# First multiply-imaged Type Ia supernova discovered

Gravitational lensing, the bending of light which travels near massive objects, can result in distortion, magnification, and even multiple images of the original light source. The closer the light travels to the massive object the stronger the lensing effect. The magnification that results from this process allows scientists to observe objects which would normally not be visible due to their distance and brightness.

Researchers at the Oskar Klein Centre, and their international collaborators, have observed for the first time a type Ia supernova that is strongly gravitationally lensed producing multiple images. The light from the original supernova explosion (at a redshift z=0.4) is bent as it travels through a galaxy (at a redshift z=0.2) which is almost perfectly aligned between the supernova and the Earth. This alignment produced a magnification of the light from the supernova as well as four images of the source.

Type Ia supernovae are thought to always have the same brightness when they explode and thus have been used to measure distances in the Universe. Distance measurements to a large number of Type Ia supernovae led to the discovery that the expansion of the Universe is accelerating. The team will now use the difference in the time it takes the light from each of the four images of the single supernova to measure the current expansion rate of the Universe, the Hubble Constant.

These results are published in the journal Science in an article titled, iPTF16geu: A multiply-imaged gravitationally lensed Type Ia supernova.

Ariel Goobar ( ariel@fysik.su.se)
Rahman Amanullah (rahman@fysik.su.se)

# Star ripped apart by supermassive black hole

It’s a star!
It’s a supernova!
No, it’s … a Spinning Black Hole Swallowing a Star!

In the very first issue of the new journal Nature Astronomy, former Oskar Klein Centre (OKC) post-doc Giorgos Leloudas (now at the Weizmann Institute of Science and Dark Cosmology Centre) suggests a new interpretation for the event ASASSN-15lh, formerly known as the most luminous supernova ever.

Leloudas and colleagues performed follow-up observations after the luminous event and found that with ten months of additional data the event no longer resembled that of a supernova. Co-investigator, Christoffer Fremling from the OKC, did all of the image subtraction to extract the light curve for this event.  Instead, they suggest, ASSASN-15lh was a star ripped apart by a supermassive black hole – a tidal disruption event.

One interesting aspect is the suggestion that this supermassive black hole might be rotating rapidly, says co-investigator Jesper Sollerman from OKC. Maybe these kind of tidal disruption events will become a way to explore the rotation of supermassive black holes billons of light years away.

The research paper discussing these results is available here in Nature Astronomy (arXiv version).  The video below shows a spinning supermassive black hole as it rips a star apart (artist’s impression).

[Top image: This artist’s impression depicts a sun-like star close to a rapidly spinning supermassive black hole, with a mass of about 100 million times the mass of the sun, in the centre of a distant galaxy. Photograph: ESO, ESA/Hubble, M. Kornmesser]

# Measuring the supernova rate in the early Universe by using galaxy clusters as gravitational telescopes

Supernovae are very rare phenomena in the Universe and their transient nature made them difficult to find for a long time. So, it is not surprising that the discovery rate was around two supernovae per month 30 years ago. Today, we are able to find supernovae daily. For example, the Intermediate Palomar Transient Factory, in which our group at the Oskar Klein Centre is involved, has discovered almost 3000 supernovae in the last few years. However, these supernovae are all relatively nearby, since the survey is not sensitive to the very distant ones.

Supernova rates, particularly at high distances, are important for several reasons. For example, core-collapse supernovae originate from the deaths of massive stars, and their rate can be used to trace the history of star formation in galaxies. Also, supernovae are the one of the major producers of metals in the Universe, so measuring supernova rates informs us about the chemical enrichment of galaxies over time.

However, measuring the supernova rate in the distant Universe is difficult. Even though supernovae are one of the brightest explosions that exist, at distances bigger than four billion light years they are hard to find simply because they become too faint. This has been especially problematic for the study of the rate of core-collapse explosions since they are on average the faintest objects in the supernova family and often embedded in dusty environments. Furthermore, due to the expansion of the Universe, the visual light from all distant objects is shifted to longer wavelengths. From the ground, near-infrared observations are particularly challenging due to the brightness of and variability of the atmosphere at these wavelengths.

Instead of waiting for more powerful telescopes to come online, we used the existing facilities and the magnification power of galaxy clusters as gravitational telescopes to take a peek at the supernovae in the early Universe. Galaxy clusters are the most massive gravitationally bound objects in the Universe, distorting and magnifying objects behind them. As predicted by Einstein’s general relativity, gravitational lensing magnifies both the area and the flux of background objects, thereby increasing the depth of the survey. In this way, the ability to find very distant supernovae is enhanced. It was Fritz Zwicky who suggested the use of gravitational telescopes nearly 80 years ago, but it is only recently that systematic supernova searches have been performed in background galaxies behind clusters. The supernova group here in Stockholm was the first one to explore this possibility using ground-based facilities in 2003.

As a continuation of this effort, during 2008-2014, we surveyed the galaxy cluster Abell 1689, which is one of the most powerful gravitational telescopes that nature provides. The results are presented in a recent publication  in the journal Astronomy & Astrophysics. We used a near-infrared instrument on the Very Large Telescope in Chile, with obtaining supporting optical data from the Nordic Optical Telescope at La Palma. Our search resulted in the discovery of five very distant and magnified supernovae. Notably, we discovered a supernova located nearly 10 billion lightyears away that was magnified four times by the galaxy cluster, which makes it among the most distant supernovae yet observed. Using these discoveries, we measured the supernova rates up to the time when the Universe was only two billion years old, without requiring any expensive space-based follow-up facilities.

Galaxy cluster Abell 1689 observed through the years 2008-2014 where the position of the discovered supernovae are shown. The red contours are the magnifications of the background sources from the cluster. Prepared by Rahman Amanullah.

Monitoring the foreground galaxy cluster also offers the opportunity to detect supernovae that originate from galaxies which are cluster members. Since clusters are dominated by galaxies where star formation has ceased, these are not core-collapse supernovae, but so-called thermonuclear or supernova Type Ia. We discovered two of this kind in Abell 1689 which allowed us to estimate the rate of supernova explosions in the cluster. Cluster rates are important since they have can be used to study the origin of the mysterious progenitors of supernovae Ia (read also A shocked neighbour) and are essential in understanding the iron abundance in medium between the cluster galaxies.

Another prediction of Einstein’s relativity is that strong gravitational lensing like the one from galaxy clusters, can give multiple images of the same background galaxy. If a supernova explodes in one of these multiply-imaged galaxies, its images will appear with certain time delays relative to each other due to the fact that the light from each image has to travel a different path. Given this event is very very rare and that our observing program was modest, we did not discover such an event. However, in the paper, we estimated the number that can be expected for upcoming transient surveys. We found that LSST , and in particular WFIRST , can be expected to find tens of strongly lensed supernovae that would allow the time delays between the multiple images to be measured, which can be measure the Hubble constant but also other cosmological parameters.

– Tanja Petrushevska (tpetr@fysik.su.se)  and Rahman Amanullah (rahman@fysik.su.se)

Article:  http://dx.doi.org/10.1051/0004-6361/201628925
Preprint of the manuscript: http://arxiv.org/pdf/1607.01617.pdf
Poster: https://www.dropbox.com/s/xovh10qx0wcklm3/EWASS_poster.pdf?dl=0

# A shocked neighbor?!

Discovering exploding stars, supernovae, within hours from explosion opens new windows to study their nature. Last year, our group at the intermediate Palomar Transient Factory (iPTF) was involved in the study of the closest SNIa explosion in several decades, SN2014J.

We have now a new exciting result – an early glimpse of ultraviolet light from a Type Ia supernova, iPTF14atg, reveals what appears to be a shocked neighboring star. The results published in the journal Nature uncover the nature of the kind of objects that are used as standard candles of cosmology.

Type Ia supernovae, one of the most dazzling phenomena in the Universe, are produced when white dwarfs, faint stars that have run out of fuel, explode with ferocious intensity. They were used as accurate distance estimators to measure the accelerated rate of expansion of the Universe, awarded the Nobel Prize in 2011.

Although thousands of supernovae of this kind were found in the last decades, the process by which a white dwarf turns explosive has been unclear. This lack of understanding has often been raised as a source of distrust in the use of Type Ia supernovae for high precision cosmology in the future.

However, there are two main hypothesized avenues for how explosions could arise from white dwarfs. A thermonuclear runaway explosion could ignite from either a merger of two white dwarfs or, by transfer of mass from a large neighboring star until the total accreted mass approaches the Chandrasekhar instability limit.

It has been theorized that the telltale signature of the presence of a large donor star would be an ultraviolet pulse preceding the main rise of the supernova light curve. This added flux is thought to be the result of collision of the supernova ejecta with its companion star. The energy released from this shock peaks at short wavelengths, X-rays and UV.

It appears that is what has been seen in iPTF14atg for the first time using the Swift satellite, triggered soon after the SN was discovered by iPTF. Although very exciting, it is not yet clear if this detection solves the mystery of the SN Ia progenitors, especially since iPTF14atg turned out to be less luminous at optical wavelengths than what expected for a “standard candle” supernova.

The work was led by Caltech PhD student Yi Cao, with contributions from OKC researchers Joel Johansson, Rahman Amanullah and Ariel Goobar (Fysikum) and Jesper Sollerman and Francesco Taddia (Astronomy). Of particular value were the early spectroscopic studies carried out at the Nordic Optical Telescope.

This kind of measurements should happen on regular basis once the Zwicky Transient Facility (ZTF) comes online in 2017. We look forward to that!

Read the article: A strong ultraviolet pulse from a newborn type Ia supernova or here: http://arxiv.org/abs/1505.05158

# The 2015 Breakthrough Prize for the accelerated universe

The discovery of the accelerated universe keeps receiving a well deserved attention. On November 9, the Breakthrough Prize Foundation announced the recipients of the 2015 Breakthrough Prize in Fundamental Physics, and all members of the Supernova Cosmology Project and the High-z Supernova Team were awarded the prize “for the most unexpected discovery that the expansion of the universe is accelerating, rather than slowing as had been long assumed.”
Nobel laureates Saul Perlmutter, Brian P. Schmidt, and Adam Riess received the prize in behalf of their collaborations, 3 million US dollars to be shared with 51 team members.

We gratulate Oskar Klein Centre member Ariel Goobar which is one of the recipient of the prize, and all other team members

Supernova Cosmology Project Team Breakthrough Prize winners: Greg Aldering, Brian J. Boyle, Patricia G. Castro, Warrick J. Couch, Susana Deustua, Richard S. Ellis, Sebastien Fabbro, Alexei V. Filippenko, Andrew S. Fruchter, Ariel Goobar, Donald E. Groom, Isobel M. Hook, Mike Irwin, Alex G. Kim, Matthew Y. Kim, Robert A. Knop, Julia C. Lee, Chris Lidman, Thomas Matheson, Richard G. McMahon, Richard Muller, Heidi J. M. Newberg, Peter Nugent, Nelson J. Nunes, Reynald Pain, Nino Panagia, Carl R. Pennypacker, Robert Quimby, Pilar Ruiz-Lapuente, Bradley E. Schaefer and Nicholas Walton.
High-Z Supernova Search Team Breakthrough Prize winners: Peter Challis, Alejandro Clocchiatti, Alan Diercks, Alexei V. Filippenko, Peter M. Garnavich, Ron L. Gilliland, Craig J. Hogan, Saurabh Jha, Robert P. Kirshner, Bruno Leibundgut, Mark M. Phillips, David Reiss, R. Chris Smith, Jason Spyromilio, Christopher Stubbs, Nicholas B. Suntzeff and John Tonry.

The annual Breakthrough Prizes in fundamental physics, life sciences and mathematics, are sponsored by Google co-founder Sergey Brin and his wife, Anne Wojcicki, a founder of the genetics company 23andMe; Alibaba Group founder Jack Ma and his wife, Cathy Zhang; Russian entrepreneur and venture capitalist Yuri Milner and his wife, Julia; and Facebook founder Mark Zuckerberg and his wife, Priscilla Chan. The goal is to celebrate scientists and generate excitement about the pursuit of science as a career. [1]

The discovery of the acceleration of the universe is an unprecedented breakthrough that marked the direction for all research in modern cosmology, and it was awarded the Nobel Prize in Physics in 2011.

If you want to know more about the work done by the two teams check the -behind the scenes video. Unfortunately the quality is not the best, but it is still interesting to hear all the stories told by both team members while in Stockholm for receiving the Nobel Prize back in 2011.

# A close look a the nearest standard candle supernova in several decades

Supernova 2014J in the nearby galaxy M82 -less than 12 million light-years away- exploded on January 14, 2014 and was the closest ”standard candle” supernova since (at least) 42 years. An impressive coordinated observational effort orchestrated by the intermediate Palomar Transient Factory (iPTF) team and led by Ariel Goobar from the Oskar Klein Centre at Stockholm University (Goobar et al. 2014, The Astrophysical Journal Letters, 784, L12) provides important new clues into the nature of these explosions, as well as the environments where they take place. The proximity of SN2014J allowed the iPTF team to study this important class of stellar explosions, known as Type Ia supernovae, over a very wide wavelength range, starting just hours after the deduced explosion time.

Furthermore, Goobar and collaborators used pre-explosion images of the region of M82 where the supernova went off, both from the Hubble Space Telescope and from the Palomar Oschin Telescope, to search for a star in the location of the explosion, or possible earlier nova eruptions. The lack of pre-explosion detections suggests that the supernova may have originated in the merging of compact faint objects, e.g., two white dwarf stars, i.e., the kind of Earth size stars that our sun will evolve to once it runs out of nuclear fuel.
“Until very recently, the leading model for standard candle supernovae was thought to include a companion star from which material was stripped by the white dwarf until the accumulated mass could no longer be sustained by the outwards pressure, leading to a runaway thermonuclear explosion. The observations of SN2014J are challenging for this theoretical picture”, says Goobar.

Type Ia supernovae are among the best tools to measure cosmological distances. Thanks to their consistent peak brightness, these ”standard candles” are used to map the expansion history of the Universe. In 1998 distance measurements using supernovae lead to the a paradigm shift in cosmology and fundamental physics: the expansion of the Universe is speeding up, contrary to the expectations from the attractive nature of gravitational forces: a mysterious new cosmic component, ”dark energy”, has been invoked to explain this unexpected phenomenon. This discovery was awarded the 2011 Nobel Prize in physics.

“Since Type Ia supernovae are very rare, occurring only once every several hundred years in a galaxy like ours, there have been very few opportunities to study these explosions in great detail. SN2014J in the nearby galaxy M82 is a very welcome exception”, says Rahman Amanullah a researcher at OKC.

A better understanding of the physics behind Type Ia supernovae and the material surrounding the explosion and dimming some of the light is crucial to further refine the measurements of the expansion history of the Universe. Joel Johansson, a PhD student at OKC that played an essential role in the analysis fills in “many supernovae explode in clean environments, free of dust in the line of sight. This is not the case for SN2014J, which gives us a unique opportunity to study both the properties of the supernova explosion but also of the intervening dust”.

The lessons learned by the studies of SN2014J may be very useful for the analysis of the large Type Ia SN sample that scientists have collected over decades, especially the astrophysical corrections needed to make accurate distance estimates. Only then may we be able to tell what is causing the accelerated expansion of the cosmos.

The iPTF project 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.

The observations were carried out using multiple astronomical facilities. Besides the Palomar telescopes, data of SN2014J and M82 were obtained at the Nordic Optical Telescope, the Keck Telescope, the Faulkes Telescope North, the Mount Abu 1.2m Infrared telescope in India, the1.93m telescope of Haute-Provence Observatory, CNRS, France, the Spitzer Space Telescope and the Hubble Space Telescope.

Contact: Prof. Ariel Goobar (ariel@fysik.su.se)

CONTACTS:
Prof. Ariel Goobar, Dept of Physics, Stockholms universitet, tfn +46 8-55 37 86 59, e-mail ariel@fysik.su.se
Rahman Amanullah, researcher, Dept of Physics,, Stockholms universitet, tfn +46 8-55 37 88 48 e-mail: rahman@fysik.su.se
Joel Johansson, PhD student, Dept of Physics, Stockholms universitet, tfn +46 8-55 37 86 61, e-mail joel@fysik.su.se

Read also: Hubble Space telescope images of a supernova in nearby galaxy M82

# Hubble Space Telescope images of a supernova in nearby galaxy M82

A new bright supernova exploded in the nearby galaxy M82 on January 14 this year, at a distance of approximately 11.5 million light–years from Earth, that makes it to the nearest “normal” Type Ia supernova discovered in the past 42 years. Its small distance together with the fact that the first observations were carried out only a few hours after the explosion, makes it in itself a very important astronomical object, since it allows to study the details of many aspects of these kind of objects that are so important for cosmology.
Type Ia supernovae, used as distance indicators, lead to the the discovery of the accelerated expansion of the universe in 1998, an unexpected result awarded the Nobel Prize in physics in 2011.
The nature of the accelerated expansion is attributed to a repulsive force, called dark energy.

- You might want to see our video about Dark Energy Problem -

However, though they are readily used in cosmology, the explosion mechanism behind Type Ia supernovae is still unclear, mainly due to the difficulty of catching the explosion at early stages and the ability to study these explosions over a wide range of wavelengths.

Ariel Goobar and Rahman Amanullah from the Oskar Klein Centre realized the importance of this object and applied for the Hubble Space Telescope (HST) director’s discretionary time to observe the supernova in ultraviolet (UV) wavelengths, which are otherwise absorbed by the earth’s atmosphere and not observable from ground based telescopes. Thanks to these measurements one can study the immediate surroundings of the supernova, an important part of the puzzle in understanding the progenitor system. Furthermore, the UV observations
are critical to study what it is that absorbs some of the light in the line of sight in the interstellar medium of the host galaxy. This study will have implications for the precision that can be obtained on the measurements of
the properties of dark energy.

The Hubble Space Telescope news center published today the composite image of this supernova explosion, SN2014J, in the galaxy M82.

A detailed paper about SN2014J has been written by Ariel Goobar and collaborators and accepted for publication in the Astrophysical Journal Letters, and we will soon blog again about this exceptional supernova.

Contact: Ariel Goobar ariel.goobar@fysik.su.se
Hubble Heritage Realease

# Large grant for supernova research at OKC with the iPTF

It is the Knut och Alice Wallenbergs foundation that grants a 5-year long project for finding and studying supernovae. The OKC are already since the beginning of this year members of the intermediate Palomar
Transient Factory (iPTF) – a supernova search aimed at finding supernovae soon after explosion. This is a pathfinder for the next generation of this project – the Zwicky Transient Facility (ZTF). The 30 million grant from KAW will now enable OKC astronomers and physicists to play a leading role in that project.

Jesper Sollerman, from the department of astronomy, is leading the application:
– Previous supernova surveys have often discovered supernovae days or weeks after explosion. We want to find them on the very first night. In this way we hope to learn more about their progenitors, the stars that actually exploded.

The deaths of massive stars is the focus for the supernova astronomers at the department of astronomy, including both observers such as Sollerman and modelers as co-applicant Claes Fransson.

The supernova group at the department of physics are more interested in thermonuclear supernovae and their use for cosmology. Co-applicant Ariel Goobar:
Continue reading Large grant for supernova research at OKC with the iPTF

# The missing star or the Supernova 2011dh

A bright supernova appeared in the fairly “nearby” (24 million light years) Whirlpool galaxy (M51) in late August 2011. It was named Supernova 2011dh. Images taken using the Hubble Space Telescope of the galaxy before the explosion showed a star at the position of the supernova, but astronomers argued whether this was indeed the star that exploded. Now we know!

New images taken once the supernova has faded reveal that the star is gone. This animation is made of B, V and R band images taken before – and long after – the supernova explosion using the 2.6m Nordic Optical Telescope on La Palma. The zoomed in part is blinked to highlight the star that is now gone (click on the image to enlarge it).

It is pretty cool that we can spot a single star, among 100 billion stars in a remote galaxy, says Jesper Sollerman, that participated in the study. This is because supernovae come from the most massive and luminous stars we know of.

The paper, lead by M. Ergon, that describes the supernova in some detail is now submitted to Astronomy & Astrophysics, and it is also available from the ArXiv.

# The intermediate Palomar Transient Factory

In February this year the iPTF (intermediate Palomar Transient Factory) program was started.
This is a survey searching for optical transients using a robotic 1.2 meter telescope in California, and the Oskar Klein Centre is one of the participating institutes for the next 2 years. The aim is to discover transients – in particular supernovae – at an earlier stage than hitherto possible, hopefully within hours after the explosion. The concrete scientific question we want to address is the nature of the progenitor systems of supernovae, and this requires very early observations of these explosions, before the memory of the initial configuration gets lost.

In the summer, a new spectrograph at the nearby 1.5m telescope will also be able to automatically classify the new transients found by the search telescope. iPTF builds upon the previous Palomar Transient Factory that successfully discovered almost 2000 supernovae since 2009. The plan
is to move via the {\it intermediate} search to the full-scale Zwicky
Transient Facility in mid-2015, where a five times larger area of the sky
will be monitored.

Members of the OKC have submitted applications to join also that effort – fingers crossed.