Alessandro De Angelis on High-Energy Gammy Ray Astronomy

Scientist Interview: November 2011

Alessandro De Angelis According to a recent analysis of Essential Science IndicatorsSM data from Thomson Reuters, the work of Professor Alessandro De Angelis entered the top 1% of researchers publishing in the field of Space Science over the past decade with the highest citation count. His current record in this field includes 128 papers cited 2,939 times between January 1, 2001 and June 30, 2011.

De Angelis is currently professor of physics at the faculty of Sciences of the University of Udine, Italy. He is Deputy Chair of the managing board of the MAGIC gamma-ray telescope in La Palma (Canary Islands), and member of the collaboration managing the Fermi gamma-ray satellite). His main interests are high-energy particle astrophysics and fundamental physics with accelerators.


SW: Your highly cited research in space science is dominated by high-energy gamma-ray astronomy. However, in the first part of your career you were at CERN, Geneva, where you worked on software development for the electron-positron collider.

I studied physics at the University of Padova, where one of my professors gave me the opportunity to visit CERN. I really liked the place so much I felt I should aim for particle physics. Thus in the early 1980s I became a particle physicist and spent some years at CERN as an experimentalist whose principal skills were in computation and data analysis.

SW: What decided you to part company with CERN?

I obtained a position at CERN at a time when they were compelled to focus on the starting development of the Large Hadron Collider (LHC). The LHC was not much fun for me personally because my future role would inevitably have been in management and bureaucracy. So in 2000 when I got an offer of a research professorship in Italy I left CERN to work in high-energy astrophysics.

SW: Although particle physics and cosmology have a lot in common, you must have found the transition involved a steep learning curve.

Particle physics is actually a good grounding for high-energy astrophysics, and in any case I was strongly motivated by what astronomers were discovering. The highly technical "know how" that I had acquired at CERN was valuable when I applied it to high-energy astrophysics.

SW: You became an astrophysicist at an exciting time for the future of gamma-ray astronomy.

"…what's exciting is that the particle energies out there in the distant universe are far greater than anything we can achieve in an accelerator."

Immediately after my transition I spent a couple of years learning the field, so that was not productive. Initially my group and I contributed technical work to NASA's Fermi project, a gamma-ray detector in space, and also to MAGIC, the large atmospheric imaging Cherenkov telescope. When I started in gamma-ray astronomy with satellites, its practitioners were not using in-depth computer modeling, and they were not using silicon cells as detectors; however these were quite standard techniques at CERN.

In 2000 there were only five or six sources known with emissions above 100 GeV, and a couple of hundred below that, so 100 GeV was then the threshold of what was possible. We knew about the technology that could improve the detection sensitivity by about two orders of magnitude, and that's what we started doing.

SW: What shows up clearly in our analysis of your high-impact papers is that you spent the years 2000-2005 learning the field, and then from 2006 you are bringing in a harvest of highly cited papers.

In 2004–2005 MAGIC started working, and then in 2008 we had the launch of the Fermi mission with its Large Area Telescope. I was rather lucky to become the scientific coordinator of the MAGIC telescope, one of the three experiments which increased by a factor of 10–15 the number of known gamma ray sources.

SW: Several of your papers report important discoveries you have made with MAGIC. What has been the impact of these discoveries on astrophysics?

I should first explain that MAGIC is a system of two optical telescopes for gamma-ray astronomy. Each mirror is 17 m in diameter. When a gamma ray hits the Earth's atmosphere it generates a flash of Cherenkov radiation with a duration of 3 ns. The telescope is at an altitude of 2,000 m on La Palma in the Canary Islands, where the conditions are very good for detecting these brief flashes. MAGIC is the largest optical surface in the world.

MAGIC is particularly good for discovering active galactic nuclei, that is to say the supermassive black holes of 106 solar masses or more in the centers of galaxies often far away from us. It is the leading experiment for this. The impact of this detection of very remote galaxies is important because these gamma ray sources are highly variable. That variability is important from the point of view of modeling the origin of the gamma rays and for their propagation through the universe. The MAGIC Collaboration has discovered more than a dozen of these objects, and we continue to discover extragalactic sources at the rate of about one a month.

SW: In addition to studying gamma ray sources at cosmological distances, how has MAGIC enabled you to make important discoveries in our Galaxy?

We found that the pulsar in the Crab Nebula emits pulses at high gamma ray energies, which was completely unexpected. Another recent example is the morphology of the emission of gamma rays from a supernova remnant. We are now convinced that supernova remnants are the sites of acceleration of particles up to 1 PeV (1015 eV). We can image these sources to see exactly where these high-energy particles are produced.

SW: The papers from your collaboration dealing with the Fermi gamma-ray space observatory are commanding a huge following. Why is that?

Fermi is a superior tool to study extreme phenomena and it brings together the astrophysics and particle physics community: it is able to study giant gravitational collapses and subatomic particles at energies far greater than those produced in ground-based particle accelerators. And it gives cosmologists new information on the early evolution of the Universe. It satisfies the needs of many customers.

SW: What you are saying in effect is that the papers have large citations because the Fermi Large Area Telescope is so successful. To what extent are the source list (De Angelis A, et al., "FERMI/LARGE AREA TELESCOPE BRIGHT GAMMA-RAY SOURCE LIST," Astrophys. J. Suppl. Ser. 183[1]: 46-66, July 2009), and the catalog (De Angelis A, et al., "FERMI LARGE AREA TELESCOPE FIRST SOURCE CATALOG," Astrophys. J. Suppl. Ser. 188[2]: 405-36, June 2010) simply a record of "here is what we can see?"

"Fermi is a superior tool to study extreme phenomena and it brings together the astrophysics and particle physics community: it is able to study giant gravitational collapses and subatomic particles at energies far greater than those produced in ground-based particle accelerators."

First of all, one has to stress the fact that the increase in the source count was more than tenfold. But the source list is much more than just a phonebook of source positions. These papers present initial results for characteristics of the emission above 100 MeV for the most significant gamma-ray sources in the Fermi data.

SW: How does the source list relate to the catalog?

The Fermi bright source list is a list of the most significant sources based on the first three months of survey data. This list was eventually superseded by the first catalog, when after one year of data taking the sensitivity of the telescope was better known. The source catalog gives characteristics of the emission of each source, such as spectral energy distribution. The catalog is fundamental because it has a lot of information on source properties, such as variability, and it tells us via extrapolation the sources that are likely to be emitting at the highest energies. It will take years to analyze fully the sources in this catalog, which is a reference on cosmic gamma ray sources, and is subject to update: a second catalog just came out.

SW: So are the list and the catalog just the first step in doing science?

Yes, the very important first step. And although the catalog gets the most citations, the research papers can be somehow more fundamental.

SW: There are several papers about gamma-ray bursts. What's special about Fermi and these bursts?

Fermi is able to react fast to unexpected triggers like gamma ray bursts (GRBs) or sharp changes in source variability. It does that much better than its predecessors in that respect. Our interest in GRBs is twofold. From the point of view of astrophysics Fermi can reach GeV energies and that allows the testing of emission mechanisms. There is now a good model for the long duration (long meaning 10 seconds!) GRBs that is based on the implosion of a massive supernova.

SW: The papers indicate that in addition to learning more about the astrophysics of GRBs, Fermi is a tool for exploring fundamental physics.

Now consider the fundamental physics: the fact that GRB sources are extremely distant allows us to study the propagation of photons at different energies over distances of billions of light years. So over distances comparable to the radius of the observable universe we can investigate how photons interact with the vacuum.

In physics my main interest is the transparency of the universe and the interaction of high-energy photons with the vacuum. When we started observing GRBs and galaxies far away we detected a handful of sources further away than expected. Our horizon is limited because the highest energy gamma rays interact with optical and infrared photons emitted by galaxies; we needed to improve the way we model this interaction to explain a transparency larger than initially expected.

SW: Do you have ideas about what's going on?

Our discovery raises the fundamental question of whether or not gamma rays are interacting with the vacuum energy. The level of interaction is very low but it raises the issue of new—that is, undiscovered—elementary particles. So I'm talking of seeing effects of the interaction of particles and photons on a cosmological scale. And what's exciting is that the particle energies out there in the distant universe are far greater than anything we can achieve in an accelerator.End

Alessandro De Angelis
Dipartimento di Fisica
Università di Udine
Istituto Nazionale di Fisica Nucleare (INFN)
Gruppo Collegato di Udine
Udine, Italy


Atwood WB, et al., "THE LARGE AREA TELESCOPE ON THE FERMI GAMMA-RAY SPACE TELESCOPE MISSION," Astrophys. J. 697(2): 1071-102, 1 June 2009. 259 cites. Source: Essential Science Indicators from Clarivate Analytics.



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