COSMICMAG

The magnetic field of our Galaxy (GMF) can be estimated using astrophysical observations, in particular the Farraday rotation of polarized radio emission from astrophysical sources and synchrotron radiation of cosmic-ray electrons spiralling in the GMF. From these observations, parametric models of the GMF can be constructed as for instance the one of Jansson and Farrar (2012) which is visualized in the left figure above.
The main result of the COSMICMAG project is a thorough evaluation of the uncertainties of estimates of the GMF, resulting from different functional forms used for the modeling, different data sets used for the tuning of parameters and different auxillary assumptions used to interpret the data. The effect of GMF uncertainties on our ability to estimate the arrival directions of extragalactic ultrahigh-energy cosmic rays is illustrated in the right panel of the figure above, where two sky maps are shown in Galactic coordinates. A grid of representative arrival directions at Earth is indicated as colored points. These directions were back-tracked through different models of the GMF to obtain the arrival direction at the edge of the Galaxy indicated as lettered squares. The different letters identify different models of the GMF. Each of them gives a reasonable description of the available astrophysical data. As can be seen, for cosmic rays with a large energy-to-charge ratio (60 EV, top panel) the overall amount of deflection and correspondingly also the model differences are small, confirming the long-speculated possibility of charged particle astronomy with protons at ultrahigh energies. Even for energy-to-charge ratios as low as 20 EV, the different deflections are mostly confined within well-defined regions so it seems plausible that a correction for the spatially varying average deflection based on all models, can still be used to enhance the capabilities to identify the sources of ultrahigh energy cosmic rays. More details can be found in our ICRC contribution.

The spectral "ankle"-feature in the flux of ultra-high energy cosmic rays as well as the proton-dominated composition observed at the presumed transition from Galactic to extra-galactic cosmic rays can be naturally explained by postulating a suitable source environment around the sources of ultra-high energy cosmic rays. This was shown in collaboration with L. Anchordoqui and G. Farrar. The left picture illustrates our model for extragalactic cosmic rays. Their sources (denoted by yellow stars) are situated in an environment such that the ultrahigh-energy cosmic rays partially photodisnitegrate via interactions with the ambient photon field, before they escape. The escape time depends on the strength of the random magnetic field around the sources. The right picture shows a comparison of the model prediction to the flux measured by the Pierre Auger Observatory. The black solid line is the total model prediction and the colored lines show the contributions for increasingly massive particles starting from protons (red) to silicon (cyan). More details can be found in our paper.

The interstellar medium permeates the space between stars in galaxies. Its properties are of scientific interest on their own, but in the context of the COSMICMAG project we are mainly interested in its influence on the rotation measures that we use to estimate the strength of the Galactic magnetic field. Previous attempts to model the Galactic magnetic field relied on the NE2001 model for the thermal electrons in the interstellar medium. The figure shows the stage of our development of an improved model of thermal electrons. On the left a sky map of the measured Hα emission is shown (composite of VTSS, SHASSA, WHAM data). On the right the emission simulated with our preliminary thermal electron model is shown, including the emission of known HII regions. The gray area masks regions with large obscuration from dust. See this presentation for more information.

The study of the magnetic field of our Galaxy is complicated by the fact that we are observing it from the inside. To judge whether the modeled magnetic field gives a fair representation of observations of other galaxies, a simulated view from an imaginary outside observer is useful. Previous simulation programs such as Hammurabi could only produce simulated sky maps for an observer sitting at the location of the Earth. This restriction does not apply to the RUQI framework developed for the COSMICMAG project. This feature is illustrated in the above figure. The three panels show simulated observables for the JF12 magnetic field model viewed from 45 degree above the Galactic plane. One the left the rotation measure is shown and the Stokes parameters Q and U are displayed in the middle and right panel.
In addition to the free choice of observer position, the RUQI framework supports adaptive line-of-sight integration and dynamic loading of astrophysical models.

The idea to detect cosmic rays with a cheap citizen-science approach was put forward by the DECO and CRAYFIS groups. On the left the population density is shown using data from the Global Rural-Urban Mapping Project . On the right the amount of user participation needed to obtain a similar area as "professional" cosmic ray observatories can be seen. From these figures it can be concluded that it may indeed be possible to detect a few events with a network of smartphones, but a large enough area to collect enough events to perform high-quality science seems difficult to reach, even under favorable assumptions. See our preprint for more information.