the point where solar gravity focuses the light of objects on the other side of the Sun as seen from a spacecraft. Note two things: At 550 AU, electromagnetic radiation from the occulted object is boosted by a factor of roughly 10 8 . Secondly, gravity-focused radiation does not behave like light in a conventional optical lens in one important sense. The light does not diverge after the focus as the spacecraft continues to move away from the Sun. Indeed, the focal line extends to infinity.

The Italian aerospace company Alenia Spazio (based in Turin) began investigations into inflatable sail technologies as long ago as the 1980s. Since then, physicist Claudio Maccone has continued to investigate a mission he calls FOCAL, a probe to the gravity focus. Maccone sees such a mission as inevitable, for it takes advantage of an asset every technological civilization will ultimately want to exploit. Here’s how he puts it in his 2002 book The Sun as a Gravitational Lens: Proposed Space Missions:

As each civilization becomes more knowledgeable they will recognize, as we now have recognized, that each civilization has been given a single great gift: a lens of such power that no reasonable technology could ever duplicate or surpass… This lens is the civilization’s star in our case, our Sun. The gravity of each star acts to bend space, and thus the paths of any wave or particle, in the end creating an image just as familiar lenses do….Every civilization will discover this eventually, and surely will make the exploitation of such a lens a very high priority enterprise.

Maccone is the FOCAL mission’s most eloquent spokesman his continuing travels and presentations on its behalf are part of the discovery process for our own civilization as we begin to see the possibilities opening up in nearby interstellar space. Another part of the discovery process is the distribution of news like the recent Hubble Space Telescope findings, which have demonstrated 67 new gravitationally lensed galaxies. Clearly, we are only beginning to understand the power of lensing to help us make sense out of even the most distant parts of the cosmos.

Hubble’s latest finds are part of a survey of a single 1.6 square degree field of sky with various space-based and Earth-based observatories. A team of European astronomers using Hubble’s Advanced Camera for Surveys (ACS) identified the new lenses, which were found around massive elliptical or lenticular galaxies without spiral arms or discs.

Here again we run into the issue of scale. Many of the gravitational lensing observations made thus far have involved not just single galaxies but whole clusters. Jean-Paul Kneib (Laboratoire d’Astrophysique de Marseille) notes the difference:

“We typically see the gravitational lens create a series of bright arcs or spots around a galaxy cluster. What we are observing here is a similar effect but on much smaller scale – happening only around a single but very massive galaxy.”

Four of the discovered lenses have produced ‘Einstein rings,’ which occur when a complete circular image of a background galaxy is formed around a foreground galaxy. Finding such lenses isn’t easy, and involves going through a catalog of more than two million galaxies by eye to identify possible candidates. Given the odd shapes that lensed galaxies can assume, filtering out a genuine lensing event from the observation of an unusually shaped galaxy takes time. But the European team now plans to train robot software on the lenses thus far found, hoping to identify still more.

Image: An Einstein ring can be seen in this image from the COSMOS project. An Einstein ring is a complete circle image of a background galaxy, which is formed when the background galaxy, a massive, foreground galaxy, and the Hubble Space Telescope are all aligned perfectly. Credit: NASA, ESA, C. Faure (Zentrum für Astronomie, University of Heidelberg) and J.P. Kneib (Laboratoire d’Astrophysique de Marseille).

The universe, it would seem, continually tries to tell us about itself through lensing inherent in the effect of mass upon spacetime. We’re a long way from such an infrastructure, but imagine the consequences for astronomy of having a wide variety of observational tools moving on spacecraft within the Sun’s gravity focus to examine targets ranging from the earliest galaxies to nearby stars and their planets. Such an outcome would depend, of course, upon our mastering propulsion technologies that can reach such distances within a single human lifetime. But it’s clear that the benefits of going interstellar won’t be found just around other stars. There’s plenty of work to do within 1000 AU of home.

Comments on this entry are closed.

The plenty of work to do within 1,000 AU of home could be accomplished by nuclear electric propulsion schemes. I could see first generation fusion rockets getting us there in a timely manner. Other possibilities obviously include nuclear fission reactor powered electron or ion rockets. The required Isp would be greatly reduced compared to the maximum levels of perhaps as high as 3,000,000 for very effiecient fusion rockets to our stellar niegboors with 1/3 C terminal velocity.

By the way, I like photos of Einstein rings. I like to call them Mother Nature’s eyes.

I suppose lensing will also work in the other direction: put a radio source in the 550 AU focus and create a 100 million times amplified transmitter.

Or communicating with an Alpha Centauri probe?

Hans, communications come immediately to mind, a topic Dr. Maccone has explored as well. I must say that the METI notion never entered my thoughts on this, but of course you’re right — the focus would make such possible.

In Stephen Baxter’s novel Manifold:Space the gravitational focus was used to host a galactic transportation system.

Hubble Telescope 2.4m (diameter) in the 550 AU focus – its 100 million times amplifier – will be have power like telescope 24000m (diameter).

I should read more fiction

I suppose lensing will also work on gravitational waves and gravitational field of stars (black holes). Gravitational
pull most close stars, like Alpha Centauris A and B (total mass = 2 mass Sun) will be 822.56 times more strong than gravitational pull of Sun in 550 AU focus!

Alpha Centauri distance = 49681440000000km (4,3 light years).
550 AU = 82500000000km (0,000872 light years or 7,639 light hours).
Its 100 million times amplifier in the 550 AU focus.

Analysis of the radio tracking data from the Pioneer 10 and 11 spacecraft at distances between 20–70 AU from the Sun has consistently indicated the presence of a small but anomalous Doppler frequency drift. The drift can be interpreted as due to a constant acceleration of (8.74 ± 1.33) × 10−10 m/s2 directed towards the Sun. Although it is suspected that there is a systematic origin to the effect, none has been found.
The nature of this anomaly could be the gravitational pull of stars, when the Pioneer 10 and 11 spacecraft is going more close to the border Suns focus in 550 AU.

You might be right – I do wonder about what the effect of focussed gravity waves would be. I did see a paper or two a few years ago that was wondering what happens at the Sun’s neutrino focus – out near Uranus’ orbital radius. Could be some interesting effects if Franklin Felber’s relativistic repulsive gravity affects neutrinos as he predicts. Neutrinos have such huge gamma factors their repulsion effect should be huge, relative to their tiny rest masses (

One thing that occurred to me is that a gamma ray burst might be amplified many times due to a chance accultation by an intermeadiate star, blackhole, or other body with disasterous results. Supposedly, as I am sure some of the readership has heard, Eta Carena could go supernova at any time. If models of gamma ray burst that suggest that such bursts originate from supernova are correct, then if Eta Carena was occulted by another stellar body, Earth could be in for a real bad time, and likewise, any ETI civilization in the way. Even unmagnified by such an occultation, such a burst could potentially fry the Earth side of the planet even at 7,500 lightyears if only in terms of the damage or destruction of the atmospheric chemisty that maintains the habitability of our ecosystem. Luckily, the probability of Eta Carena projecting a gamma ray burst beam at Earth is small when it goes supernova and much less is the probability that it will be occulted by a gravitational lensing body at the suitable distance such that if the beam was pointed at us, it would be significantly magnified. Still, just one more potential danger we need to be aware of as we consider all of the stars that will produce gamma ray burst, assumming this model is correct, in the future.

Such occultation should not only be able to produce magnfied gravitational wave flux densities at Earth as suggested by AlfaCentavra, but should also be able to produce magnified nuetrino burst should a cosmic phenomenon producing such be appropriately occulted by a stellar body or other object. Luckily, neutrinos interact only very weakly with baryonic matter. Still, this is a neat prospect if only a remote chance occurance for observational purposes.

Something I’ve at times dreamed about – having not just a telescope but a colony, or several of them, at that distance from the sun. Colonies that could build not just the small telescopes that a probe might carry but telescopes with lenses in the hundreds of meter range. Something that would really let us peer within other solar systems.

For a single probe I wonder how many near by stars would it be able to focus on. And how much detail would be possible.

As for actually getting there. Maybe in 20 or 30 years time we will be willing to invest the resources into it. In getting there we will need to do better than the voyager spacecraft. At the 3.5AU per year that voyager I is traveling it would take it 157 years to get 550AU out. DS1 I seem to recall reading somewhere would have a final velocity of 4.5k/s if it burned all its fuel. It would take that probe 581 years (without any gravitational assists). If you increased the fuel to 100 times what the DS1 had and used the same gravitational assists that voyager 1 used you might get a final velocity of 20AU per year. It would still be over a 30 year journey.

However if we were willing to invest in the research and the development propulsion methods giving 100,000 isp might be possible relatively soon (compared to 157 years anyway). I’m just not sure we humans are willing to make the commitment.

Can’t help but wonder just what sort of detail a telescope at that location would see if focused on the centauri system. If we could detect saturn like planets at 5000LY then imagine what sort of detail we might see at 4.37LY.

The view of Alpha Centauri from a FOCAL-style mission would presumably be spectacular. Eventually, to exploit a gravitational lens like this we’d need a fleet of telescopes moving about beyond 550 AU to target various stars, positioning themselves with the needed precision to get the lensing effect. But I would imagine a sufficiently advanced culture would put an operation like that into effect, learning a great deal about any target system before thinking about sending a probe in that direction. Surely this is in the toolbox of any Type II civilizations. As Dr. Maccone puts it, the ultimate lens is available for the taking.

Maybe Alpha Centauri is not the most logical target.

rough calculation:
1 AU = 8 light minutes, so 550 AU = 4400 light minutes = +- 0.2 ly

If we have the technology to go with sufficiently large telescopes to 550 AU, we probably also can go with a small probe to Alpha Centauri.

But it’s nice to hava a goal in between.

I don’t know, Hans. The gap beween 550 AU and 270000 AU (to Centauri A and B) is a mighty big one!

paul,yes sir! i thought exactly the same thing! but hans,keep thinking that is ALWAYS a good idea! thank you paul,thank you hans, your friend george

Oops, forgot the hours. 24 times off.

It was 73 light hours, not days, so 0.008 ly
Yes, that’s only a tiny fraction of 4.3 ly (0.2 %)

It would be a very inflexible observatory. How does one train such a lens on more than one object? At a radius of 550 AU you’d have to drive the sail many AUs to look at a second object even just a few degrees away from the first.

I have to disagree, Kevin. A single observatory would still be able to view carefully chosen targets as it refined the technology for this kind of work. But think long-term: A culture with deep space propulsion capabilities can aim for a network of such observatories circling its star and using this amazing lens for observations that would be unavailable from any other source. I’m going to assume that a sufficiently advanced technology will almost certainly take this step, as it’s a logical way to push the envelope on astronomy to the max.

To take an example, if we want to scan, say, Alpha Centauri A and B, which have an average separation of 17.59 arcseconds, it involves moving the scope about 7 million km to aim at the other star in the system.

The habitable zones of both stars are approximately 1 AU in radius, which at a distance of 4.36 light years means the angular diameter would be 1.5 arcseconds across, which for a scope at 550 AU would mean moving 600,000 km to traverse the HZ.

Assuming we don’t constrain the orbits of habitable planets, we need to work out the area through which our telescope should sweep to ensure it can observe planets at any orientation within the habitable zone. If my calculations are correct, this turns out to be about 300 billion square kilometres, or about 550 times the total surface area (including oceans) of the Earth, per star.

So to conduct a good survey of the habitable zones of both of the Alpha Centauri stars, it would seem you need to have scopes over an area about 1100 times the surface area of the Earth…

All this assumes the scope is at 550 AU. Obviously as you go outwards you have to move further to re-aim the scope.

Of course, if you know where the planets are to begin with (and for that you’d need some pretty good astrometric measurements), things become rather less daunting, but on the face of it, I’d say Kevin’s got a good point here.

I’ve sent both comments (Kevin’s and andy’s) on to Dr. Maccone in hopes of a comment for further clarification. He may be traveling at the moment, but if not, I’ll hope to have more on this shortly.

GRAVITAS: Portraits of a Universe in Motion

Authors: John Dubinski, John Kameel Farah

Abstract: GRAVITAS is a self-published DVD that presents a visual and musical celebration of the beauty in a dynamic universe driven by gravity. Animations from supercomputer simulations of forming galaxies, star clusters, galaxy clusters, and galaxy interactions are presented as moving portraits of cosmic evolution. Billions of years of complex gravitational choreography are presented in 9 animations – each one interpreted with an original musical composition inspired by the exquisite movements of gravity.

The result is an emotive and spiritually uplifting synthesis of science and art. The GRAVITAS DVD has been out for two years now but I am now making the DVD disk image freely available for personal and educational use through a bittorrent download. Download and burn at your leisure. The animations are also downloadable in various video formats.

Comments: Link to animations and burnable DVD image at this http URL

Subjects: Astrophysics (astro-ph) Popular Physics (physics.pop-ph)

Cite as: arXiv:0802.3664v1 [astro-ph]

From: John Dubinski [view email]

[v1] Mon, 25 Feb 2008 17:44:52 GMT (10kb)

Dr. Maccone does seem to be traveling, as I feared, so I won’t try to make his arguments for the FOCAL mission as much as reiterate my own thought that exploiting the 550 AU gravitational focus will demand propulsion breakthroughs that would allow these observatories to move as necessary to do their work. A non-trivial task, to be sure, with today’s technology, but we’re always working toward better solutions, and once we do have the capability of moving such spacecraft effectively (and over the necessary distances), a series of observing stations could do astronomy at unprecedented levels. No one is arguing this is near-term, but an initial FOCAL mission as proof of concept is a natural follow-on to projects like Innovative Interstellar Explorer. Long-term, there is much to do at 550 AU and beyond.

Galactic globular clusters contribution to microlensing events?

Authors: Fabiana De Luca, Philippe Jetzer

Abstract: In this note we perform an analysis of the large set of microlensing events detected so far toward the Galactic center with the purpose of investigating whether some of the dark lenses are located in Galactic globular clusters. We find that in four cases some events might indeed be due to lenses located in the globular clusters themselves. We also give a rough estimate for the average lens mass of the subset of events being highly aligned with Galactic globular cluster centers and find that, under reasonable assumptions, the deflectors could most probably be either brown dwarfs, M-stars or stellar remnants.

Comments: 11 pages, 3 figures, accepted for publication in International Journal of Modern Physics D

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0802.3827v1 [astro-ph]

From: Philippe Jetzer [view email]

[v1] Tue, 26 Feb 2008 15:12:48 GMT (346kb)

Gravitational Lens Systems to probe Extragalactic Magnetic Fields

Authors: D. Narasimha, S. M. Chitre

Abstract: The Faraday rotation measurements of multiply-imaged gravitational lens systems can be effectively used to probe the existence of large-scale ordered magnetic fields in lensing galaxies and galaxy clusters.

The available sample of lens systems appears to suggest the presence of a coherent large-scale magnetic field in giant elliptical galaxies somewhat similar to the spiral galaxies.

Comments: 11 pages, 1 figure

Subjects: Astrophysics (astro-ph)

Journal reference: Current Science, Vol 93, 10th December 2007, 1506-1513

Cite as: arXiv:0802.4044v1 [astro-ph]

From: Delampady Narasimha [view email]

[v1] Wed, 27 Feb 2008 16:55:06 GMT (21kb)

Reading through the posts, a thought comes to mind – if we could drop a telescope far away enough from the sun to take advantage of gravitational lensing, don’t we already have that opportunity with nearby stars, i.e. use Alpha Centauri as a lens for something in it’s distant path? Apologies if this seems a bit of a simple approach to this question.

This is a good point, Daniel, and you’re right — we live in a universe that is filled with potential lensing effects due to the effect of mass on spacetime. The fact that the gravity-focused radiation from the Sun remains along the focal axis (i.e., the focal line extends to infinity for separations greater than 550 AU) is interesting, in that it implies other stars can also be used in a similar way, since we would be able to take advantage of that focused radiation. Here’s the problem (and now I’m quoting Greg Matloff): “…the off-axis gain decreases with the inverse square root of the off-axis distance.” In other words, there is a spot-radius, or distance from the center line of the image, where the image intensity gain falls by a factor of 4, and it’s quite narrow. Matloff points out that a telescope at 2200 AU from the Sun would work with a spot radius of 11 kilometers. I take this to mean that the farther we are away from the gravitational focus of another star, the narrower that spot radius is going to be (I think I’ve got that right, but I’m just an amateur at this kind of optics, and hope anyone will jump in if corrections are needed).

For now, it appears that the best use of other stars is in ‘micro-lensing,’ where a star moving in front of another, more distant one creates lensing effects (changes in light intensity in particular) that can demonstrate the presence of planets around the nearer star. That’s just occurred with the discovery of a Jupiter and Saturn-class planet orbiting a star about 5000 light years from Earth:

So we have much to learn about how we can use distant objects for lensing, but it’s clear that this science is developing rapidly. I hope we see a FOCAL-style mission set out one day to explore its possibilities.

LensPerfect: Gravitational Lens Massmap Reconstructions Yielding Exact Reproduction of All Multiple Images

Authors: D. Coe, E. Fuselier, N. Benitez, T. Broadhurst, B. Frye, H. Ford

Abstract: We present a new approach to gravitational lens massmap reconstruction. Our massmap solutions perfectly reproduce the positions, fluxes, and shears of all multiple images. And each massmap accurately recovers the underlying mass distribution to a resolution limited by the number of multiple images detected.

We demonstrate our technique given a mock galaxy cluster similar to Abell 1689 which gravitationally lenses 19 mock background galaxies to produce 93 multiple images. We also explore cases in which far fewer multiple images are observed, such as four multiple images of a single galaxy. Massmap solutions are never unique, and our method makes it possible to explore an extremely flexible range of physical (and unphysical) solutions, all of which perfectly reproduce the data given. Each reconfiguration of the source galaxies produces a new massmap solution. An optimization routine is provided to find those source positions (and redshifts, within uncertainties) which produce the “most physical” massmap solution, according to a new figure of merit developed here.

Our method imposes no assumptions about the slope of the radial profile nor mass following light. But unlike “non-parametric” grid-based methods, the number of free parameters we solve for is only as many as the number of observable constraints (or slightly greater if fluxes are constrained). For each set of source positions and redshifts, massmap solutions are obtained “instantly” via direct matrix inversion by smoothly interpolating the deflection field using a recently developed mathematical technique. Our LensPerfect software is straightforward and easy to use and is made publicly available via our website.

Comments: 17 pages, 18 figures, accepted by ApJ. Software and full-color version of paper available at this http URL

## Distant supernova split four ways by gravitational lens

The many red galaxies in this Hubble Space Telescope image are members of the massive MACS J1149.6+2223 cluster, which strongly bends and magnifies the light of galaxies behind it. A large cluster galaxy (center of the box) has split the magnified light from an exploding background supernova into four yellow images (arrows), which form an Einstein Cross. Credit: Image courtesy of Z. Levay at NASA's Space Telescope Science Institute and ESA. Patrick Kelly and Alex Filippenko at UC Berkeley contributed to the discovery and analysis.

Over the past several decades, astronomers have come to realize that the sky is filled with magnifying glasses that allow the study of very distant and faint objects barely visible with even the largest telescopes.

A University of California, Berkeley, astronomer has now found that one of these lenses - a massive galaxy within a cluster of galaxies, both of which are gravitationally bending and magnifying light - has created four separate images of a distant supernova.

The so-called "Einstein cross" will allow a unique study of a distant supernova and the distribution of dark matter in the lensing galaxy and cluster.

"Basically, we get to see the supernova four times and measure the time delays between its arrival in the different images, hopefully learning something about the supernova and the kind of star it exploded from, as well as about the gravitational lenses," said UC Berkeley postdoctoral scholar Patrick Kelly, who discovered the supernova while looking through infrared images taken Nov. 10, 2014, by the Hubble Space Telescope (HST). "That will be neat."

Kelly is a member of the Grism Lens-Amplified Survey from Space (GLASS) team led by Tommaso Treu at UCLA, which has worked in collaboration with the FrontierSN team organized by Steve Rodney at Johns Hopkins University to search for distant supernovae.

"It's a wonderful discovery," said Alex Filippenko, UC Berkeley professor of astronomy and a member of Kelly's team. "We've been searching for a strongly lensed supernova for 50 years, and now we've found one. Besides being really cool, it should provide a lot of astrophysically important information."

One bonus is that, given the peculiar nature of gravitational lensing, astronomers can tune in for a supernova replay in the next 10 years. This is because light can take various paths around and through a gravitational lens, arriving at Earth at different times. Computer modeling of this lensing cluster shows that the researchers missed opportunities to see the exploding star 50 and 10 years ago, but images of the explosion will likely repeat again within the next 10 years.

"The longer the path length, or the stronger the gravitational field through which the light moves, the greater the time delay," noted Filippenko.

The light from the underlying supernova is deflected by the gravity of a large collection of galaxies and an elliptical galaxy, which thus acts like a magnifying glass and amplifies the light from the distant supernova. This special phenomenon, called gravitational lensing effect, works like nature's own giant telescope and the supernova appears 20 times brighter than its normal brightness. Credit: NASA/ESA/GLASS/FrontierSN team

Kelly is first author of a paper reporting the supernova appearing in a special March 6 issue of Science magazine marking the centenary of Albert Einstein's General Theory of Relativity.

Kelly, Filippenko and their collaborators have dubbed the distant supernova SN Refsdal in honor of Sjur Refsdal, the late Norwegian astrophysicist and pioneer of gravitational lensing studies. It is located about 9.3 billion light years away (redshift = 1.5), near the edge of the observable universe, while the lensing galaxy is about 5 billion light years (redshift = 0.5) from Earth.

Albert Einstein's General Theory of Relativity predicts that dense concentrations of mass in the universe will bend light like a lens, magnifying objects behind the mass when seen from Earth. The first gravitational lens was discovered in 1979. Today, lensing provides a new window into the extremely faint universe shortly after its birth 13.8 billion years ago.

"These gravitational lenses are like a natural magnifying glass. It's like having a much bigger telescope," Kelly said. "We can get magnifications of up to 100 times by looking through these galaxy clusters."

When light from a background object passes by a mass, such as an individual galaxy or a cluster of galaxies, the light is bent. When the path of the light is far from the mass, or if the mass is not especially large, "weak lensing" will occur, barely distorting the background object. When the background object is almost exactly behind the mass, however, "strong lensing" can smear extended objects (like galaxies) into an "Einstein ring" surrounding the lensing galaxy or cluster of galaxies. Strong lensing of small, point-like objects, on the other hand, often produces multiple images - an Einstein cross - arrayed around the lens.

"We have seen many distant quasars appear as Einstein crosses, but this is the first time a supernova has been observed in this way," Filippenko said. "This short-lived object was discovered only because Pat Kelly very carefully examined the HST data and noticed a peculiar pattern. Luck comes to those who are prepared to receive it."

In the large square to the right in the image you see the four light representations of the supernova that was spotted on Nov. 11, 2014. The blue circle shows another location in the galaxy cluster where you probably would have been able to see a single image of the supernova 20 years ago and the red circle shows where the supernova will appear again in a few years, according to calculations. This will give the astronomers a rare opportunity to get backward glance at the supernova and will also enable the researchers to improve their calculations of the amount and distribution of dark matter -- both in the galaxy cluster and in the one elliptical galaxy. Credit: NASA/ESA/GLASS/ FrontierSN team

The galaxy that is splitting the light from the supernova into an Einstein cross is part of a large cluster, called MACS J1149.6+2223, that has been known for more than 10 years.

In 2009, astronomers reported that the cluster created the largest known image of a spiral galaxy ever seen through a gravitational lens. The new supernova is located in one of that galaxy's spiral arms, which also appears in multiple images around the foreground lensing cluster. The supernova, however, is split into four images by a red elliptical galaxy within the cluster.

"We get strong lensing by a red galaxy, but that galaxy is part of a cluster of galaxies, which is magnifying it more. So we have a double lensing system," Kelly said.

Looking for transients

After Kelly discovered the lensed supernova Nov. 10 while looking for interesting and very distant supernova explosions, he and the team examined earlier HST images and saw it as early as Nov. 3, though it was very faint. So far, the HST has taken several dozen images of it using the Wide Field Camera 3 Infrared camera as part of the Grism survey. Astronomers using the HST plan to get even more images and spectra as the HST focuses for the next 6 months on that area of sky.

"By luck, we have been able to follow it very closely in all four images, getting data every two to three days," he said.

Kelly hopes that measuring the time delays between the phases of the supernova in the four images will enable constraints on the foreground mass distribution and on the expansion and geometry of the universe. If the spectrum identifies it as a Type Ia supernova, which is known to have a relatively standard brightness, it may be possible to put even stronger limits on both the matter distribution and cosmological parameters.

## Supermassive Black Hole Dissected With Natural Magnifying Glasses: 1,000 Times Clearer Than Best Telescopes Can Do

Combining a double natural "magnifying glass" with the power of ESO's Very Large Telescope, astronomers have scrutinised the inner parts of the disc around a supermassive black hole 10 billion light-years away. They were able to study the disc with a level of detail a thousand times better than that of the best telescopes in the world, providing the first observational confirmation of the prevalent theoretical models of such discs.

The team of astronomers from Europe and the US studied the "Einstein Cross", a famous cosmic mirage. This cross-shaped configuration consists of four images of a single very distant source. The multiple images are a result of gravitational lensing by a foreground galaxy, an effect that was predicted by Albert Einstein as a consequence of his theory of general relativity. The light source in the Einstein Cross is a quasar approximately ten billion light-years away, whereas the foreground lensing galaxy is ten times closer. The light from the quasar is bent in its path and magnified by the gravitational field of the lensing galaxy.

This magnification effect, known as "macrolensing", in which a galaxy plays the role of a cosmic magnifying glass or a natural telescope, proves very useful in astronomy as it allows us to observe distant objects that would otherwise be too faint to explore using currently available telescopes. "The combination of this natural magnification with the use of a big telescope provides us with the sharpest details ever obtained," explains Frédéric Courbin, leader of the programme studying the Einstein Cross with ESO's Very Large Telescope.

In addition to macrolensing by the galaxy, stars in the lensing galaxy act as secondary lenses to produce an additional magnification. This secondary magnification is based on the same principle as macrolensing, but on a smaller scale, and since stars are much smaller than galaxies, is known as "microlensing". As the stars are moving in the lensing galaxy, the microlensing magnification also changes with time. From Earth, the brightness of the quasar images (four in the case of the Einstein Cross) flickers around a mean value, due to microlensing. The size of the area magnified by the moving stars is a few light-days, i.e., comparable in size to the quasar accretion disc.

The microlensing affects various emission regions of the disc in different ways, with smaller regions being more magnified. Because differently sized regions have different colours (or temperatures), the net effect of the microlensing is to produce colour variations in the quasar images, in addition to the brightness variations. By observing these variations in detail for several years, astronomers can measure how matter and energy are distributed about the supermassive black hole that lurks inside the quasar. Astronomers observed the Einstein Cross three times a month over a period of three years using ESO's Very Large Telescope (VLT), monitoring all the brightness and colour changes of the four images.

"Thanks to this unique dataset, we could show that the most energetic radiation is emitted in the central light-day away from the supermassive black hole and, more importantly, that the energy decreases with distance to the black hole almost exactly in the way predicted by theory," says Alexander Eigenbrod, who completed the analysis of the data.

The use of the macro- and microlensing, coupled with the giant eye of the VLT, enabled astronomers to probe regions on scales as small as a millionth of an arcsecond. This corresponds to the size of a one euro coin seen at a distance of five million kilometres, i.e., about 13 times the distance to the Moon! "This is 1000 times better than can be achieved using normal techniques with any existing telescope," adds Courbin.

Measuring the way the temperature is distributed around the central black hole is a unique achievement. Various theories exist for the formation and fuelling of quasars, each of which predicts a different profile. So far, no direct and model-independent observation has allowed scientists to validate or invalidate any of these existing theories, particularly for the central regions of the quasar. "

This is the first accurate and direct measurement of the size of a quasar accretion disc with wavelength (colour), independent of any model," concludes team member Georges Meylan.

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