For a long time, scientists believed the highest-energy gamma rays in the universe came from distant, supermassive black holes. But a recent discovery has pinpointed a source much closer to Earth: a black hole system called V4641 Sagittarii, only about 20,000 light-years away in the direction of the constellation Sagittarius.
In this system, a black hole about six times the mass of our Sun is pulling material from a nearby giant star. As it feeds, it releases incredible amounts of energy, similar to a cosmic particle accelerator, creating gamma rays with extraordinary power.
Astronomers have now measured photons—particles of light—from V4641 Sagittarii that carry a whopping 200 teraelectronvolts (TeV) of energy. For context, that’s 200 trillion times the energy of visible light, making these photons some of the most energetic particles observed in space.
High-energy gamma rays like these are common in the cosmos, but they were thought to come mainly from quasars—extremely bright centers of distant galaxies powered by supermassive black holes. However, V4641 Sagittarii is what’s known as a microquasar, a smaller but surprisingly powerful version of a quasar. In microquasars, the black hole and surrounding matter are much smaller, so scientists expected them to produce lower-energy photons.
“Usually, gamma rays from microquasars have energies only in the tens of gigaelectronvolts,” says researcher Sabrina Casanova from the Institute of Nuclear Physics Polish Academy of Sciences. “But data from the High-Altitude Water Cherenkov (HAWC) observatory show that photons from this microquasar have energy levels thousands of times higher.”
The HAWC observatory, located on the Sierra Negra volcano in Mexico, specializes in detecting high-energy particles from space. The observatory uses 300 large tanks filled with purified water to capture particles moving faster than light can travel through water. These particles produce Cherenkov radiation, a flash similar to a sonic boom but with light. Sensitive detectors inside each tank then record these flashes, helping scientists trace where the particles came from.
HAWC continuously scans a large portion of the sky, covering two-thirds of it every 24 hours and creating a “cosmic map.” It was in one of these maps that V4641 Sagittarii showed up as a bright, unexpected gamma-ray source.
While analyzing the sky map, physicist Xiaojie Wang noticed a new bright spot five degrees away from the Milky Way’s central plane. “No gamma-ray source had been noted in this area before, so I took the chance and led the investigation,” Wang says. This analysis revealed that V4641 Sagittarii was the surprising source of these ultra high-energy gamma rays.
Until now, only one other microquasar, SS 433, had been observed with gamma rays over 25 TeV. But V4641 Sagittarii’s energy levels are far higher, even rivaling the power of much larger quasars. This finding challenges earlier beliefs, showing that microquasars can produce gamma rays on par with those of quasars, something scientists didn’t think possible.
This discovery provides new insights not only into cosmic radiation but also into the physics of quasars. Massive quasars evolve slowly over millions of years, but microquasars, like V4641 Sagittarii, undergo similar processes on much shorter timescales. This makes them excellent “natural simulators” for studying cosmic phenomena in action.
The research was published in the journal Nature.
