2014

17 January: Jifeng Liu (National Astronomical Observatories of China, Beijing)

"Puzzling accretion onto a black hole in M101 ULX-1"

Ultraluminous X-ray sources could be the long-sought intermediate mass black holes as necessary for current supermassive black hole formation theories; alternatively they could be stellar mass black holes somehow managing to radiate at near- or even super-Eddington luminosities.

The key of ULX studies is to measure the black hole mass via dynamical means, which turns out to extremely difficult. Our dynamical mass measurement for M101 ULX-1 shows that it is a stellar mass black hole instead of an intermediate mass black hole. This fact makes it hard to understand the supersoft X-ray spectrum of M101 ULX-1.

2 May: Rolf Bühler (DESY, Zeuthen)

"The surprising Crab pulsar and its nebula"

The Crab nebula and its pulsar (referred to together as “Crab”) have historically played a central role in astrophysics. True to their legacy, several unique discoveries have been made recently. The Crab was found to emit gamma-ray pulsations up to energies of 400 GeV, beyond

what was previously expected from pulsars. Strong gamma-ray flares, of durations of a few days were discovered from within the nebula, while

the source was previously expected to be stable in flux on these time scales. In this seminar I will review these intriguing and suggestive

developments. I ll give an overview of the observational properties of the Crab and our current theoretical understanding of this system.

12 September: David Eichler (Ben Gurion University, Israel)

“Relative Abundances in Energetic Particle Populations as a Diagnostic for Basic Plasma Processes”

19 September

Rahoul Kumar (Ben Gurion University, Israel)

“Electron Heating in Relativistic Shocks”

Martin Weidl (Max-Plank Institute for Plasma Physics, Munich)

“Magnetic Field Amplification by Cosmic Ray Currents”

26 September: Benoît Cerutti (Princeton University)

"Particle acceleration in pulsars: New insights from kinetic simulations"

Pulsar magnetospheres are often considered as ideal astrophysical sites for particle acceleration in relativistic pair plasmas. The particle-in-cell (PIC) technique is well-suited to study

both the evolution of the fields and particle acceleration from first principles. I will present a series of global 2D spherical PIC simulations of the aligned pulsar. The results clearly support that the current sheet is an efficient accelerator of particles, and possibly the main source of energetic radiation in pulsars. I will discuss these results in the context of young gamma-ray pulsars.

17 October: Makoto Takamoto (Max-Plank Institute for Nuclear Physics, Heidelberg)

"Turbulent Effects on Relativistic Magnetic Reconnection in Poynting-dominated Plasmas"

In this talk, we report on our recent findings of the enhancement of the magnetic reconnection rate by turbulent processes in a Poynting-dominated plasma. First, we discuss the evolution of the plasmoid-chain created in a Poynting-dominated plasma, and second, the effects of 3-dimensional relativistic turbulence. In both cases, as is reported in non-relativistic works, turbulence greatly enhances the reconnection rate, which becomes independent of the Lundquist number. We also discuss the relativistic effects and possible astrophysical applications.

24 October: Tanmoy Laskar (CfA, Harvard)

"Gamma-ray bursts as probes: from relativistic shocks to the high-redshift universe"

Gamma-ray bursts (GRBs) are the most energetic explosions since the Big Bang, thus providing a unique opportunity to study the most extreme conditions in the Universe. At the same time, the large luminosities resulting from the interaction of the ejecta with their environment also makes these events premier probes of star-formation in galaxies out to the highest redshifts. I will highlight the versatility of GRBs through three related studies: i) using GRBs to measure the first galaxy mass-metallicity relation at 3<z<5, ii) searching for differences in the progenitor population at z>6 through afterglow observations and modelling, and iii) probing GRB shock physics through the first multi-wavelength detection and characterisation of the reverse shock. I will conclude with a discussion of future directions in the study of the GRB progenitors and their use as probes in the ALMA and JWST era.

5 December: Jonathan Katz (Washington University, St. Louis, MO)

"Fast Radio Bursts"

Fast Radio Bursts were discovered by Thornton, et al. (2013), and possibly earlier by Lorimer, et al. (2007), in the Parkes Multibeam Pulsar Survey at

high Galactic latitude. They show classic pulsar-like pulse dispersion that, if attributed to intergalactic plasma, indicates cosmological distances

and extremely high (~10^{37}K) brightness temperatures that can only be explained as the result of coherent emission. The degree of charge clumping required implies relativistic potentials. The de-dispersed pulses of two FRB are broadened to several ms with a frequency dependence that indicates scattering along the paths between source and observer. The scattering cannot be attributed to the general intergalactic medium and appears to have taken place close to the sources. I will discuss implications for the origins and distances of FRB and their astronomical environments, and consider the possibility that FRB are local interference rather than an astronomical phenomenon.

12 December: Robert Wicks (NASA-GSFC)

"Coherent waves, turbulence and anisotropy of proton distribution functions in the solar wind"

The solar wind is a highly ionized plasma flowing away from the Sun between 250-1000 km/s, making it highly supersonic and super-Alfvenic. The flow is turbulent and mostly collisonless, this means that dissipation of energy from large scale motions to ions and electrons is mediated by field-particle mechanisms rather than collisions. The lack of collisions also allows anisotropic ion and electron distribution functions to persist and, perhaps, for pressure instabilities to grow. I will present a summary of recent observations from the solar wind showing how the plasma and the turbulent magnetic field fluctuations are anisotropic in different ways. I will show that the turbulent cascade generates anisotropy in the amplitude, wave vector direction and scaling of the turbulent power spectrum about the mean field direction. The proton distribution functions observed in the solar wind are also anisotropic about the magnetic field direction and so can have different temperatures parallel or perpendicular to the field. This may lead to pressure instabilities and thus to the generation of coherent modes, such as Alfven ion cyclotron waves, mirror and firehose modes. I will show results identifying each of these modes superposed on the background turbulence and investigate the spectra and anisotropy of the waves generated by each mechanism. I will also play solar wind time series data as audio files and show how each mode has its own "sound" in the solar wind.