Timur Delahaye is one of the OKC fellows working at the Cosmology, Particle astrophysics and String theory group (CoPS) since this summer. let’s get to know him better.
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.
The Alpha Magnetic Spectrometer (AMS) collaboration announced its first physics result in Physical Review Letters on 3 April 2013 [AMS Collaboration, Phys. Rev. Lett. 110 (2013)]. This was a long awaited event for the astroparticle physics community. Indeed, this large particle detector was first proposed by Nobel laureate Samuel Ting in 1994, to study primordial cosmic-ray particles in the energy range from 0.5 to 2 TeV. A proof-of-principle spectrometer (AMS-01) flew successfully for 10 days on the space shuttle Discovery during flight STS-91 in June 1998. The encouraging results have strengthen the undertaking of building the first high-precision astroparticle detector (AMS-02) to be installed on the International Space Station (ISS). Following the loss of the space shuttle Columbia and its crew on 1 February 2003, the space shuttle programme was suspended by NASA, cancelling a number of flights, including the AMS-02 one. On 15 October 2009, hence over six years later, President George W. Bush signs the bill authorising NASA to add another space shuttle to launch AMS-02 on the ISS. On 16 May 2011 the space shuttle Endavour finally takes the AMS detector to space. The crew of the STS-134 mission successfully installed AMS three days later on the ISS S3 truss (see picture), from where AMS is taking continually data ever since. Continue reading First results of the AMS-02 experiment→
Particles with TeV energies, like those produced at the LHC, seem exotic. But once outside the protection of our atmosphere, these “cosmic rays” (CR) become exceedingly common. The Fermi Telescope, for example, encounters a hundred thousand CR for every gamma ray it detects. These particles have an impressive scope of local effects, from damaging electronics and inhibiting manned space travel to possibly triggering lightning strikes. And although we have been aware of their existence since the early 1900ʼs, their exact origins remained unclear. Now a study by
the Fermi Collaboration claims to have solved the mystery.
The list of mechanisms capable of accelerating so many particles to high
energies was already a short one. Strong electromagnetic fields, like those surrounding pulsars, could do the job. So could the process for which the gamma-ray telescope got its name – Fermi acceleration. Less intuitive, it amounts to trapping particles in a region where they are repeatedly reflected by inhomogeneous magnetic fields. Each reflection supplies them with more energy, and the series of small enhancements adds up. One place there such conditions exist is at the boundary between slow and fast moving groups of particles, sometimes known as a “shock.” Supernova remnants (SNR), the
aftermath of massive astrophysical explosions, are expected to shock where the expanding sphere of stellar material encounters the surrounding gas. Continue reading Fermi observations proves Supernova Remnants produce Cosmic Rays→
The past year, 2012 has been the hundredth anniversary of the discovery of the cosmic-rays by Viktor Hess. It was also the tenth anniversary of the High Energy Stereoscopic System, in short H.E.S.S., named after him and last but not least 2012 also marked the start of the second phase of the experiment with the inauguration in September of a fifth and much larger telescope.
It was 1912, when Viktor Hess performed several flights in air balloon, and discovered the extra-terrestrial origin of ionizing particles bombarding the Earth. The cosmic-rays were discovered. Since then, they have attract attention of a large community of scientists: physicists, astrophysicists and astronomers. A new field of research, astro-particle physics, emerged, spanning several orders of magnitude both in energy and particle rate, with the aim of studying their origin and properties.
At lower energies, where fluxes are high, charged particles are deflected by different electromagnetic fields before they can reach our detectors – on ground or in space – making it impossible to trace back their sites of emission. At ultra high energies, 1 billion times the energy of the particles accelerated at Large Hadron Collider (LHC), the events recorded by the Pierre Auger experiment in Argentina, are less affected by magnetic fields and point at their acceleration site. However, at such energies, rates are very low (1 particle per km square per century!), and even if Auger is one of the largest experiment on Earth, the number of events detected does not allow to study their acceleration mechanisms.
Charged cosmic-rays however can produce gamma-rays, most commonly by inverse Compton scattering of accelerated electrons on low energy photon fields or by decay of neutral pions, for instance arising from the collisions of very energetic hadrons on dense molecular clouds. These gamma-rays travel in straight line from their sites of production and their spectra can also serve to determine the nature and acceleration mechanisms of their charged cosmic-rays progenitors. Observations of gamma-rays have been so far the only way to locate high-energy cosmic-rays accelerators and they have been used successfully to establish catalogs of astrophysical objects with extraordinary acceleration power by experiments such as Fermi and H.E.S.S.
The Large Area Telescope (LAT) aboard the Fermi satellite, had provide us a tremendous amount of information in the Giga-electron-Volt (GeV) domain, with more than 2000 sources contained in its second year catalog, and a spectacular map of the entire sky in this energy range. But the LAT, launched via a space rocket in 2008, is size limited and does not have the possibility to study the sky in the Tera-electron-Volt (TeV) domain, due to very low rate of events at these energies.
Study of TeV emitters are therefore performed from the ground. This adventure started already a long time ago, with the detection of the first source, the Crab Nebula, by the Whipple collaboration in 1989. Few other sources were discovered afterwards, either by the same collaboration, or by other experiments (Hegra, CAT, Celeste, …) built in different countries of the Northern hemisphere. In the 90s, German groups involved in the Hegra experiment and French institutes from the former CAT experiments, decided to join their efforts and established the H.E.S.S. array of imaging Atmospheric Cherenkov telescopes in Namibia, where the Galactic centre and its surroundings are observable in ideal conditions.
H.E.S.S. design was a new step with respect to the previous existing ones. The array is formed by several telescopes providing a larger effective collection area. It also provides multiple views of the same event and improves the background rejection as well as the overall instrument response functions (angular resolution, energy resolution…). The telescopes have large reflective area, to collect more Cherenkov light per showers and lower the energy threshold. Finally, they are equipped with fine pixellised camera coupled with fast acquisition electronics. The fine pixellisation improves the determination of the nature and origin of the incident primary particles, whereas the fast electronic reduces night sky background noise and dead-time.
Here you can find the characteristics of the four telescopes of H.E.S.S. phase I. Continue reading A great year for H.E.S.S.→
Antje Putze is an Oskar Klein Fellow since october 2009. She is working in cosmic-ray physics and indirect dark matter detection.
You have been an Oskar Klein Fellow for more than 2 year, how is it going so far? I am very much enjoying working at the OKC. In particular, the inspirational atmosphere within the centre is very fruitful for my work. I adjusted easily to the Swedish climate (especially the short winter days) and I love living in Stockholm.
Why did you choose the OKC for doing a postdoc? The most compelling feature of the OKC is the broad spectrum of astroparticle physics subjects addressed by the OKC researchers. In particular, the interplay between experimental, phenomenological, and theoretical physics is very appealing to me. My Ph.D. focused on experimental cosmic-ray physics and phenomenology. The OKC gives me the opportunity to continue working in both areas (within the astroparticle group at KTH and the CoPS group at SU) while simultaneously broadening my knowledge base to other fields, such as indirect searches for dark matter.