Tag Archives: talk

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Talk: Extragalactic B-fields from Gamma Rays

Speaker: Tom Weisgarber (Chicago)

Voids: <1nG; filaments: ~0.3 µG; galaxies: ~1 µG; clusters: 10 µG. No detections of B-fields in voids.
Gamma rays interact with the extragalactic background light (EBL) and create electron-positron pairs, which then cascade down to create more gamma rays. This cascade is then observed with some opening angle. Pair production and inverse Compton scattering are the important processes in creating the cascades.
Semi-analytic model for the cascade: Huan+ (2011, ApJL 735, 28). Using the blazar RGB J0710+591, they constrain the B-field to be <3e-16 G (<3e-18 G for a 3 year livetime).

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Lionel London (CRA) – GPU Computing in Matlab

Matlab Parallel Computing Toolbox (PCT) vs. Jacket
PCT allows for multi-cpu and GPU computing. Limited to 12 cores on the local machine. Very high level.
GPU computing with PCT requires an nVidia card with v1.3 compute capabilities. GPGPU with Jacket has relaxed requirements but is very expensive ($4k for 5 licenses!)
PCT GPU can run external .cu files.
Jacket has 10x more CUDA-enabled functions than Matlab. It’s cluster capable.

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Michael Nielsen

Yesterday I attended a discussion with Michael Nielsen and the GT faculty, where open science was the subject. We covered various venues for promoting collaborative science projects. Some were completely new to me. Let’s cover a few examples that were discussed. Continue reading Discussion: Doing Science in the Open →

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Kostas Kokkotas (Tübingen)

Magnetars exhibit regular gamma-ray flares that are preceded by star quakes or glitches in the pulsations. The largest flares have L ~ 10^47 erg/s (3 since 1979).
No natural explanation for (1) sudden stop of activity P ~ 12 sec and (2) AXP-SNR association but no SGR-SNR association
Thompson & Duncan (1990’s) – developed a fireball model that erupts from the NS surface to explain these largest bursts.
Can QPOs be explained by global Alfvèn waves? (see KK publications)
NS models, using perturbation theory, with crust and B-fields to predict QPO frequencies. No crust oscillations at B > 4 x 10^14 G.
Gravitational wave amplitudes are unknown and must be calculated with simulations (3D GRMHD).
Simulation of B-field instability (Lasky et al. 2011, ApJL) and prediction for GW (Zink et al. 2011)

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Matt Kinsey: Porting the 2D Wave Equation to the GPU

Optimal number of threads per block is 32*n-1, where n is an integer. The best performance in the example shown was 63 threads per block.
Minimum number of blocks per grid is 32, according to the user’s guide.
Every time a kernel is called, the memory needs to be pushed from the CPU to the GPU. Thus it is optimal to minimize the kernel calls.
In the 2D wave equation problem, Matt utilized texture memory to reduce the number of kernels to one. The memory is indexed in a space-filling curve. This results in better cache locality.
With texture memory, one can take advantage of built-in linear interpolation and boundary conditions. Texture memory can be addressed in 1D, 2D, and 3D.