Posted on January 14, 2014 | by Serena | No Comments
The Fermi satellite was launched in 2008 and since then it has continuously monitored the sky at gamma-ray energies above 100 MeV. Most of the sources detected at these energies are blazars, Active Galactic Nuclei in which the accretion onto a supermassive black hole also leads to the launching of two opposite relativistic jets. If a jet is pointing close to our line of sight we will see intense high energy emission due to strong Doppler boosting.
Fermi has so far detected well over 1000 blazars. One of these is B0218+357, which is known from optical and radio observations to be gravitationally lensed by a foreground spiral galaxy. The lens forms two closely spaced images of the blazar. The Fermi Large Area Telescope (LAT) can not spatially separate the two images so it can only measure the sum of both. However,
with timing analysis it is still possible to separate the signal of the individual components. This is because the path length from the blazar to us is different for the two images so we measure all blazar variability twice, with some time separation. Read more
Posted on December 23, 2013 | by Lars | No Comments
Hello and Happy Holidays to all friends of the OKC
As the year 2013 is now nearing its end, it is time to recapitulate the main events of the year from the OKC perspective. If I temporarily put on my Nobel hat (being the scientific secretary of the Nobel Committee for Physics) the main event from the Stockholm horizon is without doubt the Nobel Prize to Francois Englert and Peter Higgs for their almost 50-year old prediction from the early 1960’s that was so spectacularly confirmed by the ATLAS and CMS experiments at CERN’s LHC accelerator last year. Hats off for Englert and Higgs, and also for the many clever and hard-working experimentalists, in particular the ATLAS people of OKC such as Jonas Strandberg, who has been directly involved in the discovery of the Higgs particle in ATLAS. Of course we now look forward to the energy upgrade of LHC, which will increase the chances substantially to find the much awaited effects beyond the Standard Model that, hopefully, could give an indication of what the dark matter may consist of. At the Oskar Klein Centre we also have been searching in gamma-ray, positron and neutrino signals, without positive results (yet), but producing some of the best limits.
The OKC has now been in existence for 5 years, and we will soon encounter the international mid-review panel of the Science Council (VR) of Sweden. By September 1st we had to submit our self-assessment report, containing a detailed description This meant a lot of work for me and the OKC Steering Group: Christophe Clement, Jan Conrad, Claes Fransson, Ariel Goobar, Klas Hultquist, Garrelt Mellema, Mark Pearce, Sara Strandberg and Göran Östlin (and of course our great communications manager, Serena Nobili). We hope that we managed to convey our great enthusiasm for the scientific outcome of the OKC during its first 5 years, and that the evaluation committee will agree that it has been a great success. In fact, when we meet them January 30th, we will have quite a number of recent sucesses to report:
- A generous grant of SEK 32 million from the K&A Wallenberg Foundation was given to groups in OKC (with J. Sollerman as PI) for contributing to the Zwicky Transient Facility (with S. Kulkarni of Caltech leading the team).
- OKC Steering Group member Sara Strandberg has obtained both a young researcher’s grant from the VR and a Wallenberg Academy Fellow (WAF) grant – both in very strong competition.
- Jan Conrad (also OKC Steering Group member) has in addition to his previous WAF grant also been given one of the new excellent junior investigator grants from VR. Congratulations to Sara and Jan!
- The IceCube experiment has finally detected high-energy (PeV) cosmological neutrino events, with a surprising energy distribution. This was declared the discovery of the year of the Physics World magazine. Congratulations to the OKC IceCube group (Chad Finley, P.O. Hulth, Klas Hultquist & al.) and the Uppsala group (with present IceCube spokesperson Olga Botner and her colleagues)!
- The Fermi satellite project with large OKC contribution continues to make important discoveries. One concerns the discovery of a gamma-ray spectrum of two supernova remnants which clearly shows a hadronic origin (from decays of neutral pions), and thus is a proof that these sources accelerate protons, and are thus the long-sought-for sources of the Galactic cosmic rays. This was one of the runner-ups for the discovery of the year of Science magazine.
To conclude, with all the interesting science produced by OKC during its first half-life, one may only anticipate with great expectations what will come out of the second half!
Merry Holidays and a Happy New Year to all in the OKC research environment and all our followers!
Posted on December 2, 2013 | by Serena | No Comments
Where have you studied or did research before coming to the OKC?
I did my undergraduate studies at École Polytechnique near Paris. I then completed my Mas ter degree at the theoretical physics department of École Normale Supérieure in Lyon and did my Ph.D. with the IDAPP (International Doctorate on AstroParticle Physics) program both in Annecy and Turin under the supervision of Pierre Salati and Nicolao Fornengo. Autumn 2010, I moved to Madrid to do my first post-doc at the Instituto de Física Teórica (IFT) of the Universidad Autónoma where I stayed for two years. Last year I worked at the Laboratoire d’Annecy-le-Vieux de Physique Théorique (LAPTh) and the Institut d’Astrophysique de Paris (IAP).
What is your field of research?
I work in modelling the propagation of Galactic Cosmic Rays and Dark Matter indirect detection. Cosmic rays are high energy particle that are accelerated by exploding stars, by high magnetised stars called pulsars, and maybe by the annihilation or decay of Dark Matter particles. Even though cosmic rays have been discovered more than 100 years ago we still do not understand precisely where they come from nor how they propagate in the interstellar medium. In spite of being a rather young science, cosmic ray physics are is a wonderful way to look at things in the sky that do not emit light and cannot be probed by usual astronomy.
What I really enjoy in this field is that it is at the intersection of cosmology, astrophysics and particle physics and allows to interact with many people from very different fields.
Posted on November 22, 2013 | by Serena | 1 CommentToday’s issue of Science highlights a breakthrough in astroparticle physics many decades in the making. After tantalizing hints in the past year, the IceCube Collaboration now reports on a follow-up analysis, leading to a larger event sample and compelling evidence that the first high-energy neutrinos from outer space are starting to be seen.
The IceCube Neutrino Observatory is a strange kind of telescope, because it searches for neutrinos—rather than photons—from space. It’s situated at the South Pole, and it consists of thousands of detector modules buried up to 2.5 km deep in the glacial ice there. At that depth, the ice is very clear and very dark. What the modules see is the passage of subatomic particles zipping through the ice, emitting a well-known, faint blue light called Cherenkov light. When enough modules detect this light from a passing particle, the information is sent to the surface where a computer cluster starts crunching the `event’ to make a rough estimate of the direction and energy of the particle.
In the analysis reported today, 28 events are in the final sample, sifted out of over 10ˆ11 (one hundred thousand million) events in total, collected over two years. The overwhelming majority of these events are background particles from cosmic rays striking the atmosphere. Finding the handful of neutrinos that potentially originated in space is extraordinarily challenging. One of the innovations of the new analysis is to use the outermost detector modules as a `veto’, to tag any particle observed coming into the detector through the outer layer as a background event. All that remains are events which first appear, mysteriously, in the middle of the detector. This is the tell-tale signature of a neutrino event. In fact, famous for being “ghost” particles, almost all the neutrinos which pass through the detector never leave a trace at all. But fortunately a tiny fraction do happen to bump into an atom in the ice. When that happens, many other particles (like electrons, and their heavier cousins, muons) are created, and these particles create the Cherenkov light that IceCube detects.
It turns out that this veto design, selecting only those events where the light emission begins inside the detector, is extremely efficient for separating high energy neutrinos from background. Applying this technique and selecting only very energetic events, the two years of data would be expected to boil down to only about ten surviving background events (including some background neutrinos called atmospheric neutrinos). The fact that 28 events survive is thus strongly inconsistent with an explanation purely in terms of background. There are, moreover, many additional details such as their energy distribution and their distribution over the sky that are consistent with an origin in space but not with any known backgrounds.
Posted on November 22, 2013 | by Serena | No CommentsAbout once a day, a gamma-ray burst is detected. When this happens, e-mails get sent around and scientists scramble to detect whatever few photons might have been sent our way. But sometimes things are different…
On April 27th this year, an e-mail alert was sent around signifying the detection of yet another GRB. Yet this event was like no other. Rather than fighting to catch photons, there were suddenly too many to detect! The main emission episode was so bright that the GBM instrument on Fermi became saturated. And not only that – the GeV emission lasted for more than a day!
Posted on October 8, 2013 | by Christophe | 2 Comments
Today’s Nobel Prize awarded jointly to François Englert and Peter W. Higgs “for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN’s Large Hadron Collider”.
Here at OKC we are so delighted to see this prize. It confirms the importance of last year’s discovery of the mechanism and the particle imagined by Englert and Higgs. To tell the truth, although the Higgs particle was only discovered recently it has been part of some of our calculations here at OKC for some time. Some theories of dark matter assume the existence of a Higgs particle. So it was important to confirm this with the ATLAS and CMS experiments, since the discovery we know we are on the right track.
But not until a short time before the discovery annoucement did we really know that the Higgs particle existed. Not so long before the discovery some experimentalists and theorists would get a bit nervous, wondering what would we do if no Higgs particle was found… one would have to start from scratch, change the theory, go back to the drawing board, invent something new but what?
Thanks to the hard physical discovery of the Higgs particle at CERN we can now move forward, while many other theories without a Higgs particle have faded away into history.
That’s science at work. The Higgs boson is the last missing piece of the so-called standard model of particle physics. Good we got that sorted out!
But we know that the standard model is not the full story, the Higgs particle does not give mass to the neutrinos, nor do we know what is dark matter, as the standard model does not contain any such particles.
The CERN programme with the ATLAS, CMS and LHC experiments is still to provide about 200 times more data than was needed to find the Higgs particle. This is by no mean a guarantee that we will find something new, but it is only by covering new ground with some ingenious new instruments, that there is a chance to learn something new about Nature. The LHC project and the ATLAS and CMS experiments are just fantastic instruments built for that purpose. It is a great privilege to work on the ATLAS experiment and see the Nobel Prize going to particle physics today, after a bit of excitement, here at OKC, we will go back to analysing the data from the ATLAS experiment and see if we can solve another mystery of Nature.
Christophe Clement – researcher at the Oskar Klein Centre
Posted on October 3, 2013 | by Serena | 1 Comment
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:
Posted on September 16, 2013 | by Serena | No Comments
On June 3rd 2013 at 15:49 UT NASA’s Swift satellite detected an intense flash of γ -rays known as a short γ-ray burst. Follow-up observations by the Hubble Space Telescope revealed infrared emission that was present 9 days after the burst, but had faded away after 30 days. This infrared transient is likely the first ever observed example of a “macro-nova”, emission that is produced by the radioactive decay of very heavy nuclei that have been freshly synthesized in the merger of a compact binary system consisting of either two neutron stars or a neutron star with a black hole. If this interpretation is correct, the observation could have profound consequences for high-energy astrophysics, cosmic nucleosynthesis and detections of gravitational waves.
γ-ray bursts (GRBs) come in two flavors of different duration. Long bursts (longer than about 2 seconds) are produced in the death of a rare breed of massive stars, whereas short bursts (shorter than 2 seconds) are thought to result from compact-binary mergers. To date, we know 10 systems containing two neutron stars— extremely densely packed objects with masses around 1.4 times the mass of the Sun, but only about 12 kilometres in radius, and that consist predominantly of neutrons. As the stars orbit around each other they emit gravitational waves and therefore slowly spiral in towards one another until they finally merge. Such orbital decays have actually been observed2, and they agree remarkably well with the predictions from Einstein’s theory of general relativity.
Posted on August 27, 2013 | by Mark | 1 Comment
After a pioneering circumpolar journey lasting almost 14 days, the PoGOLite flight ended on the Siberian tundra. The gondola was cut from the balloon in the early hours of 26th July and touched down by parachute approximately 1 hour later. The gondola landed near, but luckily not in, a lake (this seems to be a recurring theme for us…). The landing site was close to the Siberian city of Norilsk which houses a large nickel and copper mine, as well as good infrastructure for a helicopter-based recovery of the gondola.
Photographs provided by the Russian recovery team show that the gondola is in good shape. Recovery operations are still on-going with the ultimate aim of returning the gondola to Stockholm once customs issues are solved – hopefully during the next couple of weeks. While it was hoped that PoGOLite would make a full circumpolar transit and return to Scandinavia, the stratospheric winds pushed the gondola too far to the North. Read more
Posted on August 21, 2013 | by Lars | No Comments
Hello everybody in the OKC!
Welcome back after a well-deserved vacation for most of you. For OKC this is a rather hectic period, as we have our mid-term evaluation requested from Vetenskaprådet (VR) just starting. The International Advisory Board (Katie Freese, Bengt Gustafsson, Wolfgang Hillebrandt, Hugh Montgomery, John Peacock and Larus Thorlacius) will visit us August 29 – 30, and you have been informed by Serena about some of the events taking place.
The review process will take place during the autumn, with a face-to-face meeting during a site visit January 30, 2014. The IAB has promised to act as a “mock panel”, so that we get feedback before submitting our self-evaluation with deadline September 1. When the verdict of the review comes, sometime during spring, we will know whether our Linnaeus Centre OKC gets increased funding (by up to 20 %), decreased funding (by up to the same amount), or a constant budget. It is a zero-sum game between the 20 Linnaeus Centre that received grants in 2008, so if we win some others have to lose, and vice versa. Quite exciting, in other words! Read more« go back — keep looking »