Binary star system found in the Milky Way is the new cosmic ray monster
A pair of objects 20,000 light‑years away has just broken the energy ceiling for light inside our galaxy. These twin bodies (a massive star and a black hole), known as a microquasar, are generating cosmic rays containing photons with more energy than any previously seen from within the Milky Way.
The binary system V4641 Sagittarii has fired photons above 200 teraelectronvolts (TeV), turning a once‑obscure star‑black‑hole partnership into a new “cosmic ray monster.”
Dr. Sabrina Casanova of the Institute of Nuclear Physics of the Polish Academy of Sciences helped spot the outburst.
Understanding microquasars – the basics
Stellar systems that mimic the behavior of quasars on a much smaller scale are known as microquasars. They consist of a normal star orbiting a compact object, usually a black hole or neutron star, which pulls matter from its companion.
As the material spirals inward, it forms an accretion disk that heats up and emits intense X-rays. At the same time, twin jets of high-energy particles shoot out from the poles at nearly the speed of light.
This jet activity, driven by the interplay of magnetic fields and extreme gravity, resembles what we see in distant quasars, but within our own galaxy.
Astronomers value microquasars because they offer a close-up view of processes that govern much larger and more distant cosmic phenomena.
Observations of their rapid changes in brightness and jet structure provide insight into the mechanics of black hole accretion and relativistic jet formation.
Unlike quasars, which exist in distant galaxies and evolve over millions of years, microquasars can change on timescales of days or hours.
Microquasars and cosmic rays
V4641 Sagittarii sits in the direction of the Sagittarius constellation but, unlike distant quasars, it belongs to the Milky Way.
Because this microquasar – a stellar‑mass black hole that is siphoning gas from a companion star – lies thousands, not millions, of light‑years from Earth, telescopes can track its jets in real time.
High‑energy jets from such local sources let researchers watch particle factories evolve over days instead of over geological time spans.
That immediacy gives fresh leverage for testing models that were built largely on blurred views of far‑off active galactic nuclei. It also forces a rethink of where the most energetic charged particles in our galaxy actually come from.
Microquasars are small but mighty
The system’s black hole weighs the same as about six Suns, while the donor star tips the scale at roughly three. Every 2.8 days they swing around a common center of mass, carving spiral streams of gas that feed an accretion disk no wider than the orbit of Mercury.
Magnetic fields near the hole’s poles launch twin plasma jets that are hundreds of light‑years long; one points almost straight toward us, producing apparent super‑luminal motion nine times faster than light.
“We have observed something quite incredible … photons coming from a microquasar lying in our galaxy and yet carrying energies tens of thousands of times higher than typical!” exclaimed Dr. Casanova.
Until now, photons from microquasars rarely topped a few tens of gigaelectronvolts.
That jump means that V4641 Sagittarii can accelerate particles to within a factor of two of the energies reached at the Large Hadron Collider, but without the need for an underground ring.
HAWC caught the monster cosmic rays
The High‑Altitude Water Cherenkov Observatory (HAWC) in Mexico uses 300 steel tanks filled with clear water and instrumented with four photomultiplier tubes apiece.
When a gamma ray from space strikes the upper atmosphere it triggers a cascade of particles that race through the tanks faster than light can traverse water, producing the familiar blue Cherenkov radiation.
HAWC records roughly 25,000 air showers every second and reconstructs their direction with degree‑scale precision.
For V4641 Sagittarii, repeated passes over the zenith added up to a clear, persistent signal extending past 200 TeV. Those energies dwarf the trillion‑electron‑volt photons that once defined the upper end of the gamma‑ray spectrum.
What pushes the energy ceiling
At 200 TeV, electrons would cool too fast to survive the trip down the jet, implying that protons, or even heavier ions carry the load.
Shock fronts inside the outflow likely operate like natural synchrotrons, flinging charged particles back and forth across magnetic discontinuities until they outrun energy losses.
Similar physics was glimpsed in SS 433, another microquasar whose lobes shine at tens of TeV.
But V4641 Sagittarii’s harder spectrum suggests even more efficient acceleration, perhaps boosted by its nearly face‑on jet orientation.
Ultra‑energetic protons can collide with local gas or starlight near the jet, spawning neutral pions that promptly decay into the detected gamma rays.
Fresh look at cosmic rays
For over a century, supernova remnants were cast as the main producers of galactic cosmic rays, a narrative launched when Victor Hess rode a balloon to three miles and found ionizing radiation rising with altitude.
The HAWC result shows that compact binaries can also fill the high‑energy end of the spectrum, joining supernova shocks and pulsar wind nebulae in the accelerator club.
Support comes from China’s Large High Altitude Air Shower Observatory, which has begun spotting 100 TeV photons from other microquasars.
If many such systems point their jets even partly toward Earth, their combined contribution could rival that of supernovae above 10 TeV.
That possibility reshapes strategies for interpreting cosmic‑ray anisotropies measured by detectors on balloons, satellites, and Arctic ice.
HAWC continues to monitor V4641 Sagittarii, looking for day‑to‑day changes that would tie gamma‑ray flashes to radio flares seen by interferometers.
Upcoming instruments like the Cherenkov Telescope Array will sharpen images to arc‑minute scales and hunt for corresponding neutrinos that would clinch the hadronic scenario.
The findings also motivate renewed optical vigilance, since brightening episodes could signal fresh accretion spurts that reset jet conditions.
The study is published in Nature.
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