Written by Roland Crocker for Conversation
A glowing bubble known as a “cocoon,” which appears to be inside one of the massive gamma-ray emission from the center of our galaxy dubbed “Fermi bubbles,” has baffled astronomers since its discovery in 2012.
In a new research published in natural astronomy, we show that the cocoon is caused by gamma rays emitted by fast-rotating extreme stars called “millisecond pulsars” found in the Sagittarius dwarf galaxy, which orbit the Milky Way. While our results clarify the mystery of the cocoon, they overshadow attempts to search for dark matter in any gamma-ray glow it might emit.
vision with gamma rays
Fortunately for life on Earth, our atmosphere blocks gamma rays. These are particles of light with energies a million times higher than the photons that we detect with our eyes.
Because our ground-level view is obscured, scientists had no idea the richness of the gamma-ray sky until the instruments were lifted into space. But, starting with the surprising discoveries of the Vela satellites (which entered orbit in the 1960s to monitor the nuclear test ban), more and more of this richness has been revealed.
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The most modern gamma-ray instrument in operation today is the Fermi-ray Gamma Space Telescope, a major mission for NASA that has been in orbit for more than a decade. Fermi’s ability to resolve fine details and discover faint sources has revealed a number of surprises about our Milky Way and the wider universe.
One such surprise popped up in 2010, shortly after the Fermi launch: something in the center of the Milky Way blasted what looked like a pair of giant gamma-ray-emitting bubbles. The most unexpected “Fermi bubbles” cover 10 percent of the sky.
The prime suspect in the bubbles’ source is the galaxy’s resident supermassive black hole. This giant, four million times more massive than the Sun, lurks in the galactic core, the region from which bubbles pop.
Most galaxies host such giant black holes at their centers. Sometimes these black holes are actively sucking in matter. And so they are fed, simultaneously spewing out giant “jet” flowing outward visible across the electromagnetic spectrum.
And so the researchers asked a question after discovering the bubbles: Can we find a smoke hexagon that connects them to the supermassive black hole of our galaxy? Soon, tentative evidence emerged: inside each bubble there was a hint of a thin jet of gamma rays pointing back toward the galactic center.
But with time and more data, this picture has become muddled. While the jet-like feature was confirmed in one of the bubbles, the apparent jet in the other appeared to evaporate under scrutiny.
The bubbles seemed oddly unbalanced: one contained an elongated luminous spot – the “cocoon” – with no counterpart in the other.
The cocoon and where did it come from?
Our last work in natural astronomy It is a deep examination of the nature of the “cocoon”. Remarkably, we found that this structure has nothing to do with the Fermi bubbles or, in fact, with the galaxy’s supermassive black hole.
Instead, we found that the cocoon is actually something else entirely: gamma rays from the Sagittarius dwarf galaxy, which just so happens to be behind the Southern Bubble as seen from Earth’s location.
The Sagittarius dwarf is so named because its location in the sky is in the constellation Sagittarius, which is a “satellite” galaxy orbiting the Milky Way. It is a remnant of a galaxy so much larger that the Milky Way’s powerful gravitational field has literally been torn apart. In fact, stars pulled from a Sagittarius dwarf can be found in “tails” that wrap around the entire sky.
What makes gamma rays?
In the Milky Way, the main source of gamma rays is when high-energy particles, called cosmic rays, collide with the very weak interstellar gas.
However, this process cannot explain the gamma rays emitted by the Sagittarius dwarf. She has long since lost her gas to the same oomph annoyances that have drawn so many of her stars.
So where do gamma rays come from?
We examined several possibilities, including the intriguing possibility that it is a sign of dark matter, the invisible matter known only through gravitational influences that astronomers believe makes up a large part of the universe. Unfortunately, the shape of the cocoon closely matches the distribution of visible stars, ruling out dark matter as an origin.
One way or another, the stars were responsible for the gamma rays. However, Sagittarius dwarf stars are old and quiet. What type of source produces gamma rays among this population?
We are satisfied that there is only one possibility: rapidly rotating objects called “millisecond pulsars”. These are the remnants of certain stars, much larger than the Sun, which also closely orbit another star.
Under just the right conditions, such binary systems produce a neutron star – an object as heavy as the Sun but only about 20 km wide – that rotates hundreds of times per second.
Because of their fast rotation and strong magnetic field, these neutron stars act as natural particle accelerators: they shoot particles with extremely high energy into space.
These particles then emit gamma rays. We found that millisecond pulsars in the Sagittarius dwarf were the ultimate source of the mysterious cocoon.
Searching for dark matter
Our findings shed new light – pun intended – on millisecond pulsars as gamma-ray sources in other ancient star systems.
At the same time, they cast a shadow over efforts to find evidence of dark matter through observations of other satellite galaxies of the Milky Way. Unfortunately, there is a gamma-ray “background” of millisecond pulsars in these systems that is stronger than previously realized.
Thus, any signal they emit may not be unambiguously interpreted as being caused by dark matter.
The search for dark matter signals continues.
(The author is an associate professor at the Australian National University)