2015

27 February: Randy Jokipii (University of Arizona)

"The Interaction of the Heliosphere with the Interstellar Medium"

6 March: Julia Mikhailova

13 March: Andre Gontijo Campos (Princeton University)

"Excess of Positrons in Cosmic Rays: A Lindbladian model of Quantum Electrodynamics"

The fraction of positrons and electrons in cosmic rays recently observed on the International Space Station unveiled an unexpected excess of the positrons, undermining the current foundations of cosmic rays sources. We provide a quantum electrodynamics phenomenological model explaining the observed data. This model incorporates electroproduction, in which cosmic ray electrons decelerating in the interstellar medium emit photons that turn into electron-positron pairs.

11 September: Yuri Cavecchi (University of Amsterdam)

"Fast and slow propagation of thermonuclear flames during Type I Bursts"

The Type I Bursts, thermonuclear explosions on the surface layers of accreting neutron stars, produce extremely bright X-ray flashes that outshine all the other emission for tens of seconds.

Their lightcurves encode information about star parameters such as spin, mass and radius that are key to constraining the long sought for equation of state of the matter in the interior of the neutron stars. However, to be able to fully disentangle that information from the observations, we need a solid understanding of how the burning flame propagates across the surface.

The mathematical complexity of the problem makes non-approximate analytical solutions impossible and we have to rely on numerical simulations. I will present the results of ab initio calculations of the flame spreading, describing the physical mechanisms behind the propagation and their dependence on the star parameters.

25 September: Paz Beniamini (Hebrew University of Jerusalem)

"Towards a prompt emission model in Gamma-ray bursts"

Although Gamma ray bursts have been studied for almost 50 years, their prompt emission phase is still very poorly understood. Recent Fermi and Swift observations, provide us with important clues regarding the physical conditions during the prompt emission phase. In particular they impose large energy requirements and rule out isotropic models in which the prompt emitting region is magnetically dominated, regardless of the actual emission process. I will present a reconnection motivated anisotropic emission model and show that it can reproduce many of the observed features in GRB light-curves.

2 October: Tsvi Piran (Hebrew University of Jerusalem)

"Rethinking tidal disruption events"

30 October: Gregory Fleishman (New Jersey Institute of Technology)

"Solar Flares and Active Regions: Reconstructing the 3D Reality"

Solar activity, in particular, solar eruptive events (SEE), such as flares or coronal mass ejections (CMEs), is a uniformly recognized driver of the Space Weather—one of the strategic national priorities (http://www.nswp.gov/nswp_docs.htm). In addition, the relevant physical processes—magnetic reconnection, plasma heating, and particle acceleration—are common to many astrophysical conditions, so, given the proximity of the Sun, the solar events can be taken as proxies for remote ones. In this Talk I am going to outline a research activity that integrates observations with modeling to provide reliable, observation-based three-dimensional (3D) models of such drivers of Space Weather as Active Regions at the Sun and Solar Flares. The emphasis is made on synthesis of highly disparate inputs such as generic 3D MHD models, nonlinear force-free field (NLFFF) extrapolations, parametric thermal models of chromosphere and corona, and observations made over the entire electromagnetic spectrum, particularly, radio, EUV, and X-rays, made with VLA, EOVSA, ALMA, SDO, and RHESSI. In particular, I will describe, how the NLFFF extrapolations can be substantially improved by adding microwave magnetic diagnostics at the level of Transition Region, how the 1D empirical chromospheric models can be generalized using 3D MHD modeling results, and how all these inputs can be packaged together to provide a realistic, observation-based modeling of active regions and solar flares. I will show a few representative examples of this modeling performed for a few selected active regions and flares and discuss further steps and perspectives of this modeling approach.

9 October: Patrick Crumley (University of Amsterdam)

"On the Nature of Emission from Relativistic Jets"

Several longstanding questions in astrophysics center on the make up of relativistic astrophysical jets seen in microquasars, blazars, gamma-ray bursts, and super-Eddington tidal disruption events. What carries the energy in these jets? Is the majority of the energy carried by Poynting flux or by the baryonic matter? How is this energy converted into the non-thermal gamma-rays and X-rays seen in these systems? While there are many different theoretical models for launching a relativistic jet and producing the non-thermal emission observed in these astrophysical systems, often times the observational data are not good enough to convincingly discriminate between models. Super-Eddington tidal disruption events (TDE) represent a unique opportunity to test different emission mechanisms in relativistic jets. I will also discuss using the radio monitoring of the diffuse cloud in the galactic center object G2's to distinguish between different models of G2. Early observations of G2 after periapse passage suggests this prediction was correct.

20 November: Raul Jimenez (University of Barcelona - CfA)

"On the role of GRBs on life extinction in the Universe"

As a copious source of gamma-rays, a nearby Galactic Gamma-Ray Burst (GRB) can be a threat to life. Using recent determinations of the rate of GRBs, their luminosity function and properties of their host galaxies, we estimate the probability that a life-threatening (lethal) GRB would take place. Amongst the different kinds of GRBs, long ones are most dangerous. There is a very good chance (but no certainty) that at least one lethal GRB took place during the past 5 Gyr close enough to Earth as to significantly damage life. Assuming that a similar level of radiation would be lethal to life on other exoplanets hosting life, we explore the potential effects of GRBs to life elsewhere in the Galaxy and the Universe. We find that the probability of a lethal GRB is much larger in the inner Milky Way (95% within a radius of 4 kpc from the galactic center), making it inhospitable to life. Only at the outskirts of the Milky Way, at more than 10 kpc from the galactic center, this probability drops below 50%.

When considering the Universe as a whole, the safest environments for life (similar to the one on Earth) are the lowest density regions. Remarkably, a cosmological constant is essential for such systems to exist. We find that Lambda universes favour the survival of life against GRBs. Within a LCDM cosmology, the parameters that govern the likelihood of life survival to GRBs are dictated by the value of Lambda and the age of the Universe.

4 December: Elisabete M. de Gouveia Dal Pino (IAG - Universidade de Sao Paulo)

"Particle Acceleration by Magnetic Reconnection around Black Holes"

Particle acceleration in magnetic reconnection discontinuities is currently regarded as an important mechanism to accelerate particles in a broad range of astrophysical sources and environments. In this talk, I will review this mechanism discussing both the first and second order Fermi acceleration processes, particularly in the framework of collisional fluids highlighting the differences with regard to collisionless ones. I will also discuss recent applications of this process to the surrounds of the black holes in binary systems (microquasars) and active galactic nuclei (AGNs). Applications to relativistic jets (of AGNs and GRBs) and to more diffuse turbulent interstellar and intergalactic medium will be also briefly addressed.

11 December: Gareth Roberg-Clark (IAG - Universidade de Sao Paulo)

"Kinetic Modeling of Electron Heat Flux Instabilities in High-beta Collisionless Plasmas"

We use 2D kinetic (particle-in-cell) simulations as well as semi-analytic theory to map out the growth and impact of electromagnetic waves on heat conduction in a high-beta collisionless plasma. In these simulations we keep ions frozen and look at purely electron-driven dynamics, which to our knowledge is the first effort of its kind in the high-beta regime. Understanding these processes is a basic plasma physics goal and sheds light on the thermodynamic properties of the Intracluster Medium (ICM). The work is thus relevant to any process involving conduction in these systems, such as AGN feedback, conduction driven instabilities like the MRI and HBI, and the ion mirror instability in high-beta plasmas.