Since its discovery in 2012, a glowing blob known as a “cocoon” that appears to be located in a giant gamma-ray emission from the center of the Milky Way, known as a “Fermi bubble,” has puzzled astronomers.
published in natural astronomy, we show that this cocoon is caused by gamma rays emitted by rapidly rotating extreme stars known as “millisecond pulsars” in the Sagittarius dwarf galaxy orbiting the Milky Way. While our findings demystify the cocoon, they also cast a shadow over attempts to find dark matter in any gamma-ray glow it might emit.
Observation with gamma rays
Thanks to life on Earth, our atmosphere blocks gamma rays. These light particles are more than a million times more energetic than the photons we detect with our eyes.
Because our ground-based view is blurred, scientists don’t know the richness of the gamma-ray sky until instruments are sent into space. But more and more of this abundance has been revealed, starting with the serendipitous discovery of the Vela satellite, which was put into orbit in the 1960s to monitor the nuclear test ban.
The most advanced gamma-ray instrument in operation today is the Fermi Gamma-ray Space Telescope, a large NASA mission in orbit for more than a decade. Fermi’s ability to resolve fine details and detect faint sources has revealed many surprises in our galaxy and the wider universe.
One of the surprises came in 2010, shortly after Fermi’s launch: something at the center of the Milky Way was blowing what looked like a pair of giant gamma-ray-emitting bubbles. These completely unexpected “Fermi bubbles” cover 10% of the sky.
The prime suspect for the source of the bubble is the Milky Way’s resident supermassive black hole. This behemoth, 4 million times larger than the sun, lurks in the heart of the Milky Way, where the bubbles originated.
Most galaxies have such massive black holes at their centers. In some cases, these black holes are actively gobbling up matter. So fed, they simultaneously spew huge, outflowing “jets” visible on the electromagnetic spectrum.
So after discovering the bubbles, the researchers asked a question: Can we find a handful of conclusive evidence linking them to our galaxy’s supermassive black hole? Soon, tentative evidence did emerge: Inside each bubble, hinted at a thin jet of gamma-rays directed toward the center of the galaxy.
However, with time and further data, the picture became blurred. While a jet-like feature in one of the bubbles was confirmed, the apparent jet in the other bubble seemed to evaporate on closer inspection.
The bubbles look oddly lopsided: one contains a slender bright spot — a “cocoon” — while the other has no counterpart in it.
Cocoon and its source
Our most recent work natural astronomy It is a profound investigation of the essence of “cocoon”. Remarkably, we found that this structure is not related to Fermi bubbles or even the supermassive black hole of the Milky Way.
Instead, we found that the cocoon was actually something else entirely: gamma rays from the Sagittarius dwarf galaxy, which, from Earth’s position, lies just behind the southern bubble.
The Sagittarius dwarf, named for its location in the sky in the constellation Sagittarius, is a “satellite” galaxy orbiting the Milky Way. It is the remnant of a larger galaxy torn apart by the Milky Way’s powerful gravitational field. In fact, stars pulled from Sagittarius dwarfs can be found in “tails” that surround the entire sky.
What produces gamma rays?
In the Milky Way, the main source of gamma rays is the collision of very thin gas between high-energy particles (called cosmic rays) and stars.
However, this process cannot explain the gamma rays emitted by the Sagittarius dwarf. It lost its gas long ago in the same gravitational disturbance that also pulled away many of its stars.
So where do gamma rays come from?
We considered several possibilities, including the exciting prospect that they are a hallmark of dark matter, the invisible matter known only through its gravitational effects that astronomers believe make up most of the universe. Unfortunately, the shape of the cocoon closely matches the distribution of visible stars, ruling out an origin for dark matter.
Either way, stars are responsible for gamma rays. However: the stars of the Sagittarius dwarf are ancient and stationary. In such a population, what type of source would produce gamma rays?
We’re happy there’s only one possibility: rapidly spinning objects called “millisecond pulsars.” These are the remnants of a particular star, much more massive than the Sun, that also closely orbit another star.
Under the right circumstances, such a binary star system would produce a neutron star — an object as heavy as the sun but only about 20 kilometers in diameter — that spins hundreds of times per second.
Because of their fast rotation and strong magnetic fields, these neutron stars act as natural particle accelerators: they fire particles into space at extremely high energies.
These particles then emit gamma rays. We found that millisecond pulsars in Sagittarius dwarfs are the ultimate source of the mysterious cocoon.
search for dark matter
Our findings shed new light on millisecond pulsars as sources of gamma rays in other older star systems.
At the same time, they cast a shadow over efforts to find evidence of dark matter through observations of other satellite galaxies in the Milky Way. Unfortunately, the gamma-ray “background” from millisecond pulsars in these systems is stronger than previously realized.
Therefore, any signals they produce may not be unequivocally explained as being caused by dark matter.
The search for dark matter signals continues.
Rotating stars offer new clues to strange signal from Milky Way’s center
Roland M. Crocker et al., Gamma-ray emission from millisecond pulsars in Sagittarius dwarf globular galaxies, natural astronomy (2022). DOI: 10.1038/s41550-022-01777-x
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