The minijets-in-a-jet statistical model and the RMS-flux correlation

The flux variability of blazars at very high energies does not have a clear origin. Flux variations on time scales down to the minute suggest that variability originates in the jet, where a relativistic boost can shorten the observed time scale, while the linear relation between the flux and its RMS or the skewness of the flux distribution suggests that the variability stems from multiplicative processes, which are associated in some models with the accretion disk. We study the RMS-flux relation and emphasize its link to Pareto distributions, characterized by a power-law probability density function. Such distributions are naturally generated within a minijets- in-a-jet statistical model, in which boosted emitting regions are isotropically oriented within the bulk relativistic flow of a jet. We prove that, within this model, the flux of a single minijet is proportional to its RMS. This relation still holds when considering a large number of emitting regions, for which the distribution of the total flux is skewed and could be interpreted as being log-normal. The minijets-in-a-jet statistical model reconciles the fast variations and the statistical properties of the flux of blazars at very high energies.Comment: 6 pages, 6 figures, accepted in A

Active galactic nuclei at gamma-ray energies

Active Galactic Nuclei can be copious extragalactic emitters of MeV-GeV-TeV gamma rays, a phenomenon linked to the presence of relativistic jets powered by a super-massive black hole in the center of the host galaxy. Most of gamma-ray emitting active galactic nuclei, with more than 1500 known at GeV energies, and more than 60 at TeV energies, are called "blazars". The standard blazar paradigm features a jet of relativistic magnetized plasma ejected from the neighborhood of a spinning and accreting super-massive black hole, close to the observer direction. Two classes of blazars are distinguished from observations: the flat-spectrum radio-quasar class (FSRQ) is characterized by strong external radiation fields, emission of broad optical lines, and dust tori. The BL Lac class (from the name of one of its members, BL Lacertae) corresponds to weaker advection-dominated flows with gamma-ray spectra dominated by the inverse Compton effect on synchrotron photons. This paradigm has been very successful for modeling the broadband spectral energy distributions of blazars. However, many fundamental issues remain, including the role of hadronic processes and the rapid variability of those BL Lac objects whose synchrotron spectrum peaks at UV or X-ray frequencies. A class of gamma-ray--emitting radio galaxies, which are thought to be the misaligned counterparts of blazars, has emerged from the results of the Fermi-Large Area Telescope and of ground-based Cherenkov telescopes. Blazars and their misaligned ounterparts make up most of the >100 MeV extragalactic gamma ray background (EGB), and are uspected of being the sources of ultra-high energy cosmic rays. The future "Cherenkov Telescope Array", in synergy with the Fermi-Large Area Telescope and a wide range of telescopes in space and on he ground, will write the next chapter of blazar physics.Comment: 27 pages, 28 figures, in a topical review on gamma-ray astronomy above 100 MeV, to be published in Comptes Rendus Physique de l'Acad\'emie des Sciences (CRAS

GLAST and Very High Energy astrophysics

The pioneering age of high energy gamma-ray astrophysics has come to an end for ground-based observatories as the 3rd generation of Atmospheric ˇCerenkov Telescopes (ACTs) come online. Meanwhile the next generation space-based observatory GLAST is being assembled and is scheduled for launch in 2007. At that point, gamma-ray astrophysics will enter again in a period comparable to the CGRO/EGRET epoch where the combination of ground based and space based observatories reshaped our knowledge in many ways. The instruments have evolved into more performant and efficient machines, making the two techniques very different in their conception, but getting ever closer in their energy ranges. Some aspects and constraints in the two techniques are described, focussing on a few scientific topics that would benefit from a coordinated approach

Otimismo, bem-estar psicológico e estratégias de coping em desempregados

Dissertação de mest., Psicologia Clínica e da Saúde, Faculdade de Ciências Humanas e Sociais, Univ. do Algarve, 2013O trabalho ocupa desde os tempos primórdios um lugar de primazia na vida dos homens proporcionando os meios para a sua subsistência e a atual crise económica influencia significativamente o aumento do desemprego privando os sujeitos dos benefícios de exercerem uma atividade laboral. O objetivo deste estudo visa analisar o grau de otimismo, as estratégias de coping utilizadas para uma adaptação mais adequada a circunstâncias adversas e de bem-estar psicológico em indivíduos desempregados. Os dados foram recolhidos a partir de uma amostra constituída por 65 participantes (25 homens e 40 mulheres). Os resultados obtidos revelaram que, apesar da situação desafiadora que representa o desemprego, os valores de otimismo e bem-estar psicológico mantêm-se acima do valor médio da amplitude teórica. A estratégia de coping mais adotada por estes indivíduos foi o planeamento e a menos popular foi o uso de substâncias

Lognormal variability in BL Lacertae

X-ray data from the blazar BL Lac are used to investigate the nature of its variability, and more precisely the flux dependency of the variability and the distribution of fluxes. The variations in the flux are found to have a lognormal distribution and the average amplitude of variability is proportional to the flux level. BL Lac is the first blazar in which lognormal X-ray variability is clearly detected. Lognormal variability in X-ray light curves, probably related to accretion disk activity, has been discovered in various compact systems, such as Seyfert galaxies and X-ray binaries. The light curve is orders of magnitude less variable than other blazars, with few bursting episodes. If this defines a specific state of the source, then the lognormality might be the imprint of the accretion disk on the jet, linking for the first time accretion and jet properties in a blazar.Comment: Accepted for Astronomy & Astrophysic