# 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

# 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.

# Interview with Martin Sahlén

We continue our interview series of Oskar Klein Centre fellows. Today we meet Martin Sahlén, starting his third year around. Martin works in the CoPS, Cosmoparticle Physics Group.

When did you start working for the OKC, and how it is going so far?
I arrived at the Oskar Klein Centre in September 2009, and it has been both enjoyable and stimulating. Much of my time has been spent preparing a computer code to model the cosmological distribution of galaxy clusters in great detail, to be used for a number of projects. At present I am mainly working on utilizing the code in the different projects and preparing resulting articles, so although it’s been slow going periodically things are now coming together. Some good joint projects also appear to be coming together in the near future, which I think will be excellent.

Why did you choose the OKC for doing a postdoc?
I chose the OKC knowing the excellent facilities here, the quality/profile of the group and also the rather generous funding in the Oskar Klein Fellowship. The broad approach of the centre appealed to me and was a strong reason for applying.

What is your field of research? Can you describe the project/projects in which you are involved?

My general field is cosmology, both towards the observational and towards the theoretical. The projects I am involved in centre largely around multi-consistent analysis and testing of beyond-concordance cosmology.

One area is clusters of galaxies, where I work on cosmological constraints from the XMM Cluster Survey (XCS) and the XXL Survey. Both are X-ray surveys of galaxy clusters based on XMM-Newton images. Continue reading Interview with Martin Sahlén

# Workshop: clusters of galaxies as cosmic laboratories

Galaxy clusters occupy a central role in the ongoing efforts to understand some of the greatest questions in particle astrophysics and cosmology: the nature of Dark Matter and Dark Energy. Moreover, these huge mass concentrations serve as Nature’s own telescopes, capable of amplifying faint high-redshift sources that would otherwise be beyond our reach. At the same time, the use of galaxy clusters as probes for fundamental physics requires a good understanding of the state of the intra-cluster medium.

No wonder that the Oskar Klein Centre got involved in the organization of a workshop in Stockholm, 12-14 September. Ariel Goobar, professor at the Oskar Klein Centre, explains that there is a large effort by the community to collect multi-wavelength galaxy cluster data, which present a great opportunity to make significant progress over a broad range of topics: Continue reading Workshop: clusters of galaxies as cosmic laboratories

# Time for PhD thesis defenses at the Oskar Klein Center

This spring there have been several PhD theses defenses here at the Oskar Klein Center, and as much as we hate saying good bye to some of our best students, we are proud to have been part of their professional lives.
The first to go was Jakob Nordin who defended his thesis with the title “Spectral Properties of Type Ia Supernovae and Implications for Cosmology” the 27th May 2011. During his PhD under the guidance of Professor Ariel Goobar, Jakob collected spectroscopic data of Type Ia supernovae with the purpose to study if SNIa are indeed good “standard candles” over a wide redshift range, a necessary condition to use these explosions to study the properties of dark energy. He has also investigated the nature of one of the color-brightness relation, one of the largest astrophysical corrections in the use of SNIa to measure distances. Finally, he has written a Monte-Carlo simulation package to investigate how systematic uncertainties in the use of Type Ia SNe as distance indicators propagate into our cosmological fits of dark energy parameters. Jakob is now ready to move on with a postdoc at the Berkeley, and we really wish him good luck with his life in California!

Another of Ariel Goobar students, Teresa Riehm also student at the Astronomy department, defended her thesis “Investigating the Dark Universe through Gravitational Lensing”. Continue reading Time for PhD thesis defenses at the Oskar Klein Center

# The Return of de Sitter

During the last three weeks, we were visited by an impressive list of cosmologists trying to make sense of what is driving the present accelerated expansion of the Universe. The return of De Sitter, this is the name of the NORDITA workshop organized by OKC members Fawad Hassan and Ariel Goobar, together with Stefan Hofmann from LMU in Munich.
I asked Ariel Goobar, professor at OKC, and Stefan Sjörs, a PhD student in the Cosmology, Astroparticle Physics and String Theory group, to tell us about the conference.

Continue reading The Return of de Sitter

# Welcome to the Oskar Klein Centre blog

As a way to communicate more quickly and efficiently between ourselves in the Oskar Klein Centre (OKC) and with the outside world, we have with the advice of Serena Nobili, responsible for OKC information and outreach, started this blog. Here you will now and then get updates on what is happening in the Centre Continue reading Welcome to the Oskar Klein Centre blog