A nearby microquasar within our galaxy has been discovered to emit gamma rays with unprecedented energy levels, reaching up to 200 teraelectronvolts. The discovery, made using data from the HAWC observatory, challenges previous assumptions that only supernova remnants or quasars in distant galaxies could produce such energetic particles.
Near-Earth Microquasar V4641 Sgr Emits Unprecedented Gamma Radiation, Reshaping Cosmic Ray Research
Astronomers have uncovered a startling source of high-energy gamma radiation much closer to home than previously believed.
The microquasar V4641 Sagittarii (V4641 Sgr), located within the Milky Way, has been found to emit gamma photons with energies reaching up to 200 teraelectronvolts (TeV)—an amount of energy that challenges traditional models of cosmic ray production.
The discovery, made through observations from the High-Altitude Water Cherenkov (HAWC) Observatory, is forcing scientists to reconsider how the most energetic particles in the universe are generated, shifting the focus from distant galaxies to objects within our own cosmic "backyard."
Microquasars: A New Type of Cosmic Particle Accelerator
For decades, astrophysicists assumed that the most powerful sources of cosmic rays—high-energy particles traveling through space—originated from supernova remnants or the jets emitted by quasars located in the centers of distant galaxies. Quasars, with their supermassive black holes surrounded by vast accretion disks, shoot out jets of matter moving at close to the speed of light, producing gamma radiation. It was thought that these far-off behemoths were responsible for accelerating particles to the highest known energies.
However, the recent discovery involving microquasars, particularly V4641 Sagittarii, suggests otherwise. Microquasars, unlike their distant relatives, are compact binary systems that consist of a massive star and a stellar-mass black hole. As the black hole siphons material from its companion, jets are ejected at high speeds, which, according to the HAWC data, are capable of producing radiation with energies far exceeding expectations. Dr. Sabrina Casanova from the Institute of Nuclear Physics of the Polish Academy of Sciences, a key researcher in the project, emphasized the significance of this finding: “Photons detected from microquasars have usually much lower energies than those from quasars... Meanwhile, we have observed something quite incredible in the data recorded by the detectors of the HAWC observatory: photons coming from a microquasar lying in our galaxy and yet carrying energies tens of thousands of times higher than typical!”
The HAWC Observatory, located on the Sierra Negra volcano in Mexico, uses an array of 300 water tanks to detect Cherenkov radiation—the faint flashes of light that occur when particles move faster than the speed of light in water. This setup allows HAWC to observe gamma photons with energies ranging from hundreds of gigaelectronvolts to the teraelectronvolt scale, providing unprecedented insight into the workings of microquasars like V4641 Sgr.
V4641 Sagittarii: A Microquasar with Extraordinary Jets
V4641 Sagittarii, located in the constellation Sagittarius, approximately 20,000 light years from Earth, is composed of a black hole with a mass about six times that of the Sun, and a companion star with three times the solar mass. The pair orbit each other once every three days, a rapid cycle that fuels the powerful outflows of matter observed from the system. What makes V4641 Sgr particularly notable is the orientation of its jets, which are aimed almost directly at Earth. This results in relativistic effects that make the jets appear to move faster than the speed of light, at a staggering nine times the speed of light, due to an illusion caused by their high velocity and direction toward the observer.
The discovery of such ultra-high-energy gamma rays from V4641 Sgr is transformative. While scientists had previously detected gamma radiation from microquasars, the levels observed in this case are far beyond anything previously recorded. “It therefore seems likely that microquasars significantly contribute to the cosmic ray radiation at the highest energies in our galaxy,” Dr. Casanova added, highlighting the profound implications of this discovery for understanding the origins of cosmic rays.
In fact, the observed gamma rays from V4641 Sgr are so energetic that they challenge the long-held belief that the highest-energy cosmic rays are produced exclusively by far-off sources like quasars or supernovae. Instead, this discovery points to a powerful source of radiation much closer to home, providing a rare opportunity to study these phenomena in real time.
Changing the Landscape of Cosmic Ray Research
The findings from the HAWC Observatory have broader implications for the study of cosmic rays. The Large High Altitude Air Shower Observatory (LHAASO) in China has also detected high-energy radiation from other microquasars, supporting the idea that these compact systems may play a much larger role in the generation of cosmic rays than previously understood. If this is the case, the way scientists approach the study of cosmic ray production and the mechanisms that drive these high-energy processes may need to be fundamentally reevaluated.
One of the key advantages of studying microquasars over distant quasars is that their proximity allows for much clearer observations. Unlike radiation from quasars, which must travel across millions of light years and through vast stretches of space where it can be absorbed or scattered, radiation from microquasars in our own galaxy faces fewer obstacles. As a result, scientists can study the processes that drive ultra-high-energy particle acceleration in greater detail, potentially uncovering new insights into the physics of jets, black holes, and cosmic rays.
Moreover, the time scales on which microquasars evolve are significantly shorter than those of quasars. While quasars take millions of years to change, the jets from microquasars can be observed over periods of days, making them ideal subjects for studying high-energy astrophysical processes in real time.
Dr. Casanova and her colleagues’ research, published in Nature, represents a significant step forward in understanding these energetic astrophysical systems. As more data are collected from observatories like HAWC and LHAASO, astronomers are likely to uncover even more about how microquasars contribute to the overall population of cosmic rays—an endeavor that could reshape our understanding of the high-energy universe.