X-ray Imaging and Spectroscopy Mission (XRISM) studies the hot, energetic universe by capturing X-rays to reveal details about galaxy clusters, black holes, supernova remnants and the formation of cosmic structures.
Courtesy NASA
Scientists search for “decaying” dark matter (DDM) because it offers unique signatures like specific X-ray or gamma-ray lines or neutrino signals not seen in normal matter, potentially revealing dark matter's particle nature, mass and interactions, information that could illuminate the universe's structure. DDM is a theoretical model where dark matter particles aren't perfectly stable, but slowly decay over vast cosmic timescales into lighter dark matter particles and/or massless particles, leaving behind gravitational or electromagnetic signals. Now a study published in the Astrophysical Journal Letters demonstrates this form of dark matter can potentially be detected in unidentified X-ray emission lines in the spectra of galaxy clusters.
“Eighty-five percent of mass in galaxy clusters come from dark matter, and we can model the dark matter radial distribution well,” notes Dr. Ming Sun, a professor in the College of Science at The University of Alabama in Huntsville (UAH), a part of The University of Alabama System, and the corresponding author on the project. “Thus, galaxy clusters are great targets for such a search as they are dark matter rich and we know the dark matter mass in clusters well.”
Dr. Sun’s postdoctoral student, Prathamesh Tamhane, was also involved in the work, which follows-up on a 2014 study led by UAH alumna Dr. Esra Bulbul, who now serves as the lead scientist for cluster science and cosmology at the Max Planck Institute.
Dr. Ming Sun, a professor in the College of Science at The University of Alabama in Huntsville (UAH).
Michael Mercier | UAH
X-ray emission lines are unique fingerprints of elements that appear as peaks in an X-ray spectrum when electrons drop from higher to lower energy shells in an atom, releasing energy as X-ray photons. These distinct spectral lines reveal the presence of heavy elements like iron, silicon and oxygen ejected from galaxies, allowing astronomers to map elemental abundances and measure gas temperatures and densities, all helping to understand the complex physics of these massive structures.
An unidentified X-ray emission line found at approximately 3.5 kiloelectron volts (keV) in the spectra of galaxy clusters has been subject to intense scientific debate as a persistent astronomical anomaly. Scientists have traditionally used Charge-Coupled Devices (CCDs) – light-sensitive semiconductor chips – to detect the faint tracks of ionizing particles like heavy ions or neutrinos in these spectra, allowing them to observe particle paths in attempting to resolve this “unidentified” emission line.
Researchers for the new study relied instead on data collected from the X-ray Imaging and Spectroscopy Mission (XRISM), a collaborative space telescope developed by JAXA (Japan) and NASA, with European Space Agency (ESA) support.
“Nearly all the past studies used the CCD data, which lacks the required energy resolution to resolve the unidentified line,” Sun explains. “Now XRISM provides high-energy-resolution spectra that can resolve the line. As the line signals are very weak, we combined nearly three months of the XRISM data for such a search. There are many X-ray lines detected. They originate from known atoms, such as iron, silicon, sulfur and nickel. X-ray emission lines that appear that are not at the known position of atomic lines are then the candidates for DM decay lines, which is the focus of this work.”
The leading candidate for the mysterious emission line is a particle called a “sterile” neutrino. Neutrinos are tiny, nearly massless, subatomic particles that travel near the speed of light and barely interact with normal matter, crucial to understanding the universe, despite being incredibly hard to detect.
“A sterile neutrino is a hypothetical type of neutrino that only interacts with other particles via gravity, unlike the three known ‘active’ neutrinos that also interact via the weak force,” Sun notes. “The existence of the sterile neutrino is well-motivated theoretically and can explain the very small but non-zero mass of regular neutrinos. Sterile neutrinos can decay into two photons with the same energy. Models can predict the decay rate of sterile neutrinos, which is then constrained from the data.”
In considering the future of this type of research, Weakly Interacting Massive Particles, or WIMPS – hypothetical particles that are massive but only interact via gravity and the weak nuclear force – are still considered one of the most likely places where dark matter could be hiding. But Sun is quick to note that investigating alternate possibilities remains crucial to solving the mystery.
“WIMPs are still the leading candidate for dark matter, but billions of dollars of experiments have been done, only getting stronger and stronger upper limits, so alternative scenarios have to be considered. This study provides the strongest limits from high-energy-resolution data on the sterile neutrino at the 5 - 30 keV band, subsequently limiting the models for dark matter,” the UAH researcher concludes. “With more XRISM data in the next 5-10 years or so, we will be able to either detect the line or improve the limit substantially.”