Researchers from the High-Altitude Water Cherenkov Gamma Ray Observatory (HAWC), including Miguel Mostafá, dean of the College of Science and Technology, have made a groundbreaking discovery of very high energy (VHE) gamma-ray emissions from the V4641 Sgr microquasar.

A binary star system that consists of a normal star and a compact object, such as a black hole or neutron star, that draws material from the companion star, microquasars are particularly notable for their powerful jets, which are ejected from the system at nearly the speed of light, resembling the jets found in distant quasars but on a much smaller scale. The findings, published in Nature, shed new light on particle acceleration in large-scale jets emitted from microquasars.

Observations of VHE gamma rays are crucial for understanding the most extreme physical processes in microquasars. “Studying microquasars in the VHE range enhances our understanding of high-energy astrophysical phenomena such as the mechanisms behind jet formation, particle acceleration, and the interaction of these jets with surrounding material,” said Mostafá, who joined the HAWC collaboration in 2009.

The findings show that V4641 Sgr’s gamma-ray emissions occur at similar distances from the black hole as those previously observed by HAWC in another well-known microquasar, SS 433. This makes V4641 Sgr stand out for its super-Eddington accretion—a process that supports rapid black-hole growth—and one of the fastest superluminal jets in the Milky Way.

“These results indicate that the gamma rays are likely produced by high energy protons, which has profound implications for the identification of the highest energy sources of galactic cosmic rays,” explained Mostafá. “These observations open new avenues for understanding particle acceleration in extreme environments and contribute to the broader study of high-energy astrophysics.”

To record the particles created in cosmic-ray and gamma-ray air showers, the HAWC detector uses the water Cherenkov method. In this technique, the detector is used to sample air-shower particles at ground level by recording the Cherenkov light—a shockwave of light that appears as a blue glow—produced when the particles pass through tanks full of purified water.

Mostafá joined the HAWC collaboration in 2009 and participated in the proposal for the construction of the experiment. Completed in 2015, The HAWC Observatory is located on the flanks of the Sierra Negra volcano near Puebla, Mexico, at an altitude of approximately 14,000 feet. Mostafá and his group made significant contributions to the design and operation of the water Cherenkov detectors as well as the deployment of the 300 plastic bladders that hold the water inside the steel tanks.