SST-1M: Recent results and prospects for observation of the Galactic Center region

At the very heart of our Milky Way galaxy lies a region of intense activity, a crowded neighborhood filled with dense clouds of gas, the shattered remains of exploded stars, and massive clusters of young stars. In this chaotic environment, nature acts as a colossal particle accelerator, smashing matter together to create high-energy radiation that travels across the universe. Scientists are particularly interested in a specific type of this radiation called very-high-energy gamma rays. These are the most energetic form of light known, carrying millions of times more energy than the X-rays used in hospitals. When these gamma rays strike Earth's atmosphere, they do not reach the ground directly; instead, they create a brief, faint flash of blue light known as Cherenkov radiation. To study the cosmic engines that produce these rays, astronomers build special telescopes on the ground that act like giant mirrors, catching these fleeting flashes of light to reconstruct the origin and nature of the sources high above.

A team of researchers has been working with a new type of telescope designed specifically to catch these high-energy flashes. These instruments, known as Single-Mirror Small-Size Telescopes, are currently operating in the Czech Republic. While they have already proven their worth by observing famous cosmic objects like the Crab Nebula and distant active galaxies, their location in the northern hemisphere limits what they can see. The most important target for this kind of science, the center of our galaxy, sits too low in the southern sky to be observed clearly from the north. The team is now planning to move their telescopes to a new site in Argentina, where the Galactic Center is visible for much longer periods. By simulating what these telescopes would see from this new location, the researchers have shown that they will be able to map the complex structures of the galactic core and test whether the particles there are being accelerated to energies far beyond what any machine on Earth could ever achieve.

The team's work began by proving that their current setup in the Czech Republic works exactly as intended. Since 2023, they have operated two of these telescopes together, separated by a distance of about 155 meters, allowing them to view the sky in stereo, much like human eyes provide depth perception. They spent hundreds of hours watching the Crab Nebula, a well-known remnant of a supernova, and successfully measured its energy spectrum up to 100 TeV, a level of energy that is difficult for other instruments to reach. This observation confirmed that their telescopes can detect faint, extended sources of gamma rays with high precision. They also studied other regions of the sky, such as the Cygnus constellation, where they were able to separate two distinct sources of radiation that were previously blurred together. This ability to distinguish fine details in the sky is crucial because the center of our galaxy is a crowded place where many different sources overlap, making it hard to tell which object is producing which signal.

With this performance verified, the researchers turned their attention to the future. They used computer models, validated by their real-world data, to simulate what would happen if they moved their telescopes to Malargüe in Argentina. This new site sits at a higher altitude and offers a clear view of the Galactic Center for hundreds of hours each year. In their simulation, they imagined observing the central region of the galaxy for 250 hours, focusing on the area known as the "Ridge," a band of diffuse gamma-ray emission that stretches across the galactic core. The simulation showed that the telescopes would be able to clearly detect the bright point sources in the region, such as the supermassive black hole at the center, as well as the faint, widespread glow of the Ridge itself. Crucially, the simulation demonstrated that the instruments could separate the light from the black hole and other compact objects from the broad, diffuse background, a task that has proven difficult for other observatories due to the confusion of overlapping signals.

The most significant part of this simulation was testing a specific mystery: how high in energy do the particles in the Galactic Center go? Some theories suggest that the region contains "PeVatrons," natural accelerators capable of boosting particles to one million billion electron volts, or PeV energies. If this is true, the spectrum of gamma rays should show a sharp drop-off, or cutoff, at a specific high energy. The researchers simulated the data they would collect and analyzed it to see if they could spot this cutoff. Their results indicate that the telescopes would be able to detect a cutoff with a statistical significance of between two and four standard deviations, depending on the exact energy where the drop-off occurs. This means that while the data might not be a definitive, single-shot proof, it would provide strong evidence to confirm or rule out the existence of these extreme accelerators. The study suggests that the combination of a wide field of view and high sensitivity makes these telescopes uniquely suited to solve this puzzle.

Ultimately, this work highlights the potential of moving these instruments to the southern hemisphere to unlock the secrets of our galaxy's core. The researchers have shown that their telescopes are capable of delivering high-quality data even in less-than-ideal conditions, and their simulations confirm that a move to Argentina would allow them to map the morphology and energy spectra of the Galactic Center with unprecedented clarity. By distinguishing between different types of radiation sources and searching for the spectral cutoff that signals the presence of PeVatrons, the SST-1M collaboration aims to answer fundamental questions about how the universe accelerates particles to their highest possible energies. The path forward involves relocating the hardware to a site where the Galactic Center is a dominant feature of the night sky, turning a region that was previously difficult to study into a primary laboratory for high-energy astrophysics.