Cosmic Drama

Gamma-ray bursts are caused by gigantic explosions in the depths of space. Astrophysicists from the University of Tübingen have developed the technology on board a nanosatellite that is being dispatched to precisely locate them.

Artist's Impression of gamma-rays. — Gamma-rays are caused by gigantic explosions in space – releasing more energy in seconds than our sun does throughout its entire lifespan. (Artist's Impression)

Gamma-ray bursts are among the most cataclysmic events in the universe, reaching Earth from distances of millions, often even billions, of lightyears away. They can last from a few milliseconds to several minutes and are caused by the collapse of massive stars or the collision of compact astrophysical objects such as neutron stars or black holes. While the stars merge into a black hole in their death struggle, they emit streams of matter, known as jets that accelerate particles of matter to almost the speed of light. In the process, an extremely bright flash of gamma radiation is produced – similar to X-rays – releasing more energy in seconds than our sun does throughout its entire lifespan.

Additionally, the collapse of a single massive star at the end of its life to directly form a black hole leads to an enormous conflagration, known as a supernova or hypernova, whereby again a similar bright flash of radiation is produced. Since US satellites first detected gamma-ray bursts in 1967, they have been intensively studied. However, many questions still remain unanswered. What exactly happens when stars collapse? How often are gamma rays emitted during such events? Do all jets have the same structure or do they vary from case to case? Gamma-ray bursts cannot be observed from the Earth’s surface as they are absorbed in its atmosphere. Therefore, space telescopes on board satellites play a crucial role in research. A team from the Institute for Astronomy and Astrophysics at the University of Tübingen has now provided important technology for this research.

Picture of the miniature detector, the heart of the Hermes nanosatellites. — Astrophysicists from Tübingen were involved in developing the miniature detector, the heart of the Hermes nanosatellites.

Shoebox satellite

Researchers in Tübingen have developed the technology behind a new satellite-based mission called HERMES (High Energy Rapid Modular Ensemble of Satellites) that will capture gamma-ray bursts and help to localize their sources more precisely. This involves a minicomputer for a system of crystal and silicon detectors. It detects gamma rays and transmits the data via telecommunication satellites to Earth for evaluation.

Alongside scientific research, astrophysicists at the University of Tübingen develop their own technology in-house, which is remarkable for a university. The HERMES detector system is onboard a nanosatellite called SpIRIT (Space Industry Responsive Intelligent Thermal): Not much larger than a shoebox, it weighs 11.5 kilos, and was developed at the University of Melbourne – in collaboration with the Italian Space Agency (ASI) and the Italian National Institute for Astrophysics (INAF).

On December 2, 2023, the miniature satellite launched aboard a two-stage rocket from a base in California and entered orbit 513 kilometers above the Earth. The SpIRIT nanosatellite with the HERMES detector on board is the first of seven planned nanosatellites with the same detector system that will be flown over the next year, forming the HERMES Scientific Pathfinder Constellation.

The University of Tübingen is a member of the consortium led by INAF and the Italian Space Agency. “We followed the satellite launch via a livestream and kept our fingers crossed,” recounts astrophysicist Dr. Alejandro Guzmán, a member of the six-person development team from Tübingen. “It was a tense moment. Years of work have gone into the satellite and if something goes wrong, you can’t just stop it to repair it.”

But they didn’t have to wait long for confirmation that the satellite was in the correct orbit. The ground station in Australia and amateur networks worldwide soon reported that the signal transmission was working. Currently, SpIRIT is in a commissioning phase due to last several weeks, which is intended to demonstrate whether the satellite and the research technology on board function flawlessly in space. After that, the research mission will begin.

Ride-sharing into space

Nanosatellites can have an important advantage over conventional satellites: They can be manufactured more cost-effectively because the developers – as in the Tübingen project – can use off-the-shelf components. Sending nanosatellites into space is also cheaper, as ride-sharing opportunities are used, avoiding rocket launches for each mission. Universities generally do not have the financial means of space agencies and this saving means that they can also make important contributions to research.

Another plus point of the nanosatellite project is its speed, says Guzmán: “In less than five years, we have designed, built, and launched an entirely new miniaturized detector. For doctoral students, it is very attractive to collaborate on such a project that they can accompany up to the launch.” Development and manufacturing large satellites, as is common with ESA or NASA, take significantly longer.

Once SpIRIT has completed its testing phase and proven the capability of the HERMES network, the seven nanosatellites will commence their work. Together, they will scan the sky for gamma-ray bursts and attempt to locate radiation sources more precisely. Detectors on the satellites register the same burst at different times due to their different positions in orbit, which is valuable information for the astrophysicists. Based on this difference, they can calculate where in space the collapsing stars and black holes emitting these bursts are located.

Satellites in space. — The satellites were successfully launched into space and are now locating gamma ray flashes and their sources.

Insights into the history of the universe

Gamma radiation that the HERMES satellites are designed to detect represents only part of this phenomenon. Cosmic events and their aftermath also release other electromagnetic waves: UV and infrared radiation, visible light, and radio waves. To detect them, telescopes are needed that are specialized in specific frequencies.

Another type of wave that does not belong to the electromagnetic spectrum was first detected in 2016: Gravitational Waves. GWs are generated when extremely massive stars or black holes merge. They move at the speed of light through the cosmos, warping the space they traverse. Multi-messenger astronomy aims to capture such diverse physical information through a network of specialized telescopes, identifying them as puzzle pieces of the same event, and assembling them.

Combining different methods and data promises new insights into the formation and history of the universe. For example, questions exploring the origins of the chemical elements or the speed at which the universe is expanding. Researchers at the institute specialize in one of the trickiest puzzles of cosmology, as Guzmán explains: “The special theory of relativity postulates that all observers in a vacuum measure the same speed of light, regardless of the wavelength of the light, which is related to the notion that space is a continuum.”

If space is divided into units on very small scales, it is conceivable that light could propagate slightly faster or slower in this lattice depending on its wavelength. The differences are far too small to be detected on Earth. However, by observing light rays in the cosmos over a very long time and distance, scientists can measure whether photons really move at different speeds. “Our institute is among the first to try this. The observation of gamma rays through our satellite constellation is excellently suited for this purpose,” says Guzmán.

Text: Wolfgang Krischke