Exploring high-energy astrophysics at Columbia and at l’X

Their shared passion for the study of blazars led Deirdre Horan, researcher at Leprince-Ringuet laboratory for particle physics and astrophysics /École Polytechnique and CNRS, and Reshmi Mukherjee, research professor at Barnard College of Columbia University, to meet when they both worked in research groups involved in the state-of-the-art VERITAS  ground-based imaging Cherenkov Telescopes in Arizona, in the United States. 

Both investigating the nature of the high-energy emission from the relativistic jets of blazars, the two researchers harbored the wish to work together even if their respective career paths took them to different continents.

The Visiting Professorship Program offered by the international Alliance network, including École Polytechnique and Columbia University, has provided the framework for their research collaboration. It enabled Reshmi Mukherjee to spend almost eight months as a long-term Alliance Visiting Professor at the Leprince-Ringuet Laboratory for Particle Physics and Astrophysics (LLR)  at École Polytechnique in 2013, and since then, she and Deirdre Horan have continued to work on joint projects and to co-mentor PhD students.

From Arizona to Palaiseau

After working on the Whipple and the VERITAS experiments in the United States as part of her fellowships at the Harvard Smithsonian Center for Astrophysics and the Argonne National Laboratory, Deirdre Horan joined the CNRS and the Gamma-ray Research Group of LLR/École Polytechnique to work on the Fermi Gamma-ray Space Observatory shortly after its launch in June 2008. 

A joint venture between NASA, the US Department of Energy, and government agencies in France, Germany, Italy, Japan, and Sweden, the satellite carries an all-sky telescope, the Large Area Telescope (LAT), and a gamma-ray burst monitor that allows it to observe astrophysical and cosmological phenomena including active galactic nuclei. 

“As we study blazars, a subclass of active galactic nuclei with relativistic jets oriented such that they point towards Earths, Reshmi and I focus on their gamma-ray emission to learn more about the properties of the jets and their origins and to try to understand what causes the flares that we often detect from these objects”, explains Deirdre Horan. 

Located in space, in low Earth orbit, the LAT collects data that complement those that can be collected by ground-based telescopes, such as the Very Energetic Radiation Imaging Telescope Array System (VERITAS), on which Reshmi Mukherjee primarily works. 

“The key to understanding these flares is to look at different wavelengths of the electromagnetic spectrum at the same time", Deidre Horan says, elaborating: "Since the Fermi LAT has a very large field of view, it can provide alerts to the community whenever a blazar is in a flaring state. Instruments across the spectrum, including VERITAS, can then slew to observe these sources thus allowing us to gather data from across the entire electromagnetic spectrum and carry out multiwavelength studies to try to understand the properties and processes of these galaxies".

In this way, Deirdre Horan and Reshmi Mukherjee are incorporating data from radio through optical and X-rays, all the way up to the highest energy gamma rays, from colleagues around the world in their research. Their aim is to understand not only the flares, but also the quiescent states when the blazars are not in an elevated emission state, and how these relate to the flares. 

Since they started working on joint projects, Deirdre Horan and Reshmi Mukherjee have also opened their collaboration to three colleagues PhD students they supervised at École Polytechnique and Columbia University, who in turn benefited from the support of the Alliance. Furthermore, they co-authored several articles, which have been published in scientific journals.

High energy mysteries to discover 

During Reshmi Mukherjee’s recent visit in October 2024, she and Deirdre Horan started “exploring the connection between blazars and neutrino sources, in particular with the high energy neutrinos reported by the IceCube experiment in the South Pole”, declares Reshmi Mukherjee. 

As there are still "many open questions in blazar research, such as the location of the gamma-ray emitting region, the origin of fast flares in these objects and the physics of particle acceleration", the two scientists don't risk running out of secrets to discover any time soon.

Given that “neutrinos are elementary particles that lack an electric charge and are produced in cosmic ray interactions”, studying the recently discovered high energy neutrinos, their origin, and their connection to blazars “could be the subject of future collaborations for Deirdre and me”, reveals Reshmi Mukherjee.

“The origin of the highest energy cosmic rays remains a mystery and the cosmic rays at the highest possible energies can only be produced in sources outside our own Milky Way Galaxy”, making the high-powered collimated jets, which emanate from blazars, a potential site of these ultrahigh-energy cosmic rays, Reshmi Mukherjee explains.

Immersion in the research environment at l’X

During Reshmi Mukherjee’s visit to École Polytechnique’s campus, she and Deirdre have worked in neighboring offices, enabling them to “work more intensely together, to attend workshops and potentially even expand the collaboration to other colleagues”, relates Deirdre Horan. “Having Reshmi on campus for two months helped enormously to solidify our collaboration”, she declares and adds that “this way Reshmi has also come to know other colleagues who investigate other subjects she is interested in such as polarimetry for example. The presence as a Visiting Professor has helped to reinforce the connection between our group and Columbia.”  

Of course, “a two-month Alliance Professorship is not long enough to complete a full project, but we have initiated a new chapter in our research collaboration which we can carry out in our home institutions”, says Reshmi Mukherjee. She emphasizes that the Alliance program has allowed her “not only to form a productive and invigorating scientific collaboration with Deirdre Horan’s group, but has offered an opportunity to explore French art and culture and way of life”, and she particularly enjoyed “talking to the MSc&T and PhD students, learning about their research projects, exchanging ideas with scientists in the lab on topics ranging from MeV gamma-ray detectors, the Fermi-LAT mission and advanced analysis techniques.”

Alliance Visiting Professorship

Each year, the Alliance network offers faculty members from its partner institutions - Columbia University, École Polytechnique, Sciences Po, and Paris 1 Panthéon-Sorbonne University - the opportunity to participate in a transatlantic faculty exchange, strengthening ties between the institutions and establishing collaborations on potential projects. Faculty members can spend a few weeks or an entire semester as visiting professors at the partner institution. During their exchange, participants can pursue their research, participate in seminars, or give lectures.
 

Deirdre Horan

Deirdre Horan's interest in astrophysics may well have been sparked by the clear, star-studded night skies of her native Ireland. After completing her Bachelor's degree in Experimental Physics at University College Dublin with 1st Class Honours, Deirdre Horan undertook a PhD which saw her spending most of her time as a pre-doctoral fellow at the Harvard-Smithsonian Center for Astrophysics in Arizona, USA.

Upon completion of her PhD, Deirdre ran the Whipple 10m telescope as a blazar monitor whilst VERITAS was being commissioned. Her research focused on blazars and gamma-ray bursts, and she soon became deputy leader of the VERITAS Blazar Science Working Group, in which she has remained very active during her subsequent fellowship at Argonne National Laboratory in Illinois. In 2008, Deirdre Horan joined the CNRS and the Leprince-Ringuet Laboratory for Particle Physics and Astrophysics (LLR) at École Polytechnique. 

Since 2006, Deirdre Horan, together with Scott Wakely at the University of Chicago runs TeVCat, a catalogue for TeV Gamma-ray Astronomy, which is accessible online and provides a unique and helpful tool for peer researchers' work.

Reshmi Mukherjee

Reshmi Mukherjee studied physics and mathematics at the Presidency University in Calcutta, India, and continued her graduate studies at Columbia University in New York, where she received her Ph.D. for work on the development of liquid xenon (LXe) time projection chambers (TPCs), which are now widely used as dark matter detectors in underground laboratories. 

Soon after completing her PhD, Reshmi Mukherjee joined the EGRET mission at NASA's Compton Gamma-Ray Observatory, where she explored the gamma-ray sky and worked on the discovery of new active galactic nuclei at gamma-ray energies. During her subsequent position as a visiting scientist at McGill University, she began working on the development of ground-based atmospheric Cherenkov telescopes, leading her to the VERITAS ground-based imaging Cherenkov telescope experiment in Green Valley, Arizona, where she met Deirdre Horan. 

Today, Reshmi Mukherjee is a member of the faculty at Barnard College of Columbia University, where she teaches introductory physics courses, supervises undergraduate research, and conducts research with her team as a principal investigator exploring the frontiers of VHE astrophysics.