This story is adapted from the HAWC Collaboration press release.
Microquasars—compact regions surrounding a black hole with a mass several times that of its companion star—have long been recognized as powerful particle accelerators within our galaxy. The enormous jets spewing out of microquasars are thought to play an important role in the production of galactic cosmic rays, although their contribution to the galactic cosmic-ray flux is still unknown.
In a new paper in the journal Nature, the High-Altitude Water Cherenkov Gamma-ray Observatory (HAWC) Collaboration reports the first detection of ultra-high-energy gamma rays from the microquasar V4641 Sagittarii (V4641 Sgr), with photon energies exceeding 200 teraelectronvolts (TeV).
“The HAWC observation established this microquasar, V4641Sgr, as the second galactic source with a set of large-scale jets,” says Ke Fang, an assistant professor of physics at the Wisconsin IceCube Particle Astrophysics Center (WIPAC) at the University of Wisconsin–Madison and US spokesperson for HAWC. “It suggests that mini black holes in our galaxy may power jets as large as a hundred parsec.”
Located near Puebla, Mexico, at an altitude of 13,500 feet, HAWC observes gamma rays and cosmic rays between 100 gigaelectronvolts (GeV) and a few hundred TeV. HAWC uses a network of 300 large water tanks, each filled with about 200 metric tons of water, that are nestled between two dormant volcano peaks.
HAWC detected an excess of gamma-ray emission coincident with the location of the microquasar V4641 Sgr in the Sagittarius constellation. The study revealed that microquasars like V4641Sgr could significantly contribute to the galactic cosmic ray spectrum up to petaelectronvolt energies. This discovery also suggests that large-scale jets from microquasars may be more common than previously believed, reinforcing their role as smaller analogs of active galactic nuclei.
The origin of the detected gamma rays may stem from proton interactions with ambient gas or from electrons up-scattering photons, although the leptonic scenario is challenged by their rapid energy loss at high energies. Additionally, the study shows the continuous advantages of wide-field observatories like HAWC in detecting large-scale gamma-ray emissions, providing new insights into particle acceleration in extreme environments.
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Currently, the international HAWC Collaboration consists of more than 30 institutions in the US, Mexico, Europe, Asia, and South America. The UW–Madison HAWC group at WIPAC, now led by Fang, has been involved with the HAWC experiment from the beginning of the design stage, through its construction, and into the data analysis phase. As part of her research, Fang uses data from HAWC to observe or find gamma-ray sources.