PhD06 - Probing the universe with cosmological electromagnetic cascades

Thesis title: Probing the universe with cosmological electromagnetic cascades

Laboratories involved:

- Institut de Recherche en Astrophysique et Planétologie (IRAP)– UMR 5277
  Université Paul Sabatier
  9 Avenue du Colonel Roche
BP 44346

  31028 Toulouse Cedex 4

- Laboratoire Univers et Particules de Montpellier (LUPM) - UMR 5299 
  Université Montpellier 2 - CC72 
  Place Eugène Bataillon 
  34095 Montpellier Cedex 5 

University where the PhD student will be registered:

Université de Toulouse (UPS Toulouse III) 

Lab where the PhD student will be administratively located: IRAP

Supervisors (Principle listed first):

Renaud Belmond – IRAP – tel: +33 (0)5 61 55 75 36 
Julien Malzac – IRAP – tel: +33 (0)9 70 46 65 27 
Johann Cohen-Tanugi – LUPM –, Tel: +33 (0)4 67 14 41 91 
Pierre Jean - IRAP -, Tel: +33 (0)5 61 55 67 44 
Alexandre Marcowith – LUPM –, Tel: +33 (0)4 67 14 41 89

OCEVU working group(s):

Astroparticles, Cosmology

Description of the thesis work

The very high-energy (VHE) extragalactic point sources observed with Fermi, HESS, VERITAS or MAGIC, can be used to probe the universe on cosmological scales. Indeed, the VHE radiation (GeV, TeV) from Blazars or Gamma-ray bursts may interact with the diffuse Extragalactic Background Light (EBL, mostly IR, optical and UV photons) to produce electron positron pairs. Those pairs can then produce gamma-rays through inverse Compton scattering off the diffuse radiation (mostly the Cosmic Microwave Background) and so on. This leads to the development of an extended pair cascade propagating over cosmological distances until it eventually reaches us.
Such cascades redistribute the energy of the radiation by removing photons at high energy and producing secondary radiation at lower energies. These effects must be accounted for, in order to study the intrinsic spectra of these sources. Another effect, is that extragalactic point sources will appear as spatially extended due to the inverse Compton diffusion and deflection of leptons by the Extragalactic Magnetic Field (EMF). This should also induce time-delays between different energy bands. The observable properties of the cascade (such as the size of the source projected image, or time lags between different energy bands) depend on the strength and coherence length of the EMF as well as the spectral energy distribution of the EBL. Both of them is a fundamental cosmological tracer (see Dominguez et al. 2011; Widrow et al. 2012) and there is currently no alternative to measure directly the EMF. We see that the properties of the cascade can be used to probe the intergalactic medium and constrain cosmological models (Elyiv et al. 2009).
The work of the PhD student will consist in modeling the observable effects of the cosmological cascades in order to set constraints on the physics of the sources of gamma-ray, on the EMF and EBL, on the processes leading to the formation of the Extragalactic Gamma-Ray Background (EGRB) and on the origin of the galactic 511 keV annihilation radiation.
We have very recently developed a Monte-Carlo code to simulate such cosmological cascades (David Sarria’s Masters Thesis, 2012). There are other codes that have been used to study pair cascades (e.g. Kachelriess et al. 2012), but we believe our code provides the most detailed and accurate modeling of all the radiation processes and the propagation of photons and particles in an expanding universe.
Initially, the work of the PhD student will consist in using this code to explore the properties of the cascades and make predictions regarding the spectra, size of the source image, or time lags between different energy bands as a function of the source spectrum and redshift. This may also include introducing some refinements in the code, for example regarding the treatment of propagation and energy losses of the leptons in the EMF. Then we will make detailed comparisons of the result of the simulations with HESS and Fermi observations from the archives as well as predictions for future observation with the CTA. Besides recovering the intrinsic spectra of VHE sources, the main goal will be to obtain constraints on the EMF and EBL as a function of redshift. This should take 1 to 2 years and lead to the publication of at least 2 papers.
In addition to VHE gamma-ray point sources, several processes could initiate the formation of electromagnetic cosmological cascades. This includes dark matter decay and Ultra High Energy Cosmic Rays interacting with the CMB to produce e+-e- pairs or photo-pions that then decay. In a second phase, the PhD student will estimate the respective contribution of all theses processes to the Indeed, the measured EGRB constrains all the processes that have injected energy above the e+-e- pair creation threshold over the entire history of the Universe. A detailed comparison of the results of the simulations with the data should allow us to obtain constraints on these processes.
Then, the PhD student will also use the code to estimate the number of electron-positron pairs produced in cosmological cascades initiated by these various processes. Those pairs propagate in the universe until they have lost all their energy and annihilate. Some of them may be captured by the magnetic field of galaxies and concentrated into regions that are denser than the extragalactic medium and where cooling and annihilation will be more efficient. This process may lead to diffuse 511 keV annihilation radiation such as that mapped by INTEGRAL in our own galaxy (e.g. Jean, 2010). If we find that cosmological cascades could be a significant source for the galactic annihilation radiation and if there is still some time left, we will attempt to model the transports of positrons from the intergalactic medium to the annihilation site in the galaxy. This will allow us to test whether the morphology of the diffuse 511 keV map could be reproduced. The second phase of the PhD should last for 1 to 2 years (including the writing of the thesis) and should lead to the publication of at least 1 paper.
R. Belmont, J. Malzac and A. Marcowith are experts in modeling high-energy radiation processes. J. Malzac has an extended experience in the development and use of Monte-Carlo simulations codes. P. Jean is an expert in the observation and modeling of the diffuse galactic 511 keV annihilation feature. A. Marcowith, and J. Cohen-Tanugi are part of the HESS and Fermi team respectively and will provide access to and analysis expertise in the data.
Domínguez et al., 2011, MNRAS, 410 ,2556. 
Elyiv A., Neronov A., and Semikoz D.V, 2009, PhRD, 80(2) :023010. 
Kachelriess M, Ostapchenko S., and Tomàs R., 2012, Computer Physics Communications, 183,1036. 
Jean, P., 2010, "Proceedings of The Extreme sky: Sampling the Universe above 10 keV. October 13-17 2009, Otranto (Lecce) Italy., p.72" 
Widrow, L. M.,Ryu, D., Schleicher D. R. G., Subramanian, K.; Tsagas, C. G., Treumann, R. A. Space Science Reviews, Volume 166, Issue 1-4, pp. 37-70

The DEADLINE for applications is APRIL 30, 2014.

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