This report began on a sheet of paper, somewhere on a karst plateau in the south of France. Since then it has evoveled a lot – just as I have during the five weeks I spent in Paris, France for my internship at «AstroParticule et Cosmologie» (APC).

This experience was made possible by a lot of people I’d like to thank with all of my heart: Stefano Gavici and Giada Peron for having me at APC, taking the time to explain me everything I needed to know, and for wonderful luchbreaks and evenings together. Archana Purushothaman  for always helping me when Giada wasn’t around and for fun trips through Paris. The staff at APC for happy greetings in the morning and awesome coffee breaks. My parents, for sponsoring me. And all of the friends I made for the best of memories.

But let’s start from the beginning: what is APC? The AstroParticule et Cosmologie is a reasearch centre associated with the «Centre national de la recherche scientifique» (CNRS), «Université Paris Cité», «Université Paris-Diderot»  and many more. It’s divided into multiple research groups, each with a distinct focus on Cosmology, Gravitation, Particle Physics, Theoretical Physics or High Energy Astrophysics. I was part of the latest under the supervison of Stefano and Giada.

In their group, I worked on a project trying to relate the Infrared Flux to the Gammaray Flux of star clusters. Using the data obtained, we could start drawing conclusions about the origin and acceleration of Very High Energy Gamma-Rays (a certain type of Cosmic Rays (CRs)), as well as eventually being able to point measuring devices more precicely.

The distribution of CRs follows roughly the curve shown inDiagram 1 (adopted from [2]), with the slope changing at the „knee“ at ~3 PeV and the „ancle“ at ~5 EeV. [1]

As you can see, some CRs are measured at very high energies, and the mechanisms of their acceleration are not yet fully understood. The most common theories include acceleration by shock waves during supernovae, acceleration by turbulences in the interstellar magnetic field [2], or acceleration by stellar winds in Star Clusters [3]. To verify those theories it’s important to understand where Gamma-Rays originate from – which is what my project was all about.

Using data from the WISE project [4], I developed a python script using algorithms based on Aperture Photometry for obtaining the Infrared Flux of different regions. Next, I derivated the Gammaray Flux of those regions, using data and methods Giada and Archana had obtained before as well as the fermiLAT data [5]. Of course the errors for all of those methods had to be determined and the uncertainties of the data propagated.

Then, finally, I could plot the fluxes of all the the regions and we did find a correlation (YAYY!!).

This correlation is consistent with the theory, that CRs may be accelerated by winds of star clusters. The Infrared Luminosity, which is proportional to Distance and Infrared Flux, is caused by photons emitted by the source interacting with particles of dust nearby. The dust absorbs those photons, starts vibrating and finally emits its energy again as photons in the infrared domain. More powerful sources have a higher Infrared Luminosity. At the same time, more powerful sources create stronger stellar winds, which lead to a greater Gammaray Luminosities. Thus we would expect sources with high infrared luminosity to also have high gammaray luminosity, which is exactly what we find (see diagram 2).

To summarise: in my project, I learned about Cosmic Rays and how they are measured, about exciting projects such as WISE and fermiLAT, about the determination of errors, about python (especially Astropy, Matplotlib, …) and data analysis / curve fitting.

But the time abroad taught me many more things: for the first time in my life I lived on my own, which was sometimes a challange, but in any case exciting. It also helped me improve my spoken French, altough we usually spoke English at APC as it’s very international.

During my stay, I realised that political topics are much more integrated into the everyday life in France than they are in Germany. I had the chance to be there in a very politically eventful time, learning about plenty of different struggles and modes of action.

Just before I arrived, the «soulèvement de la terre», a popular ecological movement known for mass protests, was anounced to be dissolved and at least 18 activists were arrested under the presence of that SDAT («Sous-Direction Anti-Terroriste»). In return, more than a hundred thousand people signed a petition against the dissolution. [6]

Just when I arrived, the «émeutes» in tribute to Nahel, a teenager who was shot by the police forces, and against police violence were at its peak. Traces of the protests, such as smashed windows, ashes and graffiti, were still to be found weeks later. Additionally, all over the country independent libraries held debats for anyone affected, involved or interested in the reasons behind the protests.

Just while I was there, a campaign against the «Jeux Olympiques de Paris 2024» (JOP) kicked off with an action of «désobéissance civile», covering a monument displaying the Olympic Rings in fake blood. It aimed to criticise the „social, ecological and human cost» of the JOP. [7]

Just after I left, a conference «Les Résistantes 2023» was held in Larzac, where more than 5000 people came together and discuss about the future of (ecologist) activism in France. [8]

Back to Paris. Living in the city, I was impressed by their spirit of life. For example: there are daily, free dance events held all around Paris, where strangers dance with and get to know each other. On top of that, every now and then big, festival-like events are held: On July 14th, the «fête nationale francaise», I watched fireworks at the Eiffel Tower together with 70 000 people. Aside from all of the glory, and the polished tourism hotspots, there’s lots of poverty in the «banlieues», many homeless people, and plenty of loneliness. Streets, houses and infrastructure often are poorly maintained. For privacy reasons, there won’t be pictures of people or their homes shown here.

I could have never hoped, to learn this much – academically as well as culturally and politically. Thanks to everyone who made this experience possible and supported me throughout it. And for everyone to come after me: I’d always recommend spending your internship in France,

Practical Advice

Public Transportation: The metro is really accessible in all of Paris and the easiest way to get around. Try to avoid busses, as they are very unreliable and usually delayed. If you are going to/from the «banlieues», you’ll need to take the RER. It’s most convenient to buy the «pass Navigo» for 85€ a month (as of July 2023), as it allows for unlimited rides on public transportation.

Cycling: Paris, and most other cities in France I’ve been to, have a great bycicle infrastructure with plenty of bikes to rent at every other corner, dedicated bike lanes and even streets reserved for bycicles.

Housing: I lived in an AirBnb in Bercy, a 15 minute walk away from APC. For a month you can expect the cost of an apartment to yourself to be around 1500€ (as of July 2023). Don’t underestimate the time housekeeping, cooking, and doing the laundry will take!

Food: Food, and especially vegetables, are more expensive than they are in Germany. In restaurants vegetarian options are hard to find and eating out vegan is nearly impssible. Expect to see plenty of people carrying baguettes (it’s not a clichée). If you’re in a more rural area, try eating at a local «marché».

People: Paris is a big city, and it’s easy to get lost and feel lonely. Try to join a local (sports) group, attend events at your «arrondissement»‘s independent libary (they often have debats, show movies, sell books, or just drink together), go dancing, or try out one of the many friendship applications (i.e. «Boo»).

Police: In France it is normal for the police to carry handguns and light machine guns openly. You’ll see them at train stations, in crowded areas, and sometimes on patrol in the city. Altough this can feel unsettling at first, you’ll grow more used to it as time passes on.

Language: Many people do speak some English, but it makes matters much more easy if you have basic language skills in French. Make sure to bring a travel dictionary and have a translator ready on your phone.



[1] Peron, G., „Probing the Spatial and Spectral Distribution of Galactic Cosmic Rays with High-Energy Gamma-Rays“, PhDT, 2020. doi:10.11588/heidok.00029323.

[2] Morlino, G. (2017). “High-Energy Cosmic Rays from Supernovae”. In: Handbook of Supernovae. Ed. by A. W. Alsabti and P. Murdin. Cham: Springer International Publishing, pp. 1711–1736. isbn: 978-3-319-21846-5. doi: 10.1007/978-3-319-21846-5_11.

[3] Giada Peron, Sabrina Casanova, Stefano Gabici et al. „Winds of star clusters significantly contribute to the Galactic cosmic-ray population“, 20 March 2023, PREPRINT (Version 1) available at Research Square []