Scientists may have found evidence that vampire black holes feeding on their victim stars — so-called microquasars — are the cosmic particle accelerators responsible for mysterious high-energy cosmic rays we see bombarding Earth.
These stellar mass black holes exist in binary systems with a supergiant star from which they greedily strip material. Some of that stellar matter then gets channeled toward the poles of the black hole, where it is subsequently blasted out in high-speed relativistic jets. Microquasars get their name because they are similar to quasars, which are powered by giant supermassive black holes feeding on surrounding material, but not quite as extreme.
First discovered in 1912, cosmic rays can hit our planet with staggering energies reaching 10²⁰ electronvolts (eV). That's way more energetic than the particles accelerated at the Large Hardon Collider, which is Earth's largest and most powerful particle accelerator.
To that end, supernovas and microquasars have been suggested as our universe's powerful cosmic particle accelerators. Scientists therefore believe these phenomena could be responsible for those high-energy cosmic rays. But evidence of microquasars accelerating particles to such high energies has been scarce — until now, that is.
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The team made the connection between cosmic rays and microquasars when they used the High Energy Stereoscopic System (H.E.S.S.) to detect extremely high-energy gamma rays coming from the jets of the most powerful microquasar in the Milky Way. It's named SS 433.
These gamma rays are created when the jets of SS 433 slam into surrounding matter, creating a shock front that accelerates electrons to speeds that are great enough to account for the particles witnessed in high-energy cosmic rays.
"The acceleration mechanism would be similar to that in a supernova remnant, although the shocks in SS 433 jets are faster than supernova remnant shocks and can accelerate particles to higher energies," Valentí Bosch-Ramon, an associate professor at the University of Barcelona, wrote in a perspectives paper discussing the research published in Science. "The very energetic photons detected from the large-scale jets of SS 433 are an indirect indicator that these kinds of objects should not be neglected when seeking to explain the most energetic nuclei in Galactic cosmic rays."
The cosmic manatee
SS 433 was actually the first microquasar ever discovered; its existence was first revealed in 1975. It was named "SS 433" after being included in a 1977 catalog of celestial bodies, then rising to fame when science fiction author Arthur C. Clarke named it as one of his alternative "Seven Wonders of the World."
SS 433 sits at the heart of the supernova wreckage designated W50, located around 18,000 light-years from Earth and nicknamed the "manatee nebula." Decades of intense study have revealed SS 433 to consist of a black hole with a mass of around 10 to 15 times that of the sun, and a white supergiant star. The two are separated by about 15 million miles and orbit each other around once every 13 Earth days.
With no more than around a third of the distance between Mercury and the sun between the two occupants of SS 433, the black hole's immense gravity can strip the outer layers of its stellar companion. This stripped material forms an accretion disk around the black hole, while some of it is actually fed to the black hole. Other parts of the material get funneled to the black hole's poles via powerful magnetic fields. From there, the funneled material gets blasted out at around 26% of the speed of light.
These jets twirl in a corkscrew-like pattern and are so powerful that they even shape W50.
The W50 supernova wreckage was created when a massive star exploded some 20,000 years prior to how we see it today. The microquasar within has created two bulges or "humps" that cause W50 to take on the appearance of a vast cosmic manatee, hence its colorful nickname.
The jets of SS 433 can be seen in radio waves extending out for around 1 light-year from either side of their source. Eventually, they lose energy and dim to the point where they are no longer visible. Strangely, however, these relativistic jets abruptly reappear in high-energy X-ray light around 75 light-years from the microquasar's origin.
This indicates, the team says, that something within each jet is accelerating particles to even higher energies, and thus greater speeds than they possessed when they were blasted out from around the black hole.
Using the five telescopes in Namibia that comprise H.E.S.S., the scientists investigated these strange jets from SS 433 in gamma-ray light, finding that more energetic gamma rays originate further from the binary system.
The team found the best explanation for this would be that high-speed, shock-accelerated electrons scatter infrared particles of light and transform them into gamma rays.
The higher energy gamma-rays found away from the feeding black hole indicate two points, around 75 light years from the central binary of SS 433, where shocks are reshaping jets back into a tight column and giving the associated particles an energy boost.
This also explains the jets' reemergence in X-rays: accelerated electrons produce X-ray emissions.
"This is the first-ever observation of energy-dependent morphology in the gamma-ray emission of an astrophysical jet," Laura Olivera-Nieto, team leader and a scientist with the Max-Planck-Institut für Kernphysik, said in a statement. "We were initially puzzled by these findings. The concentration of such high energy photons at the sites of the X-ray jets' reappearance means efficient particle acceleration must be taking place there, which was not expected."
There are still puzzles surrounding this intriguing microquasar that the team will now endeavor to solve. This will include discovering what the jets are striking to create these shocks so far from the binary system that launched them.
"We still don't have a model that can uniformly explain all the properties of the jet, as no model has yet predicted this feature,” Olivera-Nieto said.
The team will also attempt to apply what they have learned about the jets of microquasars to jets emerging from more powerful supermassive black hole-powered quasars.
Additionally, while suggesting a source for high-energy cosmic rays, these findings don't yet close the book on the century-old cosmic mystery.
"SS 433 cannot be the source of the very energetic, peta–electronvolt, cosmic-ray protons detected on Earth because the source is too young for its particles to reach Earth once they have escaped the source," Bosch-Ramon wrote. "However, closer and longer-lived microquasars, even if weaker and individually harder to detect, could be contributing non-negligibly to local peta–electron volt cosmic rays."
The team's research was published on Thursday, Jan. 25, in the journal Science.
