Signals Every Second – Astrophysicists Develop Telescope Alarm System for Next-Generation Gamma-Ray Observatory CTA

It is a step into a new dimension. The search for time-dependent phenomena in space is in full swing and, with it, the search for cosmic sources of high-energy gamma radiation. Researchers have already identified exploding stars, black holes, and pulsars. But they are sure that there are many more. The universe is full of extreme particle accelerators producing gamma rays and other phenomena. However, researchers still know little about how exactly the processes by which charged particles are highly energized work and what the time frame for them is. But help is on the way: A new observatory, the Cherenkov Telescope Array (CTA), is being prepared by a number of universities and research institutions, including the University of Potsdam.

They are not visible to the naked eye: energetic particles raining down on Earth. But they are there, millions of them, everywhere, in the Milky Way and beyond. Among other things, they become visible by gamma rays that can actually be measured by telescopes, albeit only indirectly, since gamma rays cannot penetrate our planet’s atmosphere. But when they hit the earth’s atmosphere and collide with atoms and molecules, they produce particle showers that can be investigated using certain detectors. In the future, CTA will be one such detector. “The observatory will allow us to explore the cosmos with unprecedented precision,” explains Dr. Kathrin Egberts, head of the “Experimental Astroparticle Physics” team at the University of Potsdam. Her team is involved in the large-scale project bringing together some 1,400 researchers and engineers in 30 countries. In the coming years, more than 100 telescopes of three different types will be erected and in operation for at least two decades on Cerro Paranal (Chile) and the Canary Island of La Palma. “The first of these in the northern hemisphere has been built,” Egberts reports. Together with her team, she has long concentrated on the HESS experiment for research purposes (see box on page 43). “The technologies developed here can now be used for CTA,” she explains. Their special expertise in developing automated warning systems for telescopes is very much needed for CTA, too. The main focus is on the criteria according to which the telescopes automatically switch to other parts of the cosmos where very scientifically interesting events are taking place.

Proven technology becomes more flexible

“The alarm system we ‘built’ for HESS is a good basis for the new project,” explains Egberts’ colleague Clemens Hoischen, “even though it cannot be transferred one to one.” The challenge is that CTA is an observatory and not purely experimental like HESS. A system for CTA, therefore, requires a higher level of professionalism, including easier configurability and higher flexibility. Aspects such as monitoring the stable operation of the autonomously acting system as well as an environment in which alarm responses can be simulated need to be expanded. This is a major challenge. “We are currently in the design stage, which is a critical one. The focus is mainly on architectures,” Hoischen explains.

Just like its predecessor, the new system will be able to receive, process, and send alarms. Incoming parameters include positioning data of a current event, but also information on measurement parameters of the delivering telescopes. These form the basis for the decision whether to switch to the respective phenomenon. To ensure that this can be done, a processing pipeline will be set up which will play a central role in the entire technology. Inside the pipeline, very complex process steps are carried out: alarm prioritizing, alignment with celestial maps, and calculation of optimum observation positions. Next, the alarm reaches the central data acquisition system, which will then manage the observation.

That’s a lot of theory, and before it can be applied in practice, the researchers have to do a lot of detailed work. CTA key systems need to be combined and basic criteria for the research program defined. “This is what we are doing now,” Hoischen describes the situation. What other components does the alarm system need to be connected with? Which alarms should be reacted to and under what conditions? And what happens if several alarms come in at once? These are some of the questions that need to be clarified, yet it is quite clear what the CTA alarm system is able to pass on to the experiments connected with it: everything discovered by its own telescopes. The systems searching other wavelength ranges could then focus on them. “Another possibility is to communicate ‘findings’. You quote the original alarm and the result,” Hoischen explains. “The community is already doing this, and it facilitates each other’s decision whether to point the telescope to the event.”

Meanwhile, Hoischen is pushing to speed things up. “We must finish the planning stage now and design a prototype system,” he says. “It should demonstrate that it can do all that CTA requires.” But important interfaces have yet to be sufficiently defined, and responsibilities need to be made clear. Ultimately, a handover process will determine whether the Potsdam system will be adopted.

Alarm systems on the rise in astrophysics

Alarm systems are not new in the world of observational astronomy. However, experts see a rising demand for them in astrophysical research. The systems are capable of receiving and sending thousands of signals a night in rapid succession which requires efficient selection and processing. Events were also filtered in the past, but filtering has improved with new technologies offering more differentiated solutions. Even today, a large global community uses warning systems to follow phenomena in the shortest possible time. The existence of alarm systems is due to a simple fact: The fields of vision of HESS – and the planned CTA telescopes, too – are limited to the size of ten full moons on average. Therefore, they depend on information from wide-angle instruments capable of capturing a larger section of the sky to observe events they would otherwise miss.

HESS heightens expectations of CTA

Major hopes are resting on the Cherenkov Telescope Array. Comprising many more than the 5 telescopes used at HESS and distributed across both sides of the globe, CTA could actually become a game changer in space research. After all, the comparatively small HESS, with its integrated alarm system, has demonstrated what observations modern systems are now capable of. For instance, its gamma telescopes reacted quickly when alarms indicated the upcoming merger of two neutron stars in 2017. In fact, HESS was the first ground-based instrument to have the decisive spot at the sky in its field of view, yet without discovering a gamma-ray source. Still, it was a key element in what was probably the biggest observation campaign in modern astronomy. Soon after, the detection of a neutrino from the direction of a known gamma-ray source attracted similar attention in the community. Again, HESS received an alarm – and observed the object for weeks, like all Cherenkov telescopes did, to study its behavior. “The results of the worldwide research found their way into many publications and events, which testifies to the strong interest in observations in the time domain,” Hoischen underlines. “Systems like the one we are developing here in Potsdam open the doors to these kinds of observations.”


The High Energy Stereoscopic System (HESS) is one of three key projects in gamma-ray astronomy today, the other two being MAGIC and VERITAS. It went into operation in 2002 and is located in Namibia. Altogether, HESS consists of five Cherenkov telescopes measuring cosmic gamma radiation in the range of 30 GeV and 100 TeV. The major result of the three experiments is the demonstration of a multitude of different gamma-ray sources. So far, over 200 have been detected, but researchers are sure that this is just the tip of the iceberg. In order to discover significantly weaker sources and study them in detail, CTA is needed, for which experts from all three experiments have joined forces.

The Researcher

Dr. Kathrin Egberts studied physics at the University of Heidelberg, where she also earned her PhD in 2009. She has been head of the working group “Experimental Astroparticle Physics” at the University of Potsdam since 2013.

E-Mail: <link>kathrin.egberts@uni-potsdam.de

Dr. Clemens Hoischen studied physics at RWTH Aachen and obtained his doctorate from the University of Potsdam in 2018. He is a research assistant in the working group “Experimental Astroparticle Physics”.

E-Mail: <link>clemens.hoischen@uni-potsdam.de

Gamma-ray astronomy focuses on high-energy processes in nature. The unit of measurement is electron volt (eV). Sources in space that can be made visible by gamma radiation can accelerate particles much faster than, for instance, the largest particle accelerator on Earth, the Large Hadron Collider at the European Acceleration Center (CERN) near Geneva. Here, protons reach a maximum of 6.5 TeV, whereas researchers using modern instruments measure 100 TeV or higher for particles reaching the earth’s atmosphere. So the key question is: How does nature do it? Gamma-ray astronomy will help find answers.

CTA is the acronym for Cherenkov Telescope Array, an observatory for gamma-ray astronomy. Its methodology is based on the fact that gamma rays produce particle showers when entering the earth’s atmosphere. These particle showers emit Cherenkov radiation, i.e. short flashes of light measured by telescopes. The obtained data are used to determine the energy as well as the direction of the incoming gamma-ray particles. Because the electrically neutral rays are not deflected by a cosmic magnet field, the direction from which they arrive leads directly to their source. For the next two decades, the CTA observatory will be the most important observation instrument for very high energy gamma-ray astronomy. Its planning and construction are supervised by a major international consortium. A central role in the planned observatory will be played by the Zeuthen-based DESY (Deutsches Elektronen-Synchrotron) research center, with which the University of Potsdam cooperates and is connected via a number of joint appointments. In the future, part of the project will be coordinated at its Science Data Management Centre.

The Project

Development of an Alarm System for CTA
Duration: 2017–2020
Funding: Federal Ministry of Education and Research (BMBF)

Text: Petra Görlich
Translation: Monika Wilke
Published online by: Alina Grünky
Contact to the online editorial office: <link>onlineredaktion@uni-potsdam.de

Published