My interests are quite broad, but generally speaking I'm a theorist who specializes in the study of High energy
phenomena associated with Active Galactic Nuclei (AGN) from a plasma physics perspective.
Specifically, I focus on the fundamentals of particle acceleration to understand the most energetic observables, whether photons, cosmic rays,
or neutrinos.
Currently, I am mostly interested in coronae, jets, and magnetized
disks of AGNs because they are an excellent window into the properties of the environs of supermassive black holes, holding galaxies together.
High-Energy observables are quite interesting in informing us on the extreme electrodynamics of supermassive black hole surroundings,
and thus have a better grasp of black hole accretion, jet formation,
inner disk properties, wind launching, and potentially feedback.
I use theoretical tools, first-principle plasma simulations, and magnetohydrodynamic simulations to study these phenomena depending on
their relevancy. Below, you can find a non-exhaustive list of projects I am working on.
I received my PhD in 2022 from the University of Chicago under the supervision of Damiano Caprioli and Scott Wakely.
Projects
Plasma Properties of Black Hole Coronae and Particle Acceleration Therein
Black hole coronae are the main emitters of x rays in AGNs. They can be quite magnetized and are great candidates to host efficient particle acceleration,
while bathing in quite strong radiation fields.
This makes them natural candidates to produce neutrinos through proton-photon interactions. Even if the exact
geometric attributes of coronae are not known, we can still infer some of their plasma properties and thus coronal x-ray and neutrino
emission. Check out this paper for more details.
Particle Acceleration in AGN Jets
Understanding particle acceleration properties in AGN jets from radio, x ray, and gamma ray emission could be a great tool to understand
the basic plasma properties of these magnetic field fountains. New x ray polarization offers a new gateway into the
properties of such jets. This is all the more exciting as these magnetic field fountains emanate from the supermassive black holes in the nuclear parts
of these galaxies.
Cosmic Rays: Acceleration, Propagation, and Detection
I worked on frameworks for the acceleration and propagation of the highest-energy CRs, also known as Ultra-High-Energy-Cosmic-Rays (UHECRs),
which are so energetic that an UHECR proton can have the energy of tennis serves. Remember that a proton is a billionth of a
millionth of a meter in size! I mainly focus on the origins of these UHECRs. We know that they must be extragalactic (from outside of our galaxy,
the Milky Way), but we do not know where exactly they come from. However, AGN jets are great candidates to produce such particles.
I was also an active member of the HELIX experiment, a balloon-borne cosmic ray detector to
probe the confinment time of galactic cosmic rays. I was building electronics and equipment for the detector, in addition to taking care of data
analysis frameworks. Image Credit: UChicago.
High-Energy Neutrinos
Astrophysical High-Enery neutrinos are weakly interacting particles that are mainly produced through proton-proton or
proton-photon interactions in the context of AGNs. The great thing about them is that they are not charged (not affected by magnetic fields) and interact weakly
which means that they don't get stopped before reaching us.
High-Energy neutrinos could be either galactic or extragalactic and in both cases, they are quite interesting. The hope is that neutrinos would
enable us to probe regions that photons cannot escape from. The image above shows an artist rendition of the IceCube
experiment which is located under the ice in Antarctica.
Image Credit: IceCube/NSF.
Particle Acceleration Frameworks
Relativistic Asymmetric Reconnection
Magnetic reconnection refers to the breaking and reconnecting of parallel oppositely directed magnetic field lines in a plasma. The current sheet
does not have to form across two homogeneous regions as has been generally assumed in the relativisitic regime. Check out our
first paper which discusses the theoretical framework and compres
it with PIC simulations. The picture above is part of ongoing
work and is a zoomed-in version of a 3D PIC simulation of a relativisitic asymmetric reconnecting layer. Asymmetric reconnection is prominent in the heliosphere, so should
be studied in the relativisitic regime, which is relevant for AGNs. This could be interesting for the initial energy kick of ions into AGN jets!
Turbulence in Magnetically-dominated plasma
I am currently working on a framework for particle acceleration in magnetically-dominated plasma. Stay tuned for more details and please reach out if you want to discuss. The image
above shows the expected density variations in kinetic simulations, which have been quite useful in thinking about this problem. Understanding this phenomenon could be greatly interesting
for particle acceleration in jets and coronae.
One shot Acceleration (Espresso)
I developed a framework, agnostic to particle acceleration mechanisms, where trajectories are integrated via standard Particle-In-Cell techniques in relativistic
3D magnetohydrodynamic (MHD) simulations. I find that particle acceleration at UHECR energies are accelerated through
espresso, an original UHECR acceleration mechanism where particles tap the motional electric field
in the ultrarelativistic jet spine (Check out this paper and this
paper), and generally receive a boost of a factor of Lorentz factor square in energy.
The figure above shows the propagation of a particle in a 3D MHD simulation of a jet.
Outreach
Astronomy Podcast
Together with Lina Necib (MIT), we started the first astronomy podcast in the Tunisian dialect. It has been quite successful with the community, and we hope to be
able to expand. Stay tuned!
Other Activities
I am quite interested in learning about ways of making seemingly-complex science concepts more accessible to a wide audience. These days, I am working with designers to make
artist renditions of some of our plasma work to make it less opaque to a general audience.
I also gave talks about my research and astrophysics in general at multiple venues including the Adler planetarium in Chicago.
If you are interested in outreach projects in the US, Tunisia, or even France, let me know.
We can discuss involvement in current projects or start a new one.
Get In Touch
If you have any questions or would like to get involved either in the science or outreach opportunities, please do not hesitate to reach out.
You can also check out more details on other ongoing astroplasma projects at UMD, or as part of the
SCEECS (Simons Collaboration
on Extreme Electrodynamics of Compact Sources) collaboration.
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Address
PSC 1120, University of Maryland
College Park, MD
United States
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Email
rmbarek@umd.edu
