When postdoctoral scholar Xiaojie Wang of the Michigan Technological University Physics Department went looking for her next research topic, she found a previously unexplored region and a path to publication.
Wang is lead corresponding author of the article, “Ultra-High-Energy Gamma-Ray Bubble around Microquasar V4641 Sgr” recently published in Nature journal. The findings highlighted in the article offer new insights into how microquasars might contribute to the cosmic-ray energy spectrum—a long-standing puzzle in astrophysics.
“While reviewing the sky maps in search of my next project, I noticed a region five degrees away from our galactic plane with bright emissions that had not been visible in previous datasets,” said Wang, who works with Petra Huentemeyer, a distinguished professor of physics at Michigan Tech. “No gamma-ray source has been identified nor analyzed in this region—so I seized the opportunity and led the analysis.”
Huentemeyer’s group focuses on high-energy astrophysical phenomena and low-level data analysis at the High Altitude Water Cherenkov Gamma-ray Observatory (HAWC). The map that caught Wang’s attention was produced by Dezhi Huang, a ’23 Michigan Tech alumnus and previous graduate student in Huentemeyer’s group. Huang was responsible for producing the maps from newly available datasets. Now a postdoc at the University of Maryland, Dezhi continues to work closely with HAWC.
“I shared the discovery with Dezhi and Dr. Huentemeyer, and we were all excited about the potential significance of this new source, especially since no other gamma-ray instruments had reported it. This made the project both groundbreaking and challenging,” said Wang.
“Dr. Huentemeyer has been very supportive, and with the guidance of Dezhi’s current supervisor, Dr. Jordan Goodman, they have contributed greatly to the project’s success,” said Wang. “My role has been central in analyzing the HAWC data, interpreting the results, and collaborating with colleagues from UM-Madison and the Institute of Nuclear Physics Polish Academy of Sciences. Together, we’ve worked to better understand the high-energy emissions from this fascinating source. This project provides new insights into particle acceleration mechanisms in binary systems, and I’m thrilled to have played a key role in this groundbreaking research.”
High-Energy Emissions from a Newly Sighted Microquasar
Wang said the most exciting aspect of the results is the discovery of ultra-high-energy gamma-ray emissions from the microquasar V4641 Sgr. “Microquasars are systems where a black hole or neutron star is pulling material from a nearby star, creating intense radiation and shooting out jets of particles at nearly the speed of light. They’re smaller versions of quasars but still incredibly powerful,” Wang explained. “This is something no other gamma-ray instruments had detected before and marks a major leap in our understanding, as V4641 Sgr is now the first microquasar to show emissions above 200 TeV.” TeV is the abbreviation for teraelectronvolts, a unit of energy equal to one trillion electron volts.
Wang said the discovery pushes the boundaries of what is known about particle acceleration in such systems. “The fact that we detected this using HAWC’s unique capabilities—its wide field of view and continuous sky monitoring—raises new questions about how particles are accelerated in extreme environments like this one,” she said.
Wang said a peanut-shaped emission region researchers observed was another surprising aspect of the findings. “When we analyzed the two parts of this shape, we found nearly identical energy spectra, which strongly suggests they share a common origin—likely the jets or lobes of the microquasar. The photon energies are incredibly high, exceeding 200 TeV, challenging existing models of particle acceleration in microquasars and opening new possibilities for understanding these systems.”
The most current discovery follows previous revelations, including HAWC’s detection of the first discovered microquasar, SS 433, in 2018. That discovery was also led by a Michigan Tech physics research group.
Tracing the Trajectory of the Study
The study started with data from HAWC, the gamma-ray observatory that continuously scans the sky to pick up signals from objects in space, including cosmic rays. When the rays reach Earth’s atmosphere they collide with particles in the atmosphere and create showers of smaller particles known as EAS, or extensive air showers. “Our detectors—large tanks filled with pure water—capture the “Cherenkov lights” produced by these particle showers,” said Wang. “By recording the time and charges, and using advanced techniques like neural networks, we can figure out the type, direction, and energy of the original particles. Once all this is done, tools developed by our team at HAWC help create sky maps, showing where the signals came from and how strong they are. The people responsible for making these maps in HAWC are called map-makers, and Dezhi is one of them.”
Discovering a region in the sky where no gamma-ray studies had been conducted was both exhilarating and challenging. “I applied different statistical models to get a better understanding of the region, estimating how powerful the signal might be. I also spent a significant amount of time searching for possible counterparts—other objects or signals detected in wavelengths like radio, X-ray, and infrared. After carefully reviewing all available data, the microquasar V4641 Sgr emerged as the most likely source responsible for the emissions,” said Wang.
She noted that collaboration was crucial throughout the process. Researchers worked closely with theoretical experts including Huang, Brenda Dingus and Goodman, from the University of Maryland; Ke Fang from the University of Wisconsin-Madison; and Sabrina Casanova from the Institute of Nuclear Physics of the Polish Academy of Sciences sharing data and insights to refine their interpretations.
Wang was recently a guest on the Nature podcast. Listen to the episode “Star-eating black hole could power cosmic particle accelerator.”
“Our findings highlight how collaborative efforts and cutting-edge observational tools can push the frontiers of astrophysics,” said Wang. “But we definitely encountered a few surprises and challenges along the way.”
One of the biggest surprises was the extreme energy of the gamma-ray emissions from the microquasar. Wang said it forced researchers to rethink their understanding of how particles are accelerated and transported in such systems. “On the challenge side, the complexity of the emission region was a major obstacle,” she said. Researchers needed more data in order to confidently determine the best-fit statistical model for the peanut-shaped emission. “The HAWC outrigger array—an upgrade to the current detector—or the future SWGO observatory could provide the additional data needed.”
Impact of the Findings Now and In the Future
“Our findings contribute a new piece to the cosmic-ray puzzle, offering valuable insights into how particles are energized and transported across vast distances,” said Wang. “Understanding these processes is crucial not only for advancing astrophysics but also for unraveling the origins of cosmic rays, which have intrigued scientists for over a century.
“While we don’t see immediate real-world applications, the discoveries we make could have long-term and unforeseen impacts. Often, breakthroughs in basic science lead to technological innovations in surprising and unpredictable ways.”
Next steps for the research include more detailed physical modeling of V4641 Sgr’s emissions, using multi-wavelength data from X-ray, radio, and gamma-ray observatories. “I successfully proposed follow-up observations with the APEX radio telescope in Chile and am now working on another proposal for follow-up observations using NASA and ESA’s X-ray instruments. We also plan to analyze more extensive observation data with the HAWC outrigger array,” said Wang. “These efforts will help refine our understanding of the source’s emission mechanisms and bring us closer to answering long-standing questions about the origins of the universe’s highest-energy particles.”
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