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
Posted on July 13, 2013 | by Mark | No Comments
On July 12th at 0818 UTC, PoGOLite was successfully launched from the Esrange Space Centre.
The balloon trajectory can be seen here.
Greetings from Esrange,
The PoGOLite team
Posted on June 18, 2013 | by Serena | No Comments
In order to celebrate a substantial allocation of time on the Hubble Space Telescope (HST) in cycle 21, the LARS project invited all of OKC for a drink on wednesday 12 june. This time, eLARS, the extension of the LARS project was awarded another 54 spacecraft orbits. In total the LARS project has so far been awarded 110 orbits, and is the biggest Swedish led project on HST ever.
The aim of the LARS project is to improve our understanding of how the Lyman alpha emission line is formed and transported out of galaxies. This is vital since Lyman alpha is the most accessible and most commonly used spectral probe of the distant universe. LARS will make extremely detailed studies of Ly alpha emission from 42 galaxies in the nearby universe, obtaining a physical resolution more than 2 orders of magnitude better than achievable at high redshift. One of the results that have emerged so far is that galaxies appear systematically larger when seen in Lyman alpha than in the continuum or in other recombination lines such as Halpha, a consequence of Lyman alpha photons resonantly scattering on neutral hydrogen atoms.
- Göran Östlin – email@example.com
Posted on May 13, 2013 | by Serena | 1 CommentOn 27 April, an incredible opportunity was given to GRB science detectives. As the spring was outbursting here in Stockholm the explosion of a distant star almost blinded the Gamma ray Burst Monitor (GBM) detectors on board the Fermi satellite. GRB130427 is the brightest GRB ever detected in the keV – MeV band and the longest lasting in the GeV energy range: Fermi Large Area Telescope (LAT) could detect it for hours after the trigger.
Gamma ray bursts (GRBs) are cosmological flashes of which the prompt emission, lasting for 0.01-100s, is in the gamma ray band. Their late emission can be detected at lower energy ranges like optical and radio. One or two GRBs per day are typically observed, but their origin and the particle acceleration mechanisms involved remain nowadays unknown. The favourite hypothesis on their origin is the collapse of a supermassive star, while there is not a leading hypothesis for the acceleration mechanisms involved in the outflow responsible for the prompt emission.
This burst was also detected by other experiments such as Swift and Integral which allowed a rapid and precise localization which enabled optical, infrared and radio follow-up observations. The redshift was measured within hours from the original trigger and revealed that the outbursting star was quite close (for this kind of objects): z= 0.34.
With nearly 1000 photons per second and square centimeter in the 10-1000 keV band and 14 photons per second per square meter in the 100 MeV – 10 GeV band (see attached figure), this burst is a unique occasion for the scientific community to probe models for particle acceleration and photon emission in the outflow.Soon the Fermi and Swift collaborations will publish their papers and hopefully more news and more papers will follow. We expect to be able to take a step further in understanding the physics of the GRB thanks to this record breaker burst. Keep an eye on it!
The figure (source: arXiv:1303.2908) shows the fluence in two energy band of the Fermi LAT detected burst, the star indicate the position this burst would have in this plot.
The event fluence in the first 20 seconds in the 10-1000 keV band is (1.975 +/- 0.003) E-03 erg/cm^2, while in the fluence in the first 140s in the 100 MeV – 10 GeV band is (1.1 +/- 0.1)E-4 erg/cm^2.
- Elena Moretti, (OKC fellow) – firstname.lastname@example.org« go back — keep looking »