Extreme and powerful objects in the Universe, such as supermassive black holes and supernovae, emit gamma rays. The Cherenkov Telescope Array Observatory (CTAO) will become the world’s most powerful ground-based observatory in high-energy gamma-ray astronomy. Its Co-Spokesperson Rene Ong, who visited FZU this spring, described the scientific high-energy odyssey.
While you were at FZU in Prague and Olomouc, we talked a lot about Prof. James W. Cronin. He won the Nobel Prize in Physics for discovering CP (Charge conjugation Parity) symmetry breaking and was founder of the Pierre Auger Observatory, who influenced a lot also the birth of astroparticle physics in the Czech Republic. How did your collaboration with Prof. James Cronin at the University of Chicago affect your early scientific career?
In the 1980s, several scientific papers presented intriguing results, suggesting high-energy gamma rays from the X-ray binary Cygnus X3. Although these findings were likely incorrect, they inspired Professor James Cronin at the University of Chicago to develop the Chicago Air Shower Array (CASA) in Utah to investigate ultra-high energy gamma rays. CASA operated in the 1990s with an energy threshold of 50 to 100 TeV. This is above the energy for atmospheric Cherenkov telescopes, but this is where previous experiments in the 1980s saw evidence from X-ray binary systems like Cygnus X3.
I was a postdoctoral fellow at the Enrico Fermi Institute of the University of Chicago. There were different projects at Fermilab like The Collider Detector at Fermilab (CDF) or the CP violation experiments. But there were also all these wonderful and different experiments related to astrophysics which looked for something which nobody knew existed -- it was the start of particle astrophysics or astroparticle physics. I had the unique opportunity to work on something new with James Cronin, a wonderful person and a great scientist. We worked together for almost 10 years on CASA and the early developments of the Auger project. It was a lucky combination of events and the timing and the excitement of building something new.
The CASA detector did not find evidence of gamma ray sources from Cygnus X3 or the Crab nebula. It wasn't sensitive enough to detect them. If we had carried out a significant upgrade, we might have detected sources, but we moved on to other projects. In the 1990s, I was fascinated by experiments that detected gamma rays in the TeV energy range using the atmospheric Cherenkov technique pioneered by Trevor Weeks and his team at the Fred Lawrence Whipple Observatory in southern Arizona, and so I decided to switch to a new energy region using a different technique
Your first lecture at FZU concerned the high energy photons coming from the Universe to the Earth. There are different types of sources of these particles identified in the sky, but the most intriguing ones are the sources, where the type is still classified as “unknown”. What do we do know about them?
Essentially, all of those unknown sources are congregated in the Galactic plane. We can thus be almost certain that they are Galactic sources because they come from the plane.. We know many types of Galactic sources now -- the most common type are Pulsar Wind Nebulae (PWNe), in which relativistic winds are accelerated from the spinning neutron stars within a pulsar. PWNe are the largest number of TeV sources and so probably a significant fraction of these unknowns are PWNe that have not been identified by other wavelengths, so they remain unknown – put they are probably not a completely new category. Some of the unknown sources can be possibly supernova remnants, which are the shells of material expelled during a supernova explosion. High energy particles are commonly produced in supernova remnants , but they could also be created by winds of outflow from different objects like microquasars and binary systems. We hope there are new source categories, but we cannot identify them with the angular resolution of current instruments.
Is this the motivation for building the new observatory, CTAO?
CTAO with greater sensitivity and greater field of view will be able to do surveys of the Galactic plane much better than the previous instruments (H.E.S.S., MAGIC and VERITAS). The CTAO will detect on the order of several hundred or more new sources in our Galaxy and also identify them with an individual photon precision of a few arcminutes instead of 0.1–0.2 of a degree. So every photon will be detected with better angular resolution and we will know the positions of the sources better to around 5–10 arcseconds for stronger sources. The combination of the better sensitivity, more sources and better angular resolution will be a major factor in allowing CTAO to pin down a large number of these unidentified sources. On the other hand, it will almost certainly see new unidentified sources, at lower fluxes than we were able to detect earlier, so the mystery will continue.
Separately, CTAO will do a rich astrophysics program outside of the Galactic plane, including looking at extra-galactic objects like quasars, , radio galaxies, starburst galaxies, gamma-ray bursts and gravitational-wave and neutrino follow-ups. There will actually be an extra-galactic survey as well. CTAO will survey 1/4 of the overhead sky, about 10,000 square degrees, which could very well lead to something totally new.
Cookies necessary for playing this content, hosted by a third party, are not enabled in your browser. By viewing the external content, you agree to the Terms of Service of the service provider, YouTube.
You are very strongly involved also in the new project called General AntiParticle Spectrometer which is searching for antideuterons and antinuclei. How is it connected with CTAO?
We don't know what dark matter is, so we need different ways to look for it. CTAO and the General AntiParticle Spectrometer (GAPS) are looking for a certain type of dark matter called the Weakly Interacting Massive Particle (WIMP), which is an important possible candidate source for dark matter. It could explain some mysterious aspects of the Standard Model of particle physics. Both CTAO and GAPS employ an indirect detection technique to identify WIMPs. Annihilation of WIMPs with other WIMPs results in the creation of Standard Model particles. These particles subsequently generate unique signatures that can be detected. For CTAO this signature is gamma rays coming from locations in the Galaxy or outside where there is high concentration of dark matter, with the most favourable location being near the Galactic Centre. CTAO can uniquely probe a set of dark matter models which are not easily testable by other experiments using the direct detection technique or even LHC. These are typically relatively high mass models which have not been explored at all and there could very well be a major discovery. GAPS is looking for primary antimatter which is not produced from secondary cosmic interactions in our Galaxy. If GAPS detects any signal for antideuterons, WIMP annihilation is a very likely source. Although GAPS can see the general flux of antimatter above Earth's atmosphere, it cannot determine the origin of these antideuterons since they are charged particles and lack directional information.
Do we need CTAO to know where dark matter comes from?
CTAO would show where dark matter would be concentrated, but GAPS could provide evidence for it, maybe even before CTAO. A GAPS detection would argue strongly that the WIMP existed, that the indirect detection technique is valid and would highly motivate CTAO, for example, to actually pinpoint where the dark matter was coming from. The mass scales of the detection techniques are somewhat different, but very complementary and the two experiments are searching for the same particle. GAPS is generally a lower mass experiment, CTAO could still detect lower masses, but CTAO has unique capabilities at the very highest masses.
What are the reasons for building CTAO in both hemispheres?
CTAO aims to be built in two hemispheres, to be able to have full sky coverage and especially to be sensitive in the direction of the Galactic Centre, which is in the south. To improve gamma-ray capture and achieve a comprehensive view of the night sky, the CTAO will deploy over 60 telescopes across two sites: CTAO-North on La Palma in the Canary Islands, and CTAO-South in the Atacama Desert, northern Chile. The southern site is larger and has all three types of telescopes to allow us to span the whole energy range right up to the highest energies. The reason to reach the highest energies is that the southern site has better visibility into the central regions of the Galactic plane, what we call the inner galaxy, where we believe the most energetic accelerators exist.
The CTAO's southern hemisphere array in Chile will cover medium to high gamma-ray energies (100 GeV to 300 TeV), while the northern hemisphere array will focus on low to mid energies (20 GeV to 10 TeV), specializing in extragalactic physics. Consequently, the northern site will not include Small-Sized Telescopes (SSTs) designed for capturing the highest-energy gamma rays.
The northern hemisphere site can see the Galactic Centre, but not as well as in the south. So at the southern site, CTAO will definitely be able to image all of the central regions of the Galactic plane. Much of the Galactic plane survey will be done in the central regions from the southern site. The real wealth of Galactic sources will be observed by CTAO-South, and the small telescopes will allow us to go to the very highest energies, which is important because we want to find the origin of the PeV cosmic ray particles, the so-called PeVatrons.
And then as part of that survey, there's a dedicated observation programme to look very deeply into the Galactic Centre region, not only to understand the astrophysics of the Galactic Centre, but also to look for the possible signature of dark matter annihilation.
The Galactic Centre, a powerful source of TeV gamma rays, cannot be included in the search for dark matter because of its extensive astrophysical production. But when you get a degree or 2° off the Galactic plane, the production of astrophysically produced gamma rays drops off dramatically and we can use that region, which we call the dark matter halo of our Galaxy, to look for dark matter annihilation.
What is the status of the of the CTAO construction projects at the northern and the southern hemispheres?
For CTAO in general, the project is moving along well in terms of the design of the detectors, the telescopes themselves, the cameras, the electronics, and the software. The science case is very strong, and the project is well funded to building the first arrays on both sites at CTAO-North and CTAO-South. The northern site has progressed more rapidly and there will be very likely within the next two years four of the large size telescopes, the 23-metre diameter telescopes, completed and starting initial operations. Sometime after that the medium sized telescopes, the 12-metre diameter telescopes will start arriving at the northern site. So we very much expect that around 2026 to 2027 the northern site will be making initial observations for CTAO, taking initial commissioning data and calibration data, and then eventually turning into initial science data, which will be a very exciting time.
In the South, there are no telescopes installed yet, but later this year there will be a major increase in the funding for the construction of the south and a fair bit of the infrastructure will start to be developed. And the agreements are all in place to start installing telescopes in the south over the next few years as well. The hope is again on a time scale of three to four years to have some of the telescopes there to start some initial operations. But again, the southern site will probably take longer to reach its full capacity than the northern site.
So you believe that at least some of the questions related to high energy photons can be answered before 2030?
Yes, definitely.
How have the contributions of Czech groups impacted the development and implementation of the CTAO project?
For over a decade, I have collaborated with colleagues from FZU on various aspects of the CTAO project, including site selection and atmospheric monitoring. Czech groups have made significant contributions to CTAO, most notably in developing a candidate SST for the southern site. Two prototype telescopes are located outside Prague, and our work with these has been crucial for advancing the SST project overall. Additionally, Czech teams have played a key role in atmospheric monitoring, using various devices to measure the transmission of the atmosphere, starlight through all-sky cameras, and cloud presence. These detectors have been tested at candidate sites and are now installed at both CTAO North and South. The first official CTAO installations at both sites largely feature Czech equipment, alongside contributions from other institutions. Their pioneering efforts in establishing on-site communication and installation have been invaluable. I have had the chance to visit both sites and work with the detectors. In 2019, I was in Chile to refurbish the atmospheric monitoring station, which was a rewarding experience working in the field.
Professor Rene A. Ong is a of the Cherenkov Telescope Array Consortium and also holds a joint appointment in the Department of Physics and Astronomy at the University of California, Los Angeles. His research interests include astroparticle physics and high-energy astrophysics.