MYSCE, a new Monte Carlo code for star cluster simulations. Axion-like particles and very-high energy astrophysics

This thesis focuses on two different topics: (i) star clusters, as favourite environments for the formation of black holes of very high mass, and (ii) the implications of axionlike particles for very-high energy astrophysics by considering -ray observations from blazars. In the first part of this thesis we develop a new Monte Carlo code MYSCE (Montecarlo Young Star Cluster Evolution) which improves and solves some problems of the original scheme presented by H\ue9non. We use our code to simulate star clusters and to inquire if an episode of gas infall during the cluster lifetime can lead to the formation of an intermediate mass black hole and/or a super massive black hole seed. In the second part of this thesis we explore axion-like particles in an astrophysical context. We describe their origin and properties within extensions of the Standard Model of elementary particle physics. We show how axion-like particles can in principle solve the cosmic opacity problem for distant blazars and naturally explain the emission of very-high energy photons from at spectrum radio quasar

Importance of axion-like particles for very-high-energy astrophysics

Several extensions of the Standard Model predict the existence of Axion-Like Particles (ALPs), very light spin-zero bosons with a two-photon coupling. ALPs can give rise to observable effects in very-high-energy astrophysics. Above roughly 100 GeV the horizon of the observable Universe progressively shrinks as the energy increases, due to scattering of beam photons off background photons in the optical and infrared bands, which produces e+e- pairs. In the presence of large-scale magnetic fields photons emitted by a blazar can oscillate into ALPs on the way to us and back into photons before reaching the Earth. Since ALPs do not interact with background photons, the effective mean free path of beam photons increases, enhancing the photon survival probability. While the absorption probability increases with energy, photon-ALP oscillations are energy-independent, and so the survival probability increases with energy compared to standard expectations. We have performed a systematic analysis of this effect, interpreting the present data on very-high-energy photons from blazars. Our predictions can be tested with presently operating Cherenkov Telescopes like H.E.S.S., MAGIC, VERITAS and CANGAROO III as well as with detectors like ARGO-YBJ and MILAGRO and with the planned Cherenkov Telescope Array and the HAWC-ray observatory. ALPs with the right properties to produce the above effects can possibly be discovered by the GammeV experiment at FERMILAB and surely by the planned photon regeneration experiment ALPS at DESY.Comment: 4 pages, 5 figures. Proceeding of the workshop "TAUP2011", Munich 5 - 9 September 2011 (to appear in the Proceedings

Transparency of the Universe to gamma rays

Using the most recent observational data concerning the Extragalactic Background Light and the Radio Background, for a source at a redshift z_s < 3 we compute the energy E_0 of an observed gamma-ray photon in the range 10 GeV < E_0 < 10^13 GeV such that the resulting optical depth tau_gamma(E_0,z_s) takes the values 1, 2, 3 and 4.6, corresponding to an observed flux dimming of e^-1 = 0.37, e^-2 = 0.14, e^-3 = 0.05 and e^-4.6 = 0.01, respectively. Below a source distance D = 8 kpc we find that tau_gamma(E_0,DH_0/c) < 1 for any value of E_0. In the limiting case of a local Universe (z_s = 0) we compare our result with the one derived in 1997 by Coppi and Aharonian. The present achievement is of paramount relevance for the planned ground-based detectors like CTA, HAWC and HiSCORE.Comment: 5 pages, 2 figures, MNRAS (2013) - in Pres

Very-high-energy quasars hint at ALPs

One of the mysteries of very-high-energy (VHE) astrophysics is the observation of flat spectrum radio quasars (FSRQs) above about 30 GeV, because at those energies their broad line region should prevent photons produced by the central engine to escape. Although a few astrophysical explanations have been put forward, they are totally ad hoc. We show that a natural explanation emerges within the conventional models of FSRQs provided that photon-ALP oscillations take place inside the source for the model parameters within an allowed range.Comment: 4 pages, 3 figures. Proceeding of the workshop "9th Patras Workshop on Axions, WIMPs and WISPs", 24 - 28 June 2013, Schloss Waldhausen, Mainz, Germany (to be published in the Proceedings