Simulating the Gamma Ray Sky

Andrew McLeodSASS

August 12, 2009

Astrophysics

Measurable Quantities

Particle Flux from a given region of the sky

Particle Energies (binned)

Astrophysics

“Modeling of the Galactic diffuse continuum gamma-ray emission”, Igor V. MoskalenkoSource: http://galprop.stanford.edu/web_galprop/galprop_manual/manual2.html

EGRET gamma ray sky, > 100 MeV

GALPROP

Simulates the gamma ray and cosmic ray sky given a set of initial conditions and physical parameters

Allows a priori predictions to be compared to astronomical data

GALPROP

Method

Generates a field of cosmic rays given a cosmic ray source distribution (SNR, pulsars) by solving the transport equation:

GALPROP

MethodComputes the interaction of these cosmic rays with the interstellar medium and radiation field Gas model based on 21-cm (atomic H) and CO

(H2) surveys 3-D or 2-D options; 2-D models have radial

symmetry and model gas in 17 concentric rings

GALPROP

“Propagation of cosmic rays: nuclear physics in cosmic ray studies”, Igor V. MoskalenkoSource: http://galprop.stanford.edu/web_galprop/galprop_manual/manual2.html

GALPROP

Method

The contribution of these processes (and others) to the gamma ray spectrum are calculated as a function of radial distance

GALPROP

Output Cosmic ray spectra Bremsstrahlung gamma ray spectrum Neutral Pion Decay gamma ray spectrum Inverse-Compton Scattering gamma ray spectrum

All output in FITS format, as a function of radius (or gas ring) and energy

GALPROPPredicted gamma ray sky from Bremsstrahlung (gas ring 13)

GALPROPPredicted gamma ray sky from Neutral Pion Decay (gas ring 13)

GALPROP

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GaDGET

Optimizes GALPROP models to best-fit observed gamma ray sky (Fermi data)

Fit-weights are computed for each component’s energy bins

Indicates how GALPROP models can be improved

GaDGET

Method

Sums over gamma ray contribution of inner, local, and outer galaxy (relative to earth, ~8.5 kpc)

Assigns re-normalizing fit-weights to each region in incremental energy bins

GaDGET

For resulting fit-weight adjusted sky-maps

Produces residual sky-maps

Calculates likelihood of model being correct

GaDGETResidual Map, normalized to 1σ

GaDGET

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Optimization

GALPROP parameters can be varied Galactic Dimensions Cosmic Ray Injection Spectra Source Distribution Diffusion Coefficient

~ 40 dimensional parameter space

Optimization

Propagation processes can be altered

Reacceleration

Convection

Optimization

With a full year of Fermi data, models can now be refined with an unprecedented level of precision

Potential Applications

Indirect determination of Milky Way parameters

Better understand the processes by which cosmic rays propagate

Study extragalactic gamma ray spectrum

Works Cited

Moskalenko, Igor. “Modeling of the Galactic diffuse continuum gamma-ray emission” 6th INTEGRAL Workshop, Moscow, Russia. 2006.

Moskalenko, Igor. “Propagation of Cosmic Rays and Diffuse Galactic Gamma Rays” Nuclear Data for Science and Technology, Santa Fe, New Mexico. 2004.

Strong, Andrew. “GALPROP: a Cosmic-ray propagation and Gamma-ray code” Tools for SUSY, Annecy, France. 2006.